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Title: Problems in astrophysics
Author: Agnes M. Clerke
Release date: July 24, 2026 [eBook #79177]
Language: English
Original publication: London: A.&C. Black, 1903
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*** START OF THE PROJECT GUTENBERG EBOOK PROBLEMS IN ASTROPHYSICS ***
PROBLEMS IN ASTROPHYSICS
[Illustration:
PLATE I.
Photograph of the η Argûs Nebula. Taken by Sir David Gill in March
1892.
(_Knowledge_, vol. xvi. p. 70).
[_Frontispiece._
]
PROBLEMS
IN
ASTROPHYSICS
BY
AGNES M. CLERKE
AUTHOR OF
‘A HISTORY OF ASTRONOMY DURING THE NINETEENTH CENTURY,’ ‘THE SYSTEM OF
THE STARS,’ AND OTHER WORKS
“_In solidis et veris aspiramus ad ultima et summa._”
Novum Organum, ii. 51.
CONTAINING 81 ILLUSTRATIONS
LONDON
ADAM & CHARLES BLACK
1903
THIS WORK IS DEDICATED
BY PERMISSION
TO
SIR DAVID GILL, K.C.B.
WHOSE SUGGESTION AND ENCOURAGEMENT PROMPTED ITS COMPOSITION AND
ANIMATED ITS PROGRESS
PREFACE
The object of the present work is not so much to instruct as to suggest.
It represents a sort of reconnaissance, and embodies the information
collected by scouts and skirmishers regarding practicable lines of
advance and accessible points of attack, with a view to annexing for the
realm of knowledge some further strips and corners from the territory of
ignorance. Its inspiring motive, in short, is the desire for a
rectification of the frontier in the interests of science. Material
resources for the furtherance of such encroachments are not lacking. The
globe is studded with observatories, variously and admirably equipped.
Yet innumerable objects in the sidereal heavens remain neglected, mainly
through inadvertence to the extraordinary interest of the questions
pending with respect to them. In the following pages it has been sought
to indicate some of these individually, and in their relations to the
larger meanings of cosmical research. But this could be done effectually
only from the vantage-ground of our actual acquirements; hence the book,
although primarily designed to stimulate the progress of astrophysics,
necessarily includes an account of its present state. Before attempting
to add to our store of learning, we must realise what is already
possessed.
The unknown, it is true, is indefinitely vast, and the rays of light
which we can project into its darkness penetrate but a short way. Our
programme of inquiry must accordingly be limited to what is now
practicable, or promises to become so in the immediate future. The
unforeseen, too, will have something, perhaps much, to do with
prescribing directions for fresh researches. Queries, in the coming
years, will be put to the skies very different from those here
propounded; and answers of a surprising kind will doubtless be afforded
to our present interrogatory. The keen delight of such revelations will
reward those who, loving truth for its own sake, have laboured for its
promotion; and if these pages should, in any degree, help to quicken and
guide their noble enthusiasm, they will have amply served their purpose.
They do not, however, cover the whole field of astrophysics. Planetary
and cometary astronomy are deliberately, although for different reasons,
excluded from treatment in them. The moon and planets still belong to
the theoretical and descriptive departments of the elder celestial
science. Nearly all that is known about their condition has been learned
by direct telescopic observation. The items of information added through
the aid of the camera and the spectroscope, though valuable, are very
few. The orbital characteristics, on the other hand, of comets and
meteors are too prominent to be set aside in any profitable discussion
of their nature. They are of the very essence of the phenomena; yet they
would be out of place in a book strictly limited to the consideration of
the heavenly bodies under their physical aspect.
The writer has received much courteous help from various quarters in
preparing the illustrations, and desires in particular to acknowledge
her obligations to Sir William and Lady Huggins, to Sir David Gill, to
Dr. Roberts, F.R.S., to the Rev. W. Sidgreaves, S.J., Mr. W. E. Wilson,
F.R.S., M. Deslandres, Professor Hale, Professor Barnard, to Professors
E. C. and W. H. Pickering, and to Professor Campbell.
LONDON, _December 1902_.
CONTENTS
INTRODUCTION
PAGE
RISE AND SCOPE OF ASTROPHYSICS 1
PART I
PROBLEMS IN SOLAR PHYSICS
CHAP.
1. PROGRESS OF SOLAR PHYSICS 13
2. THE CHEMISTRY OF THE SUN 21
3. PECULIARITIES OF THE SOLAR SPECTRUM 36
4. THE REVERSING LAYER 44
5. HYDROGEN, HELIUM, AND CORONIUM 52
6. THE PHOTOSPHERE AND ITS DUSKY VEIL 62
7. STRUCTURE AND MOVEMENTS OF SUN-SPOTS 73
8. THE SPECTRUM OF SUN-SPOTS 88
9. FACULÆ AND PROMINENCES 98
10. THE CHROMOSPHERIC SPECTRUM 112
11. THE CORONA 123
12. THE SUN’S ROTATION 142
13. THE SOLAR CYCLE 150
14. THE SUN AS A WHOLE 161
PART II
PROBLEMS IN SIDEREAL PHYSICS
1. PROGRESS OF SIDEREAL PHYSICS 171
2. THE CLASSIFICATION OF STELLAR SPECTRA 179
3. HELIUM STARS 189
4. HYDROGEN STARS 197
5. SOLAR STARS 203
6. STARS WITH FLUTED SPECTRA 209
7. CARBON STARS 215
8. STARS WITH FLUTED SPECTRA SHOWING BRIGHT LINES 222
9. HELIUM STARS WITH BRIGHT LINES 229
10. WOLF-RAYET STARS 237
11. THE GENERAL QUESTION OF BRIGHT LINES IN STELLAR SPECTRA 243
12. ANOMALOUS AND VARIABLE SPECTRA 246
13. COLOUR VARIABILITY 253
14. THE SPECTRA OF DOUBLE STARS 263
15. THE EVOLUTION OF THE STARS 271
16. ROTATION OF THE STARS 280
17. SPECTROSCOPIC BINARIES 286
18. ECLIPSING STARS 299
19. SHORT-PERIOD VARIABLES 319
20. SHORT-PERIOD VARIABLES—_continued_ 328
21. THE PROBLEM OF BETA LYRÆ 337
22. STARS VARIABLE IN LONG PERIODS 347
23. PECULIAR AND IRREGULAR VARIABLES 363
24. TEMPORARY STARS 375
25. DARK STARS 399
26. THE GENERAL QUESTION OF STELLAR VARIABILITY 404
27. IRREGULAR STAR CLUSTERS 409
28. NEBULOUS CLUSTERS—THE PLEIADES 415
29. NEBULOUS CLUSTERS—_continued_ 423
30. GLOBULAR CLUSTERS 428
31. WHITE NEBULÆ 439
32. DOUBLE NEBULÆ 452
33. NEBULOUS STARS 460
34. PLANETARY NEBULÆ 469
35. ANNULAR NEBULÆ 484
36. THE ORION NEBULA 495
37. OTHER IRREGULAR NEBULÆ 506
38. NONDESCRIPT NEBULÆ 516
39. VARIABLE NEBULÆ 522
40. THE NATURE OF NEBULÆ 531
41. THE PHYSICS OF THE MILKY WAY 538
APPENDIX 547
INDEX 553
LIST OF ILLUSTRATIONS
PRINTED SEPARATELY
PLATE
I. Photograph of the Argo Nebula _Frontispiece_
II. Photograph of A-Band in Solar Spectrum
(McClean) _to face page_ 22
III. Flash and Cusp Spectra compared „ 46
IV. The Sun portrayed in Calcium Light (1);
Photograph of the Chromosphere, 31st May
1894 (2) „ 100
V. Types of the Corona (Hansky) „ 128
VI. The Corona of 1900 (Jewell) (1); Reversal and
Distortion of the C-line (Hale) (2) „ 96
VII. The Corona of 1896 (Morin) (1); the Corona of
1898 (Wesley) (2) „ 134
VIII. The Corona of 1900 (Wesley) „ 136
IX. Photographs of Typical Star-Spectra (Huggins) „ 180
X. Photographed Spectrum of ε Canis Majoris
(Lunt) (1); Photographed Spectrum of α_{2}
Centauri (Gill) (2) „ 182
XI. Spectrum of α Herculis (Espin) (1);
Photographed Spectrum of 152 Schjellerup
(Hale) (2) „ 184
XII. Photographed Spectra of Carbon Stars (Hale
and Ellerman) „ 186
XIII. Photographed Spectra of Mira and of α
Herculis (Sidgreaves) „ 188
XIV. Spectrum of γ Cassiopeiæ. Photographed by the
Rev. W. Sidgreaves „ 190
XV. Photographed Spectra of Types II., III., and
IV. compared „ 220
XVI. Sections of the Spectra of ß Lyræ and Nova
Persei „ 344
XVII. Visible Spectrum of Nova Aurigæ (Campbell) „ 376
XVIII. Nebulosity round Nova Persei (Ritchey) (1);
Spectrum of Nova Persei, 11th August 1901,
with corresponding Intensity-Curve (2) „ 394
XIX. Photograph of Messier 11 (Roberts) „ 410
XX. Photograph of a Spiral Nebula in Cetus
(Roberts) „ 442
XXI. Photograph and Drawings of a Spiral Nebula in
Pegasus (Keeler) „ 444
XXII. Photograph of Whirlpool Nebula (W. E. Wilson) „ 446
XXIII. Fan Nebulæ (Barnard) (1); Planetary Nebulæ
with their Spectra (Keeler) (2, 3, 4) „ 448
XXIV. Photograph of Owl Nebula (Roberts) (1);
Photograph of the Orion Trapezium
(Pickering) (2) „ 472
XXV. Photograph of the Ring Nebula in Lyra (W. E.
Wilson) „ 484
XXVI. Photograph of the Orion Nebula (Pickering) „ 494
XXVII. Nebulous Formation in Orion. Photographed by
W. H. Pickering „ 496
XXVIII. Photograph of the Trifid Nebula (Roberts)
(1); Photograph of Messier 77 (Roberts) (2) „ 510
XXIX. Photograph of a Nebula in Perseus (Barnard) „ 514
XXX. Photograph of the Dumb-Bell Nebula (W. E.
Wilson) „ 516
XXXI. Photograph of the Milky Way in Ophiuchus
(Barnard) „ 540
PRINTED IN THE TEXT
FIG. PAGE
1. General View of the Atmospheric Spectrum 24
2. The Infra-Red Spectrum (Langley) 24
3. Oxygen-Triplet in the Solar Spectrum 29
4. Curves representing the Distribution of Light
in Fraunhofer and Arc Lines (Jewell) 41
5. Diagram of the Helium Spectrum 57
6. Sun-spot photographed by Janssen, 1st April
1894 73
7. Photograph of a Bridged Sun-spot (Janssen) 76
8. Sun-spot drawn by J. de M. Pereira, 18th June
1894 78
9. Group of Sun-spots drawn by Miss E. Brown 78
10. Portion of a Sun-spot Spectrum photographed
by Professor Young 89
11. Reversal of the D-Lines in the Spectrum of a
Sun-spot 94
12. Prominence observed at Kalocsa, 19th
September 1893 104
13. Prominence observed at Kalocsa, 3rd October
1892 107
14. Forms of a Prominence in Hydrogen, Helium,
and Pseudo-Coronium (Fényi) 117
15. Dark Markings in the Corona of 1871 (Wesley) 134
16. Dark Markings in the Corona of 1896 (Wesley) 134
17. Spoerer’s Curves of Sun-spot Latitude 152
18. Curves of Sun-spot Frequency and Magnetic
Agitation (Ellis) 155
19. Intensity-Curves of Ηγ in Spectrum of Mira
(Campbell) 225
20. Hydrogen-Envelope of a Wolf-Rayet Star
(Campbell) 241
21. Photometric Curves of Algol Variables
(Pickering) 309
22. Light-Curve of η Aquilæ (Schur) 322
23. Orbit of η Aquilæ (Wright) 323
24. Light-Curve of S Sagittæ (Yendell) 326
25. Light and Velocity Curves of ζ Geminorum
(Campbell) 329
26. Orbit of ζ Geminorum (Campbell) 330
27. Photometric Curve of U Pegasi (Pickering) 332
28. Light-Curves of Cluster-Variables (Bailey) 336
29. Light-Curve of β Lyræ (Argelander) 337
30. System of β Lyræ (Myers) 341
31. Distribution of the Periods of 208 Variables 347
32. Light-Curve of U Orionis (Porro) 351
33. Light-Curve of S Ursæ Majoris in 1894 354
34. Mean Light-Curve of S Ursæ Majoris 355
35. Mean Light-Curve of T Ursæ Majoris 355
36. Mean Light-Curve of T Cassiopeiæ 355
37. Light-Curve of T Andromedæ 357
38. Light-Curve of S Cephei 359
39. Light-Curve of R Scuti 364
40. Long and Short Maxima of U Geminorum 365
41. Light-Curve of S^2 Cygni 367
42. Light-Curve of Nova Aurigæ 380
43. Photographs before and after the Apparition
of Nova Persei 389
44. Light-Curve of Nova Persei, 22nd February to
6th March 391
45. Light-Curve of Nova Persei, 17th March to 3rd
May 1901 391
46. Exterior Nebulosities of the Pleiades 419
47. Photograph of ω Centauri (Bailey) 430
48. Drawing of a Pair of Spindle Nebulæ
(Spitaler) 457
49. Sketch of T Tauri and Hind’s Nebula (Barnard) 524
50. Drawing from Photographs of T Tauri and
Hind’s Nebula (Keeler) 525
INTRODUCTION.
RISE AND SCOPE OF ASTROPHYSICS.
The astronomy of the ancients was purely formal. It did not profess to
look beyond appearances. Its aim was reached, provided that phenomena
were—in the old Greek phrase—mathematically “saved,” at whatever cost of
material impossibility. Kepler first speculated on the causes of
celestial movements, and introduced the term “physical astronomy” with a
full sense of what it implied. Its establishment as an effective branch
of knowledge was a prime desideratum with Francis Bacon. While rejecting
the Copernican system and ignoring Kepler’s laws, he aspired towards a
science of the skies that should be no simulacrum, like the “stuffed ox
of Prometheus,” but should have in it the breath of life and the
instinct of progress.[1] Anticipating with imaginative insight the
prosecution of researches which Comte, two centuries later, declared to
be, in the nature of things, futile, he broadly laid down the lines of a
new astronomy, indistinguishable from modern astrophysics. As the
province of this “philosophy” of the heavenly bodies, he assigned
inquiries into the nature of their substance, of their qualities,
properties, and influences, as well as into the source of the motive
power acting upon them. Vitalised, as it were, by contact with
motherearth, it was destined in his prevision to a community of advance
with terrestrial science, one imparting to the other novel truths
tending to mutual profit and simultaneous development. Thus the
long-divorced sublunary and translunary worlds were conjoined, and their
material unity—the essential principle of astrophysics—was definitively
proclaimed. This daring forecast transcended the scope even of Newton’s
discovery, and left Kepler’s prescience far behind. For Kepler, as an
astronomer _ex professo_, took full account of apparent impossibilities,
while Bacon’s was “the golden guess”—
That’s morning star to the full round of truth.
Yet it might have been derided as that of an uninformed amateur. It rose
indeed out of sight of ordinary minds.
The establishment of the law of gravity was the first step towards its
realisation. Thereby the terrestrial regimen was, in part, extended to
the skies. The moon assumed the aspect of a projectile in perpetual
flight, tangentially launched _ab initio_, and hence keeping its
distance from the earth, while the planets, including our own globe,
proved to be similarly related to the sun. Thus celestial movements lost
the mystical character long ignorantly attributed to them, and were
found to own a common cause with movements at the surface of the earth.
They became predictable, since the cause acts uniformly and simply;
theoretical astronomy, with practical astronomy at hand to provide its
raw materials and test its results, took rank as the most perfect of the
sciences; the idea of a definable force put to the rout the old vague
notions of “tendencies,” “appetites,” “passions,” or “potencies,” and a
dynamical was substituted for a merely kinematical system.
Gravity, however, is a force of the utmost generality in the way it
affects matter. It takes no notice of distinctions of kind or quality.
The substances acted upon may be hot or cold, dense or rare, elementary
or compound; they may be of any imaginable chemical or mineralogical
constitution; they may be in any state of aggregation; they may be
organic or inorganic; no difference is perceptible; gravity is concerned
solely with mass, and is measured strictly by movement; and from
gravitational inquiries, accordingly, mass and movement can alone be
learned. So far, then, only one principle of unification was introduced.
One fundamental property of matter was known from 1687 to belong equally
to the earth and planets; and Herschel’s discovery in 1802 of mutually
revolving stars virtually made the “writ to run” throughout the sidereal
world as well. The universality of an apparent mass-attraction was a
great fact, but seemed destined to remain isolated; for Olbers’s
“electrical theory” of comets amounted to no more than a suggestive
speculation. Then in 1852 the triple identification by Sabine, Wolf, and
Gautier of the sun-spot and terrestrial magnetic periods showed the
reality of solar influences exercised in a manner not easy to apprehend,
but capable of being brought to the test of experimental investigation.
Cosmical physics began to separate out and take recognisable shape. The
spring of its most rapid growth, however, lay in another direction.
The discovery (in Professor Keeler’s words[2]) “that the light which
reveals to us the existence of the heavenly bodies also bears the secret
of their constitution and physical condition,” afforded a solid basis
for a science of far-reaching import. “The spectroscope placed new and
hitherto undreamtof powers in the hands of men. It is to the
astrophysicist what the graduated circle and the telescope are to the
astronomer.” Observations of the heavenly bodies by means of their
_analysed_ light came to the aid of observations through their
_integrated_ light. Their radiations, visible and invisible, were
brought within the range of detailed study.
Of study, not only visual, but photographic. The sensitive plate has
three leading prerogatives. It _sees_ where the eye is blind; its
impressions are cumulative to an indefinite extent; they are permanent;
they constitute documentary evidence of incontestable validity, which
can be produced or referred to at pleasure. Spectroscopic photography,
or “spectrography,” dates from Sir William Huggins’s adoption of the dry
gelatine process in 1876; and his discovery, three years later, of the
ultra-violet series of hydrogen-lines in stellar spectra started it on
its career amid acclamations. Nor has the promise been belied. The
efficiency of the camera is of so high an order that direct visual
observations of prismatic light are now only by exception made. This is
perhaps unfortunate, since the two kinds of results are, to some extent,
supplementary, and can often be most usefully compared and correlated.
The superiority of the chemical method, however, is nowhere more
conspicuous than in the motion department of the new astronomy. Its
powers in this direction were tested with striking success by Vogel in
1888, and their development, rapid as it has been, does not seem to be
near its term. The determination of radial velocities through changes in
the refrangibility of light emanating from the bodies actuated by them,
has made astrophysicists free of a territory which belongs equally to
the domain of traditionally equipped astronomers. Here we get back to
elementary facts of mass and motion, ascertained, however, not
_frontally_ by measures of position, but _strategically_ by inference
from radiative modifications. They are, indeed, of a nature unaffected
by position, and hence undiscernible with the micrometer. A body shown
spectroscopically to be in swift movement might be absolutely immobile
telescopically; or the conditions might be inverted, each method taking
cognisance of only one component of the total velocity. An immense
extension was accordingly given to the field of research in sidereal
dynamics by the application, through Sir William Huggins’s initiative,
of “Doppler’s principle.” It supplied not alone the means of completing
investigations which could otherwise be pursued only in a one-sided
manner, but of setting on foot entirely new ones of incalculable
significance. Thus the rate of the solar translation through space,
valued little better than conjecturally from the _proper_, or thwartwise
motions of the stars, can be derived securely and at once from their
_radial_ motions. Of the movements of nebulæ nothing is known—and very
little is likely to become known for some centuries yet to come—except
through spectroscopic measurements; for they are so remote that their
positions change with extreme slowness, while the evidence of radial
speed is tendered immediately, without regard either to time or
distance. But the most curious discoveries afforded by it are of double
stars revolving in such close contiguity as to be permanently
inaccessible to telescopic observation. And these are precisely the
systems of highest cosmogonic interest, as being, most likely, at the
outset of their evolutionary careers. They are surprisingly numerous,
and will doubtless prove to be linked on to telescopic binaries by an
uninterrupted succession of couples farther and farther apart.
This common ground of the two astronomies, where motion in the line of
sight is the object of research, has already proved fruitful of varied
novelties, and its yield is not within view of being exhausted. It
could, however, never have been worked to advantage but for the timely
assistance of photography. The living retina is here conspicuously
inferior to the chemical retina; for aerial disturbances are eminently
baffling to eyeestimates of line-shiftings, while the sensitive plate,
ignoring momentary fluctuations, records true mean positions. Visual
measures are hence rarely trustworthy; advantageous occasions for
securing them are few; so that they must always be either poor in
quality or scant in quantity; and accumulated data are needed as the
bases of systematic inquiries. The use of the camera is accordingly
indispensable, and has become all but exclusive.
Astrophysics widens in scope year by year, and as it wins _extension_ it
gains _intension_, each advance carrying it deeper into the secrets of
nature. Towards this result the alliance with photography has
contributed with signal effectiveness. The impersonal method confers a
certainty and power in dealing with obscure phenomena which can only in
special cases be claimed for the eye. Moreover, it is of larger
application. It can be employed on an expanded scale both of time and
space. It is thus the fittest means for collecting statistics of the
heavens; and statistics are urgently in demand for the ultimate purposes
of celestial science. The whole future of astronomy has indeed come to
depend upon the validity of photographic evidence, and specialties of
manipulation and development, the idiosyncrasies of variously prepared
plates, the shrinkage of gelatine films, the effects of graduated
exposures, have to be studied no less diligently than the theories and
errors of brass and glass instruments where immediate determinations of
celestial situations are in question. Astrophotography is an art, and
has a technique of its own needing labour for its mastery.
The ramifications of astrophysics are numerous and intricate. To trace
them out in detail would be to unroll an elaborate chart of the
sciences. Celestial chemistry is in itself an all but limitless
department. It includes terrestrial chemistry, thermotics,
thermo-electricity, and slides inevitably into the wonderland of
molecular physics and ethereal powers and qualities. For the
interpretation of spectra demands acquaintance with the nature of the
vibrating systems originating them, with their relations to
“imponderable” agencies, with their perturbations, modifications, and
disruptions under the stress of circumstances at present scarcely
definable. Here there are worlds to conquer. One phase of these
inquiries is marked by the recognition of harmonic line-series in the
spectra of the chemical elements. Another by the discovery that
wave-length is a function of density, that an increase of pressure
slightly shifts the rays emitted by a glowing vapour downward towards
the red. A third, still more significantly, by the “Zeeman effect,” with
its barely conjectured implications. It consists in the distension and
subdivision of lines normally slender and single, when the radiation
takes place in a strong magnetic field; and the specification of the
laws of its production, whether close at hand under controlled
conditions, or far out of reach at the surface of celestial spheres,
allures the imagination with possibilities of far-reaching consequence.
Above all, there seems to be a reasonable chance of learning from it
something about the electrical state of the stars. The relative strength
and brilliancy, moreover, of spectral lines afford criteria of
temperature, density, and modes of electrical action, but not with
satisfactory explicitness. There is much difficulty in duly apportioning
the effects. Thermal and electrical conditions are rarely separable;
degrees of density and of temperature again need very careful
discrimination. Electricity is the indispensable agent for exciting
luminosity; precisely, however, what part it reserves for itself in the
matter—whether heat, as generally assumed, is its plenipotentiary, or
merely a delegate with limited powers—is, so far, unknown. Nor is it
easy to define what takes place in the path of the discharge, yet it is
from the carrying molecules only that the light examined is derived, and
it is their state only that is indicated by its peculiarities. Still,
beginnings have been made in the experimental disentanglement of this
web of interdependent operations, and specific inferences of value
regarding the heavenly bodies have already been drawn from some
preliminary ordering of the various classes of facts.
The rotation of the heavenly bodies is a department annexed, while their
chemistry has been created by the new astronomy. No longer treated as a
simple geometrical datum, it is studied as an index to their physical
constitution. Spectroscopic observations of axial movements in the sun
and planets are among the most delicate and curious that have been made.
They may possibly be extended to stars, nebulæ, and comets, but the
prospects here are dubious. Nor has the old direct mode of determining
rotation been superseded by the novel method. Its employment, in some
cases supplementary, is rendered in others, by the force of
circumstances, exclusive.
Moreover, nearly the whole “descriptive” section of astronomy is
embraced by astrophysics. It is now extensively yet not altogether
worked by photographic means. The camera has so far succeeded very
imperfectly in depicting planetary surfaces; but the required special
conditions are being carefully studied, and will perhaps before long be
realised. The difficulties attending lunar photography have of late
been, in the main, overcome, as the magnificent Paris and Lick Atlases
of the moon testify. They nevertheless record essentially what was known
before; they elucidate no perplexity; selenology has been adorned and
illustrated, but not greatly promoted by their compilation. The
self-portrayal of recent comets, on the other hand, has been accompanied
by remarkable disclosures. They need, however, skilled interpretation,
and experts in this branch are to seek. The pictures are there, full of
rapidly changing and significant detail; yet patience must be exercised
before we can read in explicit terms what they implicitly convey
regarding the constitution of the bodies they represent.
The photographic study of the Milky Way—pursued systematically by
Professor Barnard—has been more definitely and distinctly communicative.
For his plates not only bring clearly to view the mixed stellar and
nebulous nature of that gigantic assemblage, but also afford grounds for
inferences of great moment as to the general distribution of the stars.
This indeed is a subject which might seem expressly reserved for
treatment by the older astronomy. Yet the all-pervasive physics of the
skies has a lien upon it. Spectroscopic considerations come into play.
The modes of stellar scattering in space are different for the various
stellar types, and the connection suggests queries, not readily
answered, regarding the origin of those types, and the very genesis of
the sidereal system itself. Abysses of speculation open before us as we
contemplate the surging galactic cloud-forms depicted through the simple
instrumentality of a portrait-lens and a sensitive-plate.
In the photometric branch of astronomy there is a similar concurrence of
claims. The arrangement of the stars in light-ranks serves primarily as
a test of their arrangement in space; the test, however, is illusory
unless the nature of their spectra be taken into account. Again, while
measurements of the brightness of individual stars are essentially
physical in their import, they are also carried out for the geometrical
purpose of determining occultation-phases. The photometric observation
of the eclipses of Jupiter’s satellites is a corresponding example in
the solar system. Otherwise, in its varied applications to the sun and
moon, to planets, asteroids, and comets, photometry may be said to have
purely physical aims. These have to do, not only with integral, but also
with analysed light. The “spectrometric” division of photometry consists
in the comparative estimation of ray-intensities, in balancing one
against another the differently refrangible beams from a given source of
luminosity, in constructing, that is to say, its spectral energy-curve.
In both departments the camera proves an invaluable ally. Photographic
photometry occupies, indeed, a place apart among the arts and crafts of
astronomy. It has its own laws, its own problems, its own difficulties,
and it furnishes data which can be interpreted on principles valid for
them alone.
The specialties of solar physics are too numerous to be particularised.
Among stars, perhaps an insignificant star, the sun is nevertheless by
its comparative vicinity to ourselves brought within range of a whole
series of observations impracticable elsewhere. In solar research,
accordingly, novel devices abound; such as the “double-slit method,” so
happily availed of by Hale and Deslandres for the spectrographic
portrayal of “flames,” facular and chromospheric. The complex operations
conducted under shelter of eclipse are equally peculiar in their objects
and in their system; by them only is the unique problem of the corona at
present accessible to attack; that of the “reversing layer” is even more
elusive in its momentary presentations. Sun-spots, on the contrary, are
open to leisurely daylight inspection; yet the perplexities connected
with their structure and spectra grow rather more than less acute as
facilities for their scrutiny are increased. But this is no uncommon
experience in the arduous walks of science.
The pliancy and generality of astrophysics contrasts singularly with the
austere exclusiveness of gravitational astronomy. The new mode of
celestial inquiry follows every indication, lays hold of every clue; it
promises much, it often performs more; yet its advance is at times
hampered by the very circumstances which make it brilliant and
surprising. For it “deals,” as Professor Mendenhall said in 1892,[3]
“with a matter of many properties, some of which are but little
understood. While its conclusions are of vital importance and of intense
interest, they result from deductions in which the premisses are
insufficient, and are proportionately uncertain. The new astronomy must
for a long time abound in contradictions and controversies, until, and
largely through its development, we shall possess a knowledge of the
properties of matter when subjected to conditions differing enormously
from those with which we are now quite familiar.”
Here indeed lies the fundamental peril, and at the same time the
essential prerogative of astrophysics. Its concern is with phenomena
falling partly within, partly without the range of ordinary experience.
It has to do with matter in transcendental states. Hence the necessity
for having recourse to the risky expedient of “extrapolation”—that is,
of applying unrestrictedly to the unknown, rules gathered from
observation over a comparatively narrow area. The indefinite continuity
of natural laws is assumed by it, but certainly on no sufficient
warrant. There is indeed no help; no other means are available; the line
and plummet that have proved serviceable for sounding the estuary must
be used likewise for the ocean. The upshot, however, is merely a “first
approximation,” to be subsequently corrected and controlled. And it may
be of immense importance as an index to consequences or possibilities
which could not have been foretold, and defy even imaginative
realisation. But just here resides the exploring faculty of
astrophysics. It often acts as the pioneer of terrestrial science. “The
discovery of unknown laws” (in Professor Keeler’s words), “as well as
the explanation of phenomena by laws already known, is one of its most
important objects.”
A great future is reserved for it. It postulates a law of order, the
same always and everywhere, and its primary function is to verify that
postulate, step by step, point by point, under continually widening
horizons of knowledge. There is no such thing as chaos, it tacitly
asserts, in the sidereal world or outside of it. For chaos is the
negation of law, and law is the expression of the Will of God.
PART I
PROBLEMS IN SOLAR PHYSICS
CHAPTER I.
PROGRESS OF SOLAR PHYSICS.
Solar Physics is the science of the sun as an individual body. It is not
concerned with the sun as the ruler of the planetary system, or as a
member of the sidereal system. The questions which it seeks to answer
relate exclusively to the “thing in itself.” And these questions,
through the effectiveness of modern methods, have become _answerable_.
Few of them, it is true, have yet been _answered_, and all can never be
set at rest, since each reply marks only the starting-point for a fresh
interrogatory. This must be so; the prospect inevitably widens with the
attainment of a higher point of view. Nor is it likely that the ascent
will soon terminate. It is indeed towards a summit cloud-wrapt and
self-withdrawn. The essential point, however, is that stagnation has
given way to progress, surmise to inquiry, and barren wonder to
stimulating curiosity.
Solar research made a threefold start about the middle of the last
century. First came Schwabe’s discovery of a decennial sun-spot
period, followed up by Sabine’s announcement of a coincident
terrestrial-magnetic period. Then, in 1860, Kirchhoff published his
momentous chemical interpretation of the Fraunhofer lines, showing the
presence of familiar metals as glowing vapours in the sun’s
atmosphere. Finally, on 18th July of the same year, the “red
prominences” were photographically referred to their true location,
and the whole marvellous eclipse-garniture was at once annexed to the
domain of solar physics. The investigations corresponding to these
three beginnings were pursued at very unequal rates of advance, and
with considerable disparity of success. The chief triumphs were those
of the prismatic method. From the spectroscope single-handed, the old
order of ideas received its death-blow. Glaring incongruities
notwithstanding, it might have survived a couple of decades longer had
it not been for the reading of the strange Fraunhofer inscription. But
the subversive effect of the attack delivered in 1860 was too palpable
to be ignored. At last, unmistakably, the Herschelian theory of the
sun was in ruins, and it only remained to clear away the rubbish of
the structure preparatory to erecting a modern edifice on new
foundations.
Its corner-stone was the principle of the conservation of energy. This
obtained its first solar application from Helmholtz in 1853. His
gravitational hypothesis explained the enormous outflow of heat from the
focal hearth of the planetary household with a directness and simplicity
that compelled conviction of its truth. Energy of position is, in this
view, the store drawn upon by radiation; and it is a store so vast that
millenniums of thermal expenditure will make no perceptible encroachment
upon it. As the great globe cools, it contracts; and each one of its
constituent particles falls, day by day, infinitesimally nearer to the
centre, heat being thus mechanically evolved. Potential energy is in
this way converted into actual energy, and we are warmed and lighted
because the sun shrinks, and is raised by shrinkage to a surpassing
pitch of incandescence. His constitution must then be such as to meet
these requirements. For an ideal body, endowed at pleasure with fanciful
properties, a machine has to be substituted, definitely adapted to the
fulfilment of a recognised function. This change in the point of view is
characteristic of astrophysical aims. It makes all the difference
between antique and modern science. A great deal is involved in it. The
demands of the novel situation are multitudinous, and can be met, not by
speculative efforts, but only by toilsome experimental comparisons.
These will need time and much patience, and can never be wholly
satisfactory in view of the contrast between terrestrial and solar
conditions. Yet the efforts towards their assimilation, unremittingly
prompted by the new astronomy, lead to a continual growth of knowledge,
and are unlikely to be relaxed until the torch has finally dropped from
human hands.
The absence, then, of a satisfactory all-round theory of the sun need
not be taken as an implication of failure. On the contrary, progress is
necessarily attended by incompleteness. Facts, when research is most
active and successful, accumulate too rapidly to be at once collocated.
Finality means stagnation. Compare the map of the world drawn by
Hecataeus with that by Herodotus. The earlier is by far the more
finished production. Neat and trim, with its circumfluent Ocean Stream,
it pictured the earth mainly from ideas of what it ought to be. The
later chart, on the other hand, as the upshot of wider experience,
admitted ignorance by abolishing limits and leaving room for the
unknown. A true theory must always be somewhat expansible. It must be
capable of accommodating new facts. Otherwise their intrusion will
speedily rive it asunder.
Only the broad lines of solar theory can then at present be laid down.
Details must be filled in gradually with the progress of research. The
preliminary ideas, however, already acquired are unlikely to be
subverted; we can represent to ourselves a sun which is a reality, and
no figment of the brain.
Our luminary is neither solid nor liquid. It is mainly, perhaps
entirely, gaseous; but its gaseity is of the “critical” kind, due to the
combination of intense heat with enormous pressure. The thermal supplies
needed to meet its vast emissive expenditure must be continually and
rapidly brought from the central parts to the surface; and this can only
be accomplished by the actual transport of the heated materials,
conductive processes being much too slow to meet the exigencies of the
situation. We thus recognise in the sun a globe riddled with
convection-currents, of which the shining cloud-shell of the photosphere
constitutes the limit. At the photospheric level the uprushing torrents
deliver their cargo of radiative energy, and from the photospheric level
the corresponding subsidence of cooled matter starts for the
unimaginable furnace below. This course of exchange, however, must be
greatly complicated by the rotation of the plastic mass in which it
progresses. A true vertical circulation is rendered by it impossible;
the ascending and descending currents must be variously and incalculably
deflected. Incalculably, since the state of the sun’s interior lies, in
some respects, beyond the range even of conjecture.
The very remarkable circumstance has been emphasised by recent inquiries
that the photosphere fixes a boundary between two solar regions scarcely
less strongly contrasted—to speak illustratively—than the terraqueous
globe and its encompassing atmosphere. The sun has several distinct
envelopes, but none, apparently, in the condition of atmospheric
equilibrium. There is first a shallow, veil-like covering by which the
disc is reddened and darkened. Next comes the “reversing layer,” a bed
of mixed incandescent vapours, some hundreds of miles in thickness, the
absorptive action of which mainly produces the dusky lines in the
Fraunhofer spectrum. It is overlaid, to a depth of four or five thousand
miles, by the chromosphere, a gaseous ocean incarnadined by the crimson
blaze of hydrogen. The irregularities of its outline develop, locally
and temporarily, into “prominences,” often of gigantic size, but
belonging to the chromosphere as essentially as mounting waves and
tossed spray do to the ocean. Finally, we reach the far-spreading
corona, a mere lustrous phantom, approaching the absolute zero of
density, yet of astounding decorative effect during total eclipses.
Between the corona and the chromosphere there seems to be absolutely no
material continuity, although structural relationships have been traced.
These appendages are distinguished by two peculiarities, rendered more
obvious at each step forward in research. The first is that they contain
an extremely small quantity of matter. The second, that the effect of
the sun’s gravity upon them is, in some way, neutralised. We have only
to consider that at an elevation of three and a half miles air is
reduced to one-half its sea-level density, while the corresponding
height at the surface of the sun—where gravity is twenty-eight times
more powerful than it is on the earth—is but one-eighth of a mile. Into
the compass of a shell just one furlong thick, accordingly, half the
substance of the reversing layer, chromosphere, and corona should be
compressed, if the sway of gravity over them were undisputed. Its
comparative impotence is attested not only by their vast extent and
excessively slow rate of luminous degradation, but still more
emphatically by the almost total absence from them of spectroscopic
symptoms of internal compression.
The various “claims” into which the wide field of solar physics has
inevitably come to be divided are marked by curious differences of
productiveness. Some are thickly sown with “pockets” of bright ore;
others have hitherto yielded little beyond the “sparkle of golden
splendour” on the surface. Thus the geometrical relations of sun-spots
are not now more surely known than in the days of Derham and Cassini. A
consensus of opinion that lasted a full century has given way to
notorious disagreement. The elementary question as to whether spot-umbræ
are elevations or excavations, is once more actively debated. True, the
overthrow of an artificial unanimity often preludes a forward movement;
yet it might have been expected that the immense mass of photographic
records accumulated during thirty years would have amply sufficed to
settle this matter once for all. It must indeed be admitted that direct
sun-pictures, notwithstanding the exquisite perfection to which the art
of taking them has been brought, and the striking nature of the details
they often exhibit, have contributed only in a minor degree to the
promotion of definite knowledge. Super-eminent among them are the long
series due to M. Janssen’s skill; yet after twenty years the _réseau
photosphérique_, a phenomenon of “blurring” manifested by their means,
continues enigmatic as to its nature, and open to doubt even as to its
solar origin.
The swiftest advances in solar physics have been along the various
routes opened by light-analysis. Four of these are broadly separated by
differences of aim and method. The inquiries they have made practicable
relate to the chemistry of the sun itself, the daylight study of
prominences and faculæ through a selected element of their emissions, to
radial movements in the sun, and to spectroscopic disclosures during
eclipses. The Fraunhofer spectrum has been studied year by year with
minuter accuracy, and similar refinements in the treatment of the
arc-spectra compared with it have assured real, and annulled fictitious
correspondences, largely, as may readily be imagined, through
photographic agency. Spectra, to be exactly collated, must be durably
imprinted. The fine measurements now executed upon them would be an
impossible task for the eye. Professor Rowland’s invention of concave
gratings in 1883 has also contributed very notably to the development of
solar chemistry. They simultaneously focus impinging rays and disperse
by diffracting them. No lens needs to be interposed, and thus the
disturbing effects of selective absorption and unequal deviation are
avoided. Improvements in the technical processes of photography—the
substitution of gelatine for collodion as the vehicle of the
decomposable salt; the intensification of sensitiveness in plates;
modifications in their colour-susceptibility—have been equally
essential. Mechanical contrivance has not been behindhand. Without
faultless screws, for instance, there could be no perfectly ruled
gratings. The chemistry of the sun has indeed drawn upon many and
unexpected resources for its promotion.
Spectroscopic work at the sun’s edge was carried on steadily for
twenty-three years after its initiation in 1868. Its outcome was the
collection of a mass of valuable information regarding the chromosphere
and its jutting eminences. Their forms, movements, and duration were
registered, the law of their distribution was ascertained, the mode of
their conformity to the spot-cycle inferred. So rich a harvest was, in
fact, gathered at once that the soil began to show signs of exhaustion;
the prospect seemed dim of detecting any further essential novelties in
this direction; the routine task of daily promenading the slit of the
spectroscope round the limb lost its zest. Then in 1891 a novel
commencement was made, and made in duplicate by Professor Hale at
Chicago and M. Deslandres in Paris. They transferred the business from
the eye to the sensitive plate, definitively and with splendid success.
The photography of prominences, although tried on a correct principle by
Professor Young in 1870, remained in an abortive experimental stage
until recourse was had to the device of isolating the K-line of calcium,
and depicting them in this single element of their light. It proved
applicable to faculæ as well, and in one minute a complete picture of
the disc and its appurtenances, as shown in the violet ray profusely
emitted by them, can be secured with the spectroheliograph whenever
the sun shines on either side of the Atlantic. The very
name—“spectroheliograph”—of the instrument invented for the purpose
comprises a history of changing methods—of the supersession by
photography of eye-and-hand delineation, and of the replacement in turn
of direct photographic portrayal by impressions of spectral images. And
there has been a corresponding modification of ideas. New conceptions
are gaining ground, not through the broaching of startling theories, but
under the steady guidance of undeniable, and often surprising facts.
Doppler’s principle was applied by Sir Norman Lockyer about 1870 to
_meteorological_ investigations (as they might be called) in the sun.
They disclosed the not infrequent occurrence there of portentous
cyclonic agitations. Distortions and displacements of the hydrogen lines
attested the rushing of incandescent whirlwinds at speeds up to 250
miles a second. Much has yet to be learned regarding these extraordinary
phenomena, their relationships having scarcely received the detailed and
particular attention that they deserve. Professor Young’s use of the
same method in 1876 to measure the solar rate of rotation served both as
a test of its validity, which it established beyond cavil, and as a
prelude to important refinements in the treatment of that intricate
subject. Notwithstanding its anomalous retardation north and south from
the equator, M. Dunér obtained in 1887–9 spectroscopic evidence of axial
movement up to fifteen degrees from either pole, and thereby brought a
widened range of its complexities under observational control.
Line-displacements, too, similarly produced, have become the standard
criterion for discriminating between the genuine solar, and the merely
telluric constituents of the Fraunhofer spectrum; and it need scarcely
be pointed out that to set them decisively apart is a pre-requisite to
solar chemical progress.
Each favourable eclipse since 1842 has furnished to science its quota of
new facts and inspiring suggestions. In 1851, the solar status of the
“sierra” and “red protuberances,” demonstrated in 1860, was recognised
by all except a few obstinate sceptics. In 1868, the hydrogen and helium
spectrum of these wonderful objects came into view; in 1869, the green
coronal ray was detected. Then on 22nd December 1870, Young’s tangential
slit was momentarily lit up by the “flash spectrum” of the reversing
layer, which, after twenty-six years, was photographically captured by
Shackleton, and so became the subject of definite and critical
investigation. This was greatly promoted by the multiplied records of it
obtained along the line of totality which crossed India, 22nd January
1898. The cyclical variation of coronal types, indicated by the
substitution of luminous “wings” for the more familiar “glory,” during
the Rocky Mountains eclipse of 29th July 1878, was verified by a
splendid series of coronal photographs taken in Egypt in 1882, at the
Caroline Islands and Grenada in 1883 and 1886, in California and at
Cayenne during the January and December totalities of 1889, at Novaya
Zemlya in 1896, in the Deccan in 1898, in Sumatra and Mauritius in 1901.
Sir William Huggins’s device for photographing the corona in daylight,
invented under the stimulus of the Nile-eclipse disclosures,
unfortunately remains in abeyance. Its realisation is a prime
desideratum in solar physics.
The progress of science in this branch might with substantial accuracy
be described in the condensed statement that a fabulous luminary has
made way for a working machine—a machine, it is true, of infinite
complexity, yet in touch with, although transcending, the common order
of things. Just here reside the extreme interest and value of such
inquiries. They deal with what is concrete; they can be pushed on by
experiment, but by experiment always straining to widen its resources.
Limits are accordingly pushed back little by little—limits of
temperature, of rarefaction, of ethereal stress as manifested by
electric and magnetic intensity. The end of the process is not within
view. The way, arduous though it be, lies open, and is securely
travelled by those who, relying on the unity and continuity of nature,
confidently hope to attain by it to the knowledge of higher truths.
CHAPTER II.
THE CHEMISTRY OF THE SUN.
Knowledge of solar chemistry is based exclusively upon the analysis of
solar light. It advances _pari passu_ with the interpretation of the
Fraunhofer lines. And by their interpretation is signified the process
of identifying them, one by one, with the rays of known substances, made
to glow artificially in the laboratory. They are the characters of a
script in the main decipherable, and already, to a satisfactory extent,
deciphered. Their reversal from bright to dark simply implies that the
prismatic background upon which they are projected represents a hotter
source of radiation than theirs. In other words, the temperature of the
photosphere is above that of the ignited vapours through which its light
is sifted, and by which it is selectively absorbed.
Fraunhofer’s survey of the solar spectrum was necessarily confined to
its visible section, and was executed with very imperfect appliances.
Yet the lines laid down in his map had the importance of permanent
landmarks. The following is a list of the chief among them, their
wave-lengths on Rowland’s scale, and the chemical origins ascertained
for them, being added:—
Designation. Wave-length in ten-millionths of a Origin.
millimetre.
A 7594·059 (upper edge of a band) Terrestrial oxygen
B 6867·461 „ „ „ „
C 6563·054 Hydrogen
D_{1} 5896·156 Sodium
D_{2} 5890·182 „
E_{1} 5270·495 Iron
_b__{1} 5183·792 Magnesium
F 4861·496 Hydrogen
G 4308·034 Iron
H 3968·620 Calcium
K 3933·809 „
The first and last lines in this table approximately define the range of
dispersed sunlight visible to ordinary eyes. It extends over nearly an
octave; and a higher half-octave in the ultra-violet is disclosed
photographically. The infra-red, however, offers a far vaster scope for
exploration. Using a “bolographic” method, in which the camera registers
what the bolometer[4] _feels_, Professor Langley has surveyed a stretch
of dark radiations eight times longer than the bright strip mapped by
Fraunhofer; nor was his advance downward in the spectrum checked by any
insurmountable barrier. There is, indeed, much probability that long
heat-waves and short “Hertzian” waves are really indistinguishable, and
that the luminous spectrum passes without a break through the thermal
into the electric spectrum.
In the ultra-violet region, on the other hand, a peremptory stop is put
to research by the interposition of the air. It excludes by absorbing
ether-waves shorter than about λ 2950. Cornu found sunlight to be
arrested just at this point. Sir William Huggins fixed the limit for the
photographic spectrum of Vega (α Lyræ) at λ 2970.[5] It does not
fluctuate with meteorological conditions. Dampness and drought are
equally ineffective in shifting the atmospheric barrier against the
entry of quick vibrations. Cornu ascertained in 1881[6] that it is
affected only by the height of the barometer. Nor is the reduction of
impermeability through ascent above the earth’s surface nearly as great
as it would be if aqueous vapour were the producing agent. That oxygen
is chiefly concerned is rendered certain by converging proofs. Nitrogen
seems to be, in this respect as in others, nearly inert.
[Illustration:
PLATE II.
Fraunhofer’s A-Band in the Solar Spectrum. (_Photographed by Frank
McClean, F.R.S._)
]
The general enfeeblement, by transmission through our atmosphere, of the
violet and blue sections of sunlight becomes obvious in the redness of
the sinking sun. On the lower radiations telluric absorption acts more
specifically. They are interrupted by a multitude of dark bands and
lines certainly referable to it. The question as to the terrestrial or
solar origin of such effects is evidently of fundamental importance to
solar chemistry. It can be answered in two distinct ways. The earlier
and simpler method is by comparing the spectra of the high and low sun.
The groovings that gain strength with approach to the horizon stand
self-declared as atmospheric, while lines unaffected by altitude tell
plainly of exotic conditions. The latter class are much the more
numerous. Of 3200 lines mapped by Thollon, 2090 are purely solar, 866
telluric, and 246 of compound production.[7] And the proportion is not
very different in Dr. Becker’s catalogue of 3637 spectral lines,
published in 1890.[8] Among them 928 came out blackened in “low-sun”
observations, and proved in the main due to the selective absorption of
water-vapour. A considerable proportion, indeed, belonged to the
“rain-band,” and varied hygroscopically. Dry-air absorption is almost
exclusively an oxygen product. It takes effect chiefly in three wide
bands, Fraunhofer’s “A” and “B” and Ångström’s “_a_,” all relieved
against a crimson background. They are characteristic of cool oxygen.
The molecules, whose vibrations they in a manner reflect, are broken up
at high temperatures. They survive, however, the liquefaction of the gas
at −181° C. Professors Liveing and Dewar observed the atmospheric A and
B in light that had been transmitted through three inches of this frigid
fluid.[9] The rhythmical flutings composing the former are shown in
Plate II., from a photograph by Mr. McClean. The work, of which it is a
specimen, portrays the solar spectrum in seven sections, from D to below
A (λ 5800 to λ 7700), the dispersion having been effected by means of a
Rutherfurd grating of 17,296 lines to the inch.
Fig. 1 gives a general view of the atmospheric spectrum, so far as it
can be seen, but it is largely invisible. Langley found the immense
tract of the heat spectrum, down to wave-lengths of nearly six
“microns,”[10] thronged with “cold” rays, 652 of which[11] were
accurately determined from “bolographs,” but remain, with few
exceptions, chemically unidentified.
[Illustration:
FIG. 1.—General View of Atmospheric Spectrum (Schemer’s _Astronomical
Spectroscopy_, translated by Frost).
]
[Illustration:
FIG. 2.—The Infra-Red Spectrum (Langley).
]
Fig. 2 reproduces Langley’s drawing of part of the infra-red spectrum.
The blank strip to the left shows the comparative brevity of the visible
part of the scroll. The invisible part includes the distinctive
signature of one other atmospheric constituent besides oxygen and
water-vapour. Two strong bands in the infra-red are assigned by Knut
Ångström to the absorption of carbon dioxide,[12] a substance of which
four volumes are present in ten thousand of air at sea-level. The huge
nitrogen envelope of our globe, together with its argon-ingredient,
appears to be perfectly transparent to rays of all refrangibilities. The
unexplained fact of its spectral nullity emphasises the inadmissibility
of negative conclusions regarding the chemistry of the heavenly bodies.
The second peculiarity which distinguishes telluric lines is a negative
one. They do not shift as the sun rotates. But lines genuinely emanating
from the equatorial edges of the sun are displaced towards the blue by
the advancing movement of the left or eastern limb, towards the red by
the recession of the western limb. The juxtaposition, accordingly, of
spectra from these two opposite sources serves as an unfailing test of
the origin of their constituent markings, those claimed by the sun being
perceptibly notched at the points of junction, while their telluric
associates run on continuously.
Little has been added to knowledge of the sun’s constitution by
researches in the infra-red part of the spectrum. They are as yet
crippled by the lack of metallic comparison-lines. Sir William Abney
obtained in 1879 a modification of bromide of silver sensitive to slow
heat-vibrations, and thus succeeded in directly photographing the solar
spectrum between the wave-lengths λ 7600 and λ 10,750. Of 590
absorption-lines measured by him in this region in 1886,[13] only an
insignificant fraction have been identified. All these belong to metals
with low melting-points.[14] Further, certain bands which Becquerel
succeeded in rendering visible by phosphorescence proved assignable to
magnesium, calcium, sodium, and potassium.[15] This confirmation of the
presence in the sun of potassium was far from superfluous, as only one
line due to it can ordinarily be seen.
The recent era in solar chemistry may be said to date from Rowland’s
production of a perfect screw in 1882. This minor feat of ingenuity
opened the way for vital improvements. Through its means, gratings ruled
with almost ideal regularity became widely available, and the
difficulties impeding the diffractive mode of light-analysis were
removed or diminished. Now observations are mutually comparable only
when the _absolute_ wave-lengths of the observed rays are known; and
they are derivable immediately from the diffraction spectrum, while in
the refraction spectrum several complicating circumstances come into
play. Hence the supreme value of gratings. For in the spectra afforded
by them the positions of rays depend simply and solely upon the distance
from crest to crest of the minute ethereal undulations they represent.
Rowland’s photographic map of the solar spectrum[16] was a document in
advance of the time. The amount of detail shown in it may be exemplified
by the statement that 150 lines of absorption could be separately
reckoned between H and K, the great calcium pair in the violet. No
comparable delineations of terrestrial spectra (apart from that of iron)
were, however, then extant; coincidences between the rays in them and
Fraunhofer lines might, accordingly, be often apparent only, and devoid
of chemical significance. Dr. Scheiner gave expression to a general
sense of discouragement when he wrote in 1890: “It is unfortunately the
case that less is known to-day as to the meaning of the Fraunhofer lines
than was supposed to be known ten years ago.”[17]
The need for fresh efforts was, however, promptly met. Photographic
investigations of metallic spectra, fully coming up to the new standard
of accuracy, were set on foot, among others, by Kayser and Runge at
Hanover, by Hasselberg at Stockholm, above all, by Rowland and his
coadjutors at the Johns Hopkins University. Here the spectra of nearly
all the chemical elements have been photographed with high dispersion
for the purpose of solar comparisons. And the end of the process is well
within view. Measurements have already been carried far enough to give a
multitude of identifications. Between 1895 and 1897 Professor Rowland
published in the _Astrophysical Journal_ a “Preliminary Table of Solar
Wave-Lengths,” extending from λ 7331 to λ 2976—that is, from dusky
crimson up to the highest ultra-violet ray capable of penetrating the
aerial barrier. He unhappily did not live to make the list definitive;
but it comprises, as he left it, nearly 20,000 lines, about a third of
which, by a rough estimate, may be confidently referred to absorption by
various terrestrial substances. These are enumerated below, according to
the number of lines associated with them in the sun. The corresponding
atomic weights are given in a second column.
ROWLAND’S TABLE OF SOLAR ELEMENTS.
Element. Atomic Weight.
Iron (about 2750 line-coincidences) 56
Nickel 58
Titanium 48
Manganese 55
Chromium 52
Cobalt 59
Carbon (about 240) 12
Vanadium 51
Zirconium 65
Cerium 140
Calcium (over 75) 40
Scandium 44
Neodymium 140
Lanthanum 139
Yttrium 89
Niobium 94
Molybdenum 96
Palladium 106
Magnesium (about 24 coincident lines) 24
Sodium (13) 23
Silicon 32
Hydrogen 1
Strontium 87
Barium 137
Aluminium 27
Cadmium 112
Rhodium 103
Erbium 166
Zinc 65
Copper (2) 63
Silver (2) 108
Glucinum (2) 9
Germanium 72
Tin 117
Lead (1) 207
Potassium (1) 39
Of the following substances no traces could be found the solar
spectrum:—
Element. Atomic Weight.
Antimony 120
Arsenic 75
Bismuth 208
Boron 11
Cæsium 133
Gold 197
Indium 113
Lithium 7
Mercury 200
Nitrogen (vacuum tube) 14
Phosphorus 31
Praseodymium 144
Rubidium 85
Selenium 79
Sulphur 32
Thallium 232
Argon may now be included among the non-apparent elements, and the
presence in the sun of platinum and the metals cognate with it is still
an open question. A metal belonging to a very different class was added
to the number of solar ingredients by Messrs. Hartley and Ramage in
1897.[18] They convincingly identified two blue rays of gallium with
Fraunhofer lines, pointing out that the proportion to iron of the new
metal indicated as existing in the reversing stratum was by weight only
one to thirty thousand. This accords well with its terrestrial
relations. Gallium, discovered by Boisbaudrin in 1875, seems to be
widely, but very minutely, diffused throughout the earth’s crust. It
occurs also in meteorites. It has an atomic weight of 70, is singularly
volatile, and melts almost as readily as butter.
No substance has been more eagerly looked for in the sun than oxygen.
But the search was long in vain. Henry Draper’s recognition, in 1877, of
_bright_ lines of oxygen in the solar spectrum created a sensation, but
proved illusory. J. C. Draper’s _dark_ lines were a still less plausible
personation. Eisig ascertained in 1894 that none of the eighty-one
emission-lines measured by himself occur in the solar spectrum.[19]
Janssen demonstrated, by observations from the summit of Mont Blanc,
that the _cool_ oxygen-absorption conspicuous in it is of purely
telluric origin;[20] and his conclusion was ratified by Dunér’s
application of the motion-displacement test.[21] Oxygen, however, is a
substance of most complex, and perhaps unstable molecular structure. No
less than six distinct spectra characterise it, two of them produced at
low temperatures, and known through their absorptive effects alone; four
derived from vacuum tubes, under varying degrees of electrical
excitement. Moreover, one of these forms of emission is a series
spectrum of the most intricate kind, comprising six different sets of
harmonic vibrations, three made up of triple, three of single lines.[22]
And here at last a significant coincidence was found. A triplet in the
red part of the Fraunhofer spectrum, photographed by Higgs and McClean,
was in 1897 clearly identified by Runge and Paschen as a fundamental
oxygen group[23] (see Fig. 3). The representation of the element,
although certain and authentic, is reduced to a minimum.
[Illustration:
FIG. 3.—Oxygen-Triplet in the Solar Spectrum (_Astroph. Journ._ vol.
iv. p. 318).
]
The spectrum of helium, which is analogous to the “series spectrum” of
oxygen, makes no show in analysed sunlight, but appears bright above the
limb. In the case of this substance, moreover, the usual order of
detection was reversed. Its recognition as a chromospheric material
preceded by nearly a quarter of a century the expulsion in Professor
Ramsay’s laboratory of an identical gas from clevite. But about helium
more will be said presently.
So far, then, thirty-nine of the chemical elements are known to be
common to the earth and sun, and the remaining forty may very well be so
likewise. The absence from among the solar ingredients of any single
terrestrial species of matter is unproved, and perhaps unprovable. The
Fraunhofer spectrum sums up the combined absorption of a heterogeneous
mixture of vapours. But the aggregate is widely different from what
would be obtained by simply adding together the separate effects. For it
is the outcome, so to speak, of struggle and survival. In a medley of
ignited substances, the rays of certain among them predominate, while
those of others are effaced. Thus non-metals, as a rule, make a poor
figure in the spectral competition with metals, and this is doubtless
one reason for their inconspicuousness in the sun. Apart from hydrogen,
the properties of which are exceptional, only three metalloids, silicon,
carbon, and oxygen, contribute to produce the Fraunhofer lines, and
their contributions are feeble and fragmentary. That other similar
substances—selenium, sulphur, nitrogen, argon, and the rest—may be
there, yet exercise no perceptible absorption, is amply possible.
The metals themselves, too, differ widely as regards conditions of
visibility. Some are rich in strong lines, favourably situated for
observation. Iron is an example. It emits thousands of rays, widely
distributed over the spectrum, although most crowded in its higher
sections; and they hold their own vigorously against the adverse
influences of dilution. Again, the rare metal cerium is extraordinarily
prolific of blue rays. No less than 400 were measured by O. Lohse in
1897 in the comparatively narrow region between λ 4000 and λ 4600.[24]
Most of them, however, are quite feeble, and only twenty-nine have yet
been identified as Fraunhofer lines. Those of bismuth, on the other
hand, being all compound, are too diffuse (as Rowland observes) to be
detected in sunlight. And most of the radiations of lithium are so
highly refrangible as to fall under the ban of atmospheric exclusion.
Their reversal in the solar spectrum can thus only be a matter of
inference. That the inference should be negative is suggested by the
absence of a strongly characteristic line in the carmine red. It _ought_
to be readily seen, if lithium be a solar constituent. Its visibility
should be promoted by the small atomic weight—only seven times that of
hydrogen—and low fusing-point of the metal; and the persistence of the
carmine beam is actually shown by its emergence in the spectrum of the
Bessemer flame. Yet it is unlikely that lithium is, in fact, missing
from the sun. The case deserves particular attention.
Heavy substances are obviously at a disadvantage as regards the
production of absorptive effects. Their vapours must tend to lie low,
like carbonic acid in the earth’s atmosphere. Hence the average
lightness of the solar elements is only what we should expect. The mean
atomic weight of the thirty-five metals represented in the Fraunhofer
spectrum is, in fact, just 72, while that of the non-apparent metals is
159. Atomic weight, however, is only one of many conditions affecting
this result. The inclusion of lead-absorption in the scroll, and the
exclusion from it of lithium, sufficiently prove that vapour-density is
far from being alone concerned.
The detection of carbon in the sun was difficult and noteworthy.
Originating with Sir Norman Lockyer in 1878,[25] it was ratified by
Trowbridge and Hutchins in 1887,[26] and still more decisively ten
years later by Rowland. It was, and could only have been made,
photographically. The visible carbon bands are barely discernible in
the Fraunhofer spectrum. One, however, in the ultra-violet (beginning
at λ 3883) comes out unmistakably on sensitive plates. It is due,
according to the best authorities,[27] not to elementary carbon, but
to cyanogen—that is, to carbon in its combination with nitrogen. The
fact is not easy to explain. No other compound body is known to exist
in the sun; and it might have been judged _à priori_ impossible that
any could prove capable of resisting the enormous temperature reigning
near the photosphere. Sir Norman Lockyer attempted to get rid of the
anomaly by locating the absorbing cyanogen in coronal regions, where
relative coolness must prevail; but recent observations point rather
to its presence as a shallow, deeplying stratum. It is certain,
moreover, that the line-spectrum derived from free carbon through the
exciting influence of a powerful electric spark has no counterpart in
the sun. We are then bound to admit, at least provisionally, that the
ultra-violet solar band genuinely indicates absorption by cyanogen.
There is a further complication. The green fluting, a few _shreds_ of
which were measured by Rowland among the Fraunhofer lines, makes part
of the typical hydro-carbon spectrum given primarily by acetylene. The
whole subject is indeed thick-set with embarrassing considerations;
they need careful sifting out. Carbon molecules are remarkably
sensitive in their modes of vibration, four of which have been
separately distinguished.[28] The conditions, however, prescribing the
replacement of one by another are still in large measure obscure.
Temperature is concerned, but it is not alone concerned; density,
admixture with foreign substances, perhaps variations of electrical
state, come into play. Yet the broad certainty has been gained that
carbon, in one, if not in several of its many forms, exists in the
photospheric neighbourhood. And this has an important bearing, not
only upon theories of the solar constitution, but also upon questions
of great interest regarding solar relationships with the stars.
Professor Rowland disbelieves in any fundamental difference between
solar and terrestrial chemistry. The earth, heated to the solar pitch,
would give, he affirms, a spectrum virtually identical with that of the
sun. Yet we cannot well ignore evidences, apparently valid, of some real
diversity. Even if all our “elements,” without exception, are found in
the sun, they are unlikely to occur in the same proportions there as
here; _quantitative_, if not _qualitative_ dissimilarity must be
recognised. Thus certain metals, so scarce that their ores rank as
mineralogical curiosities, produce marked effects of absorption in the
sun. Zirconium, yttrium, cerium, lanthanum may be instanced. Titanium
and vanadium are multitudinously represented in the solar spectrum.
Hasselberg ascertained for the former substance in 1896 no less than 562
coincidences with Fraunhofer lines out of a total of 718 photographed by
him from the metal.[29] On the exclusion of the feeblest rays on his
plates as being of quite uncertain origin, the percentage of agreement
rose to 88 per cent. The Swedish spectroscopist might well claim that
“the presence of titanium in the solar atmosphere is confirmed, with
even superfluous evidence, by these investigations.”
Vanadium—first registered as a solar constituent by Sir Norman
Lockyer—might be called the satellite of titanium. Where one is, the
other is sure to be not far off. Hasselberg’s recent discovery of
vanadium in the Scandinavian mineral rutile[30]—a form of titanic
acid—accentuates the relationship. Both occur, too, although very
scantily, in lead and iron ores, and just traceably in trap and basalt.
This close association may be accounted for by inherent resemblance. The
atomic weight of titanium is 48, that of vanadium 51. Both are eminently
infusible. They share the unusual peculiarity of exhibiting a strong
high-temperature affinity for nitrogen.[31] They exemplify, moreover, a
transition-stage from metals to metalloids, titanium approximating to
silicon, vanadium to phosphorus. Both, it may be added, have been
detected in meteorites. A large proportion of the numerous rays emitted
by vanadium are reversed in the sun, but somewhat faintly reversed,
except in cases of special disturbance, and with these we are not at
present concerned. It is worth remarking that titanium and vanadium,
notwithstanding their near kinship, physical and chemical, show no
coincident spectral lines. In these twin elements, if in any, a common
material substratum might be looked for. There is not the slightest
sign, however, that it exists.
The new metal germanium is an obvious solar constituent; yet it only
_lurks_ in one uncommon terrestrial product. Winkler recognised it in
the mineral argyrodite in 1886. And the sun appears to be much richer
than the earth in Sir William Crookes’s “meta-elements.”[32] The number
of these is almost indefinite; their individualisation, resting upon the
dubious principle, “one band, one element,” is often imperfect or
misleading; and many of the evasive substances, ranked for a time as
separate entities, have failed to make good their footing, and relapsed
into the condition of “sub-aggregates of atoms.” The chemistry of “rare
earths” has of late assumed a kind of departmental importance. It began
in 1794 with the extraction by the Finnish chemist Gadolin of “yttria”
from a jet-black material picked up at Ytterby, near Stockholm. “Ceria,”
detected in 1803 in the “heavy stone” from Bastnäs, was named after the
first asteroid; “lanthana,” obscurely associated with it, came to light
in 1839; “didymia,” “terbia,” “erbia,” successively followed.[33] The
opening, in 1878, of a fresh and fairly abundant source of supply in the
American mineral samarskite started vigorous inquiries into the nature
of these remarkable bodies; and Cleve enumerated in 1895 nine fully
characterised metallic bases, most of them emitting, under electrical
excitation, a brilliant array of spectral beams. The nine “rare” metals
are scandium, yttrium, lanthanum, cerium, erbium, praseodymium,
samarium, gadolinium, and ytterbium. The first four absorb strongly in
the sun, where the presence of erbium and of neodymium, a constituent of
the original didymium, is also evident. Twenty-two additional
meta-elements swell Sir William Crookes’s “suspense account,” but only a
minority are at all likely to obtain ultimate recognition as substantive
forms of matter. Perhaps the surest test of their quality will be found
in the appearance among the Fraunhofer lines of their characteristic
emissions. Those, at any rate, of terbium, holmium, and thulium should
be carefully looked for.
The light of glowing metallic vapours tends, as we have seen, to
suppress or efface the rays of non-metals. Professor Trowbridge made
some experiments in 1896 with a view to determining the conditions of
obliteration. Photographing on a single plate the spectra of pure carbon
and of an electric arc between carbons containing 28 per cent of iron,
he found that the iron ingredient sufficed very nearly to wipe out the
carbon bands in the arc.[34] “This proportion, therefore, of iron,” he
remarked,[35] “in the atmosphere of the sun, were there no other vapours
of metals present, would be sufficient to prevent our seeing the full
spectrum of carbon.” An interesting illustration was thus afforded of a
fundamental principle in solar interpretations. The principle, indeed,
has scarcely yet begun to be applied. Hitherto the absorptive effects of
each of the forty substances vaporised above the photosphere have been
considered apart. But they are not independently produced. They are
often profoundly modified by extraneous action. A systematic
investigation of the various modes in which it comes into play is
desirable, although likely to prove arduous. “There is at the present
time,” Dr. Ames wrote in 1895,[36] “no more fruitful field open to
research than that of the study of the influence of the presence of one
substance upon the spectrum of another.” And Mr. Percival Lewis’s recent
treatment of the subject has had the preliminary result of showing that
“very small traces of an impurity in a gas may cause considerable
changes in its spectrum, whether this impurity be chemically active or
not.”[37] The changes, too, are in many ways perplexing. They are
governed by no traceable rules, depending apparently, in each case, upon
intimate, and to us unknown, molecular relations with electricity.[38]
The explanation of their anomalies is evidently needed for the
satisfactory future progress of solar chemistry.
CHAPTER III.
PECULIARITIES OF THE SOLAR SPECTRUM.
The solar spectrum is densely thronged with unidentified lines. Of these
upwards of twelve thousand have been measured and registered, but lack
chemical interpretation. They are, however, on the way to receive it.
Their recognition will doubtless attend the gradual progress of
acquaintance with metallic spectra. Thus cerium, scandium, and other
bases of “rare earths” may satisfactorily account for a considerable
proportion of them, these substances emitting crowds of rays, as yet
only in part recorded. But besides, say, twelve thousand catalogued
“unknown” lines, an inestimable number remain unnoticed.[39] They are
still, as it were, “in the street”; they have not been admitted even to
the antechamber of science; the preliminary steps to their
identification have not been taken. They will of course be taken in due
time, little by little, as the photography of the Fraunhofer spectrum is
brought nearer to perfection; and to many of them chemical meanings full
of interest will certainly be assigned. Nevertheless, it can scarcely be
expected that the significance of all can ever be made plain. To the
very end, probably, a residuum will keep the secret of an origin due to
forms or conditions of matter strange to terrestrial experience.
That the _type_ of the sun’s spectrum becomes modified in the course of
ages—that it has been, and will again be different from what it now
is—may be admitted without hesitation. But this evolutionary change is
effected imperceptibly at more than millennial leisure. It might,
however, have been expected that transient alterations would manifest
themselves—alterations caused by tumultuous movements in the “reversing
layer,” or connected, possibly, with periodical outbreaks of spots and
prominences. Yet almost none of this definite and obvious character have
been noticed. Only a few lines may be set down as somewhat vaguely and
indeterminately variable. To take a few examples. In 1891 Father
Sidgreaves of Stonyhurst obtained several photographs of the group b
(magnesium and iron) in the green part of the spectrum. They showed with
excellent definition more faint lines than are contained in Rowland’s or
Thollon’s maps; yet one relatively strong in them—“Winlock’s No. 17”—was
barely discernible.[40] This indication of change does not seem to have
been followed up. Again, Sir Norman Lockyer noted in 1873 the
disappearance of a zinc line in the red (Ångström λ 6361·16), which,
nevertheless, was seen as usual in 1878, and has not since been
missed.[41] The most recent instance of the kind was vouched for from
Baltimore. To a faint, slightly nebulous line of unknown origin in the
ultra-violet (λ 3719·796) Rowland attached the note, “Variable, though
not atmospheric.” Jewell[42] describes it as situated within the shading
of a strong iron line, and as “quite distinct upon some plates, while
not visible upon others showing lines closer to the iron line, and much
weaker than the variable line,” as it originally appeared. It has also
been photographed in an intermediate condition, so that its fluctuations
of intensity may be said to be ascertained, although their law and cause
remain wholly obscure. The only hope of learning anything about these is
by continuous and minute observation, which should extend to other
suspicious cases of the same kind. Certain interesting questions might
thus be answered. For instance, are the alleged alterations connected
effects of some general disturbance, or do they occur sporadically, each
on its own account? Can they, in any way, be brought into relation with
the spot cycle? Are they visible in light taken indiscriminately from
all parts of the sun, or are they confined to special localities? These
may serve as specimens of the inquiries suggested by phenomena, perhaps
none the less significant for being inconspicuous. With the camera at
hand the task of daily comparison becomes easy and simple. As Professor
Hale wrote in 1896,[43] “Every photograph of the solar spectrum taken
with high dispersion must now be regarded as a document of great value,
which may ultimately reveal irregular or periodic changes in the
condition of the gases and vapours of the solar atmosphere.”
The Fraunhofer lines have of late forfeited their early reputation
as “constants of nature.” They are not really “fixed”; their
positions in the spectrum are affected by several minutely modifying
causes, and they cannot, accordingly, be depended upon as standards
for the most refined measurements. It is true that only the extreme
accuracy of modern methods has caused them to “step down” from the
high level of invariability, for their deviations are very small,
and might, superficially regarded, appear negligible. They are of
two kinds, physical and kinematical, the former being produced in
the very act of emission, the latter in the course of transmission.
Pressure-displacements and motion-displacements are, in fact,
respectively concerned.
Symptoms of a persistent shift of the Fraunhofer lines towards the red
were first detected in 1890 by Professor Lewis E. Jewell of the Johns
Hopkins University. Persistent, although unequal. It is not the same for
the lines of different elements; it is not even the same for all the
lines of the same element. Motion, then, is not its cause. Fortunately,
a clue was supplied by laboratory-experiments. Attentive study of the
behaviour of metallic lines under varying conditions showed that “with
an increase in the amount of material in the arc there was increasing
displacement towards the red.” “Considering the subject carefully,”
Professor Jewell adds, “there seemed no reason to doubt that the
wave-length of a line depended, to a certain extent, upon the conditions
under which the material producing the line was present in the electric
arc, the vacuum tube, or the solar atmosphere; or, in other words, the
vibration period of an atom depends to some extent upon its environment.
An increase of the density of the material, and presumably an increase
of pressure, seemed to produce a damping effect upon the vibration
period.”[44] Confirmatory results were obtained by Messrs. Humphreys and
Mohler,[45] and the assumption of a constant vibration-frequency as an
essential attribute of the ultimate particles of matter had to be
finally abandoned.
The observed changes are clearly distinguishable from ordinary
temperature effects. Lines are often broadened; under peculiar
circumstances they may be unsymmetrically broadened by thermal
influences; but simple displacements are never due to heat. Moreover,
they can be produced artificially by condensing the air about an
electric arc, so that their immediate cause is not doubtful. “It was
often easy,” according to the Baltimore investigators, “to observe a
line gradually change its position while the pressure was being let off
without alteration in width or other appearance.”
The general upshot of their inquiries[46] was to show that the spectral
shifts in question, far from being an isolated phenomenon, stand in
close relationship to all the most intimate properties of matter. Their
amount, _cœteris paribus_, is proportional to the pressure and to the
wave-lengths of the shifted lines. It differs, however, for each series
in a given spectrum. For different substances it is usually large or
small in the inverse ratio of the absolute temperatures of their
melting-points. Again, it is largest for those substances which expand
most readily with heat. Finally, and most significantly, line
displacements are, in the same group of elements, proportional to the
cube roots of their atomic weights. Or, as Mr. Humphreys expresses it,
“The shift of similar lines is a periodic function of atomic weight, and
consequently may be compared with any other property of the elements
which itself is a periodic function of their atomic weights”—that is to
say, the measured displacements show recurring maxima and minima in
passing from one to the next of Mendeléef’s elemental families. Their
gradations thus correspond with those of other physical attributes of
material species, and plainly imply that the retarded vibrations are
executed by “ultimate” atoms. The confirmatory fact should be noted that
band-spectra, universally associated with aggregates of atoms, display
no sensitiveness to pressure. And by pressure in this connection is to
be understood, not the separate density of the vapour emitting the
damped rays, but the total pressure of all the substances promiscuously
diffused throughout the stratum or enclosure.
Its complex effects add, in some respects, to the difficulty of
interpreting spectral appearances; but they lend to them, on the other
hand, new and unlooked-for significance. In solar inquiries more
particularly, they have started a fresh lead, sure to be followed up.
Thus indications may be gathered from them as to the relative altitudes
in the sun’s atmosphere at which different Fraunhofer lines originate,
no less than as to the absolute pressures to which they correspond.
These are lower than might have been anticipated. They range, according
to Professor Jewell, “from little more than zero to only two or three
atmospheres, though the shading of the stronger lines may be produced at
a greater pressure.”[47] The subject, however, has not got beyond the
stage of inception. One important branch of it, the discrimination of
lines belonging to the same series, is barely sketched. The phenomena of
displacement through pressure evidently involve much more than is yet
apparent. They must be present in stars and nebulæ, and may afford
curious disclosures regarding their states of density and rarity.
The Fraunhofer lines are, as a rule, narrow and sharp; but minute
photographic study reveals, in a certain proportion of them, singular
complexities of structure. These are illustrated from Professor Jewell’s
observations in Fig. 4, which shows graphically, by four typical
examples, the comparative distribution of light in corresponding solar
and arc lines.
[Illustration:
FIG. 4.—Curves representing the Distribution of Light in Fraunhofer
and Arc Lines (_Astroph. Journ._ vol. iii. p. 100).
]
No. i., a green ray of iron, is bordered in the sun (where it is of
course reversed) by a filmy illumination, “the remains of an emission
line, either produced at the photosphere, or lower down in the solar
atmosphere than the absorption line.”[48] The notch at the summit of the
same line gives evidence of radiation at a high level. It is, in fact,
an _abortive_ bright iron line, superposed upon a strong absorption
line, itself superposed upon a faint effusion of light of identical
quality from underlying vapour. Thus this single line is built in three
stages, although the foundation and coping are barely discernible as
traces of luminosity. In Nos. ii. and iii., “shaded lines” are depicted
in the same manner as the “sharp line” in No. i. They belong
respectively to iron and magnesium (λ 5183·8 = _b__{1}). Their
characteristic feature is the outlying obscurity, which deepens from the
edges towards the central shaft. Professor Jewell remarks that the gas
producing these shadings “extends through a much greater range of
pressure” than that giving rise to the green iron line (No. i.), while
the clean-cut line in the middle must be due to absorption “much higher
up in the solar atmosphere, where the pressure is very much less.”
Similar appearances are conspicuous in the great calcium pair H and K
(see Fig. 4, No. iv.). Here the abnormal breadth of their wing-like
appendages proves that the “absorption must persist through an extreme
range of pressure, or that the amount of calcium gas varies enormously
in the solar atmosphere where this absorption is produced.” These lines
are obviously twice reversed. A stratum of radiative calcium is
apparently interposed, in the sun’s neighbourhood, between two
absorptive strata of the same material. That their arrangement is,
however, subject to some kind of disturbance is indicated by the
irregularity of the diagram; nor is it always disturbed to the same
extent. “Upon some plates,” Professor Jewell says, “the central
absorption line is almost symmetrical with respect to the emission line,
while upon other plates its unsymmetrical character is very marked, the
central line being displaced considerably towards the red, and the part
of the emission line on the violet side of the central line being much
the strongest.” Motion-displacements due to ascending and descending
currents are thought to be in question,[49] but there are obstacles to
be removed before this explanation can be unreservedly accepted. The
extreme difference of velocity suggested by the observed dissymmetry of
the calcium lines amounts to no more than 75 miles a minute, but is
notably variable. All the “shaded lines” in the spectrum appear to be
similarly affected, though in a minor degree. Thus the descending motion
corresponding to the narrow central components of the sodium “D” are at
the rate of barely one-fifth of a mile per second. Most of the fainter
lines, on the contrary, indicate ascending currents over the solar
surface at an average speed of about a third of a mile a second.[50]
Motion-displacements, besides, due to the earth’s rotation and the
eccentricity of its orbit, can be detected, and have been allowed for.
These latter minute corrections naturally apply to all the solar lines
without distinction.
Enough has been said to give an idea of the manifold considerations
which have to be taken into account in estimating the _true_
wave-lengths of the solar absorption rays. They are changed, according
to a special and complex law, by the sun’s rotation; they are changed by
the movements of approach or recession of the earth as a whole, as well
as of each particular spot on the earth; they undergo alteration through
the solar atmospheric circulation; they are affected by pressure,
perhaps by other undetected influences, and each of these modifying
causes acts variably, either in time, or according to locality on the
solar surface. Happily, most of them act only to an infinitesimal
extent; but their unquestionable, although slight effectiveness
illustrates very strikingly the subtlety which every increase in
accuracy necessitates in the methods of science.
An embarrassing peculiarity of the Fraunhofer lines is their virtually
uniform intensity all over the sun’s disc. Just as the telluric bands
develop with the sinking of the sun, they ought to become strengthened
near the limb; yet they remain sensibly the same, notwithstanding the
greatly augmented depth of the absorbing strata traversed by the light
before reaching the eye. This is really a glaring anomaly, and one
almost forgotten through sheer hopelessness of getting rid of it.
In concluding this brief chapter we would once more draw attention to
the curious individualities of the Fraunhofer lines. They are
constructed, in many cases, at successive levels; they are modified by
various influences. Some are of hair-like fineness; others, emanating
from an identical substance, have nebulous edges. Moreover, the sharp
and the diffuse lines respond differently to pressure, so that their
characteristic aspects are significant of profound distinctions in their
mode of origin. The more closely, in fact, these mysterious rulings are
examined, the less trivial or casual their slightest diversities appear.
They are charged with meaning, transcending, in part, our actual powers
of interpretation, but challenging efforts towards that end, which
cannot fail to breach, if they do not wholly raze, the ramparts of
ignorance.
CHAPTER IV.
THE REVERSING LAYER.
During the eclipse of 22nd December 1870 a new phenomenon came into
view. Professor Young of Princeton, New Jersey, was the fortunate
observer. With the slit of his spectroscope tangential to the sun’s limb
and perpendicular to the moon’s advance, he awaited the moment of second
contact. The thin solar crescent narrowed second by second; at last it
vanished; then “all at once, as suddenly as a bursting rocket shoots out
its stars,” the ordinary Fraunhofer spectrum previously visible was
replaced by a serried array of bright lines on a dark background. They
seemed a complete reversal of the familiar absorption-rays, and the
impression was also conveyed to Mr. Pye, a member of the same party, of
“all the dark lines being converted into bright ones.”[51] The “flash”
at the edge of the eclipsed sun was not unlooked for. Something of the
kind had been anticipated as the due accompaniment of the beginning and
end of totality. For Kirchhoff’s explanation of the Fraunhofer lines
implied the interposition, between the eye and the sun, of a screen of
glowing vapours, which should be separately, if only instantaneously,
visible on the withdrawal of daylight glare—visible, that is to say,
spectroscopically; with the telescope nothing more distinctive than a
silvery shimmer[52] corresponds to the dazzling variotinted fireworks
disclosed by the prism.
But their disclosure was not enough; they demanded close investigation.
The question is fundamental in solar physical theory whether the flash
is the true reversal of the Fraunhofer spectrum, and no conclusive
answer could be given to it except by photographic means, visual reports
as to the details of so intricate and evanescent an apparition counting
for very little. Twenty-six years, however, elapsed before a permanent
record of it was secured. The result ensued from a skilfully-timed
snapshot by Mr. Shackleton at Novaya Zemlya during the Arctic eclipse of
9th August 1896. He gave an exposure of half a second with a “prismatic
camera”—a simple form of spectrograph, destitute both of slit and
collimating lens, the employment of which in eclipse work has been
vigorously promoted by Sir Norman Lockyer. An impression was thus caught
of singular interest and value. We may quote Professor Young’s
description of it. “The photograph,” he writes,[53] “shows a long range
of several hundred bright curved images, of which there are nearly 250
in the blue portion of the spectrum between F and H. About 25 are much
more extensive and conspicuous than the others, and are images of the
chromosphere and prominences. They are due to hydrogen, calcium, helium,
strontium, and one or two other elements which often appear in the
chromosphere. The rest are simply reversals of the Fraunhofer lines, as
Mr. Shackleton has shown by developing the flash spectrum into a
bright-line spectrum of the usual form (which is easily done by a simple
mechanical contrivance), and comparing it with an ordinary dark-line
solar spectrum photographed with the same camera and prisms, but with
the addition of a collimator and slit. The agreement is practically
complete, although there are two or three somewhat conspicuous
Fraunhofer lines which are missing in the flash spectrum, probably
because they originate, not above the surface of the photosphere, but in
its depths, as probably also do the wide, hazy shadings that accompany
the H and K lines and some others; but this is a matter for further
investigation.”
The solitary success of 1896 was manifolded a year and a half later.
“Reversing-layer” photography stood in the forefront of the programme of
work for the Indian eclipse of 22nd January 1898, and the documents
collected during its hundred seconds of obscurity showed that a complete
mastery of the art had been attained. Sir Norman Lockyer and Mr. Fowler,
Captain Hills, Mr. Evershed, and Professor Naegamvala secured
photographs of the flash, not only in its full development, but also
when incipient and vanishing, so that the phenomenon could be traced at
leisure throughout its brief phases. Two specimens are reproduced in
Plate III., both taken instantaneously by Mr. Evershed with a prismatic
camera of 2¼ inches aperture and 36 inches focus.
The conventional straight appearance of the lines results from the
employment of a cylindrical lens to give breadth to narrow slices of the
curvilinear originals. The upper section represents the spectrum of the
last thread of sunlight just as the accustomed dark lines were fading
out before the incoming of their bright correlatives. The range is from
below H and K—the strong pair to the left—to λ 3350 in the
ultra-violet.[54] The lower section corresponds to a moment twenty
seconds later, when the continuous light was gone, and vivid rays
dominated the field. Amid the throng, twenty-seven members of the
hydrogen series are recognisable, and three titanium lines rival them in
importance.
Ample materials were provided during the eclipses of 1898, 1900, and
1901 for at least a preliminary discussion as to the true character of
the reversing layer, although the difficulties still remaining to be
encountered are neither few nor trivial.
[Illustration:
PLATE III.
Flash and Cusp Spectra compared. Ultra-Violet Region λ 4100 to λ 3350.
]
To begin with, the fact has been ascertained that a shell of mixed
incandescent vapours, five or six hundred miles thick, encloses the
photosphere on every side. We see it, however, in projection. The line
of sight penetrates it tangentially at the edge of the sun, and to an
extreme depth near the base of about forty thousand miles. Between the
reversing layer and the chromosphere there is no solution of continuity;
to some extent, undoubtedly, the lower merges into the upper formation;
yet they are essentially distinct. Each has its own spectrum apart,
notwithstanding a certain amount of community, apparent, casual, or
partial. Thus since the reversing layer is visually accessible only
through the enveloping chromosphere, the spectrographic prints taken at
sun-and-moon contacts are inevitably composite. They include the
chromospheric together with the “flash” rays. Discrimination is,
however, facilitated by the notably greater length of the arcs
representative of the former corresponding with the higher extension of
the substances emitting them. The class of discrepancies between the
flash and the Fraunhofer spectrum thus accounted for are discrepancies
by excess. The flash includes helium rays; the Fraunhofer spectrum has
none. The flash exhibits the complete hydrogen series up to its extreme
limit in the ultra-violet; the Fraunhofer spectrum reverses only its
visible members. Titanium lines strong in the flash are feeble in the
Fraunhofer spectrum; besides other analogous dissimilarities. Yet they
do not affect the claim of the reversing layer to be, speaking broadly,
the locus of solar absorption. Rather they bring us face to face with
the totally different question, Why do the chromospheric gases exercise
no appreciable arresting effect upon the light transmitted through them?
Later on we shall attempt to answer it; here we need only remark that
the rays from the chromosphere cannot be excluded from photographs of
the flash. They necessarily appear in them, and it was known beforehand
that they had no counterparts in the Fraunhofer spectrum.
There are besides discrepancies by defect. Many solar absorption lines
do not show bright at the first and last instants of totality. But this
is easily understood. Some are doubtless too faint to assert their
presence photographically. Others may be supposed, with the utmost
probability, to originate out of sight at the base of the reversing
stratum. The shadings of H and K certainly do, for the central lines
start out clear, though strong, in the flash, and their hazy appendages
are indubitable products of augmented pressure. Nor do the denser
vapours rise high enough to make any perceptible display. Mr. Evershed
tells us[55] that, while nearly all the metals with atomic weights less
than 60 are represented in the sun’s marginal spectrum, none, of which
the ultimate particles are heavier than 92, make any assured
contributions to it. A formation at least five hundred miles in vertical
extent must vary widely between top and bottom both in composition and
density. So at least we should reasonably anticipate. In point of fact
the changes indicated are surprisingly slight. One criterion is
available by which chemical differences can be correlated with
differences of depth. This consists in the various lengths of the curved
lines representing the emissions of the sundry constituents of the
“layer.”[56] Substances attaining high altitudes, like the chromospheric
gases, give long arcs because their visibility extends over a wide
section of the sun’s circumference, while low-lying materials,
illuminating a narrow verge, are characterised by short arcs. They are,
moreover, the most difficult to catch as the moon goes by. Now the great
majority of the flash lines are of a nearly equal length, corresponding
to an arc of about 40° on the solar limb, and this equality implies a
considerable approach to uniformity of constitution throughout the
greater part of the momentarily exposed layer. But its basal stratum,
perhaps not more than a few miles in thickness, should be that most
effective in absorption,[57] and it forms a crescent much too fine to be
directly seen. Here below, then, down near the photosphere, missing
Fraunhofer lines with no apparent corresponding radiations may be
produced; nay, _must_, since there is absolutely no evidence of the
corresponding light-stoppage taking place in, or above the
chromosphere.[58] Mr. Evershed’s conclusion is indeed fully warranted,
that “the flash spectrum as a whole appears to represent the upper more
extensively diffused portion of a stratum of gas, which, by its
absorption, gives the Fraunhofer spectrum.”[59] The appellation
“reversing layer” would then seem to be no misnomer, but to indicate
correctly the seat of the linear absorption which serves as our alphabet
for spelling out the secrets of solar chemistry.
The density of this vaporous envelope is measurable by the “pressure
shifts” of the Fraunhofer lines. It would seem to be nowhere _less_ than
that of our atmosphere at sea-level; otherwise displacements towards the
_blue_ end should occur, and none such are perceptible. Nor, on the
other hand, is there proof of its exceeding, even in the lowest depths,
three or four times the standard value. So that the increase of pressure
downward is exceedingly slow—a fact to be carefully noted. The
distinction (already adverted to) between the effects of total and of
partial pressure is also most important. Through the former the
positions of spectral rays are modified, through the latter their
characters. In other words, the shifts of the Fraunhofer lines
correspond to the sum of incumbent vapours, while the quantity of each
separately present determines their width and diffuseness. Most are
associated, by their fineness and sharpness, with _individually_ tenuous
substances. The hydrogen lines, for instance, represent, according to
Mr. Maunder,[60] a pressure of only one-hundredth of an atmosphere. But
the indications in this respect, as in others, vary greatly for the
different vapours.
The most refractory substances, such as titanium and vanadium, are
volatilised in the reversing layer. It is, then, enormously heated. But
it is cooler than the photosphere, since its rays show dark against the
vivid background they are projected upon. Now the temperature of the
photosphere, by the most authentic recent determination, is about 6600°
centigrade,[61] and this marks an upper limit for the temperature of the
reversing layer. A lower limit is fixed by the temperature of the
electric arc, estimated at 3500°. The much higher grade of the spark is
almost certainly not attained. The inverse behaviour of two magnesium
lines, first commented upon by Professors Liveing and Dewar,[62] led Dr.
Schemer to this conclusion.[63] One at λ 4352 is prominent in the sun
and strong in the arc, but fades out in the spark; the other, at λ 4482,
of which a mere trace is perceptible in the sun, is a characteristic
spark-product. It must, however, be borne in mind that comparative
temperatures are subject to great uncertainty where electricity is the
exciting agent. Dissentients are even to be found from the broad
proposition that the spark is hotter than the arc; nor is it one capable
of direct demonstration. Qualifying circumstances come in, and their
separate effects are not easily unravelled.
M. Deslandres made some curious experiments at Paris in 1894 in
photographing the sun by means of the dusky rays in its spectrum.[64]
For their isolation he used a “double slit”; and since they are only
comparatively dark, no difficulty was encountered through want of
actinic power in the rays dealt with. He thus succeeded in obtaining
with each a monochromatic picture of the sun delineated exclusively with
emissions from some particular ingredient of the reversing layer. The
uniformity of elemental distribution was, by this ingenious device, put
to the test. Photographs of the disc, for instance, taken on an iron
line might be expected to show different features from those taken on
calcium or magnesium lines if local accumulations of those vapours were
present; but no divergences of the kind became perceptible. The
composition of the absorbing envelope did not seem to vary _regionally_.
The investigation, however, was not carried far, and would be worth
prosecuting.
The _fact_ of the existence of a true reversing layer may now be looked
upon as established; yet the _mode_ of its existence remains in several
ways perplexing. The slightness of its absorptive action needs
explanation at the outset. One notes with amazement that the miniature
atmosphere surrounding an electric arc is equally effective for
light-stoppage with this ocean of vapours. Then there is the singular,
and perhaps related circumstance that the spectrum from the limb is not
more deeply grooved than the spectrum from the central parts of the
disc. The results upon light of being sifted through six hundred and
through twenty thousand miles of the mixed materials glowing near the
sun are virtually the same. Their comparative tranquillity, too, is
unexpected. The reversing layer lies between two agitated structures.
Beneath are the photospheric clouds, rent and whirling under the stress
of cyclonic disturbances; above, the chromospheric flames, driven hither
and thither by influences of fantastic violence. Yet a region of peace
seems to intervene. The Fraunhofer lines indicate a steady vertical
circulation, but scarcely ever a temporary commotion. By a rare
exception, Father Fényi observed at Kalocsa, 27th July 1887, the dark C
in the neighbourhood of a spot-group, displaced alternately towards the
blue and the red, indicating, he supposed, a powerful disturbance of the
reversing stratum by an irruption of hot hydrogen.[65] Such invasions of
its precincts, however, are under the ban of some prohibitive decree, or
they encounter unknown difficulties. They occur, at any rate, with
remarkable infrequency.
The reversing layer is heated from below, and gravitates downward.
Thermal equilibrium is doubtless maintained by the convective transport
of material, but the due effects of superincumbent weight are
unapparent. Evidence is not indeed wanting of _some_ increase of density
with descent, but of an increase relatively insignificant. Gravity at
the sun’s surface possesses nearly twenty-eight times its terrestrial
power; hence a true solar atmosphere should double its density with each
furlong of approach to the sun’s surface,[66] and the total increase of
pressure in an envelope five or six hundred miles deep would be
“inexpressible by numbers that have name.” Actually there is, at the
most, a quintupling of pressure. This formidable discrepancy is
altogether unexplained. We are debarred by it from considering the
reversing layer to be in statical equilibrium. Its successive strata do
not rest one upon the other under the sole dominion of gravity. Some
counteracting influence is brought to bear. This problem of _levity_—so
to call it—is one that perpetually recurs in studying the solar
surroundings.
CHAPTER V.
HYDROGEN, HELIUM, AND CORONIUM.
Three tenuous gases—hydrogen, helium, and coronium—are of essential
importance in solar physics. The first plays also a leading part in
terrestrial and vital economy. The second exists on the earth merely as
a chemical curiosity. The third must for the present be classed as an
exclusively solar product.
Solar hydrogen was discovered by Ångström in 1862. He recognised it by
the identity of its three least refrangible rays with the Fraunhofer
lines C, F, and G[67] (now designated Ηα, Hβ, and Hγ), to which, in
1865, he associated the indigo line _h_ (Ηδ). A fifth line (Hε),
photographed by H. W. Vogel in 1879, is situated quite close to the
calcium H—so close that, like Teucer behind the shield of Ajax, it
lies concealed, in the sun, under covert of its neighbour’s broad
shadow, if indeed it be present in the Fraunhofer spectrum at all; for
it is so effectually hidden that the point remains uncertain. Shortly
afterwards, Sir William Huggins’s spectrographic investigations of
Sirian stars gave the key to the true character of the hydrogen
emissions. Nine ultra-violet lines came out on his plates, and their
rhythmical arrangement at intervals continually lessening upward left
no doubt of their forming a connected series. That this included the
visible lines was manifest at sight. Its law was stated by Balmer in
1885.[68] The relations expressed by his formula are not those of
wave-_lengths_, but of their reciprocals, wave-_frequencies_. These
quantities obviously bear to each other an inverted proportion. Deep
crimson light, for instance, consists of undulations about twice as
long as those of violet light; only half as many of them, accordingly,
enter the eye in a given time. A doubled length corresponds to a
halved frequency, a tripled length to a frequency of one-third, and so
on. Now oscillation-frequencies are, for several reasons, more
important natural constants than wave-lengths; hence until they were
made the basis of investigation, no real progress was effected in the
detection of spectral series.
Balmer’s law has the following form: N = N_{0} − (4N_{0})/(_m_^2), where
N is the wave-number ((1)/(λ)), N_{0} is a constant to be determined by
trial, and _m_ is any integer greater than 2. By assigning to N_{0} the
empirical value 27418·75, the places in the spectrum of each individual
ray emitted by hydrogen may be calculated with approximate accuracy.
That of C (Hα) corresponds to _m_ = 3, and the series has been
photographed up to _m_ = 34, its constituent lines growing fainter and
more crowded as the scale is ascended. They approach, in fact, with the
increase of _m_, indefinitely near to a definite limit, marked by the
constant N_{0} minus 0 (the second term having disappeared). This limit,
known as the “convergence frequency,” is a distinctive feature of
spectral series.
Many have contributed to their elucidation. Johnstone Stoney,[69]
Alexander Herschel,[70] Hartley,[71] and Cornu[72] prepared the ground,
and the subject was treated, in its larger bearings, and with more
definite results, by Liveing and Dewar,[73] Schuster,[74] Rydberg of
Lund,[75] Runge and Paschen[76] of Hannover, Kayser[77] of Bonn, and
Ames[78] of Baltimore. Their labours have been unexpectedly successful
in educing partial order out of all but total emissive confusion.
Harmonic series of identical type were marshalled from promiscuous
throngs of rays, and their association into sets of three, or even into
double sets of six, simultaneously given forth by a single element,
proves the extraordinary complexity of the molecular systems through the
movements of which they originate. That some of these movements are of
an orbital nature is strongly indicated, and they not improbably show
perturbative effects analogous to those manifested in lunar and
planetary revolutions. “The final impression,” M. Balmer writes,[79]
“which our mind involuntarily receives in contemplating these
fundamental relations is that of a wonderful mechanism of nature, the
functions of which are performed with never-failing certainty, though
the mind can follow them only with difficulty, and with a humiliating
sense of the incompleteness of its perception.”
Until 1897 the spectrum of hydrogen was thought to be of unique
simplicity. It apparently consisted of one individual series resembling
that formed by a musical note and its overtones. No outstanding lines
interrupted the perfect regularity of the progression. All this,
however, was changed by Professor Pickering’s discovery, in a few
peculiar stars, of a second hydrogen series.[80] It is associated with
the first in such a manner as to indicate that both are subordinate to a
principal series, the three together forming a triple group on the
normal pattern. Of the principal series, one member has been probably
identified as a blue band in certain “bright-line” stars,[81] the rest
being placed inaccessibly high up in the ultra-violet. They would be cut
off by atmospheric absorption. None of the new hydrogen rays occur in
the sun, and none have, so far, been rendered visible in the laboratory,
possibly because the temperatures available are inadequate for their
production. This indeed is a matter of conjecture; what is certain is
that hydrogen affords the only known example of a spectral series
capable of isolation from its fellows. Here evidently we have a clue to
some specialty of intimate structure, the guidance of which may lead to
surprising disclosures.
Hydrogen has other singularities. In some respects it is solitary among
the elements. The “periodic law,” by which their properties are
connected with their atomic weights, does not apply to it. Chemically
and electrically it behaves as a metal; reduced to the liquid state,
however, it definitely ranges itself with non-metals. Its condensation
is effected with the utmost difficulty, physical and mechanical agencies
being only just competent to vanquish the elasticity of this lightest of
terrestrial substances. But what force can barely compel, affinity
readily obtains. United to oxygen under the form of water, it can exist
as a liquid up to a temperature of 100° C., and it is of all gases the
most readily “occluded.” Imprisoned thus in metallic masses, it remains
inert for unlimited periods, but recovers freedom and activity by heat.
Meteoric irons bring to the earth no inconsiderable supply of occluded
hydrogen, and palladium can take it up to the extent of six hundred
times its own volume. In this quasi-combination it is, by a curious
anomaly, six times denser than when liquefied by sheer cold.[82]
The volatility of hydrogen perhaps transcends the earth’s power of
control. By a necessary consequence of the kinetic theory, adverted to
by Dr. Johnstone Stoney in 1870,[83] light gases in a free state can be
permanently retained only by massive globes. For atmospheric particles
no sooner attain a speed just overbalancing the holding power of gravity
than they irrevocably fly off into space, and the process being
continued unintermittently, eventuates in the total dissipation of the
envelope they once constituted. It is, however, a matter of some
delicacy to discriminate between the gases that may escape from any
individual planet and those that must remain. According to a recent
calculation,[84] the earth could now maintain a hydrogen atmosphere
virtually without waste; but in former ages, when the agility of the
gaseous molecules was quickened by heat, the strength of its grasp upon
them must have been insufficient for their lasting retention. This was
nevertheless effected by their reduction to the liquid state in the form
of water. The presence of an excess of oxygen hence saved terrestrial
hydrogen.
Only the four lowest members of the hydrogen series show dark in the
sun.[85] The absence of the higher rays is enigmatical. All are ablaze
in the chromosphere; but the chromospheric gases emit sensibly as much
light as they stop. In the reversing layer it would then seem that
hydrogen glows so imperfectly as to emit vibrations of no more than four
or five qualities, the upper “notes” being somehow quenched. It might be
supposed that the temperature there is too low for their production,
were it not that they have been photographed from vacuum tubes held, on
good grounds, to be cool relatively to the electric arc. The true
explanation is probably to be found in the heterogeneous composition of
the stratum in question. Intermixed particles of different kinds of
matter mutually check each other’s oscillations, and those of shortest
periods are the most susceptible to this adverse influence. Its nature
and the laws of its action remain obscure, but much may be learned about
them by careful experimental inquiry.
A similar anomaly is more markedly visible in the case of helium. This
gas exists near the sun in scarcely less profusion than hydrogen, yet
the Fraunhofer spectrum includes no trace of its action. Absorptive
nullity is not a quality inherent in the substance, as we shall see
presently; hence it probably depends, like the partial inertness of
hydrogen, upon conditions present in the reversing layer.
Until March 1895 helium was known only as a chromospheric element. A
bright yellow ray at λ 5876, distinguished as “D_{3},” because it forms
a trio with the sodium pair D_{1} and D_{2}, was noticed in the
prominences uncovered during the eclipse of 18th August 1868, and can
always be observed spectroscopically at the edge of the sun. But the
substance emitting the yellow ray lay outside the range of our
acquaintanceship, and seemed unlikely to be brought within it. That
contingency, nevertheless, came to pass. In the course of a search for
compounds of argon, Professor Ramsay, at the suggestion of Professor
Miers, fortunately examined the reputed nitrogen occluded by the
Scandinavian mineral “clevite.”[86] This velvety-black stone, remarked
as peculiar by Nordenskiöld and analysed by Cleve, is a kind of
pitch-blende, composed of uranate of lead mixed with rare earths. The
gas evolved from it at University College gave a brilliant spectrum, in
which the prominence-line D_{3} shone conspicuous. Helium was indeed
captured! A beautiful confirmation of the identity was soon afterwards
afforded. The golden line seen in the laboratory was perceived by Runge
to have a faint close companion, and he declared that, unless the solar
D_{3} were also double, clevite-gas should be regarded as different from
helium.[87] The challenge was taken up on both sides of the Atlantic.
Professor Hale on 20th June, and Sir William Huggins independently on
10th July, succeeded in resolving the prominence-ray into a delicate,
unequal pair, and our possession of helium as a truly indigenous element
was rendered incontrovertible.
[Illustration:
FIG. 5.—Diagram of the Helium Spectrum.
]
Meantime sundry other leading chromospheric rays—four especially,
coloured deep red, green, blue-green, and intense violet[88]—had been
recognised in the complex spectrum of clevite gas.[89] The task,
however, of reducing its tangled rays to harmonic order seemed desperate
until it was performed. Without exception, they ranged themselves, at
the bidding of MM. Runge and Paschen, into six related series (see Fig.
5). These form two sets, each consisting of a subordinate pair drawing
together towards a common limit in the ultra-violet, with a principal
series “leaping over the other two in large bounds,” and ending in the
more refrangible part of the spectrum.[90]
Not only did Runge and Paschen’s formula (which may be regarded as a
modification of Balmer’s law for hydrogen) include all the perceptible
emissions of helium, but it intimated the presence of others beyond the
reach of ordinary observation. Each of the principal series, it was
inferred, should own a “leader line” far down among the heat rays, and
with the aid of the bolometer the prediction was strictly verified. Thus
“the actual spectra” (as Mr. Maunder remarked) “corresponded to the
theoretical, and were complete from their rise far in the obscure
regions of the infra-red till they died away in the darkness which lies
on the other side of the visible spectrum.”
Their number, however, suggested a twofold origin, since there was then
no precedent for assigning more than three series to a single substance.
Clevite gas was accordingly regarded as a mixture of two solar elements,
distinguished as “helium” and “parhelium,” the rays of the former, like
D_{3}, being all double, those of the latter single. Each set of three
series was, in fact, “analogous to the complete spectrum of a distinct
element.” Yet parhelium has failed to make good its footing in either
cosmical or terrestrial chemistry. Attempts to isolate it have entirely
failed, and the spectroscopic argument for its existence collapsed with
the discovery that oxygen, no less than clevite gas, claims six series,
which are certainly inseparable, and represent in combination the
vibrations of perfectly similar, highly intricate molecular systems.
“Parhelium” may then safely be treated as fictitious. Clevite gas, or
mineral helium, is the identical undiluted material of prominences. The
hypothetical companion-stuff exists neither in the sun nor upon the
earth.
The qualities of helium are most unusual. Like argon, it is monatomic;
its ultimate chemical units are the same as its ultimate mass-units.
This inference is based upon the heat-relations of the substance. Its
vapour-density is hence only half its atomic weight; for the molecule of
helium possesses nearly double the mass of the hydrogen molecule; and it
is, by hypothesis, indivisible, while that of hydrogen includes two
combining atoms. But helium has no “atoms,” or rather its molecules
_are_ its atoms. Its chemical equivalent is accordingly 3·96 on the
hydrogen scale. A value indeed rather nominal than real, since helium is
devoid of sensible affinities. It enters into no combinations. It again
resembles argon in being a “rogue” element. To both equally, one of the
ordinary properties of matter is wanting. They form with three other
inert gases a class apart as “non-valent” substances.[91] In choosing
its mineral cloisters, helium showed, nevertheless, some original
preferences. The heavy metal uranium has a special attraction for it,
and it is constantly associated with rare earths. Once released,
however, it can scarcely be re-incarcerated. Mr. Tilden’s experiments
led him to conclude that helium-yielding rocks must have been
primitively charged under a pressure of several hundred atmospheres.[92]
The earth may in those early days have possessed a vast helium-envelope,
since dissipated in space. Opinions differ on the subject,[93] and data
for precise calculations are wanting. But the probability is strong that
the helium now sparsely lurking on our globe is a mere remnant of a far
ampler store, which terrestrial potencies, whether gravitational or
chemical, were incompetent to hold.
Helium scarcely retards the passage of light. Its refractive index,
which is the smallest known, is expressed by the fraction 0·146, those
of air and hydrogen being respectively 1·0 and 0·5. Still more
unexpected than its low refractivity, is its high conductive power for
electricity. Professor Ramsay ascertained that the “sparking distance”
through helium at atmospheric pressure is nearly 300 millimetres, while
the same current is stopped in hydrogen by a gap of 40, in oxygen by a
gap of just 24 millimetres.[94] Another surprising property of this gas
is its abnormal faculty of diffusion. It has a rate of self-dispersion
ten times that of hydrogen, or fifteen times what, by Graham’s law, it
ought to be. On the other hand, it has the lowest solubility on
record;[95] water absorbs it in evanescent quantities. This led to the
anticipation, amply justified by experience, that helium would prove to
be one of the most obstinately gaseous bodies in existence. It has not
indeed yet (October 1902) surrendered to the compulsion brought to bear
by Professor Dewar in his memorable researches at low temperatures.
Still, the cooling efficacy of liquid hydrogen evaporating under
exhaustion may be expected finally to overcome its all but invincible
recalcitrance, and the “salamander gas” of the chromosphere will assume
the guise of a frigid fluid boiling five or six degrees above absolute
zero.
The reversing layer—properly so-called—emits apparently no helium rays.
A reason for their absence has been already suggested, and is tolerably
obvious. They are extremely sensitive to damping influences. Foreign
admixtures readily occasion their suppression. Thus 10 per cent of
helium just shows spectroscopically in hydrogen, and that only if the
pressure in the tube is very low; while one part of hydrogen in 100,000
of helium glows manifestly when the current is made to pass.[96]
Nitrogen has a similar adverse effect upon helium-radiation, which
would, however, gain relatively in strength with diminution of pressure
in ascending through the reversing-layer into the chromosphere.
Of the chief coronal ingredient no terrestrial trace has yet been found.
A bright green ray observed during total eclipses is its only assured
badge, for eight or nine other more refrangible associated rays may
quite possibly emanate from different substances. As the leading gaseous
constituent of a structure on the borderland of nothingness, coronium
must be an unimaginably subtle form of matter. It exists in prodigious
volumes near the sun, rising to heights altogether inaccessible to
hydrogen or helium, yet under conditions differing from those of an
ordinary atmosphere. Successive coronal strata are not mutually
superincumbent. There is no sign that their density increases downwards.
The characteristic green line is no less fine and sharp given out by the
inner than by the outer corona. No reversal of it has ever been
detected. There is no corresponding Fraunhofer line. Thus the substance
most plentifully present about the sun is, by a strange anomaly,
absolutely passive as regards its light. This is most probably a result,
not of any specific incapacity, but of the peculiar state in which it
subsists. The real qualities of coronium, however, and its entire
spectrum can only be ascertained by laboratory investigations. That
these will ever become practicable it would be rash to assume, but it is
permissible to hope. There seems, at any rate, no valid reason why
coronium should not be added to the number of unearthed or frozen-out
recondite gases. The former possession by our planet of a
coronium-atmosphere may be plausibly surmised. But it most likely
vanished still earlier than that of helium. Had its material been
endowed with chemical affinities, some compound or compounds should have
preserved it more or less abundantly. It would have been detained, as
hydrogen was detained in water, and kept available for our late
acquaintanceship. Since no compound of the kind appears to exist,
coronium presumably resembles helium in being “non-valent.”
CHAPTER VI.
THE PHOTOSPHERE AND ITS DUSKY VEIL.
The sun is virtually bounded by a spherical envelope of intense lustre.
What lies outside is negligible in mass and function. What is hidden
within has its energies concentrated, so to speak, on the maintenance of
the “photosphere” at the highest point of radiative efficiency. This
implies enormous internal activity, the slackening of which would be the
prelude to speedy extinction. The materials of a self-renewing stratum
of concentrated emission are necessarily in a state of flux. Each
constituent particle, as it delivers up its store of light and heat,
becomes instantaneously effete, and is replaced by another. Charging and
discharging processes pursue a ceaseless round, ceaseless, that is to
say, until the growth of viscosity fatally impedes them. When that time
comes, convection-currents cease to flow, superficial cooling advances
rapidly, and the sun-like stage terminates. The epoch of inertness must
in fact arrive when, for a circulatory, a fixed surface is substituted.
During some long antecedent period, again, the same body was presumably
too rare to be definitely limited, and might fitly be designated a
nebula. So that a “sun” is definable as a gaseous mass clothed with a
pellicle of dazzling luminosity, and organised for long continuance in
the capacity of a distributor of light and heat.
The shining pellicle of our sun is, to all appearance, of a cloud-like
nature. It is a locus of condensation, where uprushing gases, chilled by
expansion, momentarily change their state, and thus acquire the power of
suddenly parting with their stored energy. The “mottlings” of the
photosphere mark visibly, perhaps, this rapid course of interchange,
brilliant floccules denoting regions of _arterial_ ascent, dusky tracts
those of corresponding _venous_ descent. That it is accompanied by
violent turmoil, the evidence of the camera shows conclusively. The
reticulated areas are highly evanescent. Ridges and brilliant cumuli,
some hundreds of miles in extent, form only to be swept away. “When we
come to study the minute details of the granulations,” Professor Young
writes, “we find movements at the rate of a thousand miles an hour to be
the rule rather than the exception.”[97]
Since photospheric light is purely continuous, photospheric chemistry
remains a _terra incognita_. Only conjectures are possible regarding the
kind of matter present in the solar condensations. The idea that they
may be formed of carbon, started by Dr. Johnstone Stoney in 1867,[98] is
still very generally entertained. It is indeed hampered by difficulties
at present insurmountable; but the same may be said of every other
hypothesis on the subject. Carbon was recommended for the position
assigned to it by its refractoriness to heat and by its great radiative
power. Lampblack, we need hardly say, is, in this latter respect, the
standard substance. An unfavourable peculiarity, on the other hand, is
its inability to exist as a liquid under conditions at all likely to be
realised in the sun. Carbon has no fusing-point in the ordinary sense.
At a temperature of about 3500° C. it sublimes without melting.
Preparatory to crystallising as diamond it perhaps liquefies through the
incomparable stress of molecular forces, but the process is transitory
and obscure. It has never been observed; it is only reasonably supposed
to take place. Moreover, at or near the photosphere, pressure of the
required intensity certainly does not exist. The cumuli forming it
should then consist, not of carbon droplets, but of carbon dust, and the
analogy with terrestrial clouds would disappear. A still more serious
objection is that carbon volatilises at a temperature far below that of
the photosphere. Nor are we acquainted with any kind of matter the
condensation of which might be thought of as possible under the
conditions there prevailing.
The question of temperature is fundamental in solar physics. Everything
that regards the nature, structure, and innate activity of the solar
globe depends upon the answer furnished to it. And of late the answers
have become much more plausible than those discordant to the extent of
some millions of degrees arrived at thirty years ago. The main cause of
this wide uncertainty lay, not in the actual measurements, which can be
made sufficiently precise, but in the failure to establish on secure
grounds some definite relation between temperature and radiation. There
is no doubt that thermal outflows increase far more rapidly than the
accompanying thermometric rise—that the heat received at a distance
corresponds, in an augmented proportion, to a gain of heat at the
source; but the correspondence has, until lately, been expressed only by
empirical rules, not implicitly or unconditionally to be trusted.
Boltzmann,[99] however, supplied an _ex post facto_ theoretical basis
for a law published by Stefan of Vienna in 1879, according to which
radiation grows as the fourth power of temperature. Its agreement with
facts, so far as they are available, is besides tolerably close. Yet the
security is precarious that it continues to match them in regions of
cosmic heat, unattainable by experiment. It was, however, employed by
Messrs. Wilson and Gray, with some modification and with excellent
results, in their authoritative determination of the sun’s
temperature.[100]
They adopted a method of direct observation, involving the fewest
possible uncertainties of principle. Sun-heat, allowed to fall upon a
“radio-micrometer”—an instrument of extreme sensitiveness invented by
Professor Boys—was measured by the “balancing” of its effects against
those of a strip of platinum heated to a known pitch. This gave the
means, by the aid of Stefan’s law, of translating them into terms of
temperature. Allowance had then to be made for a double absorption,
first in the sun’s, again in the earth’s atmosphere. That only a
fraction of the heat emanating from the solar condensations reached the
apparatus in the West Meath observatory was unmistakable; how large a
fraction was less easy to decide. Langley finds that the intensity of
radiation at the centre of the disc is reduced near the limb by
one-half,[101] and the total loss is estimated by Wilson and Rambaut at
one-third of the whole.[102] Hence the sun’s thermal power would be one
and a half times greater than it is if the emitting surface were
stripped of its absorbent covering, and the correction of temperature
demanded by its action amounts to at least 1000° C.
The despoiling effect of our own air has next to be considered. It is
very large, and so are the discrepancies in its valuation. Rosetti of
Padua, who in 1879 determined the temperature of the sun to be
20,000°,[103] concluded for a zenithal heat-stoppage of 29 per cent;
Langley estimated it at 41; Knut Ångström[104] in 1890, laying stress
for the first time upon the thermal opacity of the carbonic acid
ingredient of the atmosphere, obtained 64 per cent as the ratio of
absorption. This seemingly authentic result, namely, that only 36 per
cent of the heat rays striking the earth vertically are transmitted to
its surface, was provisionally admitted by Wilson and Gray, and after
having made careful allowance for various kinds of possible error, they
arrived in 1894 at an effective solar temperature of 8700° C.
Substituting Langley’s value for terrestrial atmospheric absorption, and
working up fresh experimental data, Mr. Wilson in 1901 reduced this
figure to 6590°,[105] which probably underestimates the truth. At some
such inconceivable degree of heat the undimmed photospheric clouds glow.
This is not all. The value just given belongs to an ideal stratum in the
sun. It stands for the “effective,” not the actual temperature—the
temperature, that is to say, which should be attributed to a surface of
standard radiative capacity sending out the measured quantity of heat.
Now it is certain that the photosphere falls very far short, in emissive
power, of its imaginary substitute. There is no such thing in nature as
a “perfectly black body,” or its correlative, a perfect radiator, the
efficiency even of lampblack being only six-tenths of what it is assumed
to be for purposes of calculation. And the sun is unlikely to be as good
a radiator as lampblack. It must then be hotter in proportion to its
inferiority, but to what extent it falls short of the ideal standard
remains undetermined. It must also be very unequally hot. The brilliant
granules giving its flocculent appearance to the photosphere radiate
much more intensely than the gray interspaces. Hence computed
temperatures represent an average higher than prevails in some
formations, lower than is assignable to others. It is noticeable that
several corrections based upon recent improvements in experimental data
tend to enhance our conception of the tremendous energy of solar heat.
Le Chatelier’s method[106] of employing the intensities of selected rays
in various light-sources as a criterion of temperature gave 7600° C. for
that of the sun (uncorrected for solar absorption). It is, however, of
doubtful validity. A parallel line of research was opened by Langley’s
establishment of the principle that temperature is connected by a
definite relation with the wave-length of maximum energy in the spectrum
of a radiating body. Divergent views, nevertheless, prevail as to the
_form_ of the relation. Michelson[107] and Rubens[108] agree that the
wave-length of most powerful emission varies in length inversely as the
square root of the temperature, while Paschen and Wien[109] maintain
that the simple inverse ratio tallies more closely with facts. The
outcome in determinations of the sun’s heat differs of course vastly
with the law chosen. From Michelson’s, H. Ebert deduced in 1894 a
temperature of 40,000° C., but added the qualifying remark, “The parts
of the sun to which this value applies belong to the more interior
regions; they are at any rate deep under the reversing layer, and
therefore probably below the photosphere.”[110] Now subphotospheric heat
may be of almost any intensity; hence the result, although not very
informing, is safe to be in some sense correct. Paschen, on the other
hand, obtained the low value of 5130°. It might be added that the law
upon which he relied is suspiciously simple, “in view of the known
complexity in the radiation of a solid body, and the various rates of
increment with temperature attaching to different rays.”[111] It seems
to be one of those formulæ which cannot be trusted far out of sight.
They are _not true enough_ to bear extension into regions beyond
experience. Useful over a moderate compass, they prove treacherous
adjuncts to investigation. Difficulties, indeed, all but insuperable
hamper attempts to infer the solar temperature from comparisons of
spectral energy-curves. Unexpected peculiarities are found to
characterise the modes of emission of solid bodies. Even continuous
spectra are to some extent distinctive. Thus the same quantity of energy
is very differently distributed in the rays sent out respectively by
polished and lampblacked platinum, by carbon filaments, copper, and iron
oxides; while with an equal increase of energy, the distribution becomes
diversely modified for each substance. For each, that is to say, the
maximum ordinate of the energy-curve creeps upward at a different rate.
In the absence, then, of precise knowledge as to the composition and
condition of the photosphere, inquiries as to its temperature, based on
this principle, are futile. We should first need to be acquainted, in
Professor Very’s words, with “the selective radiating power of the solar
photosphere.”[112] Generalisations are here eminently unsafe, since laws
of radiation derived from the experiments with one kind of material are
by no means certain to prove applicable to others. Besides, the
“absolute solar spectrum” (as Langley calls it) cannot be directly
observed, and the shape of its representative curve is most materially
altered by the effects of absorption in the solar atmosphere.
On the whole, the straightforward plan of attack on the problem of the
sun’s heat seems the most promising. Messrs. Wilson and Gray’s practical
operations left little room for improvement, and the uncertainties
affecting their final result will gradually diminish with the progress
of other kinds of research. As higher temperatures, for instance, are
brought under command, the range allowed to perilous processes of
“extrapolation” can be restricted. And improvements, sure ere long to be
realised, in the value assignable to telluric atmospheric absorption,
will effectually reduce the marginal errors attached to present
estimates of the _primitive_ heat-power of the sun.
Fluctuations in the sun’s heat-power must be regarded as possible, and
they might be either irregular or periodical. Indeed, the superposition
of both kinds of change would perhaps be more likely than the occurrence
of either separately. Their detection would, in any case, be extremely
difficult, although it is not, in Messrs. Wilson and Gray’s opinion, to
be regarded as hopeless. The required measures would be simply
differential; and differential measures escape many of the snares that
hamper the execution of absolute measures. But comparisons in this
matter are rendered almost nugatory by inconstancy of weather.
Variations in the “solar constant,”[113] even if real, would probably be
masked by local and temporary changes in the diathermancy of the air.
Professor Very holds that “under these circumstances refinements in
actinometry are of small avail,”[114] and he suggests “that the problem
will have to be solved entirely by meteorological methods.”[115] “If
temperature and humidity observations could be collated from the logs of
vessels crossing the torrid zone, estimates of oceanic evaporation from
day to day, combined with rainfall measures, might lead,” he believes,
“to the detection of the variation of solar radiation.” But the chance
of their doing so appears, all things considered, to be incalculably
small. The elements of disturbance are too numerous and too strong to
permit the emergence of the slight residual effects looked for. Far
preferable appears Piazzi Smyth’s plan of earthing thermometers deeply
enough to be inaccessible to superficial vicissitudes of temperature.
And it can scarcely be without significance that the readings of those
buried on the Calton Hill showed oscillations coincident in period with
the sun-spot cycle.
Absorption in the sun’s atmosphere may also prove to be variable. And
here again differential observations should suffice to test the
question. They were undertaken by Wilson and Rambaut in 1892, but
relinquished after one series had been made. The method employed was to
pass an image of the sun across the radio-micrometer, while the motion
of a spot of limelight, reflected from the mirror of the instrument,
recorded the changing amounts of heat received from the different parts
of the disc. The intention was to obtain such “curves of absorption”
frequently throughout an eleven-year cycle, and thus determine the
question of concurrent fluctuations in depth of the absorbing envelope.
“If we find,” the authors wrote, “that such changes are taking place, as
would be shown by the alteration in the ratio of the heat from the limb
and centre of the disc, we think it will be quite possible, by an
investigation of the co-ordinates of these curves, to determine the
change in the value of the solar constant.”[116]
This theoretical possibility, nevertheless, is still a long way from
realisation. Divers indications lead almost irresistibly to the
conclusion that the sun is hotter at certain times than at others; and
Professor Young counts it as “one of the most important and difficult
problems of solar physics now pending to determine the actual amount of
these variations and ascertain the laws that govern them.” But they are,
as we have partly seen, disguised by manifold complications.
The one clear upshot of inquiries into the temperature of the sun is to
show that it stands high above the boiling points of the most refractory
among the chemical elements. The fact is embarrassing, but cannot be
evaded. Apart from its consideration, no theory as to the nature of the
photosphere is of the slightest value. And it is no easy task to frame
one bringing it into harmony with other circumstances equally well
assured, and equally rigid in their consequences. Three alternative
hypothesis may be said to exhaust the possibilities of the subject. They
are as follows:—
1. The photosphere is a surface of condensation for unknown materials
capable of maintaining the liquid or solid state at a transcendent
degree of heat.
2. It is a surface of condensation for known materials under unknown
conditions.
3. It is no true surface of condensation, the substances composing it
being, although viscous, still vaporous.
Now each of these explanations is largely an appeal to ignorance, and so
far scarcely deserves to be ranked as an explanation at all. Yet one of
them must be fundamentally true. The first may be dismissed as
contradictory of a strong consensus of evidence. The third involves
glaring incongruities, both with what can be seen and with what must be
inferred. There remains only the second. We seem bound to adopt the view
that the sun is veritably clothed in a kind of cocoon—a web of
incandescent filaments. It is perhaps of mixed composition. The surface
is irregular. It comprises “fleece-like floors” at apparently different
levels. Possibly they represent the successive condensations of various
substances—silicon, carbon, titanium, vanadium, platinum, to mention a
few of those most resistant to heat. The diversity of their emissive
powers might contribute to produce the _tonings_ brought out in
photographs of the disc, and the arrangement would be analogous to the
surmised replacement in our upper air of aqueous by carbonic acid
cloud-fields. But the postulated “unknown conditions” needed to enforce
condensation at the enormous temperature of the photosphere may long
continue to baffle the scientific imagination.
A darkening of the sun’s disc towards the limb is obvious
telescopically, and conspicuous photographically. Its amount, measured
by Bouguer in 1729, formed the basis of Laplace’s calculation that the
arrest of light indicated was no less than eleven-twelfths of the
entire. The data were correct, but the result, owing to certain mistaken
assumptions, was greatly in error. Modern authorities, nevertheless, are
far from being unanimous on the subject. Pickering finds that the
intrinsic lustre of the sun exceeds its apparent lustre four and
two-third times; the disparity, according to Vogel, is about twofold.
There are, however, distinctions to be made. The absorption in the
solar, as in the terrestrial atmosphere, is markedly selective. The
brunt of its attack falls upon the most refrangible rays. Father Secchi
noticed in 1870, and Professor Langley again in 1875, that the light
from the limb is, in consequence, tinged with chocolate brown, while
that from the central parts of the disc seems bluish by comparison. This
general indication was, in 1877, analysed by Dr. Vogel,[117] who, by
detailed measurements with a spectral photometer constructed on the
polarising principle, ascertained that 30 per cent of the red, but only
13 per cent of the violet marginal rays penetrate the solar atmosphere.
Hence an alteration in tint corresponding in its mode of origin to the
ruddy suffusion of the setting sun. Now Seeliger has pointed out that
selective absorption implies a medium of high refractive power; but
equivalent conditions might, according to E. von Oppolzer,[118] be
supplied by “a rare atmosphere in which flying particles are suspended.”
It is, beyond doubt, an exceedingly shallow one. This was inferred by
Vogel from the rapid degradation of light towards the edge of the disc,
and it is rendered patent to sense by the brilliancy of facular summits,
which, rising above the absorptive strata, shine unveiled against the
dusky limb. Obviously, then, the darkening effect is produced in the
immediate neighbourhood of the photosphere. It cannot be due to _cool_
gases, and _hot_ gases stop light distinctively in isolated beams. An
alternative hypothesis was suggested some time ago by Professor Hastings
of New Haven. The sun’s so-called “atmosphere” is, in this view, nothing
more than a smoke-laden stratum.[119] Minute solid particles of carbon
or silicon, carried upward from the photospheric clouds, are the agents
of obscuration. The assumption of a solar analogue to a London fog is
certainly a daring expedient, yet none more satisfactory is at present
available.
The sun’s “veil” is indeed particularly difficult to fit in with the
rest of its economy. It manifestly exists, and the position seemingly
prescribed for it is between the photosphere and the reversing layer,
although Dr. Scheiner prefers to place it in chromospheric regions.[120]
In some torrid locality, at any rate, it exercises a kind of action
characteristic of cool substances. Its composition out of refrigerated
materials is strongly indicated. The refrigeration, however, may be
excessively transient as regards each individual particle, although
permanent in their aggregate. The general effect is to diminish the
sun’s heat by one-third or one-half, and its light by fully two-thirds,
with an attendant change to pale primrose of its original glacier-green
tint.
Among the many enigmas of solar physics there is none more curious or
more evasive than that which confronts us in this intimate appurtenance
of the photosphere. Even the lines of approach to it are very few. Yet
some are practicable, and almost untried. Researches of a special kind
into the spectra of sun-spots should help towards its elucidation; still
more, perhaps, careful spectroscopic comparisons with the obscurer
interstitial spaces of the brilliant granules strewing the solar
surface. If these are relatively dark through a mere lowering of
temperature, then little can be learned from them in this connection.
But if they are dark through increased absorption, it will be important
to determine whether the absorption corresponds to that produced by the
problematical “veil.” Are they, in other words, sinks for solar _soot_?
Do they mark the lines of subsidence of the same refuse materials which
by their interposition dim and tarnish the shining face of the sun? A
definite reply to the question would bring us preceptibly nearer to the
goal of our inquiries into the nature of the sun’s “smoke” envelope.
What cannot be doubted is the importance of its function as a regulator
of the sun’s output of energy. This has for the first time been
adequately discussed by Dr. J. Halm in a paper incorporated with the
_Annals of the Royal Observatory, Edinburgh_.[121] His views on the
subject demand careful consideration.
CHAPTER VII.
STRUCTURE AND MOVEMENTS OF SUN-SPOTS.
A normal sun-spot consists of a round black “umbra,” garnished with a
circumferential “penumbra.” The chief member of the group shown in Fig.
6 is a good example. The ground should be almost white, with the
granular texture delicately indicated. The details of such objects,
however, are seen much better than they can be photographed even by the
consummate art of M. Janssen.
[Illustration:
FIG. 6.—Sun-spot photographed by Janssen, April 1, 1894 (from
_Knowledge_, vol. xviii. p. 108).
]
One of their characteristic features is the definite separation of their
parts. The umbra does not merge into the penumbra, nor the penumbra into
the photosphere; the lines of demarcation are as sharp as the edge of a
cascade. Their darkness is indeed accentuated by the enhanced brilliancy
of the regions invaded by them. An immense area of disturbance usually
surrounds an active spot, and this area of disturbance is also an area
of actual elevation. A series of micrometrical measures carried out by
M. Sykora of the Charkow observatory in 1895 showed that, as a very
general rule, the sun’s diameter is lengthened in the direction of a
spot on the limb.[122] The fact is most significant, for it indicates
relief of pressure as perhaps the cause, and certainly as an
accompaniment of solar outbreaks, and thereby associates them with
volcanic explosions. Now a region lifted is, on the sun, a region
brightened, the effects of absorption diminishing with the rise of
level. Hence the exceptional vividness of the photosphere in a spotted
neighbourhood.
Faculæ gain higher altitudes, and are consequently still more lustrous.
They are like the summit-ranges of a tableland. Their connection with
spots is intimate, but not inseparable. Every spot, it is true, claims a
retinue of faculæ; but faculæ exist abundantly where there are no spots.
It is a moot point whether spots can come into being apart from
preceding facular disturbance. “Which is the forerunner of the
other?”[123] is a question not to be answered off-hand. As a rule, the
embryo spot has apparent priority. The rule, however, is not invariable,
and the priority recorded may often be illusory, faculæ being extremely
evasive of observation. Their survival, on the other hand, after the
openings enwreathed by them have closed, is obvious and constant. The
complex relations of the two kinds of phenomenon must be unravelled
before the nature of either can be thoroughly understood.
Photospheric structure is very curiously modified in the penumbræ of
spots. The roundish granules of the unbroken surface seem as if drawn
out into threads, which lie side by side, pointing radially inward, and
overhang the umbra with ragged edges, compared by Dawes to those of an
untrimmed straw-thatch. And the eaves of this luminous thatch are its
brightest part, possibly because of the crowding together of materials
forced into a narrow circular space. The whole effect suggests the
subjection of viscid masses to a pulling action emanating from the
centre of disturbance, by which they are stretched and _carded_ like
wool-flocks.
The umbra of a sun-spot shows a cloudy texture, markedly unlike the
streaky aspect of the penumbra. This can be perceived, however, only
when the seeing is exceptionally good. The ordinary impression is of
uniform and very profound darkness. Contrast, indeed, greatly heightens
this effect. The obscurity is only comparative. Mr. Evershed estimates
the light-emissions from umbræ as varying from about one-twentieth to
one-hundredth those of the dazzling photosphere,[124] and when
intersected by the black advancing moon during the progress of a total
eclipse they seem dully bright.[125] Yet with differences. They are
rarely of the same tint throughout. Dawes perceived in 1852 a “black
opening” in the umbra to be a characteristic of all well-developed
spots. It can only, however, be discerned visually, and that by the aid
of special precautions; the sensitive plate takes no notice of this
deeper depth of shadow, which has, accordingly, received somewhat less
attention than it deserves. Fortunately eye-and-hand portrayals still
continue to be made, and they not infrequently afford valuable records
of the Dawes phenomenon. M. de Pereira, a Portuguese observer at the
Azores, wrote as follows of a group which came into view 20th April
1895: “The sense of a cavity in this spot is unmistakable, as though the
crust of the sun were torn and scratched, and the black, or rather dark,
under-skin were visible beneath. Definition on this day was the best I
have ever seen, enabling me to make the smallest detail reliable. On the
24th, this same spot showed a conspicuous black hole in what I may call
the centre of gravity, a dazzling white bridge crossing it from south to
north-east, and a smaller one lying on the northern edge of the abyss,
the brims being full of curiously intertwined points of photospheric
matter.”[126] The chief member of the splendid group visible in the
sun’s southern hemisphere during the last half of February 1894 had also
an inner nucleus,[127] and the same feature has been studied in numerous
examples by Father Cortie, Mr. Maw, and others. It is commonly
associated with the presence of “bridges,” and both belong
characteristically to the final stages of active spots. Bridge-building
is preliminary to the indraught of luminous matter by which photospheric
breaches are closed; it might be compared to the trickle under the dyke
that preludes the rush of inundating waters. The process is a remarkable
one. From abutments (so to call them) at opposite sides of the umbra,
segments of light protrude; then at a given moment they unite with a
leap or a flash, and the arch stands complete. A beautiful photograph by
Janssen of a spot doubly spanned is reproduced in Fig. 7. But the ground
is _altogether too dark_. The surface near the spot was dazzling,
likewise the facular masses crossing the nuclei.
[Illustration:
FIG. 7.—Photograph of a Bridged Sun-spot, by Janssen (from
_Knowledge_, vol. xiii. p. 74).
]
“On the negative,” Mr. Ranyard wrote in describing it, “the brilliant
bridge which stretches across the great spot is seen to break up into a
number of distinct elongated masses,” and these are evidently the
“rice-grains” of the photosphere laid end to end in single file for
suspension above the abyss. This kind of structure is probably always
present, although often imperceptible. Exceptional facilities are needed
to bring out the finer details in spots. Thus Professor Young tells us
that, on the rare occasions when powers of six hundred and upward could
be profitably used with the twenty-three-inch Princeton refractor, he
succeeded in resolving “the apparently club-like, almost bulbous ends of
the penumbral filaments” into “fine sharp-pointed hooks, reminding one
of the curling tips of flames, or grass-blades bending over.
Ordinarily,” he adds, “they are seen as club-like simply because of
their brightness and the irradiation and diffraction effects of
moderate-sized object-glasses.”[128]
The connection of “bridges” with “black holes” was tentatively explained
in a valuable paper presented by Father Cortie to the Royal Astronomical
Society, 11th May 1900. He considers that the latter may be the portions
of the umbra left uncovered by “faculous veils,” which, extending from
the penumbra, not unfrequently lighten up certain regions of nuclear
gloom, leaving others more profoundly dark by contrast. Now the
relationship between “veils” and “bridges” is obviously quite close.
Both represent luminous invasions, although differently organised and
conditioned, and both are heralds of decay. Their kinship is on
occasions emphasised by the development of one from the other. Twice at
least, in 1865 and 1866, the transformation into “roseate veils” of
brilliant arches spanning the umbræ of spots was observed by Father
Secchi at Rome.[129] He was quite positive about the colour of these
“veils,” which seems to intimate for them a gaseous nature, assimilating
them to prominences rather than to faculæ.[130]
Sun-spots are rarely solitary. They ordinarily appear in clusters or
processions, consisting of one or two dominating members and many
satellites, down to mere umbral dots and penumbral scraps. Individual
spots show endless varieties of conformation. The nuclei are often
multiple; as many as nine umbræ have been seen within the compass of a
single penumbra. Again, they become pear-shaped, or spiral, or caudate,
as if through the action of stresses or twisting forces of an unknown
character. The penumbra is equally subject to irregularities. It is
sometimes a mere torn strip of fringe; half the umbra may be duly
furnished with its _valance_, while the other half remains bare; or the
umbra and penumbra may be disjoined by intruding photospheric matter.
Fig. 8 shows a “fimbriated” spot from a drawing by M. de Pereira, the
partially veiled and bridged umbra of which included two conspicuous
black holes.
[Illustration:
FIG. 8.—Sun-spot drawn by J. de M. Pereira, 18th June 1894.
]
[Illustration:
FIG. 9.—Group of Sun-spots drawn by Miss E. Brown, 15th August 1894.
]
In Fig. 9 three “confluent” spots are represented, drawn by the late
Miss E. Brown, 15th August 1894. She described the group as covering “a
vast extent of surface with a mass of nuclei and penumbra partially
connected and very variable in form.” A few hours later the largest
nucleus had assumed a helical form, and seemed to be “throwing out
feelers like a jelly-fish.”[131] Strong hydrogen incandescence was
spectroscopically perceived to be an accompaniment of these rapid
changes. The great spot of September 1898 had in its declining stage a
nucleus divided by intersecting bridges into three lobes; it assumed on
3rd October the shape of an ace of clubs.[132] Mr. Maw perceived in the
same object on 11th September delicate veins of comparative brightness,
termed by him “submerged bridges.”[133] Indeed he believes this to be a
constant feature of large spots, the umbræ of which, viewed with a
suitable eye-piece, appear no longer uniformly dark, but marked with
fine traceries in chiaroscuro. These cannot at present be photographed,
and their visual detection accentuates the indispensable co-operative
functions of the eye and the sensitive plate.
The level of sun-spots has once more become a subject of active debate.
Yet it was believed to have been determined once for all in the
eighteenth century by the geometrical reasonings of Dr. Wilson. The
characteristic perspective effects of depression below the surface were
noted by him in a well-developed spot as it circuited the sun’s globe in
November 1769, and the saucer-like conformation of all such objects was
universally admitted for a hundred years and upwards, notwithstanding
frequent failures to verify the due optical consequences of their
changing situations. At last, however, this “venerable theory” (as
Professor Frost calls it) has been uprooted from the soil of conviction.
It is denied by many on geometrical grounds alone; by some on physical
grounds as well. Not that it has been finally discarded, but its credit
is gravely impaired. Certainly _all_ spots do not follow its
prescriptions; probably very few strictly comply with them.
Irregularities of form account for a good many of these deviations, but
others cannot be so readily explained away. Mr. F. Howlett, in offering
to the Royal Astronomical Society, 14th December 1894, three volumes of
drawings representing the fruits of thirty-five years of solar scrutiny,
declared uncompromisingly that the Wilsonian view must be
abandoned.[134] Father Cortie’s examination of them convinced him too
that “the phenomena presented by many spots are directly contrary” to
the current hypothesis, a mountainous rather than a cavernous structure
being, in certain cases, indicated for the umbra.[135] Nor does the
umbra usually vanish near the sun’s edge, as it should if it were simply
an excavation with sloping sides. It remains, on the contrary,
persistently visible, although foreshortened into a black line. This, to
be sure, might be a simple consequence of refraction by vapours
congested within the cavity. The explanation is tempting, since it would
avail to get rid of many anomalies; still it must not be adopted
unreservedly. Originally suggested by Proctor,[136] it has been taken
into fuller consideration by Mr. East[137] as a means of exit from the
difficulties that hamper attempts to conceive rationally of the build of
sun-spots.
There is, indeed, pressing need to conciliate opposing evidence. Thus M.
Riccò,[138] at Catania, from eleven years’ study of spots in their
geometrical aspect, derived results strongly in favour of the Wilsonian
hypothesis, computing for twenty-three especially symmetrical formations
an average umbral depth of rather more than a thousand kilometers. Yet
the discussion of the long series of Stonyhurst drawings led Father
Sidgreaves[139] to ascribe to most spots a convex rather than a concave
shape. Professor Hale[140] allowed small weight to testimony so
contradictory as that regarding the apparent width of the penumbra at
various distances from the sun’s limb, but was inclined to consider the
advocates of the Wilsonian doctrine as having rather the better of the
argument. “In any case,” he added, “they will hardly be ready to admit
that the umbra is at a higher level than the penumbra, for it cannot be
doubted that the penumbral filaments overlie the umbra, and frequently
unite to form bridges extending completely across it.”
One of the most singular details of spot-phenomena is the occasional
appearance of a large umbra as a notch on the limb. This implies its
projection in a dark mass against the sky, the encroachment upon the
bright disc being perhaps only an effect of irradiation. Its
inconsistency with a depressed form was pointed out both by Mr. Howlett
and by Father Sidgreaves.
Where, then, is truth to be found in this remarkable controversy? How
can the jostling facts be reconciled? Compromises have been resorted to.
Spots, it is averred, are cloud-like at certain stages of their growth,
crateriform at others. Or individual spots belong to one or the other
type, according to the circumstances of their origin. But these are
subterfuges; let us take a broader view. The concavity, at any rate, of
bridged spots is indisputable. The attribution to them of a
“mountainous” character would throw the arrangement of their parts into
utter confusion. Moreover, De la Rue obtained in 1861, by stereoscopic
means, ocular proof of depression in one such object. The experiment
might easily and usefully be renewed. A pair of photographs, taken at an
interval of twenty-six minutes, gives, through the sun’s rotation, just
the right amount of difference in aspect for combination into one
picture in relief. The moot question, “concave or convex,” might thus
receive a direct answer. Then if, in a long succession of instances, the
answer preserved a uniform tenour, it might safely be concluded that
anomalous appearances of lifted umbræ in spots seen obliquely are
illusory and of purely optical production.
But we cannot even so escape from the entanglements of the subject. It
has different bearings, which have all to be taken into account. Spots
are very hot relatively to their light, and their thermal radiations are
peculiarly conditioned. Professor Frost’s determinations,[141] carried
out at Potsdam in 1892, showed that absorption does not take increasing
effect upon them with approach to the limb to anything like the extent
that it does upon the corresponding radiations from the photosphere.
“The reasonable inference from this,” he wrote, “is that the spots are
at a higher level than the photosphere, and hence less subject to the
absorption of the sun’s atmosphere.” Mr. W. E. Wilson’s[142] more
delicate series of observations in 1893–4 showed likewise that “the
radiation from the umbræ of spots does not suffer absorption when near
the limb in the same manner as a point on the photosphere.” Nevertheless
Professor Langley[143] had found in 1874–5 “the decrement of heat in
approaching the limb” to be very nearly in the same ratio for
photosphere and spots. This flagrant contradiction between results
equally authoritative may not be without meaning, since they were
obtained at nearly opposite phases of solar activity: Langley’s, three
years after a maximum, when it was verging towards stagnation; Frost’s
and Wilson’s, during a period of culminating disturbance. The two latter
recommended systematic observations throughout an eleven-year cycle for
the purpose of investigating the nature of the relationship, and they
respectively threw out the alternative suggestions that, during its
course, the thermal condition of spots, or the level at which they are
formed, may undergo progressive changes.
A more promising explanation was offered by Egon von Oppolzer.[144]
Spot-umbræ, he reminded perplexed solar physicists, are surmounted by
abnormally hot chromospheric layers, certainly exempt from absorption.
Hence the indiscriminate sum of their radiations and those of the
underlying spots gains by comparison with those from the simple
photosphere, at and near the marginal parts of the disc. Now the
flame-stratum develops chiefly above spots of an active type; and spots
of an active type predominate at epochs of maximum. This consideration
at once removes the discrepancy between Langley’s results on one side,
and Frost’s and Wilson’s on the other. In the first series, absorption
produced its full and due effects, because quiet spots being presumably
in question, no appreciable overlying source of heat was present. In the
second and third, the overlying source was so strong as in great measure
to efface the gradations of heat-stoppage suffered by the object
beneath. The disproportionate thermal power of spots may be similarly
accounted for. Our instruments measure, not only their direct
radiations, but also those sent out by ignited materials, to some extent
enveloping them.
The solar rotation is a subject much too important to be disposed of in
a paragraph; it need here only be said that a by-product of its detailed
study has been to throw further doubt upon orthodox opinions as to the
location of spots. The rates of axial movement deduced by Stratonoff and
Wolfer from their progression round the sun appear, at least _primâ
facie_, to compel the inference that they are veritably situated at a
level higher than that of the photosphere.[145] Yet here again some
fallacy is likely to be involved. In view of all these complications it
is scarcely to be wondered at that the _Where?_ has almost superseded
the _What?_ in recent discussions about solar maculæ. The upshot, so
far, seems to be that they are essentially depressions, although
depressions very shallow relatively to their superficial extent. Their
abnormal geometrical behaviour is due, in part, as Mr. Maunder has
suggested, to the cavities being _over-filled_, and the umbræ
consequently dome-shaped;[146] in part to the optical elevation into
view of bottoms which should, but for refractive action, be concealed by
shelving sides. Their radiative irregularities, again, are explicable by
the influence of their coronas of hot flames. As to their rotational
anomalies, they must stand over for future consideration, with the
remark that, to deduce the position of spots from the degree of their
conformity to a supposed law of solar rotation, is to attempt the
solution of one enigma by proposing another still more arduous.
The movements of sun-spots are of three kinds. There are first those
that belong to them collectively, as objects attached to a rotating
globe. With these we are not at present concerned. Next, they have
individual “proper motions” of transport over that globe. Finally, they
show internal movements variously connected with the processes of their
development and decay. The last are mostly spiral or circular, and they
evidently ensue upon inrushes of photospheric matter. They are sometimes
performed round “black nuclei” as centres; and black nuclei are
probably, as we have seen, interspaces between obscurely luminous umbral
effusions. But the whirling of spots is not systematic or innate; it
does not characterise them essentially; it occurs incidentally, and as a
result of disturbance. No fixed rules prescribe its mode or direction.
Opposite gyrations have been simultaneously observed in different
members of the same group of umbræ, and even successively in a single
spot. They are executed in other cases intermittently by fits and
starts. A revolution is not often completed; the description of large
angles is exceptional. Spots cannot then be described, in any true
sense, as “solar cyclones”; the vorticose motions occasionally exhibited
by them spring from temporary impulses, and cease when the force of
these is exhausted.
The _proper_ motions of spots are indicative of much more than has yet
been learned from them. Three kinds of influence seem to be effective in
producing them; namely, mutual action, action from without, and action
from within. After segmentation, in the first place, umbræ repulse one
another; they separate with great velocities. They behave like similarly
electrified masses, but whether they really are such or not is an open
question. In the second place, growing spots in general move rapidly
forward. They share the common drift, but with an acceleration often
amounting to three or four hundred miles an hour. It seems as if cooled
materials, pouring down upon them from above, drove them forward with
the added speed due to a wider circle of rotation. If this were actually
the fact, however, macular increase and macular advance should always go
together; and they are not uncommonly disunited. Processes of extension
in spots may even be accompanied by retrogression over the sun’s
surface.[147] Indeed, the conduct of these strange objects is governed
by no invariable rules. Strong tendencies visibly influence it; yet none
that are irresistible. They can be annulled or reversed by
countervailing circumstances. Hence the special need for guarded
inferences in treating of this subject.
The movements of sun-spots in latitude are not visibly related to their
drift in longitude. They are highly irregular, and not often
conspicuous. Carrington, however, perceived in them a kind of inchoate
method. Spots, according to his generalisation, situated within the
solar tropics (so to speak) tend to approach the equator; spots outside
the north and south limit of twenty degrees, to depart farther from it.
But the exceptions observed are so numerous as sometimes to go near
disproving the rule. We have said that spot-movements in longitude bear
some marks of being communicated by exterior agencies. Those in
latitude, on the contrary, suggest interior action. They are connected,
most likely, with the hidden system of circulation prevailing in the
body of the sun, and reflect its local perturbations.
A remarkable feature of photospheric commotions was referred to by
Father Cortie at the meeting of the Royal Astronomical Society, 11th May
1900.[148] He termed it “alternation” in disturbance. A group of spots
generally includes two chief members, posted respectively in the van and
rear of the array. These _take it in turns_ to develop. We are reminded
of the reciprocal flickerings of the fragments of Biela’s comet.
Analogous pulsations, but on a larger scale, manifest themselves in
responsive disturbances north and south of the solar equator. Mr.
Maunder had already pointed out in 1894 that an “active train” of spots
is often “accompanied by a feebler copy of itself a few degrees north or
south. An outbreak of the first magnitude,” he continued, “will indeed
reproduce itself in several directions.”[149] Somewhat similar
correspondences are noticeable between volcanic foci on the earth; yet
the inferences they suggest might prove misleading.
The occultation of a spot 107,500 miles across was observed during the
solar eclipse of 15th March 1858. Its vast dimensions were, however,
exceeded in the same year by those of an object with the “record”
diameter of 143,500 miles. An enormous double spot, which appeared in
June 1883, covered an area of 2500 million square miles; and the great
spot of February 1892, with its dependants and outliers, spread still
further afield. But such gigantic formations are rarely stable. Their
history is one of tumults and vicissitudes. Comparatively small circular
spots possess individually a much more lasting character, although great
outbreaks are the longest lived in their successive modifications. The
maximum duration so far registered was for a “composite disturbance,”
consisting of four very large spot-groups, and thirteen others of
smaller dimensions, which appeared seriatim, and in obvious association,
on a restricted region of the solar surface.[150] The manifestation
continued for 527 days, from 25th September 1891 to 5th March 1893,
while the sun completed nearly twenty-one rotations. About two
rotation-periods represent, according to Father Cortie, the average life
of a spot.
Most of what we know about sun-spots has been learned by a statistical
method of inquiry. Nor can such methods be dispensed with in the future.
But they do not alone suffice. They must be supplemented and reinforced
by _individualisation_. Each notable spot should be studied in itself
and in all its relations, singly, specifically, and generically. Efforts
should be made to determine its nature, as though it were a solitary
specimen. Can it, without doing violence to plain facts, be regarded as
an excavation in the photosphere? Or, if apparent inconsistencies with
this view be present, are they such as might be due to refraction? The
hypothesis can only be tested by confronting it with particular cases,
and trying definitely how far it avails to meet their exigencies.
Refractive possibilities in the sun have been, until lately, almost
ignored; they are now in some quarters vastly exaggerated. Still
allowance has to be made for their realisation, in ways perhaps
corroborative, rather than subversive of received theories.
There are many other doubts to be set at rest besides those regarding
the interpretation of perspective effects. The record, indeed, of no
significant structural detail should be omitted; and what detail of
these enigmatical objects can be called insignificant? Above all,
variations in their parts and features, whether simultaneous,
successive, or alternative, claim the closest attention; since the
establishment of a course of correlated changes comes very near to the
detection of the underlying causal nexus.
The comparison of one spot with another is a natural sequel to the
investigation of each spot in itself. Do their peculiarities, it may be
asked, depend in any way upon heliographical position? Do they vary
periodically? Can certain traits in sun-spots be classed together as
inevitably associated, certain others as mutually exclusive? Wider
questions, too, suggest themselves as to the place of spots in the
general solar economy, and as to the nature of their connection with
faculæ, prominences, coronal streamers, and the totality of solar
phenomena. Attempts have been made, both by speculative and practical
means, to throw light on these obscure topics, but with results not as
yet wholly satisfactory. Meantime, additional facts are needed—facts
systematically collected, methodically sifted and compared. Isolated
observations are rarely of any considerable value in such complex
matters. Meaning accrues to them just in proportion as they can be
allied to others made in correspondence with them, but under modified
conditions. “Correlate and compare” should be the watchword of
astrophysicists.
CHAPTER VIII.
THE SPECTRUM OF SUN-SPOTS.
Sun-spots give a remarkably compounded spectrum. It appears to sum up
five different sets of effects. That is to say, the obscure longitudinal
stripe corresponding to the umbra owns a quintuple origin. Each of its
elements might be made the subject of a somewhat prolonged discussion.
For the sake of clearness we will briefly enumerate them. They consist
of: (1) A bright background of ordinary photospheric light; (2) a nearly
continuous band of dense absorption, extending from the infra-red to the
ultra-violet; (3) a select array of widened Fraunhofer lines; (4) nearly
all the Fraunhofer-lines under a normal aspect; (5) a restricted number
of bright lines.
Dunér of Upsala ascertained in 1891[151] that the _fundamental_ radiance
of spots is indistinguishable from that of the general surface of the
sun. Their darkness is then due to increased absorption, not to
diminished radiation. This fact decisively negatives some current
theories, and thus limits the field of speculation as to the nature of
spots. They are shown by it unmistakably to be regions where cooled
materials of some kind accumulate. Of what kind those materials are, we
can learn something—although not by any means all that could be
wished—from their peculiar modes of arresting light.
A section of a spot-spectrum in the yellow-green is portrayed in Fig. 10
from a photograph by Professor Young. The belt of strong absorption
which is its leading feature seems, but is not really, continuous. Young
himself discovered in 1883 that it is made up of innumerable fine dark
lines, set very closely together, or even actually overlapping. Each
individual in the multitude is, he tells us, “spindle-shaped—_i.e._
thicker in the middle where the spectrum is darkest—and tapers to a fine
hair-like mark at each end; most of them can be traced across the
penumbra-spectrum, and even out upon the general surface of the
sun.”[152] These observations, which require a high resolving power in
the apparatus employed, were amply confirmed by Dunér. He perceived
further that the lines are collected into groups, leaving chinks of
undimmed photospheric brilliancy between.[153] Within the limits of the
“_b_-group” alone, no less than 300 of these dusky _fibres_ were
counted; above F, however, they become merged together by crowding, and
below E by diffusion. Their separation and arrangement are most evident
in quiescent round spots with intensely black nuclei—in spots, that is
to say, commonly regarded as of the minimum type. Now such a spectrum as
they constitute cannot be produced by liquid or solid matter, however
minutely subdivided; it decisively claims a gaseous origin. Hence the
darkening in spots is not merely an intensification of the “smoky”
absorption veiling the entire disc; it is special and peculiar. So much
can be safely asserted.
[Illustration:
FIG. 10.—Portion of Sun-spot Spectrum, photographed by Professor Young
in 1893.
(From Young’s _General Astronomy_. By permission of Messrs. Ginn and
Co.)
]
Perhaps the most distinctive part of the spot-spectrum is the
collection, included in it, of accentuated Fraunhofer lines.[154] They
are picked out to be widened and darkened on some recondite principle of
selection, which varies from spot to spot, and from epoch to epoch. This
was early noticed by Sir Norman Lockyer, and he pursued the inquiry with
striking results. The discussion in 1886 of observations upon the
spectra of seven hundred sun-spots, made at South Kensington on a fixed
plan during six years, led him to the following conclusions:—[155]
(1) “The most widened lines in sun-spots change with the sun-spot
period.”
(2) “At, and slightly after the minimum, the lines are chiefly known
lines of the various metals.”
(3) “At, and slightly after the maximum, the lines are chiefly of
unknown origin.”
In other words,[156] “As we pass from minimum to maximum, the lines of
the chemical elements gradually disappear from among those widened,
their places being taken by lines of which we have at present no
terrestrial representatives.” “Dissociation,” in short, was the _mot de
l’énigme_. As the sun’s temperature increased with the growth of
disturbance, substances in a terrestrial sense “elementary” were
supposed to split up into exotic constituents, giving spectral lines
strange to laboratory experience.
The evidence for the progressive change thus interpreted was indeed
slight, except as regarded iron; and iron alone was taken account of in
the confirmatory Stonyhurst observations. So far as they went, however,
they were decisive, and all the more so that they covered a different
spectral range (B to D) from that (D to F) examined at South Kensington.
They showed demonstratively that, throughout the disturbed interval
between January 1884 and October 1886, iron lines were all but
completely replaced by “unknown lines” in the list of those affected in
spots, while they duly reappeared upon the restoration of photospheric
tranquillity. In connection with their behaviour, nevertheless, Father
Cortie established an important distinction. Their presence or absence
he found to be determined, not by the general flow of solar commotion,
but by the nature of individual spots. In those of rent and ragged
aspect and tumultuous proclivities, iron lines are ousted by
unidentified faint rays; but in tranquil spots the iron spectrum is at
all times prominent. And since the former sort prevail at maximum, the
latter at minimum, the statistical outcome is that the spectral
variations appear to depend simply upon the great cyclical pulsation of
the solar globe. Only on special examination they prove to be determined
more locally and particularly than this would imply. In some unquiet
spots, for instance, which developed near the minimum of 1889, the
effacement of iron lines was as complete as if the epoch had been one of
maximum. In fact (as the Stonyhurst astronomer remarked),[157] the
widening of unknown lines is common to all stages of solar activity,
provided spots of an appropriate character be at hand. This is not a
distinction without a difference. It cuts the ground from under the
assumption of periodical vicissitudes in the general chemistry of the
sun. Iron is not everywhere, and inevitably reduced there to its
elements as temperature and disturbance culminate together, but—if at
all—only as a special effect in the hottest spot-craters. And this again
brings up difficulties connected with relative temperature—difficulties
which, in one form or another, perpetually recur in the study of
astrophysics.
But there is more to be said. Further inquiries have materially altered
the aspect of the case, for they have led to the transference from the
“unknown” to the “known” class of so many spot-lines that the completion
of the process may be confidently anticipated. Rowland’s photographic
comparisons have contributed most effectually to its advance. Young and
Cortie have traced a crowd of sun-spot rays to vanadium; titanium claims
as many, or more; and others perhaps originate from allied “rare”
metals. This singular line of identification is very strongly traced.
Thus _all_ the vanadium lines, twenty-eight in number, between C and D
are by turns broadened in spot spectra, although of evanescent faintness
in the photosphere; nor does the conjecture seem unwarranted that the
high temperature compounds with nitrogen and oxygen, both of this metal
and titanium, may yet be recognised in umbral chemistry. The distension
in a spot of two vanadium lines, at λ 5728 and λ 5731 respectively, is
well shown in Fig. 10. They are of quite minor importance in the
Fraunhofer spectrum.
Father Cortie surmises that the vapours absorbing in spots may be
associated by their approximate conformity to a certain standard of
density. “The level of sun-spots,” he suggests, “is possibly the level
of the faint lines of such metals as have an atomic weight about
50.”[158] Iron, nickel, titanium, and vanadium, all assiduous
frequenters of umbral cavities, belong to this category. But the rule is
compromised by exceptions and incongruities.
The actual state of the case is this. There is no evidence of elemental
dissociation in sun-spots, but spectral diversities are obvious and
persistent. They indicate the disappearance of iron from tumultuous
formations, and the emergence in them of titanium and vanadium. There
are doubtless concomitant changes, but they await ascertainment and
particularisation. A _caveat_, however, has to be entered. The principle
upon which these inquiries have been conducted is imperfectly assured.
It is commonly taken for granted that the widened lines constitute the
spot-spectra; that they, and they alone, represent the emanations of the
constipated vapours blotching the lustrous disc. But this is a somewhat
arbitrary assumption. The theory of line-expansion by pressure is very
imperfectly understood. The phenomenon does not occur uniformly and
invariably. Lines of different substances are differently affected by
it; lines even of the same substance are unlike in their susceptibility
to its influence. The inferential building up then, of spot-spectra out
of widened lines is subject to many qualifications. These do not lessen
the importance of the observed relation, but they importantly modify it.
The iron lines intensified in spots, presumably by the specific action
of their nuclear vapours, are often unsymmetrically broadened. They are
usually diffuse towards the violet side, sharp to the red. This may
point to the presence of chemical compounds;[159] since the
flame-spectra of metals and of their oxides seem to be differentiated
just by the development, in the latter, of these peculiar shadings. The
possibility must accordingly be admitted that iron-oxides exist in the
sun. Yet the implied temperature is improbably low, since they can be
broken up here on the earth by the simplest metallurgical processes. But
they might perhaps form transiently (so to speak) in spots, as a result
of the local chilling of swiftly circulating material. It may be added
that the iron lines distinctive of spot-spectra are so-called
“low-temperature lines.” They are brilliant in the electric arc, but
tend to be outshone by others in the higher excitement of the spark.
Further complexity was imparted to spot-absorption by the appearance of
certain dusky bands, of which nine, situated below D, were observed
early in 1885 at Stonyhurst,[160] and no fewer than seventeen more
refrangible in 1880–3 at Greenwich. One proved identical with a fluting
drawn by Young in 1872,[161] and all were resolvable into densely packed
lines. Nothing is known, or can even be conjectured, as to their origin;
but they are clearly symptoms of disturbance, since, with the sudden
advent of a solar calm in October 1886, they at once utterly vanished.
Mr. Evershed considers the majority of the unaffected Fraunhofer lines
in spot-spectra to be possibly spurious;[162] they may, he thinks, be
inherent, not in umbral light, but in the photospheric glare diffused
equally over spots and the surrounding sky. Some of the ordinary dark
solar lines, however, thin out in crossing umbræ, and a few show traces
of partial brightening. Moreover, the radiations from spots cannot
escape transmission through the reversing layer, and are hence subject
to precisely the same absorption exerted upon sunlight in general, so
that the Fraunhofer spectrum in its integrity truly belongs to spots,
notwithstanding the reinforcement of some of its components and the
enfeeblement of others through influences special to them.
One of the most remarkable features of spot-absorption has still to be
noticed. Helium, as we have seen, makes no show in the Fraunhofer
spectrum. Yet a helium-envelope surrounds the sun to a depth of five
thousand miles. Every ray of sunshine sent abroad into space has been
sifted through this huge volume of gas, which, by its anomalous
inertness, bids defiance to “Kirchhoff’s law.” Emitting complex ranges
of vibrations, it nevertheless exacts no corresponding toll of
absorption. Its transparency seems absolute. Either it is so hot that it
replaces the light arrested, or its arresting power is nullified by
rarefaction. The former alternative is excluded by the consideration
that an excess of temperature should be notified by the presence of
bright helium lines in the general spectrum of the sun, and they are no
more visible in it than dark ones. Hence the absorptive incapacity of
chromospheric helium may provisionally be attributed to extreme
attenuation.[163] This view has gained plausibility through the
discovery that helium in or near spots acts at times normally upon
light, for the vapours and gases producing umbral obscurity are
assuredly, on any theory of spot-formation, denser within than outside
the apparent cavity. The shading then at D_{3}, like the fusiform shape
of the sodium pair below it (see Fig. 11), results from increased
pressure. It is usually significant of vehement disturbance. In twenty
or thirty spots with flaming appendages—and mostly in their penumbral
regions—Professor Young has seen the yellow helium ray reversed;[164] it
was similarly visible to Professor Naegamvala in the huge vortex of
February 1892,[165] and to Mr. A. A. Buss of Manchester, on 17th March
1899,[166] in a spot the incessant activity of which was the more
remarkable on account of its occurrence near an imminent minimum.
Absorption by the deep red helium ray at λ 6678 was three times observed
in spot-spectra by Father Perry during 1883,[167] but its chemical
meaning was then unsuspected. Now that the helium-spectrum has been
unravelled, further particulars might easily be learned as to the
associates of D_{3} in spots. Its isolated occurrence is improbable.
[Illustration:
FIG. 11.— Reversal of the D-Lines in the Spectrum of a Sun-spot
(Young).
]
Spot-spectra are crowned and completed by the frequent superposition
upon them of vivid rays. These originate from the gaseous effusions
often accompanying the formation and transformation of spots. They are
readily identifiable. Hydrogen lines and the H and K of calcium are the
most frequently brightened; D_{3} is sometimes bright over the umbra,
dark in the penumbra, of the same spot; and “double reversals” of the
sodium “D pair” are quite commonly observed. The phenomenon is
illustrated in Fig. 11. The brilliant ray shining at the core of the
fuzzy spindle, into which each of the coupled lines is broadened,
evidently proceeds from an overlying hotter and rarer stratum of
sodium-vapour. The magnesium group “_b_” is occasionally affected in the
same way. The “rosy veils” in spot umbræ give out, as might be expected,
hydrogen rays, and “bridges” are also loci of emission. The ultra violet
members of the hydrogen series are never present, bright or dark, in
spots; and the fifth line (Hε), which falls just within the region of
visibility, has often been looked for in vain. In some of Professor
Hale’s spectrographs, however, of the giant spot of February 1892 it
showed faintly bright beside the more conspicuous H of calcium.[168] So
far the record stands alone. It has a particular interest from the
ambiguous position occupied by this ray in the solar spectrum.
The agitation prevailing in spots is often betrayed by line-distortions,
telling of the swift recession or approach of vapours congregated in
them. The condition of the C-line, as sketched by Professor Hale, 13th
February 1892, in the same spot, is shown in Plate VI., Fig. 2. The
brilliant patch over the umbra is of normal wave-length; it was derived
from a flame radially immobile; but the hook-like appurtenance of the
dark line testifies to an extraordinary outrush of cooler gas from the
lower part of the formation. We can, to a certain extent, trace its
course. It started outward with a uniform velocity away from the earth
of about 120 miles a second. This slackened unequally, as can be seen by
the breadth of the “hook” at its junction with the line; and the whole
mass of hydrogen came to rest at a distance of thirty to forty thousand
miles from the point of issue, which (it is worth noting) was at the
very middle of the nucleus. The nature of the force raising this brief
but tremendous storm cannot readily be imagined. Its abrupt development
marked an acute crisis of disturbance, to which the earth responded with
magnetic twitches and auroral illuminations.
Motions in spots seem to be limited and local. They can be inferred to
prevail with great violence at certain levels, while complete
tranquillity reigns at others. This, at least, is the only explanation
of the chemical peculiarities of solar hurricanes. Nothing, for
instance, is commoner than the raging of hydrogen-storms amid profound
calcium-calms. Nay, lines belonging to the same substance may indicate
for it simultaneously rest and motion. Thus a few iron lines are at
times observed to be displaced or twisted through the effects of rapid
approach or recession, while the remainder maintain their usual
positions and aspect. The anomaly is most striking, and challenges
persistent attention. Sir Norman Lockyer meets it with the
dissociation-hypothesis; but this raises more difficulties than it
removes.
Enough has been said to show that numerous and most curious problems
await solution by students of sun-spot spectra. The subject is wide
enough to occupy a band of specialists, and its remoter implications can
still be only surmised. Nevertheless, definite conclusions are not
wholly out of reach. First, as to the cause of nuclear darkness. It is
certainly to be found in augmented, and (so to speak) reiterated
absorption. Spots are not simply rents in a shining veil, exposing an
obscure substratum. They are not super-heated regions, where processes
of condensation are suspended. The photosphere is screened, not
perforated, by them. Moreover, the screening is by interposed vapours.
Umbral absorption is mainly, if not altogether, of the gaseous kind. It
is essentially linear and banded. No part of it can be safely attributed
to the action of a foggy precipitate such as modifies elsewhere the
“surpassing glory” of the disc. They probably differ in this respect
from “pores” and “veiled spots,” but specific inquiries on the point
have yet to be made.
There are strong indications that spot-spectra originate under
conditions of increased pressure and diminished temperature. Still the
coolest umbræ must be hotter than the reversing layer, for otherwise the
Fraunhofer lines would show bright against them, and, as we know, they
cross them in dusky array. This circumstance is fundamental in solar
thermal relations, yet has been generally overlooked. The ordering
aright of such relations is a prime desideratum in solar physics, and
should serve as an indispensable guide to the interpretation of spectral
diversities.
[Illustration:
PLATE VI.
1. The Corona of 1900. Drawn from Photographs by L. E. Jewell.
2. Reversal and Distortion of the C-Line in Sun-spot (Hale).
]
CHAPTER IX.
FACULÆ AND PROMINENCES.
Faculæ are outgrowths from the photosphere. This is known by direct
observation. They have been seen and photographed jutting from the limb
as rotation brought them into, or carried them out of view. It is also
inferred from their superior brilliancy, which might serve to measure
the opacity of the veil spread over the surface they surmount. On the
other hand, they escape none of the Fraunhofer absorption; the whole
range of solar dark lines is invariably present in their analysed light.
Hence their position can be defined as intermediate between the “smoke
veil” and the reversing layer. This consideration affords a safe
holding-ground for reasonings about the status of these remarkable
objects. They testify to internal commotions of the same nature as those
giving rise to spots, but exempt from their heliographic limitations.
Faculæ are not confined to the spot-zones; they develop all over the
solar globe. Since, however, spots invariably claim their attendance,
they are most numerous in the latitudes frequented by such disturbances,
while showing independent maxima much nearer to the poles.[169] But
their imperfect visibility upon the disc greatly restricted their
observation, until Professor Hale and M. Deslandres almost
simultaneously invented a method for spectrographically recording
them.[170]
It depends essentially upon the use of a double slit—Janssen’s valuable
invention for isolating spectral rays. Celestial objects can in this way
be photographed in monochromatic light—that is to say, their forms in
each separate chemical element can be distinctively recorded. The
importance of the fresh start thus made is difficult to exaggerate. With
a double slit and a sensitive plate, the comparative distribution of
glowing vapours in the sun can be satisfactorily investigated, and
anomalies connected with their distribution, if not removed, at least
fully recognised and defined. All spectral rays, nevertheless, are not
equally available for all purposes. In chromospheric photography, for
instance, the calcium H and K offer immense advantages, not only because
of their actinic efficiency, but still more on account of the broad
bands of shadow rendering them conspicuous as Fraunhofer lines. These
serve to protect against atmospheric glare the bright lines superposed
upon them at the edge of the sun; and glare is the worst foe of daylight
photography. In systematic work, moreover, K is, for more than one
reason, always preferred to H. And so came to be established, in two
continents at once, a new branch of astronomical art—the art of
picturing the sun and its surroundings in calcium-light of a single
quality.
The first and immediate object in view was the day-by-day photography of
prominences; but it was very soon found practicable to extend the work
from the limb to the disc. One slit was caused to travel across the
sun’s image, which had a diameter of two inches in the Chicago
twelve-inch refractor, while the motion of the second was adjusted so
that it exactly kept pace with the K-line, admitting it alone, through a
chink just two thousandths of an inch in width, to impress the sensitive
plate. At Chicago, the first experiment of the kind was made 28th
December 1891, and a similar mode of procedure was described by M.
Deslandres before the Paris Academy of Sciences, 8th February 1892.[171]
The upshot in each case was the discovery that K is doubly reversed over
extensive tracts of the photosphere. Plate IV., Fig. 1, shows the sun
self-portrayed in calcium light, 11th April 1894. The lines crossing the
photograph are accidental imperfections, one set originating from
dust-particles in the jaws of the slit, the other from irregularities in
the movement of the siderostat. The regions of reversal are not confined
to the neighbourhood of spots, but spread irregularly over the sun’s
surface. The individual bright forms they include are often bent into
spirals or doubly curved. They correspond very closely, both in aspect
and position, with faculæ directly seen, and were at once, by Professor
Hale, identified with them. M. Deslandres, however, classing them as a
species of hybrid between faculæ and prominences, bestowed upon them the
compound name, expressive of this mixed quality, of “facular
flames.”[172] Hale’s view, in other words, was that the novelty
disclosed by his “spectroheliograph” consisted in the emission by faculæ
of bright “H and K” by way of supplement to their regular photospheric
spectrum, while Deslandres considered that the new investigation applied
to a distinct kind of objects, neither prominences nor faculæ, although
partaking of the nature of both. The question raised is difficult; let
us briefly examine its bearings.
The calcium-flames photographed at Paris and Chicago are certainly not
prominences. Their positions do not coincide with those of the
chromospheric offshoots. They appear where prominences are not, and
_vice versâ_. Nor are prominences bright enough—unless by a rare
exception—to show in relief against the sun; they are strictly objects
for marginal spectroscopic study. Finally, motion-displacements are
conspicuous in them, and are absent from the sinuous shapes on the disc.
The objections to identifying these with faculæ are less obvious, yet
seem equally insuperable.
[Illustration:
PLATE IV.
1. The Sun portrayed in Calcium Light. From a Photograph by M.
Deslandres.
2. Photograph of the Chromosphere and Prominences taken by M.
Deslandres at 5 h. 40 m. P.M., 31st May 1894.
]
Faculæ belong no less unmistakably to the photosphere than the Himalayas
do to the earth’s crust. They are flung upward from it; they subside
back into it. Their light is of the same brilliantly continuous quality;
it is marked by the same kind and amount of linear absorption. It has
then been sifted through all the vaporous strata amassed above the
surface of the sun. This gives the key to the position; for it compels
the inference that while faculæ are situated beneath the reversing
layer, the allied gaseous forms must be located above it. The argument
for their comparative elevation is cogent. The reversing layer, as we
know, is rich in calcium vapour cooler than the photosphere, and, _à
fortiori_, cooler than the flames detached by their superior lustre from
the photosphere. Their calcium-rays should, accordingly, be effectually
stopped by the reversing layer; they could not by any possibility be
transmitted through it. Hence the bright “K” in which the solar disc can
be depicted must originate above the region where Fraunhofer absorption
takes place. The wreathing forms emitting it belong to a different
locality from that of true faculæ. They are chromospheric, not
photospheric, developments. Yet, as just stated, they cannot be
assimilated to prominences. They probably lie at the base of the
chromosphere, and they follow the distribution of faculæ with close,
though not rigid exactitude. Hydrogen is at times represented in their
spectra by a crimson glimmer of C, so they are not composed of calcium
solely; but their real nature and place in the sun’s economy remain
admittedly enigmatical. We are only sure that they spring up in close
relation with faculæ as symptoms of the same disturbances. That their
relation to them is one of actual identity seemed indeed at first sight
scarcely open to doubt; yet for the reasons just assigned they must be
regarded as separate phenomena.
The connection of faculæ with spots is not, as we have seen, of the
“mutual” kind. Their reciprocity—as a typical Irishman might say—is
one-sided. Outside the spot-zones, where solar commotions have a less
vehement character than nearer to the equator, faculæ lie “extended many
a rood” in sluggish inactivity during intervals long enough for the
working out of manifold changes in the brilliant circumvallations of
spots. These are very often crowned with prominences, which, however,
develop somewhat tardily. Nascent umbræ are rarely accompanied by them,
while moribund umbræ offer a favoured site for their growth. Thus a
remarkably brilliant prominence, agitated by violent motion, towered
above the eastern limb as it was passed, on 3rd March 1892, by the
shrunken remnant of the vast spot-group which had slipped out of sight
round the western limb a fortnight previously.[173]
Prominences are related to the chromosphere in very much the same way
that faculæ are related to the photosphere. They arise from it by
effluence or eruption. Now the chromosphere itself has a markedly
eruptive aspect. It presents no billowy ocean surface, but resembles
rather a Tartarean meadow planted with stalks and grass blades of fire,
waving under some unimaginable furnace-blast. With clear definition, a
filiform texture is everywhere apparent. The “straw-thatch” effect of
penumbræ denotes a similar peculiarity in photospheric materials, and
that it is shared by prominences can be inferred from their frequent
construction, as if out of the untwisted strands of a rope.[174] The red
rim of the eclipsed sun is conspicuously jagged, and its saw-like
outline suggested the name _Sierra_, originally bestowed upon it by Airy
in 1851. It does not, then, represent a fluid envelope in a state of
equilibrium. There is another reason why this is impossible. The
chromosphere does not preserve a uniform depth. Averaging about 5000
miles, it is subject to irregular and temporary variations, as yet
imperfectly observed and entirely unexplained. General subsidences are
even affirmed to take place. One such was noted with surprise by
Trouvelot, June to August 1875.[175] He had frequently observed
extensive local depressions, but never before a universal shoaling. He
attributed to the sharper views of the photosphere afforded by this
removal of part of the interposed medium, his discovery of “veiled
spots,” imperfect umbral formations, owing their abortive character,
perhaps, to their unfavourable situation near the poles. In 1887, the
height of the chromosphere, measured thirty-two times by Dr. Fényi,
S.J.,[176] was found to diminish above the spot-zones, and sensibly to
increase outside their limits. It would be of interest to determine
whether this condition prevails commonly, or whether it is restricted to
epochs of minimum.
Prominences are of two varieties—eruptive and quiescent. These differ
chemically, visually, and heliographically. Eruptive prominences are of
a more mixed constitution than the quiescent sort; they include many
more ingredients; they give an intense carmine light, have jet-like or
upspringing shapes, and are mostly confined to the spot-zones. They are
genuine fire-fountains, while quiescent prominences frequently resemble
cloud-banks. The former are individual outbreaks; the latter unite into
communities, counterfeiting banyan groves, ranges of jungle, fields of
cirrus. Usually connected with the chromosphere by stems or trunks, they
in some cases, not only float free in isolated masses, but, still more
remarkably, are generated at great elevations above it, as if by the
spontaneous illumination of pre-existing material. The analogy with
terrestrial condensations in an azure sky is obvious, but may be quite
misleading.
Quiescent prominences develop on an enormous scale, nor always
tranquilly. Formations, at least, which are in some respects cognate
with them, become, on occasions, the scenes of striking explosive
accidents. So that their generic title must be understood in a
restricted sense. Thus the cloud-like character of a huge object
photographed by Deslandres,[177] 31st May 1894, seemed vouched for by
its vicinity to the south pole, and by the large extent of the solar
limb garnished by it. At 2 P.M., when it was first pictured—as usual, in
calcium light—its height, apart from foreshortening, was 2′ 20″, or
63,000 English miles. At 4^h 27^m it had sprung up to 135,000, and 73^m
later still to 270,000 miles (see Plate IV., Fig. 2), an elevation far
beyond any recorded for objects of the kind situated more than
sixty-five degrees from the solar equator, while this wonderful
structure spread from the seventieth to the eighty-first southern
parallel. Perceptibly filamentous, it seemed to grow by the elongation
of its component threads or ribands, and included within its vast bulk
probably the minimum conceivable quantity of matter. On the same plates,
a group of smaller but intensely active prominences registered
themselves at the solar antipodes.
[Illustration:
FIG. 12.—Prominence observed at the Haynald Observatory, 19th Sept.
1893.
The spectroscopic line-displacements due to motion are illustrated in
the upper sections of the figure.
]
An example of a transient apparition, difficult to classify, is given in
Fig. 12. It simulates a portentous conflagration. From bottom to summit
the red pillar of hydrogen measured 166,000 miles, according to Dr.
Fényi’s observation at 2^h 20^m, 19th September 1893. But it wholly
lacked any interior principle of stability. Opposite, and excessively
swift movements in line of sight of the base and shaft betrayed the
progress of destructive change. The catastrophe was not long delayed.
Within less than half an hour all was over. A rush upward set in at the
rate of 132 miles a second, carrying the frail edifice to a height of
224,000 miles. A minute and a half later, it had faded and dissolved,
“like the baseless fabric of a vision,” leaving just “a rack behind” in
the shape of an insignificant protuberance hedged in with faculæ.
“Throughout the course of its appearance,” Dr. Fényi wrote,[178] “the
entire object consisted simply of very bright luminous bands or strips
scattered one after another in ragged forms, and apparently lying nearly
at right angles to the limb of the sun. They were strikingly bright even
in the highest parts of the prominence. The form as a whole was also
like a band or stripe, which had no pronounced inclination, but stood
erect nearly in the direction of the sun’s radius.”
This amazing outburst was repeated with enhancements, nineteen hours
later, at a point on the sun’s limb almost diametrically opposite to its
predecessor. The height in this case attained was 300,000 miles, the
mean velocity of ascent being 214 miles a second, besides which, retreat
from the earth was indicated for the entire mass at the rate of close
upon 160 miles a second. A convulsion at least equally violent was
witnessed by the same observer on 24th December 1894,[179] when a
brilliant and wide-spreading, yet tolerably tranquil prominence, 56,000
miles high, suddenly began to mount, and attained in thirty-five minutes
the towering stature of 300,000 miles. Speedy disorganisation ensued.
Less than two hours after the explosion its scene was vacant. The body
affected by it had been shattered out of existence by its destructive
violence.
The Kalocsa observations were made with a visual spectroscope.
Simultaneous photographs would have been of especial value for
comparison with them, but leisure was not afforded for combining
methods. The three objects they referred to were palpably identical in
character. All showed precisely the same kind of structure. They were
made up of glowing vertical bands, collected into loose sheaves, or
scattered in outlying detachments. Each enormous aggregation, too, stood
erect during the tumultuous processes of expansion and collapse. They
were related to spot-groups, if at all, only by _diametrical
opposition_. The singular counterbalancing tendency of solar
disturbances is frequently conspicuous, and perhaps rarely or never
absent. Two gigantic, although ephemeral apparitions, thus coupled,
were, for instance, observed by Trouvelot, 26th June 1885;[180] and on
16th August of the same year he noted the apparent connection of a
prodigious chromospheric outburst with a spotted condition of the limb
180° distant. A relief of pressure may be concerned in these phenomena,
as in volcanic eruptions; if so, the lift, or diminution of gravity,
must act right across the solar globe, quickening convection-currents
and facilitating the antipodal delivery of extra supplies of heat. In
point of fact, the usual premonitory symptom of the explosive
development of prominences is a disengagement of light.[181] Exceptional
brilliancy is the forerunner of abnormal activity. No satisfactory
cause, however, can be assigned for the imagined relief of pressure,
since tidal influences are fairly out of the question. Only one
undeniable inference can be derived from the contrary symmetry of solar
commotions. It is that they are extremely deep-seated. They have their
roots in the hidden profundities of the great globe they agitate
exteriorly. They are not then mere local accidents; they make an
intimate part of the solar economy. The recognition of this
characteristic is interesting and important.
The tallest prominences are ordinarily the least coherent in structure.
An example is shown in Fig. 13, from a drawing made by M. Fényi, 3rd
October 1892. The object it portrays was almost unique in its fantastic
and colossal form. Overarching thirty degrees of the limb, it rose above
it to a height of nearly a quarter of a million of miles. Its
fragmentary composition was patent.[182] Under the eyes of the
delineating artist it was “in the act of being blown to shreds.”
Photographs happily secured at Chicago seven hours later, exhibited it
as then fallen to less than one-quarter of its high estate, while of
augmented lateral spread. These represented, of course, its calcium
aspect, the Kalocsa drawing its shape in red hydrogen; but there was no
evidence of the two vapours being differently distributed throughout
this bubble edifice. Facular patches marked, on the sensitive plates,
the points both of rise and re-descent of the materials constituting it.
Next morning only some insignificant wreckage strewed its place; virtual
annihilation had overtaken it.
[Illustration:
FIG. 13.—Prominence observed at the Haynald Observatory, 3rd October
1892. Height, 8′ 51″.
]
The tremendous velocities observed in prominences constitute a
formidable problem in solar physics. They not infrequently exceed the
critical rate of 383 miles per second; that is to say, they transcend
the limit of the sun’s gravitational controlling power, and, apart from
possible resistance by a medium, should carry the substances animated by
them finally away into space. The shapes and motions of prominences,
however, clearly indicate some retardative action,[183] although its
excessive feebleness, at least in coronal regions, can be inferred from
the unimpeded circulation of comets passing within a hundred thousand
miles of the sun’s surface. Hence it is only a surmise, not a certainty,
that chromospheric outbursts are attended by irrevocable loss of matter.
The swiftest so far recorded took place, 17th June 1891, in connection
with a spot just disappearing through rotation. A concentration of vivid
luminosity gave the signal that something unusual was impending, “and
after six P.M., Kalocsa mean time,” M. Fényi wrote,[184] “the point at
281° shone with so great a brilliancy that its reddish light seemed to
become white; an enormous displacement of the spectrum towards the blue,
at a medium height above the sun’s limb, indicated at the same time an
approach of the hydrogen in our direction with a prodigious velocity.”
The entire object, which was composed of glowing filaments, lay, in
fact, on the more refrangible side of the C-line, as the result of
approaching speed up to 550 miles a second. A vertical ascent at the
rate of 300 miles being meanwhile directly visible, a total velocity
(neglecting an uncertain third component) of about 680 miles a second
must have been attained in this amazing explosion. Trouvelot’s
observation a few hours earlier of a brilliant and peculiar facular
blaze at the place where it occurred[185] was significant of the intense
energy pent up in the spot-group, waiting release by the figurative
trigger-touch.
This prominence easily wins the prize for rapidity of movement; yet the
competition has been keen. Velocities of the same order, arising under
similar circumstances, have been frequently determined. They appear,
indeed, improbable, and have been stigmatised as “fabulous.” No actual
transport of matter, it is alleged, can be in question, but merely a
swift transference, through gases previously obscure, of a luminous
condition.[186] But this does not account for the conspicuous shiftings
of spectral lines, which could not ensue from the progress of
incandescence through stationary matter. They demand the strict
application of Doppler’s principle; real velocities must correspond to
them. Nor, even if there could be deception about the movements of
prominences _in_ the line of sight, is illusion possible about those
_across_ it. And both kinds are of the same order of speed; they are
complementary; they represent different aspects of identical
disturbances. This is not all. Prominences are often visibly twisted;
they are composed of spirally mounting flames; while to this helical
conformation correspond gyratory movements, at times disclosed by the
spectroscope. Here, at any rate, we have to do with bodily
transportations of matter; “luminescence” cannot be propagated
vortically. Besides, the measured speeds are as difficult to explain on
one hypothesis as on the other. Chemical action does not spread
instantaneously. Through the tenuous gases of the chromosphere, a
maximum rate of one mile a second might be assigned to its progress—a
rate, that is to say, some hundreds of times slower than the velocities
to be explained. That they are somehow of electrical production is a
safe assertion, likely to be true, if not in an immediate, then in a
remote sense. Yet we are little the wiser for the admission. The
“floating of an idea” in the mind does not constitute knowledge; and a
speculation is only valuable when it offers a definite starting-point
for practical research.
Total eclipses have ceased to be indispensable for the prosecution of
chromospheric studies. Day by day the red rim of the sun, with the
strange forms protruding from it, can be viewed spectroscopically; and
day by day the same objects vestured in violet can be photographed under
the broad shelter of the Fraunhofer K-line. Nevertheless, noontide
darkness, when it comes, brings very appreciable help. Differences are
noticeable between what can be seen in and out of eclipse. According to
the late Professor Tacchini,[187] the chromosphere always appears deeper
under cover of the interposing moon, because it is surmounted by a
pink-white margin, giving continuous light, and therefore
spectroscopically invisible in daylight. Some prominences are probably
of analogous composition. Only their skeleton-forms come out in the
crimson radiance of hydrogen; they are compacted and clothed with white
materials, the shining of which is effaced by the glare of common day. A
spectroscopic survey of the chromosphere and its appendages should hence
always be made immediately before and after every eclipse, for
comparison with the direct photographic records obtained during the
corresponding totalities. For the present the information acquired by
daylight work at the edge of the sun remains under a partial slur of
incompleteness.
The objects called “white prominences” belong indeed wholly to the
pageantry of eclipses. First noticed by Tacchini at Caroline Island, 6th
May 1883, they showed as lucid jets about a hundred thousand miles high,
with a surface like granulated silver. Attempts made, after the return
of daylight, to view them prismatically proved fruitless; they gave
forth no hydrogen or helium rays. Again at Grenada, 29th August 1886, a
gigantic helical structure, described by Mr. Maunder as “of the
intensest silver whiteness,”[188] towered three hundred thousand miles
above the limb of the moon. Its spectrum, photographed by W. H.
Pickering, included bright H and K, and Professor Hale accordingly
entertained the hope that such objects might come within the range of
his spectroheliograph; but so far no trace of them has been caught
outside total eclipses.
Dark chromospheric forms are not unknown. A “black protuberance,”
observed by Trouvelot at Meudon, 7th October 1892,[189] might have been
only a negative impression, like “black” flashes of lightning; yet it in
a measure falls into line with eclipse-records certainly not due to
illusion. In the great “anvil protuberance” disclosed 7th August 1869,
Dr. Lewis Swift saw “many black lines crossing in different directions,
and inasmuch,” he added, “as they must have been, at least, fifty
thousand miles long and a thousand miles broad, it would appear to be
important to understand the cause of this phenomenon, and (to ascertain)
if these markings are always present.”[190] Corroborative observations
were made by Alvan G. Clark and Professor George Davidson. With them may
be usefully compared M. Liais’s description of a black-edged but
colourless prominence watched at Paranagua, in Brazil, during the
eclipse of 7th September 1858.[191] The composite effect was not due to
contrast, since the obscure summit stood out alone when the bright lower
portion had disappeared behind the advancing moon. These singular
phenomena excited little comment, and lapsed into oblivion, so that
dusky ramifications, connecting and defining prominences in the
eclipse-photographs of 9th August 1896,[192] seemed entirely novel
features. They were explained by M. Hansky, a member of the Russian
party on the Amur, as outflows of hydrogen cooled by expansion;[193] but
inadequately. Hydrogen below the temperature of luminosity is
transparent, and the photographed veinings were densely opaque. They
afford a hint, that will certainly not be disregarded, of the workings
of unknown activities in connection with prominence-development.
CHAPTER X.
THE CHROMOSPHERIC SPECTRUM.
The spectrum of the chromosphere is almost purely discontinuous. It is
composed of detached bright lines. Some of these are always present, but
the majority come and go. We will consider first the permanent
radiations.
The chief of these are readily identified; they belong to hydrogen,
helium, and calcium. Thirty hydrogen lines have been seen or
photographed in dispersed chromospheric light, all members of the
original, or “Huggins series,” which attained nearly to its theoretical
limit on plates exposed by Mr. Evershed during the Indian eclipse of
1898. The fundamental C (Hα) is the brightest. To its intensity is due
the crimson glow of chromosphere and prominences; and prominences viewed
spectroscopically on this line appear larger than when imaged in any of
the other qualities of hydrogen-light. This is generally explained by
the influence of temperature, a higher degree of heat being needed to
give complete development to forms of shorter wave-length.[194] A
comparatively moderate chromospheric temperature would thus be
indicated. But there may be other influences in question. Professor J.
J. Thomson made the significant observation in 1895[195] that the red
and green lines of hydrogen show marked differences of intensity at the
two electrodes of vacuum-tubes, the red predominating on the positive,
the green on the negative side of a partition. The hint must indeed be
reserved for future use. We are not yet in a position to apply it
profitably.
The helium-spectrum of the chromosphere also gives rise to some
interesting considerations. For its various constituent series are not
represented indiscriminately at the edge of the sun. These series, we
may remind our readers, are six in number, and they are distributed with
beautiful precision into two corresponding triple systems. They may be
distinguished for convenience as “yellow” and “green,” D_{3} giving the
_tone_ to the former, several vivid green lines characterising the
latter set. In the laboratory, as we have seen, they are inseparable;
one set cannot, by any artifice so far devised, be procured apart from
the companion set. But it is otherwise in the sun. Yellow helium is
always present in the chromosphere and prominences; green helium only
about one-fourth as often as it is looked for. The permanent
chromospheric lines are four, namely, one far down in the red at λ 7065,
the familiar D_{3}, a deep blue ray at λ 4472 (formerly known as “_f_”),
and the ultra-violet “leader line” of the principal series at λ 3889.
These have probably many associates of still shorter wave-lengths; but
photographic data are too scanty as yet for purposes of discrimination
between the constant and the occasional elements of the spectrum. They
do not proceed, it must be borne clearly in mind, from two substances,
but from one indivisible form of matter, differently conditioned. In
what way, it is not easy to imagine. Laboratory experiments show that
the green set of lines gains relatively in brightness with rarefaction,
although the yellow set persists as well to the limit of practicable
exhaustion. This indication, however, does not open a way out of the
chromospheric difficulty. The gases near the sun are of inordinate
subtlety. Helium ought there, if this alone were the determining
quality, to be in the _green_ state. Its most fundamental emanation,
nevertheless, is D_{3}. Nor is its dominant position compromised in the
highest prominences. On the contrary, the green rays nearly always
proceed from lower lying, and therefore from denser strata than the
yellow.[196] Supplementary influences are then active—temperature, mode
of illumination, admixture of foreign materials. To this latter cause of
spectral modification, helium, we know, is abnormally sensitive.[197]
And it is quite possible that some of the series emitted by it may be
more liable to suppression than others, in which case the emergence of
the yellow without the green set would be an effect of damping, not of
density. It must, nevertheless, be admitted that what little relevant
experimental evidence there is, scarcely countenances this surmise. The
complex spectra derived by Professors Liveing and Dewar[198] from the
volatile residuum which survived the freezing-out of the main
constituents of atmospheric air, included rays taken impartially from
all the helium-series. No quantitative analysis of the contents of their
tubes was, however, possible; and the fact has been otherwise learned
that the helium-ingredient of a blend must be predominant to become
spectroscopically conspicuous. Every volume of hydrogen present in the
chromosphere (neglecting the effect of metallic vapours) should thus
probably be diluted with two volumes of helium; constituting the solar
appendage largely a helium envelope. This important piece of information
was brought within reach only by terrestrial observation of the new
element captured from clevite.
The significance of calcium in its chromospheric relations has only of
late been fully recognised. And this for an obvious reason. It is
represented by only one pair of lines—bright H and K—and these are so
near the limit of visibility that they could be effectively studied only
by the aid of photography. They were indeed registered as leading
features of the chromospheric spectrum by Professor Young in 1872; but
he was entirely incredulous as to their calcium origin, holding it
impossible that a substance with a vapour density forty times that of
hydrogen should mount to at least equal elevations above the sun’s
surface. Sir Norman Lockyer,[199] on the other hand, maintained them to
be characteristic of a subtle dissociation-product of calcium, alleging
in support of his view the progressive enfeeblement of the “blue line”
of calcium (λ 4227), concurrently with the enhancement of H and K, as
the substance was more and more completely decomposed at each addition
of intensity to the electric current transmitted through the vacuum
tubes. The case for dissociation appeared strong; it has, nevertheless,
broken down. Sir William and Lady Huggins in 1897 successfully reduced
calcium to the “two-line” condition by attenuation alone.[200] Thus the
enigmatical prominence-spectrum of calcium was at last artificially
produced, and no escape was left from the identification with true
metallic calcium of the form of matter encompassing the sun with violet
radiance.
A formidable problem, however, remained. Calcium near the sun seems to
possess a _levitating_ faculty altogether inexplicable. It floats as
high up as hydrogen, or even overtops it. H and K are the most diffusive
of all the prominence-rays; they are derived from the summits of the
tallest flames, and from every fibre of their texture. This anomalous
agility in a comparatively heavy metal must, according to the late
Professor Keeler,[201] be the index to some remarkable property
unrecognised by ordinary chemical methods. Unless, indeed, something
analogous to electrolytic action be in question. There is much to be
said in favour of M. Deslandres’s opinion that the chromosphere is
electrically luminous;[202] and if so, then “ions,” not molecules or
atoms, are presumably its constituting particles. But ions are on a
Lilliputian scale of magnitude, and they may be of nearly the same mass
for all the chemical elements. Here, however, we trench upon a region of
pure speculation. A region, nevertheless, that is likely ere long to be
annexed, in part at least, to surveyed territory, since pioneers are
actively engaged there. Much, in the interpretation of solar phenomena,
depends upon the results of their work; for here, as in every department
of astrophysics, the experimental decisions of terrestrial physics must
be awaited, not anticipated.
The three ingredients of the chromosphere so far spoken of—hydrogen,
helium, and calcium—are found as well in its eruptive outgrowths.
Gaseous prominences of all sorts and sizes are thus triply compounded.
But we have now to consider a form of matter permanently present in the
chromosphere, though rarely projected to any considerable altitude above
it. Its badge is a single green ray, which has a curious history. It was
momentarily identified with an auroral line; it was long erroneously
identified with the distinctive corona-line. It is apparently reversed
in the sun—that is to say, a Fraunhofer line falls just in its place.
This is Kirchhoff’s “1474” (known as 1474 K), which has proved, under
close scrutiny, to be triple. It results from absorption by iron, by
cobalt, and by an unrecognised substance. Now the chromospheric ray
agrees in position with the iron line, which is one of secondary
importance; yet it cannot at present be asserted confidently that it
really emanates from glowing iron-vapour. If it did, it should be
ordinarily associated with other iron-lines, and none have been
ascertained to make part of the fundamental chromospheric spectrum. The
vapour giving out “1474 K,” however, is never absent from the solar
envelope,[203] although it perhaps subsides at times into its lower
strata.[204] On the other hand, it occasionally rises in metallic
prominences to a height of about fifteen thousand miles. Mr. Lord, at
the Lick Observatory, observed the enigmatical line (λ 5316·8) as
shining vividly at the base of a violently disturbed prominence on 4th
August 1892,[205] and Mr. Evershed records similar experiences. Dr.
Fényi caught a still rarer effect on 19th February 1892 in a prominence
attending the great spot-group then visible.[206] The object was
peculiar, though not unique, in showing complete forms built up of the
various metallic and other substances injected into it from below. Fig.
14 reproduces an instructive drawing made on the spot. The bottom sketch
was taken on the C-line when the eruption was at its height. It reached
an elevation of 56,000 miles. The overarching of three filaments towards
a point at some distance from the base is noteworthy. The second drawing
in Fig. 14 is of a date twenty-four minutes later than the first. It
depicts the flame in “parhelium” light. The image viewed was constructed
on the _red_ ray of “green” helium. This was much smaller than the
simultaneous hydrogen-image, and that obtained on the “1474” line had
shrunken still further. But it was measured at half-past ten, an hour
after the drawing on “C” had been made, when the first vehemence of the
outburst had subsided. The line 1474 K is singularly exempt from
displacement effects through motion. It remains erect and undeviated in
the midst of solar storms. Nor does it widen perceptibly with increase
of pressure downward. Its invariable fineness contrasts remarkably with
the wedged shape near the photosphere of C, H, and K. Unusual agitation
is betokened by its emergence in prominences.
[Illustration:
FIG. 14.—Forms of a Prominence in Hydrogen, Helium, and
Pseudo-Coronium (Fényi).
]
Eclipse-spectrographs do not include it, while they have afforded some
other quite unexpected results. Of special consequence is Mr. Evershed’s
detection of titanium as an unfailing chromospheric element.[207] Plates
exposed by him in India, 22nd January 1898, proved to be crowded with
ultra-violet lines belonging to this metal. Some among them had indeed
been already recognised by Mr. Jewell in Professor Hale’s daylight
photographs of prominence-spectra.[208] The height to which they
extended indicates a diffusiveness for titanium-vapour equal to that of
hydrogen and helium, although inferior to that of calcium. Its atomic
weight on the hydrogen scale is 48; it is just as much lighter than iron
as it is heavier than calcium. But comparative vapour-densities are, so
to speak, impotent for the regulation of elemental distribution near the
sun. Another surprise afforded by the Indian eclipse was the conspicuous
presence in the chromosphere of scandium as well as of manganese and
chromium; while Mr. Hartley[209] has identified as another of its
constituents the rare metal gallium from two of its characteristic lines
(λ 4172 and λ 4033) recorded during the eclipse of 1893.
On 29th September 1897, Professor Hale, using a grating spectroscope on
the forty-inch Yerkes refractor, discerned the green carbon fluting
bright at the edge of the sun.[210] Two years later, he found the
corresponding yellow band, although the third in the blue remained
imperceptible. He inferred the permanent existence near the sun of a
shallow layer of carbon vapour.[211] Its rays do not come near the
surface; they have to be _dredged for_. Hence the extreme delicacy of
detective observations. The anomaly of this behaviour on the part of
carbon is glaring. Its specific lightness ought to carry it to altitudes
far beyond those attained by titanium and calcium. Yet it lies sunken
almost out of sight, while they float manifestly aloft. Some other
sorting-out influence besides that of gravity palpably comes into play
in the chromosphere and prominences. The possession by the sun of a
carbon-envelope, which, although thin visually, must really be at least
500, and may well be 1000 miles in depth, has an important bearing upon
the vexed question of photospheric constitution, and tends to strengthen
a barely indicated analogy between the sun and a peculiar class of red
stars.
The chromospheric spectrum, when its adventitious elements are reckoned
in, is highly complex. During a few weeks of 1872, Professor Young, by
vigilant watching, determined 273 lines seen, in the clear air of the
Rocky Mountains, to flash out intermittently, one by one, or in
companies together. And this first systematic enumeration was
subsequently greatly extended by its author, while crowds of
ultra-violet lines have been added by the photographic investigations of
Hale, Deslandres, Evershed, and others. For the most part the rays
brightened in eruptions are reversals of Fraunhofer lines, picked out
largely at haphazard, yet with an obvious preference, expressly noted by
Sir Norman Lockyer, for such as are vivified when the higher excitement
of the spark is substituted for the arc in laboratory experiments. This
has been taken to imply that the chromospheric is essentially a
high-temperature spectrum; but the facts may be differently interpreted.
Among the elements most apt to shine evanescently in metallic
prominences are sodium, magnesium, “green” helium, barium, iron,
vanadium, and strontium. Gigantic ragged forms, especially when they
appear in high latitudes, are of relatively simple chemical composition;
or it may be that their condition favours the visibility of only the
most persistent radiations. Small compact prominences yield, at any
rate, much richer harvests to gleaners of spectroscopic novelties. A
specimen of this class was observed by Father Sidgreaves, 10th September
1891.[212] It presented the aspect of “four blow-pipe jets intensely
bright at the bends,” was 16,000 miles high, and lasted only an hour.
Twenty-six brilliant lines were counted in the visual part of its
spectrum, the invisible part, in the absence of suitable appliances,
remaining unrecorded. The shape of this flame was equally well defined
in _both_ the red rays of helium, and appeared nearly the same in sodium
and magnesium. The spectral peculiarities of such objects, however, are
most marked in the ultra-violet. A photograph taken at the Kenwood
Observatory, 15th October 1892, registered, from an undistinguished
prominence, no less than seventy-four bright lines between the
wave-lengths 3970 and 3630,[213] most of them being of unknown origin.
But since then it has become possible to identify a dozen and upwards
with rays measured in the spectra of krypton and xenon by Professors
Liveing and Dewar;[214] and the circumstance seems to associate those
scarce atmospheric gases with helium as chromospheric constituents.
Occasionally, under circumstances not yet defined or understood,
prominences emit continuous light. The sheeny white objects sometimes
seen under cover of the moon’s shadow owe their peculiarity to this
cause. The whole gamut of prismatic radiance is derived from them, with
the addition of bright H and K and a suspicion of hydrogen lines. Thus
they are essentially calcium-forms interpenetrated with glowing _dust_.
Their light is probably original. If it were reflected, traces of
Fraunhofer-absorption which seem to be missing should be perceptible.
Distinctively “white” prominences are not common; none were observed
during the totality of 28th May 1900. But “red” prominences differ
considerably in colour-intensity, all the ruddy shades, from deep ruby
to pale pink, being represented in them. Many perhaps consist of a
crimson core veiled in almost colourless material. Certainly all are not
equally well seen in and out of eclipse. From a comparison of drawings
made during the totality of 1870 with his own simultaneous daylight
observations, Tacchini inferred that the spectroscope disclosed only the
cores of flame in such objects;[215] and the experience was repeated on
the occasion of the Egyptian eclipse of 17th May 1882. The four
prominences then measured were of a rosy tint, lightening towards the
margins, which looked as if edged with a lustrous fillet. “These
results,” Professor Hale writes, “may be accepted as establishing an
important difference between the spectroscopic and eclipse-images of
prominences.” Nevertheless the difference is not constantly present.
Some chromospheric forms are identical, viewed prismatically at the edge
of the sun, or directly in the dark beside the occulting moon. These
are, of course, purely gaseous; the others presumably give a mixture of
continuous and discontinuous light.
A very curious feature of the prominence-spectrum was ascertained by Mr.
Evershed during the eclipse of 1898.[216] It _becomes_ continuous in the
extreme ultra-violet. The range of unbroken radiance begins abruptly
just where the hydrogen series ends (near λ 3668), and extends to the
limit of the plate’s sensitiveness. Not even a guess can be hazarded at
the physical condition underlying this radiative vagary. A different
cause must be ascribed to certain local outbursts of white light in
eruptive prominences.[217] This symptom of disturbance has been
interpreted by Professor Hale as follows. “Objects of the kind,” he
says,[218] “are closely related to faculæ, and probably rise from them.
It thus occasionally happens that a violent eruption carries some of the
white-hot particles to a considerable distance above the photosphere. In
such a case the prominence gives a continuous spectrum in addition to
its bright lines.” The explanation may pass muster, since no better has
been offered.
M. Deslandres succeeded in showing, early in 1892,[219] that the sun
may, in a restricted sense, be designated a “bright line star”—that is
to say, he elicited from the aggregate of its light evidence of gaseous
emissions. Treating the sun as a star by admitting into his
spectrographic apparatus the whole of its rays simultaneously, he
obtained vivid reversals of the violet calcium lines. But this is only
possible when the calcium flames crowning faculæ are widely and strongly
developed. Ordinarily their emissions are drowned in the surging flood
of continuous radiance. But facular maxima recur, coincidently with
spot-maxima, once in about eleven years; so that the periodicity of the
sun might conceivably be determined by this one feature at distances
obliterative of all other signs of disturbance. Not that the sun viewed,
say, from Sirius, could with our actual appliances be detected, even at
culminating epochs of agitation, as a bright-line star. Some of his
fellow-stars, however, may be in a greatly enhanced stage of his
condition, and we may learn to follow their vicissitudes by
spectrographic observations of the alternate glimmering and fading of
fine rays projected upon the deep shadow of their calcium-absorption.
Thus the means may be afforded of ascertaining the flow of change in
remote and gigantic orbs; and we shall perhaps in the future be better
acquainted with the cyclical peculiarities of Capella or Arcturus than
with those of our own “particular star.”
CHAPTER XI.
THE CORONA.
The corona is exclusively an eclipse phenomenon. No sooner is totality
established than it is there. It seems to have emerged from nothingness,
to have arrived from nowhere. It starts into view with the abruptness,
the inexplicableness, of an apparition. “The sun,” Professor Langley
says, recording his impressions of the eclipse of 1869, “went out as
suddenly as a blown-out gas-jet, and I became as suddenly aware that all
around where it had been, there had been growing into vision a kind of
ghostly radiance, composed of separate pearly beams, looking distinct
each from each, as though the black circle where the sun once was
bristled with pale streamers stretching far away from it in a sort of
crown.”[220]
The corona presents various aspects, but it may always be described as
composed of extended streamers springing from a much more intensely
luminous ring, the so-called “inner corona.” There is no real
separation; the entire appendage is evidently framed on the same
constructive principle; yet the distinction is obvious visually, and
convenient descriptively. “I do not know,” Mr. Francis Galton wrote of
the corona visible 18th July 1860, “to what I can justly compare that
magnificent meteor. It differed from other objects in the remarkable
whiteness and purity of its light, and also in the definition of its
shape as combined with a peculiar tenderness of outline.”[221] Both he
and Winnecke noticed the curvature of some “long arms of light”
protruding from the ring, while other rays took “a more or less
tangential direction.” Mädler was struck with the _determinateness_ of
the formation. What he saw was no vague light-effusion, but a congeries
of sharply terminated beamy sheaves. This is a radical characteristic.
The solar corona is a texture of significant pattern. There is indeed
much difficulty in laying bare the original design. A spherical
agglomeration projected on a plane gives rise to intricate effects of
perspective, from which the true relations in solid space of the objects
originating them can be deduced only by careful and systematic
interpretation on strict geometrical principles. The problem was
attacked by Professor Schaeberle[222] with the help of an ingeniously
contrived model, photographs of which showed divergent rods inserted
over the surface of a globe as apparently intercrossing and interlacing
in the flat picture imprinted on the plates. It is, however, well to
remember that, while curved rays may be projected so as to seem
straight, straight rays can never appear curved. Beams that show flexure
are inflected. Nor can a semblance of double curvature be given by
perspective to those bent simply in one direction; moreover, rays that
are actually normal to the sun’s surface must, from all points of view,
appear radial to the limb.[223]
Coronal structure is of immense variety. It is intrinsically of a
radiated character, and the fact is of primary importance. There are no
signs of concentric arrangement;[224] coronal materials do not form
shells or envelopes, such as surround the heads of comets; they are, on
the contrary, drawn out into fibres by forces acting upon them in minute
detail. Comparisons to spun glass and to silken filaments indicate the
delicate nature of the shining tissue spread round the obscured sun.
There are indeed differences. The fibres sometimes, as in the corona of
22nd December 1870, look to have been “combed out,” but more or less of
derangement is usually prevalent. Tangled hanks of thread are often
suggested, or “masses of luminous hair in disorder.”[225] These
contorted forms, although their complexity is doubtless augmented by the
superposition of sundry groups of twisted rays presented at different
angles, afford remarkable evidence of disturbance within the corona
itself. Their photographic registration dates from the Sumatra eclipse
of 18th May 1901. On plates there taken with the Lick forty-foot
telescope, and by Mrs. Maunder at Mauritius, a tumultuous area, agitated
as if by the effects of an explosion, was strikingly depicted. “A long
thread-like prominence appeared,” it was stated, “to emanate from the
same source.”[226]
Fleecy coronal tracts are at times intermixed with regions of striation.
The corona of 1868 was perceived at Masulipatam to be “slightly mottled”
near the sun, and the “curdled” aspect of the great nebula in Orion has
often been recalled to telescopic observers. Sir Norman Lockyer, at
Baikul, 12th December 1871, was struck with an “exquisite” and
“strongly-developed structure.” “I at once,” he continued, “exclaimed
‘Like Orion!’ Thousands of interlacing filaments varying in intensity
were visible; in fact, I saw an extension of the prominence-structure in
cooler material.”[227] It may be remarked that nebular tufts, no less
than prominence-jets, are resolvable into fibres under the best
conditions of seeing. The aureola of 1871 was of such incomparable
beauty that M. Janssen could scarcely rouse himself from its delighted
contemplation to carry out his programme of work. Numerous coral red
prominences were relieved against the “velvet white” of the corona, the
exterior shape of which was rudely quadrilateral. Interiorly the
streamers leaned together in pairs so as to imitate flower petals, the
general effect resembling that of a gigantic lucid dahlia, with the
black moon for its heart.
Again and again, in descriptions of successive coronas, the
Orion-similitude recurs. In the “density, brightness, and species of its
light,” that of 1st January 1889 strongly reminded a Nevada State
observer of the nebula, and its slightly greenish tinge of colour
completed the likeness.[228] Again, in examining the coronal photographs
of 9th August 1896, Mr. W. H. Wesley detected an area “broken up by dark
channels into flocculent-looking masses, giving to it somewhat of the
_curdled_ appearance of some parts of the nebula in Orion.”[229] The
similarity is not merely superficial. Laborious photographic comparisons
by Mr. Ranyard (assisted by Mr. Wesley) emphasised the organic analogy
between the great nebula and the solar corona.[230] Synclinal forms (as
the petal-shaped structures are called) emerge in both, and the
branching effusions round the trapezium seem to mimic details legible in
many eclipse-pictures.
A chain of “pearly cones” furrowed spirally, 200,000 to 300,000 miles in
height, and rising above a long bank of red prominences, were perceived
by Professor Cleveland Abbe in the corona of 1869. And in 1893 the sun
appears to have been fringed in this manner nearly all round, the
individual peaks being projected together into such close array as to be
in many places undiscernable apart. “Systems of (approximately)
concentric arches” were also distinguished by Professor Schaeberle in
his large-scale photographs of the same eclipse.[231]
Inverted shapes are also, though less commonly, met with. Paraboloids,
convex towards the limb, now and then replace arcs and cones based upon
it. A curious instance was afforded by Schaeberle’s “coronal comet” of
16th April 1893.[232] This object seemed as if _spitted_ upon a slender,
solitary, nearly radial streamer, from which it had evidently developed.
It was not the only specimen of its class. A well-known drawing by Liais
of the corona of 7th September 1858 shows an immense double paraboloid
lying behind and partly hidden by a “dahlia petal.” The vertex seemed to
be just at the limb. A somewhat similar dusky arc was seen by Winnecke
in the corona of 1860.[233] It looked, he said, like a tracing in sepia.
Again, during the eclipse of 1868, a bright parabolic outline, “with its
vertex towards the sun,” was noted by Weiss. Finally, Homer Lane
observed at Des Moines, Iowa, 7th August 1869, two condensations of
light which “might well be compared to small telescopic comets, with
tails of some length, but without a head, and with no distinct
indication of a head at one end rather than the other. They were not far
from radial in direction relatively to the sun’s centre, and had their
origin above the limb of the moon.”[234] He estimated the length of each
at about 130″. These appearances are full of meaning. They plainly
assert the subjection of coronal matter to a dual repulsion, such as
acts upon the “crystal tresses” of comets. A local centre of
condensation throws off a filmy envelope, the constituent particles of
which, as they approach the sun, are swept backward into a train by a
counter influence proceeding from him. The only genuine “eclipse comet”
so far captured was that seen and photographed at Sohag, 17th May 1882.
It was sharply characterised as such, the effects of swift motion being
unmistakably impressed upon its curved plumage.
An eclipse, visible in the Western States of North America, 29th July
1878, disclosed a surprising spectacle. In lieu of the ordinary radiated
corona there were seen “bristles” of light at the sun’s poles, enormous
“wings” at each side of the equator. Professor Langley observed the
phenomenon from the summit of Pike’s Peak in Colorado, at an elevation
of 14,000 feet in a stainless sky. Thus favourably circumstanced, he was
able to trace one wide beam to a distance of about five millions of
miles from the sun, the other fully twice as far.[235] The direction in
which they lay proved, when carefully measured, to agree closely with
that of the zodiacal light, and “a faint central rib” emphasised the
coincidence. “With the telescope,” he says, “the whole of the bright
inner light close to the sun was found to be made up of filaments, more
definite even than those seen in sun-spots,” and apparently exempt from
the effects of spherical projection; they “fringed the sun’s edge in
definite outline, as though it were really but a disc.”
At the time of this eclipse, the sun was in a state of exceptional
tranquillity, and a search through the solar archives brought out the
notable fact that a similar apparition had, eleven years previously,
spots then too being nearly extinct, been described and depicted by
Grosch of Santiago. He inferred from it the possession by the sun of
“strong magnetic polarity.” And indeed the divergent light-fibres at the
poles, in 1867 no less than in 1878, seemed to trace precisely the lines
of force in a magnetic field. The concurrence of these phenomena with
critical epochs in the sun’s activity started the idea, due, in the
first instance, to Mr. Ranyard,[236] of varying coronal types. It was
amply borne out by subsequent experience. From eclipse to eclipse,
throughout the eleven-year cycle, the corona exhibits changes of form in
marked conformity to spot-vicissitudes. In the accompanying plate, the
original of which is by M. Hansky, the coronas of 1860, 1870, 1883, and
1893, all of pronounced maximum type, are represented in the first
column; those of 1867, 1878, and 1889 in the third. The second and
fourth show coronas of intermediate forms. The last figure in the third
column has a prophetic character. It shows M. Hansky’s anticipation of
the kind of halo due, on the theory of recurring types, in 1900. What
was actually photographed is given for comparison in Plate VI., Fig. 1.
The correspondence leaps to the eye. A definite law of variation indeed
quite obviously regulates the shape of the effluence about the sun. At
spot-maxima its component streamers issue from all latitudes
indiscriminately; they pay little or no regard to heliographic
co-ordinates. Then, as disturbance relaxes, they gradually draw away
from the poles, and tend to form “synclinals” above the spot-zones,
giving to the whole appendage the “form of a four-rayed star, the points
of which are inclined 45° to the sun’s axis.”[237] In the polar regions
abandoned by them, “magnetic” filaments meanwhile become noticeable, and
what may be called the intermediate type is constituted. It is subject,
however, to indefinite variations of detail. A good example is shown in
Plate VII., Fig. 1, from photographs taken on the Amur, 9th August 1896,
by a Russian party under the leadership of M. Bélopolsky.
One ray, it will be noticed, is greatly longer than the others, and the
same peculiarity distinguished the corona of 1898. Only when the tide of
solar agitation is dead out, is the finished type of minimum aureola
realised. We have then a symmetrical arrangement of crested poles and
equatorial extensions, with this one singular qualification to its
symmetry, that the wings are not a pair. One is formed of convergent,
the other of parallel, or even divergent rays. And they seem to be
reversed east and west at alternate epochs. Both are radically double;
they are formed by the closing down upon the equator, as spot-activity
becomes exhausted, of the synclinal groups previously visible in middle
latitudes. It is difficult to realise that these “wings” are merely the
profile-shapes of a vast luminous disc completely encompassing the sun.
Hence an extreme intricacy of structural details most baffling to
efforts towards interpretation.
[Illustration:
PLATE V.
Types of the Corona 1860–96, with an Anticipatory Sketch of the Corona
of 1900. Drawn by M. Hansky.
(From the _Observatory_, February 1898.)
]
Coronal modifications are not so entirely isolated as might at first
sight appear. Looked at more closely, they are perceived to correspond
unmistakably with the cyclical changes in distribution of surface
disturbances. This was insisted upon by M. Bélopolsky in 1897.[238]
Spots descend into lower latitudes with the approach of each minimum.
One after the other, the eleven-year waves of commotion attain their
acme in medium zones, and die out near the equator. Coronal development
pursues the same course. Its most intimate relations, however, are with
chromospheric eruptions. Tacchini[239] was the first to notice that
coronal outflows emanate from regions frequented by prominences, which
at times of maximum spread all over the sun, but near minimum withdraw
from the extensive polar tracts simultaneously denuded of far-spreading
streamers. Particular agreement frequently accentuates this general
correspondence. Thus the springing of a coronal arch has usually a
prominence for its motive. Each pearly pavilion is erected over a red
flame. Coincidences of the kind are of perpetual occurrence.
Chromospheric jets seemed (and doubtless were) appropriated individually
to the “striated cones” observed by Cleveland Abbe in 1869. In
Schaeberle’s fine photographs of the eclipse visible in South America
16th April 1893, one-sixth of the sun’s circumference came out clear of
prominences; and just over the same segment there is a gap in the
elsewhere unbroken range of coronal arches.[240] In some cases arches
are buttressed upon prominences; in others they are symmetrical as
regards them; coronal streamers appear to be vaulted into domes, or bent
together into ogives, through effects of eruptive action in the
chromosphere. These relations were especially marked in the corona of
1896. The wide polar rifts were devoid of prominences, but a prominence
lay at the root of each great streamer, and a prominence was enclosed by
each synclinal structure. These M. Hansky inferred, from their interior
darkness, to be hollow,[241] like the tails of certain comets; and he
noticed curious effects of coronal transparency, a few beams showing
traceably through the substance of those in front of them. The
correlation of prominences with coronal forms was somewhat less
conspicuous in 1898 than in 1896, and was barely perceptible in 1900.
The Lick photographs of 1901, however, showed the envelopment of a
prominence by a “series of coronal hoods,” besides other symptoms of
community in disturbance between the chromosphere and the silvery
aureola.
The more closely the spectrum of the corona is studied, the more
interesting and enigmatical it becomes. It has a triple origin.
Continuous reflected light is mixed up in it with continuous original
light, and these again with bright-line emissions. The three elements
are not easily separated and the proportions of them present vary from
time to time. The gaseous spectrum is feeble, especially near spot
minima. Its leading constituent is a green line at λ 5303, long
confounded with the chromospheric λ 5317. Their disconnection first
became apparent in Mr. Fowler’s eclipse photographs of 22nd January
1898, and was announced by Sir Norman Lockyer[242] as one result of
their examination. The green line characterises the unknown substance
designated “coronium,” the distribution of which round the sun can be
investigated by its means. Photographs taken during totalities with the
“prismatic camera” give separate images of the solar appendages in each
quality of discontinuous light emitted by them, and the “green” coronal
image proves to be approximately ring-shaped. The gas it is derived from
seems to spread through the “inner corona” to an average height of from
160,000 to 200,000 miles, but not to extend into the sheaves and
streamers beyond. There are indeed irregularities. The annulus of
coronium is far from being uniform or homogeneous. It is wider, perhaps
also more condensed, in some places than in others, and spectrograms
taken with a slit by Professor Campbell at Jeur suggested, by the
distortions of the characteristic ray impressed upon them, the progress
of radial movements, such as might well be deemed inevitable in an
aerial envelope obviously not in a state of equilibrium.[243]
Ten or a dozen coronal lines besides the green ray have been
photographed. The wave-lengths of the most authentically recorded are as
follows: 4232, 4086, 3987, 3801, 3643, 3456, 3388, and 3381. No success
has hitherto attended efforts to arrange them in a series; nor is it by
any means certain that all claim the same chemical origin. On the
contrary, the fundamental green line, together with the most refrangible
of those above enumerated, appears capable of segregation from the
violet ray λ 3987 and the first three of its ultra-visible
associates.[244] Two strange gases then, at any rate, are indicated as
co-existing in the corona. And they are unmixed with any familiar
substance. Evidence collected during recent eclipses testifies strongly
to the absence of all the chromospheric materials. Not even the
pervasive trio, hydrogen, helium, and calcium, extend into the vast
solar halo. Some of their radiations, notably H and K, have _seemed_ to
be derived from it, but only through the effects of atmospheric
scattering. They come as well from the black disc of the moon. Probably
only some peculiar forms of matter, or forms of matter in a peculiar
state, constitute the gaseous corona. So far coronium has not been
recognised elsewhere.
The continuous light from the interior halo is mainly original. It
emanates from incandescent solid or liquid particles. But their
incandescence appears to be of an unusual kind. Bolometric experiments,
carried out by Messrs. Abbot and Mendenhall of the Smithsonian
Observatory during the eclipse of 1900,[245] showed the coronal beams to
be almost wholly wanting in thermal power. Compared with them moonlight
is a potent source of heat. They include, according to the results in
question, next to no infra-red waves, and are surmised to be comparable
in quality to the glow of phosphorescent or luminescent substances.
Novel inquiries in the laboratory will be needed to ratify these
significant conjectures; while it is to be hoped that the eclipse of
1904 will afford some positive data as to the distribution of energy in
the coronal spectrum, which may serve as a basis both for photographic
investigations and for theoretical conclusions.
Photospheric light _must_ be reflected from the pulverulent materials of
the corona, and a reflected ingredient is, accordingly, contained in its
radiance. It is, however, small in quantity. Thus the dark Fraunhofer
lines are barely distinguishable in it. They were detected by Janssen in
1871; fifteen were photographed by Pluvinel at Senegal in 1893.[246] On
the same occasion, nevertheless, Deslandres could obtain no trace of
them, and Campbell was equally unsuccessful in 1898.[247] The truth is
that where the original emissions are strong they get _drowned out_.
They show, and that with difficulty, only in the less luminous sections
of the appendage. This was made perfectly obvious by Mr. Perrine’s
discussion of the plates taken in Sumatra, upon which thirty-five
Fraunhofer-lines impressed themselves, but only in regions remote from
the limb. Polarisation-effects give accordant testimony.[248] They are
slight but unmistakable, and plainly indicate the action of scattering
particles right up to the limb. Some hints as to the manner of their
distribution were obtained by Professor Turner in 1898, and again in
1900, through the ingenious device of photographing the corona across a
plate of Iceland spar, and so obtaining two pictures in oppositely
polarised light.[249]
Much remains to be learned about the nature of coronal radiance, and the
opportunities for its investigation are restricted and unsatisfactory.
Yet upon their outcome success in coronal portraiture largely depends.
Eclipse-photography is an art in itself, and one beset by subtle
difficulties. To ensure the best results, the plates employed should
have a curve of sensitiveness as nearly as possible coincident with the
energy-curve of the coronal radiations, and the form of the latter is
entirely unknown. Many questions too arise regarding the quality of the
plates, and the development proper to be given them, regarding the best
kind of instrument for exposing them with, and, above all, the length of
time that should be allotted to the process. And here the obstacle is
encountered that no single exposure is suitable to the entire aureola.
One long enough to bring out the streamers is too long for the delicate
details of the brilliant interior. The choice has to be made between
solarisation and incompleteness of representation. Two remedies have
been tried. The first is by piecemeal delineation. From photographs of
varied exposures, a picture showing the special features rendered by
each is compounded by a skilled draughtsman. But it has no longer an
autographic value; the forms embodied in it have been deliberately
selected and unconsciously emphasised. “To a certain extent,” it has
been well said,[250] “the same personality enters into the examination
of a photograph that is known to exist in naked-eye observations of the
corona.” M. Morin’s drawing of the corona of 1896 (see Plate VII. Fig.
1) is a fine example of what can be done by combining multiplied
photographic impressions. The two best of those availed of were taken
with an ordinary camera in one and two seconds respectively; the rest
with a photoheliograph, getting exposures up to thirty seconds.[251]
The alternative method is purely automatic. It was invented and
successfully applied by Professor C. Burckhalter of the Chabot
Observatory, California, at the eclipse of 28th May 1900.[252] In his
apparatus a system of revolving diaphragms is arranged so as to give
exposures graduated to correspond with distance from the sun. The image
being progressively covered at successive short intervals, time is
allowed for the imprinting of faint coronal extensions, while the bright
parts already effectively portrayed are shielded against further
prejudicial action. The photograph “controlled” in this way was a
striking record, and conveyed some curious intimations of perspective
effects in the mutual overlaying of tufts and beams.
The outlying branches of the corona usually baffle the perception of the
sensitive plate; for they merge into a dimly illuminated sky-ground,
from which they are, unless by special precautions, chemically
indistinguishable. The most conspicuous performance in this direction
was by Mrs. Maunder in 1898.[253] With a lens of only 1½ inches in
aperture, giving small but intensely bright images, exposures were
experimentally made of inordinate length, proportionately to the
shortness of the focus. The unprecedented result was achieved of
photographing rays to a distance of nearly seven solar diameters from
the limb. Mr. Wesley’s drawing from the original negatives is reproduced
in Plate VII. Fig. 2. Four long narrow divergent rays are visible in it,
each starting outward from a synclinal structure. Mr. and Mrs. Maunder
hold it probable that such structures have always rod-like extensions,
needing only protracted photographic exposures to bring them into
view;[254] but this is more than doubtful.
[Illustration:
FIG. 15.—Diagram of Markings in the Corona of 1871. Drawn by W. H.
Wesley (_Knowledge_, vol. xxiii. p. 225).
]
[Illustration:
FIG. 16.—Diagram of Markings in the Corona of 1896. Drawn by W. H.
Wesley (_Knowledge_, vol. xxiii. p. 226).
]
A very perplexing appearance is that of dark markings in the corona.
They are not mere interspaces between brilliant rays. Mr. Wesley, who is
an expert in the scrutiny and interpretation of celestial photographs,
vouches for their reality. Figs. 15 and 16 copy his diagrams of obscure
streaks and veinings in the coronas of 1871 and 1896. In the first case,
they cut right across the lustrous branches of the halo; in the second,
they are in obvious connection with prominences. Indeed, black coronal
and black chromospheric forms belong undeniably to the same order of
effect, and cannot be separated causally. They took another shape in the
corona of 1900. Mr. Wesley’s beautiful drawing from Mr. Maunder’s
photographs (see Plate VIII.) shows rifts apparently darker than the
general background of the sky, and hence of a _positive_ character.
Their substantial presence was confirmed[255] by negatives taken at
Wadesborough, U.S.A., by Miss Gertrude Bacon; but the difficulty of
accounting for them is at present insurmountable. That they are due to
the interposition of opaque bodies can scarcely be admitted. The
objections are prohibitive. Yet the phenomenon is none the less genuine
for being incomprehensible. We must wait and compare.
[Illustration:
PLATE VII.
1. The Corona of 1896. Drawn by M. Morin from Photographs taken on the
Amur.
2. The Corona of 1898. Drawn by W. H. Wesley from Photographs taken by
Mrs. Maunder. (_Knowledge_, vol. xxi. p. 108.)
_N.B._—The sun’s axis meant to be indicated is a _vertical_ line
bisecting the disc, with which the north-and-south line makes a
small angle.
]
The embarrassments attending coronal photography are enormously enhanced
by the effulgence of daylight. Success here is more earnestly desired
the less it can be hopefully anticipated; for the prospect is dim of
realising Sir William Huggins’s scheme of 1882, or any modification of
it. It is true that, during the partial phases of the last couple of
eclipses, sensitive plates were impressed by the inner corona, but it
only showed as a vague glow throwing into relief the small segment of
the moon outside the sun some forty seconds before and after
totality.[256] Still, even this scanty measure of success was welcome as
an earnest of what the incalculable future might bring. Everything
depends upon catching differential effects—upon obtaining plates capable
of _feeling_ the delicate gradation between daylight pure and simple and
daylight plus corona. And this would be greatly facilitated by
acquaintance with the law of intensity in the coronal spectrum. It ought
to be stronger in the upper reaches than the ordinary solar spectrum,
since the corona escapes the heavy toll of blue absorption exacted from
the photosphere by the “smoke-veil”; the question is, can this
presumable superiority be rendered predominant enough for the ends of
portraiture? The use of coloured screens, letting through the more
refrangible rays, and barring out those lower ones in which mere glare
has the advantage, has proved ineffectual; and the “double-slit method,”
so splendidly helpful in other departments of solar physics, has also
been tried in vain.[257] In coronal photography, no bright line can
serve the end in view because the gaseous spectrum belongs only to the
inner corona, and the record aspired after is more especially of the
outer corona, with its characteristic plumes, streamers, and aigrettes.
Until the changes these undergo can be followed day by day, little will
be satisfactorily known of their intimate relations with the different
orders of solar phenomena, and still less of the underlying cause of
variation.
It has yet to be determined whether the corona rotates with the sun.
Opposite motion-displacements above the east and west limbs, of the
green line or one of its companions, would settle the point; but they
show, if at all, most evasively. M. Deslandres first attempted such
measurements at Fundium in West Africa, 16th April 1893.[258] He chose,
however, as the object of his attack the K-line of calcium, since
ascertained to be non-coronal, so that his results were null and void.
Yet they marked a starting-point, for they sufficed to introduce the
research definitively into the eclipse-programme; nor will it be dropped
out of it, we may hope, until a substantial increase of knowledge has
been gained. Subsequent experiments, although legitimately conducted,
have been indecisive. Mr. Newall, in those tried by him at Pulgaon, 22nd
January 1898, went too far afield for their materials.[259] He directed
his spectroscope to points 8′—corresponding to upwards of 200,000
miles—from each limb, whence no bright line could be obtained, since
they lay outside the limits of the gaseous corona. Professor
Campbell,[260] on the other hand, found in spectrograms taken east and
west of the sun during the same eclipse, a difference in position of the
green ray giving an ostensible radial velocity of 3·1 kilometers per
second, suggesting rotation at half that speed; but he regarded its
genuineness as open to grave doubt.
[Illustration:
PLATE VIII.
The Corona of 1900. Drawn by W. H. Wesley from Photographs taken by E.
W. Maunder. (_Knowledge_, vol. xxiii. p. 227.)
]
More hopeful than the method of simple displacements is perhaps the
method of _inclinations_ recommended by Deslandres.[261] A small,
intensely luminous image of the corona being thrown upon the slit of a
powerful spectroscope, the varied deviations of the bright lines derived
from its different parts should tell something as to the mode of motion
prevalent throughout the appendage. If it rotates like a solid body, all
in one piece, the velocity increases outward, and the lines would lie
aslant on the plate in corresponding directions. If, however, the
regimen be analogous to that governing Saturn’s ring-system, in which
every component particle revolves as an independent satellite, then the
speed of coronal matter slackens with increase of distance from the sun,
and the spectral rays emitted by it should be deflected the opposite
way. The criterion, if it prove applicable, will be highly
discriminative as regards rival hypotheses.
These may be classified as ejective, meteoric, and electro-magnetic.
Professor Schaeberle analyses the phenomenon into “streams of matter
ejected from the lower latitudes of the sun.”[262] To the materials of
the longest rays he ascribes initial velocities up to 400 miles a
second, and excursions outward to the remote neighbourhood of Jupiter or
even Saturn; ogives and arches being due to eruptions of less violence;
while the interplay of innumerable curving jets, foreshortened in all
possible ways, explains the complex aspect of the sun’s lucent crown.
Mr. and Mrs. Maunder, again, consider the prominences to “represent
centres of strong eruptive action, and that in consequence of such
action coronal matter is driven upward from the sun over a very wide
area in domelike forms.”[263] A succession of arches results, “the outer
being less definite and complete than the inner ones. Outside all we
find the curves defining the boundaries of the synclinal group.”
There is much plausibility in these inferences. Prominences are visibly
spouted or flung upward, and the lustrous filagree vaults often rising
above them can scarcely have a totally dissimilar origin. But the whole
secret is not thus laid bare. The rationale by eruptive action is no
sooner sought to be made exclusive than it meets contradictory facts. It
offers no admissible explanation of varying coronal types; it ignores
the mysterious coronium envelope; it seems to be negatived by the
bolometric observations of Abbot in 1900; for obviously the light of the
supposed bombs, projectiles, or pulverulent ejected streams should
contain the ordinary solar proportion of heat-rays, which are
nevertheless markedly deficient.
The corona, according to Sir William Huggins, must consist of
“incandescent fog.” And Professor Newcomb,[264] following a similar
train of ideas, avers it to be made up of detached particles, wholly or
imperfectly vaporised. They might be most sparsely distributed. Intense
radiance would, he informs his readers, result from the occurrence of a
single fragment of dust in every cubic mile of space about the sun. But
how is the dust (if dust there be) supplied? Does it come from within or
from without? Here the upholders of the meteoric theory join issue with
the eruptionists. There must be rings and streams of meteors revolving
quite close to the sun in orbits of all possible inclinations and
considerably varied eccentricities. These furnish, we are told, the
materials of the corona, which—as Dr. Scheiner has recently
shown[265]—are raised to a temperature of incandescence by direct solar
radiation. This view, nevertheless, like the eruption-hypothesis, is
scarcely tenable in view of the non-thermal quality of coronal light.
There remains the electrical theory. Formally enounced by Sir William
Huggins in 1885,[266] it remains unverified indeed, yet unrefuted.
Coronal streamers are regarded by it as analogous to comets’ tails; they
issue forth under the influence of a repulsive force emanating from the
sun; they are illuminated by electrical discharges due probably to
differences of potential at their bases and extremities.[267] The
magnetic relations of the phenomenon, vividly indicated by the minimum
forms exemplified at the two eclipses of 1889, were ably discussed by
Professor Bigelow.[268] He analysed coronal structure by spherical
harmonics, on the supposition of its dependence upon some mode of action
similar to that of free electricity, “the rays being lines of force, and
the coronal matter being discharged from the body of the sun, or
arranged and controlled” by a power proceeding from it. He further
showed that the power was of a repulsive nature and varied inversely as
the square of the distance;[269] but he avoided speaking of it as
“electrical” out of “deference to the doubt that free electricity can
exist at such high temperatures as prevail on the sun’s surface,”
content to have proved “that some force is present acting on the corona
according to the laws of electric potential.”
The diagrammatic halo laid down on these principles was indeed the very
“twin Dromio” of the corona of 1889; but they stood ill the test of
prediction. The corona of 1893 failed to exhibit the special features
anticipated for it by Professor Bigelow. Plainly the assertion that an
illuminated magnetic field surrounds the sun, although scarcely
deniable, does not comprise the whole truth. The same may be said of
Hermann Ebert’s “electro-magnetic theory.”[270] He defines the corona as
“the visible reaction of the finely-divided matter in the vicinity of
the sun upon the dielectric polarisation proceeding from the different
parts of the sun.” Luminosity is evoked by Hertzian oscillations
propagated outward with the velocity of light, and its filamentous
texture corresponds to differences in dielectric stress connected with
the distribution of electricity on the sun’s surface.
The auroral aspect of the corona has often been commented upon. M. Ebert
remarks[271] that the magnetic lines of force near the earth are not
more definitely traced out by the play of polar lights than are those
about the sun by the disposition of coronal rays. The truth of this,
however, is patent only as regards aureolas of the minimum type. In
those visible at maximum it is at any rate disguised. For, as the sun’s
internal activity augments, beamy outflows predominate over tufted
effluences, although both kinds of radiance may be simultaneously
present. Professor Holden considered the sun to be _hairy_ all over, and
not merely at the poles. “There is no latitude,” he wrote, “at which we
can say that here the polar rays end and a new species—equatorial
rays—begins.”[272] This was also recognised by M. Hansky in his study of
the corona of 1896;[273] but the superposed streamers and arches can
with difficulty be included in any magnetic theory. On the other hand,
Mr. Pupin of Columbia College obtained in 1892 striking imitations of
the maximum type of corona by means of electrical discharges through
partially exhausted bulbs.[274] Effects of polarity being, however,
wholly absent, the reproduction failed to convey one fundamental
characteristic of the real phenomenon. But the deficiency was supplied,
three years later, by Ebert’s experiment of subjecting “coronoidal”
tubes to the action of a powerful magnetic field. The organising effect
upon the light-effluence was just what was needed, according to
Bigelow’s contention, to give nature’s own imprimatur to his “magnetic
theory of the solar corona.”[275]
A fact of high import in this connection is that the coronal bright
lines are not reversed in the Fraunhofer spectrum. Coronium, and the
gases associated with it, exercise no perceptible absorption upon the
light transmitted through them. Now this kind of inertness, according to
M. Cantor’s experiments,[276] is distinctive of substances glowing by
electrical stimulation, so that we have here—as the late Professor
Fitzgerald pointed out—a confirmation, absolutely _sui generis_, of the
conjecture that coronal emissions are analogous to those of an
illuminated vacuum-tube.
Yet none of the views propounded on the subject are completely
satisfactory. They have points of contact with truth, but they do not
closely embrace it. This, indeed, could hardly be expected at so
comparatively early a stage of coronal research. For the questions
involved are beyond measure baffling and intricate. We may re-enumerate
them.
One that is fundamental relates to coronal heat. Its virtual absence,
attested bolometrically at Wadesborough in 1900, must be confirmed
during future eclipses before Deslandres’s contrary inference[277] as to
the copious presence of long waves in coronal light can be finally
dismissed.
The coronium-envelope offers a problem which stands almost apart from
that of the stellate appendage surrounding it. And it is one that can
scarcely yet be grappled with. No familiar substance enters into its
composition. Its ingredients are altogether exotic. They do not diffuse
into the chromosphere, while those of the chromosphere are as strictly
excluded from the corona. The cause of this extraordinary circumstance
will perhaps long remain obscure. Meanwhile, through the attempted
apportionment of the coronal rays photographed in totalities between
sundry hypothetical substances we are led to regard coronium as only one
of a group of gases foreign to terrestrial experience.
The rotation of the corona can be measured only by the most refined
methods; but there is little doubt that they will be successfully
applied. Movements of other kinds may also be spectroscopically
determined, since they are likely, in many cases, to take directions
oblique to the limb, and therefore to have large components along the
line of sight to the earth. Notwithstanding the lasting, and, in some
respects, the growing importance of securing the best possible
picture-photographs of the corona during totalities, novel revelations
are scarcely to be expected from them. Their leading interest just now
centres, first in the structural relationship of coronal arches with
prominences, next, in the information they may afford about dark
markings in the solar appendages. For the rest, we must look to daylight
photography. When the great desideratum is attained of getting behind
(as it were) the veil of atmospheric glare, we shall be able to trace
the progress of coronal change, to follow the unbuilding and rebuilding
of the typical aureolas, to witness, perhaps, sudden coronal
developments in sympathy with chromospheric outbursts. From the
vantage-ground thus gained, in short, the true function of the corona in
the solar economy can be systematically investigated. During the few
crowded moments of eclipse this is not possible.
CHAPTER XII.
THE SUN’S ROTATION.
The mode of the sun’s rotation is perhaps the most significant feature
of his constitution. A thorough understanding of it would doubtless
bring with it an explanation of many other outstanding difficulties. But
it seems, unfortunately, a long way off. No more has been attained as
yet than the representation of the observed facts by empirical formulæ.
That is to say, a law of order has been discerned in them although their
cause remains obscure.
No single period of rotation can be ascribed to the sun. Each element of
the photosphere, probably each layer of the chromosphere, moves round
the axis in a fashion of its own. The fundamental rate, if such there
be, is so masked by local drifts as to be unrecognisable. The most
hopeful road to its eventual detection seems to be by the gradual
disentanglement of the solar influences affecting terrestrial magnetic
phenomena. The immediate task in hand, however, consists in extending
and giving precision to knowledge on the whole subject, in bringing
varied methods to bear upon it, and in linking into some kind of
sequence the circumstances ascertained.
Until spectroscopic and photographic means became fully available, the
solar rotation could be determined only by timing the circuits of spots.
And spots were very soon found to have “proper motions” precluding them
from discharging the function of points of reference. Hence there could
be no unanimity as to the sun’s rotation, the period arrived at by each
observer depending upon his choice of spots. At length Carrington’s
scrutiny during the years 1853 to 1861 showed the systematic nature of
these baffling diversities. They proved to vary with heliocentric
latitude, and the great solar swirl was brought into evidence. Once in
about twenty-five days the visible surface of the sun sweeps round at
the equator; but as the distance from it increases, the time lengthens
progressively. The rate of retardation is given mathematically in the
following expressions, which, being artificially adjusted to correspond
with observation, might be multiplied and modified indefinitely:—
FORMULÆ OF SOLAR ROTATION.
X = 865′ − 165′ sin ⁷⁄₄ _l_ (Carrington).
X = 1011′ − 203′ sin (41° 13′+ _l_) (Spoerer).
X = 862′ − 186′ sin^2 _l_ (Faye).
X = 858′ − 157′ sin^2 _l_ (Tisserand),
X signifying the daily angular motion, _l_ the solar latitude.
Formulæ of the kind, moreover, are of restricted application. Based
exclusively upon spot-measurements, they can scarcely be trusted outside
of the spot-zones—that is, beyond 38° north and south of the equator.
Carrington’s gives a period of 25^d 9^h 53^m where _l_ = 14°, of 26^d
9^h 9^m for _l_ = 30°; and he adopted as the standard period 25^d 9^h
0^m, conformed to in latitude 13½°. But what this average speed of
transport actually represents, is the mean rate of motion of the
multitude of sun-spots,[278] not the rotation of the body of the sun.
Professor Bigelow argues from meteorological analogies that this
prevails without disguise at the equator, while retrogressive currents
of a “trade-wind” character lengthen the periods derived from
observations in the spot-zones.[279] But such-like comparisons are
plainly inadmissible. More plausible, although far from decisive, is his
contention that the equatorial period must be the true one because it
lends itself to the correlation of terrestrial phenomena—auroræ,
magnetic storms, wind and weather changes—with solar outbreaks.
Solar rotational studies entered upon a new phase with the application
to them of the spectroscope. The particular importance of the innovation
lay in the change of venue which it involved. A new court, so to speak,
was constituted, a fresh set of witnesses called. These were the
Fraunhofer lines; and while requiring more delicate treatment, they
seemed likely to prove more trustworthy than their predecessors. Spots
do not float inertly on the photospheric tide. They are subject to
individual hurryings and laggings that to some extent invalidate the
record of their axial progress. Fraunhofer lines, on the contrary, are
eminently steadfast. They fall short, it is true, of the absolute fixity
formerly ascribed to them; yet after every deduction has been made,
their reputation as natural constants remains substantially intact. It
must, however, be borne in mind that what the Fraunhofer lines tell
about the sun’s rotation is not strictly comparable with the information
derived from spots. For they proceed from a different level, and may
therefore obey a different law of revolution. They indicate the
velocity, not of the sun itself, but of the absorbent strata in which
they originate.
The eastern limb of the sun advances, the western limb recedes at the
rate of 1·2 miles a second. The consequent line-displacements amount to
just ¹⁄₁₅₀ the little gap between the D-lines of sodium. In 1871 their
simple detection by Vogel was a feat of some moment.[280] Five years
later, Young was enabled, by his early possession of a grating or
diffraction spectroscope, to fix their range with approximate accuracy.
Then Langley showed how, by their means, to distinguish at a glance
lines of solar and telluric production. For in juxtaposed spectra from
opposite ends of the equator, the solar lines, being affected by
rotation, are manifestly “notched,” while atmospheric rays run straight
on without a break. They stand self-announced as of domestic production.
The differential plan of measurement thus suggested obviates many forms
of otherwise inevitable error. It was adopted by Dunér in his classic
work on the sun’s rotation, presented to the Royal Society of Upsala
14th February 1891. He selected two iron lines in the red (λλ 6301·72,
6302·72) for comparison with a pair of adjacent oxygen lines from the
“Alpha” band, known to be terrestrial, and therefore exempt from
motion-shiftings.[281] They were then safely treated as fiducial; and
the intervals between them and the solar lines differed, east and west
of the equator, to an extent corresponding with rotation in a period of
twenty-five and a half days. By the use of the utmost refinements, the
observations were carried up to within fifteen degrees of either pole,
where the period was found to be protracted to thirty-eight and a half
days, the intervening zones showing intermediate velocities. Direct
acquaintance with the sun’s axial movement was thus extended far beyond
the regions of spot-occurrence; and only direct acquaintance is, in this
matter, of any avail, since the formulæ devised to suit low latitudes
break down nearer the poles, disclosing their unsoundness by a total
want of agreement in the periods calculated from them.
“Carrington’s law” might have been true of spots only; it might have
denoted some peculiarity in their mode of production, causing a
systematic increase of backward drift, north and south of the equator.
The Upsala measures, however, proved it to apply, irrespectively of
spots, to the solar globe generally. They told, indeed, something more
than this. The period deduced from them was longer than that given by
spots. The difference amounted to about half a day, and it persisted in
all latitudes—that is to say, the vapour of iron surmounting the
photosphere gyrates more slowly than the spots _in_ the photosphere. A
variation of angular speed with elevation above the sun’s surface was
for the first time indicated. Confirmatory evidence was soon
forthcoming.
The photographic investigation of facular movements was attempted by Dr.
Wilsing at Potsdam in 1888.[282] From 1012 measurements executed upon
108 plates, he obtained a constant angular velocity of 14°·27 per diem,
equivalent to a period of 25·23 days, which is just that of spots
situated ten degrees from the equator. But the conclusion that faculæ in
all parts of the sun conform to it was certainly fallacious. M.
Bélopolsky made this apparent in 1892,[283] and M. Stratonoff of
Taschkent still more decisively in 1894–96.[284] His research included
the determination of 2158 positions of 997 faculæ on 316 Pulkowa plates,
and furnished the clearest evidence of their poleward retardation. The
Stonyhurst drawings, discussed by Fathers Sidgreaves and Cortie,[285]
yielded similar results. Concomitant increase of period and latitude is,
in fact, a rule without exception on the sun.
Faculæ, however, have a mean rate of their own. In the same parallels
they are transported more rapidly than either spots or absorbent
vapours. Their period of rotation is the shortest attributable to any
solar formations. The main facts of the case can be taken in by a glance
at the following little table:—
PERIODS IN DAYS
┌────────────┬───────┬──────┬─────────┐
│Heliographic│Faculæ.│Spots.│Reversing│
│ Latitude. │ │ │ Layer. │
├────────────┼───────┼──────┼─────────┤
│ 0°│ 24·66│ 25·09│ 25·46│
│ 15°│ 25·26│ 25·44│ 27·49│
│ 80°│ 25·48│ 25·81│ 31·83│
└────────────┴───────┴──────┴─────────┘
The equatorial facular period, it is worth noting, is nearly identical
with that ascribed by Hornstein, on the ground of magnetic observations,
to the mass of the sun.
The diversities exhibited above are as perplexing as they were
unexpected. They do not even fall readily into any satisfactory order of
progression. The slowest movement belongs to the reversing layer, or at
least to the slice of it stopping out Dunér’s iron lines; for there is
no certainty that the entire stratum rotates unanimously. It is,
however, certain that it covers both spots and faculæ. The level of
absorption is higher than the level of the photosphere with all its
immediate appendages. This fact has been already insisted upon;[286] it
may perhaps usefully be reasserted in the present connection.
Fraunhofer-absorption is stamped in the prismatic rays of faculæ and
sun-spots precisely as in those of the photosphere. They have then been
demonstrably sifted through the same screen of incandescent vapours. We
have yet to learn that they escape any minutest part of the absorptive
effects produced in ordinary sunlight. They must accordingly be
submerged beneath the whole series of strata occasioning them. At the
same time, it has to be borne in mind that Dunér’s deductions rest upon
a narrow basis. He measured only a single pair of lines, and we lack the
specific assurance that those individual lines occur in the facular and
spot spectra. Presumably they do; no reason is apparent why they should
behave exceptionally; but a direct record of their actual presence would
be satisfactory. They should also be looked for in the “flash” at the
edge of the eclipsed sun. Their detection as brilliant lines would
confirm and settle their status.
But here we encounter an anomaly. Since the reversing layer rotates more
slowly, and lies higher than the photospheric formations, a law of
retardation with altitude might be assumed. Its prevalence is,
nevertheless, contradicted by the relations of spots and faculæ. Faculæ
undeniably rise above spots, yet they wheel more rapidly; they give a
shorter period. This seems to be established by Wolfer’s[287]
confirmation of Stratonoff’s results. Thus while retardation outward is
indicated by the reversing layer, acceleration outward is forcibly
suggested by faculæ. And there is a general consensus of opinion that
this rule applies generally, so that the comparative tardiness of the
absorptive region stands over as an unexplained discrepancy. Even within
the region itself, Mr. Lewis E. Jewell has found indications of
diminishing angular velocity towards the photosphere, where also the
equatorial quickening is small compared to its value higher up.[288]
Moreover, Bélopolsky’s speculations as to the nature of the corona,
which probably adumbrate, if they do not convey truths, require that it
should rotate much more swiftly than the sun itself.[289] The forecast
will doubtless be tested ere long by eclipse-spectrograms. Indeed,
further evidence is much needed on a number of crucial points connected
with the sun’s rotation.
Dunér confessed his inability to imagine a “Why” for the singular
rotatory régime of which he had clearly expounded the “How.” “It
constitutes,” he remarked, “one of the most difficult problems in
astrophysics.” One theory after another has been proposed, and sunk out
of sight, overweighted by manifest inconsistencies. A machine “going”
like the sun the wit of man has not yet been able to devise.
Nevertheless, a means of evading the difficulty has been found. The
possibility has been perceived, and perhaps too readily admitted, of
regarding it as a legacy from chaos to cosmos. The embarrassments
surrounding the subject may be relegated to a far-distant age, and to a
state of things for which the investigators of to-day disclaim
responsibility. The anomaly, on this view, is a survival of nebulous
conditions, which, so far from having a present sustaining cause, is,
and has always been, subject to the destructive agency of friction. Why
then, it may be asked, has it persisted throughout the æons of the sun’s
growth? Should it not have been quite early abolished, if it be nothing
more than a residual inequality, begun to be smoothed away in the dim
foretime when the solar globe took shape? Mathematicians reply in the
negative. Wilczynski of Berlin gave in 1896 an apparent demonstration
that internal resistance cannot change the diurnal arc described by any
point on the sun to the extent of two minutes in twenty-seven million
years.[290] But Harzer showed it to be applicable only to cases
non-existent in nature.[291] It would be valid under ideal
circumstances; things being as they actually are, it falls to the
ground.
Starting from less questionable premisses, however, Wilsing of Potsdam
in 1891,[292] and Sampson of Durham in 1894,[293] reached practically
the same conclusion. They agreed that millions of years must elapse
before the sun comes to rotate “all of a piece,” like a solid body.
There is, indeed, one flaw in their reasoning. Both limit convective
circulation within the solar globe to a relatively thin shell of
material. They assign to radiation an essentially superficial character.
In the attempt to prove that the rotational currents flow without
impulsion, they sacrifice the functional efficiency of our great
light-giver. For assuredly its immense output of radiant energy can only
be supplied from interior stores, rendered available by powerful and
_deeply rooted_ vertical currents. But the continued activity of such a
system should speedily efface inequalities of surface drift, unless some
countervailing force maintained them.
Dr. Wilsing asserts that, in the comparatively late “phase of celestial
evolution represented by the fixed stars, radial currents are beginning
to disappear.” It might more plausibly be argued, considering the
intense brilliancy of such bodies, that radial currents are in them at a
maximum of strength and volume. “We are therefore relieved,” the author
continues, “from the difficulty of accounting for ‘Carrington’s law of
rotation’ on mechanical and physical principles, since it appears as the
result of earlier conditions of motion.”[294]
We leave a good deal to the coming time; why should we not shift an
occasional perplexity back to the broad shoulders of antiquity? Much
that is inconceivable in a sun might be possible in a spiral nebula. No
such expedient, however, will answer the present purpose. The
originating cause of the sun’s rotational anomalies, whatever it may
have been, continues to act. In a globe so profoundly disturbed, they
could not possibly have survived of themselves.
Professor Young finds the necessary driving power in falls of cooled
materials, bringing with them to the photosphere the swifter motion
appertaining to a wider circumference. The consequent accelerative
impulse would be greatest at the equator, and would diminish to nothing
at the poles, according, so far, with the observed facts. Spots,
moreover, by onrushes at epochs of reconstruction or recrudescence,
formally acknowledge the receipt of supplies from above. But
Carrington’s “law” governs all the solar formations, and the rationale
by continuous downward precipitations seems to be of very partial
applicability. Faye tried to solve the problem on an inverse principle.
Instead of descents from without, he postulated ascents from within, the
equatorial retardation being less than that in high latitudes because
the rising matter comes from nearer the surface. This, however, was a
purely artificial arrangement, with no voucher for its reality. The
explanation needed to be explained. Speculation here, as elsewhere, must
await the progress of direct inquiry.
CHAPTER XIII.
THE SOLAR CYCLE.
Solar periodicity is a most complex phenomenon. The more it is studied,
the less it seems to be understood. Its effects branch out into endless
entangled and obscure _fibres_ of fact, to trace all of which back to
their root-source would be an almost superhuman task. At present they
can only be dealt with in groups and tentatively. Their subtle, and
often disguised relationships need much patience for unravelment. They
need, above all, a free mind. Prepossessions are sure to compromise
truth.
The sun is subject to a rhythmical tide of disturbance, ebbing and
flowing in about eleven years. But the flow is irregular and
spasmodic. Both the intensity of the crises and the intervals at which
they recur vary largely and unaccountably. Probably the eleven-year
cycle is involved in others. One, there is reason to believe, brings
about alternate accentuations and partial effacements of change
comprised within a term of some sixty-five years.[295] And minor
pulsations—wavelets on the great rollers—are besides evident.
Prediction, nevertheless, remains at fault. Spot-maxima are delayed or
anticipated, they are languid or energetic, as the outcome of modes of
action defying calculation. Not even the loose fetters of an arbitrary
formula have ever been forged for them. The attempt would indeed be
hopeless, since the laws governing them, besides being highly
intricate in themselves, are plainly disturbed in their working.
Circumstances intervene which we must call “accidental.” Could we
describe them in detail the science of solar physics would lie before
us as an open book.
The “error” of the spot-period may amount to nearly half its normal
length. Thus sixteen years elapsed between the maximum of 1788 and the
next certainly ensuing, and only 7·3 years separated the culminating
points in 1829·9 and 1837·2. A characteristic feature of the
representative curve is that it mounts more rapidly than it descends.
Maxima succeed minima, on an average, after 4·5 years, while the
corresponding minima are only reached after 6·1 years. Substantially, a
disparity of this kind is probably always present, although now and
again masked by the prominence of a secondary maximum. These
peculiarities deserve the most careful attention, as sufficing in
themselves to place the sun in the category of variable stars. His
spot-curve might almost be said to be modelled on the light-curves of
such objects; and the analogy is eminently instructive. We learn from
it, for instance, that a spotted condition in the sun matches a phase of
strong luminosity in the stars; and are hence led to infer that the sun
radiates most powerfully when his disc is most maculated. The extreme
difficulty of obtaining direct proof of this relation lends especial
value to the side-wind of evidence thus brought to bear.
[Illustration:
FIG. 17.—Spoerer’s Curves of Sun-spot Latitude (from _The Sun_, by C.
A. Young).
]
Now an increase of radiation involves a quickening of the sun’s internal
circulation; and the process, when hurried, is likely to become
tumultuous. So that a connection is easily traceable between heightened
brilliancy and photospheric laceration. Spots, however, are only the
most conspicuous symptoms of an agitated state. Faculæ and prominences
follow suit. They are indeed too intimately bound up with spot-economy
to do otherwise. The corona, by a less obvious necessity, sympathises,
and the periodicity of all these formations has a double aspect. They
fluctuate in mode of distribution as well as in vigour of development.
Spots, faculæ, prominences, and corona, all vary similarly and
simultaneously in heliographic latitude as the waves of disturbance rise
and fall. The spot-zones are not stationary. They shift over the body of
the sun according to a definite law enounced by Carrington in 1859, and
confirmed by Spoerer in 1861.[296] At maxima they occupy mean positions
in about fifteen degrees of north and south latitude; then, as the cycle
advances, they close together, and the commotion finally dies out near
the equator. Meanwhile, the start of a new series in high latitudes has
anticipated the termination of the old. Feeble at the commencement, it
gains strength as it departs more and more from its native regions.
“Mobilitate viget, viresque acquirit eundo.” Fig. 17, copied from a
diagram of Spoerer’s, illustrates the nature of this progression. The
overlapping of the curves at minimum brings before us the remarkable
circumstance that, as a consequence of successive disturbances breaking
out before those antecedent to them have expired, the full duration of
each is, not eleven, but twelve to fourteen years. Moreover,
spot-production at minima, however inactive, has a twofold nidus; two
widely separated zones in each hemisphere are appropriated to it. A
further characteristic of the cyclical decline in latitude is that it
proceeds in “waves.” “Every fourth or fifth rotation,” Mr. Maunder tells
us, “there will be an effort to reach a higher level, a lift of one or
two degrees, and then a gradual slipping back until a fresh effort
brings another small lift, but a weaker one than the last. And so the
cycle goes on; the decline is continual on the whole, but is broken and
interrupted by these frequent little struggles to get back to a higher
plane.”[297] The growth in spotted area accompanying the descent of the
zones is similarly rhythmical. So closely connected, indeed, are these
two modes of periodicity that irregularities in the cyclical progression
frequently show under the double aspect of abnormal outbreaks of spots,
and abnormal movements in latitude. Theories are accordingly valueless
that fail to rationalise simultaneously both kinds of facts. Several,
indeed, profess to do so, but by constrained expedients.
The zonal law applies, with qualifications, to faculæ and prominences.
Eruptive prominences are strictly governed by it. They frequent the
spot-belts, it may be said, exclusively. The quiescent kind, on the
contrary, avoid them,[298] and have their main gathering-grounds within
fifteen degrees of either pole.[299] They may even occur, near
spot-maxima, right up to 90° of latitude. As activity decreases,
however, they too move downward, and crowd more or less closely towards
the equator, although maintaining at all times a wider range than spots.
Faculæ show a divided allegiance. They attend on spots, and their
principal maxima are hence displaced with the spot-zones, while their
affinity with quiet prominences is evinced by the occurrence of
secondary maxima in high latitudes. Not that their local arrangement is
the same with that of prominences.[300] Discrepancies are frequently
noted; and they are important as indications that the two varieties of
outgrowth do not originate under identical conditions.
The succession of coronal types is in clear accord with the law of
zones. Streamers and prominences march, on the whole, closely together.
They unanimously quit the poles after each maximum; they linger in
company over middle latitudes, where “synclinals” overarch red flames at
epochs of medium activity; finally, they descend towards the equator,
the white wings of the minimum corona meeting and spreading above the
last members of each decadent eruptive series.
The spectral periodicity of the sun is less marked than might have been
expected. It is, indeed, almost confined to spots. The ordinary
spot-spectrum at minimum (to repeat what has been already stated)
includes many broadened iron lines, replaced at maximum by vanadium and
scandium absorption. The nature of the individual spot, however (as we
have seen), not the stage of the cycle, is really the determining cause
of this diversity, which recurs periodically, simply because
minimum-formations are usually of the tranquil sort. The chromospheric
spectrum has quite other relations. It does not vary fundamentally; but
the metallic rays temporarily added to it become fewer as metallic
injections fall off. Nor is the quality of coronal light subject to
radical change. Only the relative strength of its constituents slightly
fluctuates. In the “winged” type the gaseous emissions are feebler than
in the “radiated” type; yet they are always present and always the same.
The virtual invariability of the Fraunhofer spectrum is more surprising,
since the reversing strata are in immediate contact with the
periodically agitated photosphere. They preserve, nevertheless, an
almost inviolable tranquillity, and their composition remains unaltered
from one cycle’s end to the next. A single recurrent modification is,
however, just traceable. It is that produced by the emergence, at
maximum, of the facular bright lines H and K. _Pro tanto_ and _pro
tempore_, the symptom constitutes the sun a “bright line star.”
Analogous detections in stellar spectra would afford a possibility of
determining the spot-periods of globes in the solar condition; but they
are, for the present, scarcely to be hoped for. The observation is
difficult in the sun; in a star, unless facilitated by extraordinary
facular development, it would be impossible.
The throbbings of solar agitation affect his entire system. In how many
ways, and by what hidden means, we can but vaguely surmise. Terrestrial
meteorology, as a whole, is certainly embraced in the great cycle,
although the details of its conformity baffle, by their intricacy, the
most painstaking pursuit. Only in the magnetic department there is no
room for doubt. A thoroughly satisfactory discussion of the subject was
completed in 1898–99 by Mr. William Ellis,[301] who for long years
controlled this branch of work at Greenwich. Comparing the observations
of the diurnal range of magnetic declination and horizontal force made
at the Royal Observatory during the years 1841 to 1896, with the
sun-spot numbers for the same interval determined by Wolf of Zürich, he
found between the two orders of phenomenon, not only a general
parallelism, but a correspondence in irregularities of period and
secondary variations of intensity. This is strikingly evident in Fig.
18, copied by kind permission from his paper read before the Royal
Society, 10th March 1898.
[Illustration:
FIG. 18.—Curves of Sun-spot Frequency and Magnetic Agitation (Ellis).
]
The sympathetic relation extends to auroræ. They even obey a “law of
zones” similar to that regulating the distribution of sun-spots. The
earth is circled—presumably in each hemisphere—by an auroral belt, which
advances into temperate latitudes at epochs of cosmic disturbance, but
retires towards the pole as it quiets down. The reality of the
connection was singularly affirmed by the simultaneous dearth of
sun-spots and auroræ during the seventeenth century. A prolonged solar
calm appears to have set in about 1643. Galileo and Scheiner had been at
no loss for objects of study; but the diligence of their successors,
although unrelaxed, went mostly unrequited. To Cassini, Flamsteed,
Hooke, De la Hire, the occurrence of a spot was an event of rare
interest, which rewarded perhaps a decade of fruitless watching. Yet, as
Mr. Maunder says,[302] the cycle was “submerged” rather than actually
abolished, “the crests of a sunken spot curve” being marked by the
solitary spots perceived in 1660, 1671, 1684, 1695, and 1705.
Definitively, the protracted minimum came to an end in 1716, and there
was a normal maximum in 1718. Meantime auroræ too were in abeyance.[303]
None were seen in England from 1575 to 1706, when a glimmer of polar
lights heralded the magnificent display witnessed by Halley, 17th March
1716. That there was concomitant magnetic quiescence need not be
doubted; but Gauss of Göttingen was still in the distant future, and
nothing could be known on the subject.
Individual outbreaks on the sun are often unmistakably associated with
commotions of the terrestrial magnetic system. These so-called “storms”
are world-wide in their nature, abrupt in their origin, and bear witness
to some sudden _vital_ spasm attacking the globe as a whole, and at
once. Auroras and earth-currents make part of these mysterious
affections, which commonly reach their height when a large spot-group is
nearly central on the disc—that is to say, when it is broadside on to
the earth. Instances abound. On 17th November 1882, the photosphere was,
to the naked eye, conspicuously rent. The coincident aurora and magnetic
storm were said to “beggar description.”[304] A spindle-shaped beam,
which darted that night across the sky, was indeed a unique phenomenon,
and, on its farthing-candle scale, recalled the amazing solar flambeau
of 1st September 1859. Nor can the contemporaneous twitchings of the Kew
magnets on this latter occasion be regarded as accidental, any more than
the sudden small disturbance of all three magnetic elements which
accompanied an outburst of faculous light on 17th June 1891.[305] The
magnetic turmoil raised by the transit of the enormous spot of February
1892 was exceptionally violent. Earth-currents seriously interfered with
telephonic and telegraphic communication in all parts of our busy
world;[306] the needles at Greenwich went completely off the prepared
track of photographic registration;[307] and an auroral pageant
completed the programme of response. Similar concurrences were observed
in February 1894, September 1896, and March 1898, to mention a few out
of a multitude of cases. Yet the sympathetic connection is not
invariably manifest. A hole in the sun may evoke no earth-trouble. Mr.
Maunder hence concludes that “though sun-spots are the particular solar
phenomenon most easily observed, we must not therefore infer that their
number and extent afford the truest indication of the changes in the
solar activity which produce the perturbations we remark in our magnetic
needles.”[308]
Not the spot itself, but the connascent agitation thrills the
terrestrial organism. Quiet formations pass unheeded; crises of growth
or reconstruction meet with instant rejoinders. Tacchini[309]
accordingly holds that _chromospheric_, not _photospheric_ phenomena
are, in this respect, truly influential, and that it is to the fierce
flame-rushes above spots that the magnetic nerve-system is sensitive.
The view is favoured with some hesitancy by Professor Hale.[310]
Professor Bigelow, as the result of much suggestive inquiry, affirms
that “from the sun to the earth come two great supplies of energy, both
types of radiation through the ether”—one propagated by plane waves, the
other by vortical rotation—“and possessing very different properties,
the one visible to the eye, the other visible to magnetic perceivers.”
And he computes, from various indications, a value for the normal
magnetism of the sun about one-fifth the maximum of steel, which may, at
epochs of extraordinary disturbance, be augmented fifty-fold.[311]
Variations in the sun’s electrical state assuredly accompany his more
obvious cyclical changes. “May not,” Dr. Schuster asks,[312] “the
periodicity of sun-spots and the connection between two such dissimilar
phenomena as spots on the sun and magnetic disturbances on the earth, be
due to a periodically recurring increase in the electric conductivity of
the parts of space surrounding the sun?”
The surmised alteration may be a consequence, it can hardly be the cause
of solar periodicity. Its actual occurrence, however, is far from
unlikely. Certain cometary phenomena lend it a qualified support. The
grouping together near sun-spot maxima and minima respectively, of the
bright and faint apparitions of Encke’s comet during a century
(1786–1885), forced itself upon M. Berberich’s attention in 1888.[313]
Correspondences of this unexpected form were even traceable with
displaced epochs of activity, such as the retarded maximum of 1788, and
the premature maximum of 1837. Some indications were besides gathered
that comet-discoveries become more numerous as the tide of solar energy
rises, many that would otherwise pass unseen being lifted into
visibility by accesses of transmitted excitement. But this relation, M.
Berberich admitted, might be more apparent than real.
One still more recondite and unaccountable has been lately adverted to
by Mr. J. Halm.[314] He contends that the physical condition of the sun
reacts perceptibly on the motion of the earth. Variations in its orbital
elements, which have hitherto baffled attempts at explanation on
gravitational principles, are comprised, he finds, within the “great”
spot period of about sixty-five years. The obliquity of the ecliptic,
for instance, instead of decreasing uniformly with the time, shows
subordinate fluctuations synchronising with the long waves of solar
activity. Its shorter waves, on the other hand, prove to be influential
upon the variation of latitude. Mr. Halm considers that the deviations
of the terrestrial pole conform unmistakably to the eleven-year cycle,
with, however, a lag of about one and a half years in the corresponding
epochs. “It may,” he writes, “be taken to be clearly established that
the radius of the circle described by the pole of instantaneous rotation
is greatest at times of sun-spot minima, and smallest at times of
maximum displays of solar spots.” This “holds true,” he adds, “for the
whole interval of about sixty years now covered by Dr. Chandler’s
investigations.” His explanatory hypothesis is both simple and
ingenious. It depends upon the large inclination of the earth’s magnetic
axis to its axis of figure, combined with alterations, due to solar
influence, in the total magnetism of our planet. Molecular strains along
the magnetic axis would—it is plausibly assumed—occasion bodily
distortions of the globe, whence should result displacements of the axis
of figure relative to the axis of rotation. “The outcome of this
hypothesis would” then “be that changes in the state of solar activity,
since they produce a measurable effect on the terrestrial magnetic
forces, should also be accompanied by corresponding changes in the
motion of the earth’s axis.”
The machinery by which electro-magnetic impulses are propagated from the
sun to the earth, completely evades scrutiny. Sundry conjectures on the
subject have been hazarded, but none of them rest on any sure basis.
What we know about modes of communication is chiefly negative. Thus,
Hertzian vibrations are not transmitted to the earth with sunlight. They
do not, at least, reach its surface. Wilsing and Scheiner tried for them
in vain with an electric “bridge” and a galvanometer.[315] They might,
indeed, as the experimenters noted, be atmospherically arrested. All
that seems certain is that direct magnetic action is concerned in
producing the observed perturbations of the terrestrial magnetic system,
which are not explicable like ordinary meteorological phenomena as
effects of thermal vicissitudes, or convective air currents.[316] They
stand apart, and imply special conditions which cannot, without
detriment to science, be ignored.
Little progress has been made towards ascertaining the cause of solar
periodicity. We are only assured that it is not imposed from without,
but arises from within; it resembles a “free,” rather than a “forced
vibration.” This conclusion, it is true, tends to relegate the matter to
obscurity; for the interior of the sun is a _terra incognita_, and seems
likely to remain so. His cyclical changes may belong to his original
constitution; they may date from nebular times, and be as inherent as
the tone of a bell. Or they may simply characterise a stage of growth,
and prove liable to modification and effacement. The study of variable
stars will perhaps help to guide our ideas as to the probabilities of
the case.
Its full bearings, meanwhile, can only be conjectured. The scope of the
disturbance needs to be defined. There are still many open questions
connected with it. Does the “smoke-veil” absorption vary with the
abundance of spots? Are the temperature of the photosphere, and the
depth of the chromosphere affected by it? Can periodical changes of
pressure in the reversing layer be detected? These are among the
problems of the immediate future. They are already within reach of
attack.
CHAPTER XIV.
THE SUN AS A WHOLE.
Modern science contemplates in the sun a huge sphere 867,000 miles in
diameter, bounded by a dazzling cloud-shell, and composed of materials
1·4 times heavier than water, yet heated so far above the “critical
temperature” of any terrestrial substance, that they must be regarded as
in the gaseous state. The imperious demands of radiative emission can be
met only by rapid exchanges, implying the unceasing activity of a system
of profound vertical currents; while rotational surface-drifts of a
peculiar kind notify complexities of internal movement defying
speculation or research. Incidental to them, doubtless, are the dark
spots marring, at times, the brightness of the disc, and giving
evidence, by their more or less copious occurrence, of that far-reaching
periodicity which may be called the central fact in solar physics. Spots
are garlanded with faculæ, which represent photospheric upheavals; and
above them, to a height of some hundreds of miles, rise the metallic
vapours producing, by their absorption, the Fraunhofer lines. The
reversing strata seem to be continuous with the chromosphere, although
the nearly perfect transparency of the latter establishes a noteworthy
distinction between the two formations; but the corona is a thing by
itself, sharply separated, physically and spectroscopically, from every
other appendage of the sun. It has, of course, relations with them, just
as our air has with the ocean it surmounts—relations, however, that do
not even verge towards a confusion of identity.
The two fundamental problems connected with the nature of the sun are
its rotation and its periodicity. They may be quite closely allied, and
in regard to both, “counsels of despair” have begun to prevail. The
spot-cycle, like “Carrington’s law,” is set down as a congenital
peculiarity, and the mists of the past are invoked to cover the
perplexities of the present. There seems little immediate prospect of
their being removed. Early modes of investigating the subject have had
no striking success, and fresh ones await development. So operations
have come to a pause, yet by no means to a dead-lock. The difficulty of
learning how the phenomena are occasioned should only stimulate
diligence in unmasking and tabulating them. The ramifications of the
period can be followed out, even if we cannot get at its roots, and
there are signs that they will be found to take unlooked-for directions.
Similarly as regards the sun’s rotation. Explanatory hypotheses avail
little, but the sifting of facts avails much. The case has not yet been
fully stated. Lacunæ need to be filled up, anomalies to be smoothed
away, errors to be corrected. In the doing of all this, a clue to the
labyrinth may present itself.
The nature of sun-spots must long be under discussion. Questions of
extreme interest are involved, some of them being visibly _answerable_,
since criteria are at hand to determine which way the truth lies.
Discriminative, rather than numerous observations will serve the
purpose. The relations of faculæ with prominences offer another
promising topic of inquiry, as well as those of prominences with coronal
streamers. But this last is an eclipse-problem, and so, unluckily, is at
present the entire subject of the corona. No pains, however, will be
spared in order to bring to bear upon it the full resources of daylight
investigation. As to whether they will be rewarded or no, the balance of
forecast swings pretty even. The unfolding of some condition, now
hidden, may incline it either way.
Solar spectroscopy presents a variety of aspects. It can be studied from
the chemical, thermal, electrical point of view; pressure, density,
motion, possibly magnetic stress, are concerned in it. Nothing in this
branch, however, is more instructive than the spectral diversities of
the various solar formations, or even of different parts of the same
formation. Thus each section, some few miles thick, of the reversing
layer is probably distinguished by emissive modifications; and the
Fraunhofer spectrum integrates the absorption of them all. The
spot-spectrum is a more pronounced variety; the chromospheric spectrum
is largely, the coronal spectrum, wholly peculiar. Now the task of
interpreting these several scripts is not—could not be—easy; yet solar
chemistry is, in the main, less unfamiliar than might have been
expected. It includes only one element—coronium—that can be clearly
distinguished as terrestrially unknown. For unidentified lines are not
necessarily of exotic origin, as is proved by the rapid progress of
their recognition in the Fraunhofer spectrum _pari passu_ with advances
in the photographic registry of metallic spectra. Nor is it probable
that spot-cavities harbour strange forms of matter. The vaporisation in
them of rare metals sufficiently explains what long appeared enigmatical
in their absorptive action.
With the substitution of known for unknown substances in the sun, a
leading argument for dissociation vanished. In this view, the
supramundane species giving lines experimentally unrecorded were
chemical fragments of our elements broken up by enormous heat. But if no
supramundane species exist, the elements presumably remain intact.
Absolute stability it would indeed be extremely rash to ascribe to them;
yet it is certain that their individuality survives fierce ordeals.
Evidence of other kinds tends towards the same conclusion. Mr. Jewell’s
critical examination of the Fraunhofer rays indicated for the shadings
attached to certain iron and calcium lines an origin quite close to the
photosphere.[317] This goes far towards demonstrating the integrity of
extremely complex molecules at the highest temperature prevailing near
the sun. Outbursts of heat from the interior there may be, reaching a
still more exalted pitch; but they must be transitory, since cooling by
expansion should promptly and potently affect them. Further, the thermal
relations of the terrestrial elements are distinctive and uniform. If
these bodies are not really simple, they are at least compounds of a
different order from those known to be such, and artificially
producible. Finally, the extensive detection of spectral series
invalidates the “one line, one element” principle which underlies most
of the arguments for dissociation. Unity of origin is emphatically
claimed, not only by each series, but by each connected set of series.
Researches into the essential nature of matter have, however, entered
upon a new phase through the aid of what we may call electrical
analysis, in which “ions” play the part of atoms in chemical analysis;
and their outcome may perhaps aid in the solution of many intricate
solar problems.
There is a strong temptation to transfer to the sun the conditions
prevailing on the earth, and to model solar upon terrestrial
meteorology. This was the line taken by M. Egon von Oppolzer in 1893. He
admitted, indeed, that it traversed dangerous ground, in view of the
wide divergences of opinion as to the nature of atmospheric processes
belonging to immediate and everyday experience. Still he judged it safe
to apply the kinetic theory of gases and the dynamical theory of heat to
the determination of the state of equilibrium in the sun’s aerial
appendages. The results were scarcely encouraging. At elevations of one
second of arc, or 450 miles, above the photosphere, the prevalent
temperature was found to be 14,000° C. lower than at its surface, which
should hence be extravagantly hot. And although this inconvenience was
abated by forced assumptions, it could not, on the principles adopted,
be removed. They involved, for example, a rise of temperature in
downward currents to the extent of at least 5000° for each descent of
450 miles. Spots were regarded as “places of extreme alternations of
temperature,”[318] where abnormally hot layers cover anticyclonic
regions of increased pressure and reduced thermal excitement. They are
“produced indirectly through a sinking down of masses upon the
photosphere, and directly through extraordinary radiation, brought about
by transparency of the overlying region.” To counterbalance descending
movements in the spot-zones, a continuous uprising of heated matter was
supposed to progress at the poles, constituted so far the analogues of
our equatorial belt of calms. Two great permanent cyclones were thus
centred on the axis of the sun. They had important functions assigned to
them. Upon their regulative power was made to depend the working of the
entire machine, and solar periodicity itself was referred to the
alternate relaxation and enhancement of their activity. They were,
nevertheless, a purely arbitrary creation. Not so much as a tortoise in
mid-air was provided for the earth-bearing elephant to stand upon. The
author claimed, it is true, only the merit of simplification; yet the
contrast of conditions between the earth and the sun largely vitiated
his reasonings. Atmospheric circulation on the earth is maintained by
external heat; agitations on the sun by internal heat; they depend
absolutely upon processes of cooling. Hence the impossibility of
assimilation. Trade winds and cyclones lack in the sun the driving power
by which they are kept going on the earth. They are characteristic of a
planetary body—of a globe _vitalised_ from without.
Eruptive hypotheses of the solar constitution have been proposed under
various forms by a succession of writers—by Secchi, Faye, Lockyer,
Schaeberle,[319] Young,[320] Sidgreaves.[321] Here, at any rate, we are
in touch with reality. Volcanic forces are powerful in the sun, which
might, in a sense, be described as organised upon a volcanic basis. The
conditions of upheaval are everywhere at hand; only some casual relief
of pressure, or access of heat is needed to provoke an actual explosion.
Settling down on the photosphere in a cooled, though still gaseous
state, the products of eruption would then give rise to spot-phenomena.
Dark patches would mark the effects of their general and special
absorption of light, while faculæ and flames attested the vehemence both
of the original outbursts, and of those reactively started by their
partial subsidence. Procedures of this kind on the sun appear
inevitable; they at least count for something, if they do not explain
everything.
A curiously subversive theory in solar physics was propounded in 1891 by
Dr. August Schmidt.[322] Its interest, however, is largely academic. Yet
it is no illusory speculation. It rests upon a sound foundation, and
emphasises an undeniable truth. The mode of action brought into
prominence by it necessarily plays a part in modifying the aspect of the
heavenly bodies; the only question open is whether the part is
conspicuous or insignificant.
The laws of atmospheric refraction were traced to their ultimate
consequences by Kummer in 1860.[323] Some of them are remarkable.
The visual lifting of extra-terrestrial objects results, as is
familiarly known, from the inflected character of the paths pursued by
light-rays through our air. A beam reaching a spectator at sea-level
from a horizontal direction is really bent upward with a curvature
one-seventh that of the earth itself. But on a globe of seven times the
earth’s radius—other things remaining the same—the refracted ray would
possess identically the curvature of its surface, to which it would
accordingly run parallel for ever, or until extinguished by absorption.
It would never reach the eye of a spectator. “Circular refraction” would
take effect upon it. A similar fate would befall a ray starting
horizontally from the surface towards outer space, instead of attaining
to which, it should follow an unending round within the air. On Jupiter,
this critical stage must be considerably overpassed. From a Jovian
atmosphere of the proportionate mass of the earth’s, light could only
escape at angles of elevation exceeding 3° 22′.[324]
Applying his formulæ to the sun, Schmidt found that circular refraction
would there be produced in a hydrogen atmosphere at a temperature of
10,000°C., and of one-ninth the standard density of air. The upshot was
to “explain away” most of the solar appendages. The photosphere he
showed to be an optical illusion, arising at the surface where circular
refraction just comes into operation in an incandescent gaseous globe
diffusing uniformly outward. The difficulty was thus overcome of
accounting for the visibly sharp separation between the dense body of
the sun and its tenuous surroundings; while a phantasmagoria of
granules, spots, flames, and faculæ was easily evoked on the supposition
of irregular refractions within the seeming disc.[325] To this dubious
locality, the reversing layer, too, was transferred by Dr. Knopf of
Jena, who hailed the speculation as the dawn of a new era in solar
physics. Hence, opposite displacements of the Fraunhofer lines cannot be
a rotational effect, and the singular accordance of what they should be
on that supposition with the measures actually obtained, is the outcome
of pure accident. But this is extravagant and unthinkable—a _reductio ad
absurdum_ of the optically composed sun. Plainly, the whole theory is in
the air. Its reasonings, to be sure, are mathematically valid; but they
refer, as Dr. Seeliger has pointed out,[326] to an ideally transparent
globe, from which the absorptive effects, prominent in the sun, are
absent. The solar spectrum, besides, fails to be accounted for by them
“on the accepted principles of physics.”[327] Nor can practised
observers of the solar surface readily be persuaded that they have
watched, not realities, but mirage-effects. In dismissing the hypothesis
as untenable, the important inference was, however, retained by
Professor Frost “that refraction within and on the sun itself may modify
in some considerable degree” observed phenomena.
An ingenious corollary has been added to Schmidt’s theory by M. Julius
of Amsterdam.[328] It was suggested by certain anomalies in the
dispersive action of sodium-vapour upon light, noticed by Becquerel, and
confirmed by himself. They led him far. He was conducted by them to
optical explanations of the “flash” spectrum, of the broadening of lines
in sun-spot spectra, and of line-displacements, usually interpreted on
Doppler’s principle. They are, however, given under reserve, and can be
tested by comparing the lines affected in the sun with those in the
vicinity of which special refraction is exerted. This is a laboratory
task, which M. Julius himself is well qualified to discharge. Should the
provisionallyassumed correspondences be found to subsist, the arguments
for the phantasmic character of solar appurtenances will have gained
imposing strength. But such a result cannot reasonably be anticipated.
Refraction on the sun is likely, owing to the extreme rarity of the
medium in which it should take place, to be of evanescent effect. Only
in spot-cavities, it may perceptibly affect appearances. The entire
matter, nevertheless, deserves to be thoroughly sifted, and cannot in
future be forgotten or ignored.
Here again a fresh prospect is opening into view, and many others invite
exploration by difficult and devious routes. For solar science does not
become less arduous as it advances. One of the surest marks of progress
in any branch is, indeed, the development of new and unforeseen
problems. And the sun, as we have learned by degrees to recognise, is a
body organised in too complex a method for easy apprehension. We
perceive that its energies are specially directed; the purpose of the
machine is obvious, and it is admirably fulfilled. In part we can see
how, but much remains mysterious. Right to the core of the mystery we
may never penetrate; nature is virtually invincible by man; but in
urging our way forward, we shall gain continually larger and more lucid
views.
PART II
PROBLEMS IN SIDEREAL PHYSICS.
CHAPTER I.
PROGRESS OF SIDEREAL PHYSICS.
Sidereal physics includes stellar and nebular physics; the two branches
cannot be separated. They have interlacing offshoots, and progress
during the last hundred years has tended more and more to unite them in
one main stem. Objects closely akin are dealt with in both, and they are
dealt with by methods substantially the same. Stars and nebulæ are not
only related as fellow-members of the grand galactic system, but they
coexist in numerous sky regions, and often in such close connection that
it is difficult to define them apart. A nebula with a stellar nucleus
can scarcely be distinguished from a nebulous star, and nebulæ
altogether devoid of star-like condensations are perhaps non-existent.
Chemical affinity ratifies visual conjunction. Spectroscopic
classification proceeds from stars to nebulæ with hardly a break. The
establishment of an exceedingly low standard of density for certain
varieties of stars forges an additional link between the two sidereal
orders. For it brings to our acquaintance bodies in a transition-stage
from nebular diffuseness to solar condensation—bodies attracting feebly
while radiating powerfully. Variability in light is another quality, the
common possession of which by stars and nebulæ has recently been placed
beyond doubt; and strong evidence is forthcoming that binary systems
occasionally consist of a stellar and a nebular member, united by origin
and inseparable to all time.
The methods of solar and sidereal physics do not differ fundamentally;
all alike depend in the main upon light-analysis and chemical
delineation, and resort to direct telescopic observation only as a
subsidiary expedient. The aims of sidereal science are, however,
profoundly modified by the remoteness of the objects it is concerned
with. Many kinds of inquiry, successfully prosecuted in regard to the
sun, are impracticable for application to suns deprived by distance of
sensible dimensions. Surface-phenomena are in them wholly out of reach,
no less than the paraphernalia that lend their glory to total eclipses.
No star sends us more than a single pencil of light, collected
indiscriminately from a wide hemispherical area to the obliteration of
its local peculiarities. Spots, flames, faculæ (if such there be) mix
their rays inextricably together; for stars have no parts. But while
their distance narrows in some directions the scope of possible
inquiries concerning them, their multitude immensely widens it in
others. They are not all similar, and their differences supply grounds
for a classification, the import of which deepens with every advance in
physical knowledge. Delicate spectral traits are found to be indexes to
conditions of temperature, density, magnetic strain, or electrical
excitation, in part imitable in the laboratory, in part transcending,
and hence contributing to enlarge terrestrial experience.
Classification, moreover, is dominated by the idea of development.
Comparative sidereal study leads inevitably to far-reaching speculations
on cosmical growth.
The sun is solitary; he exercises a “sole dominion.” But it is not so
with all his compeers. Nor are the mutual relations of those linked
together expressed solely in terms of motion. They do not fall within
the exclusive competence of the mathematical astronomer. They involve
constitutional modifications of profound import. Coupled stars,
clustered stars, stars immersed in or attached to nebulæ, are probably
subject to influences, the nature and modes of action of which remain
largely obscure. Their investigation has, however, been tentatively set
on foot, and may give results of peculiar interest. Thus sidereal
science extends and supplements solar science. It assigns to the sun its
status in the universe; it provides objects with which it can be
compared or contrasted. The two modes of knowledge mutually act and
react; what one acquires the other assimilates. The progress of each is,
by this comparative action, quickened and assured.
The population of the heavens is so dense that general conclusions as to
its characteristics can be attained only by statistical methods. And to
employ these effectively, the command of vast masses of information is
required. Data must accordingly be secured wholesale, and the necessity
has been met by the creation of a world-wide international organisation.
But its mills grind slowly, and individual enterprise will not be
stayed. Sir David Gill has already completed, in the _Cape
Durchmusterung_, a preliminary work designed purely in the interests of
geometrical astronomy, but fraught with importance to cosmical physics.
Professor Kapteyn of Leiden, who undertook the examination and
measurement of the plates, detected relationships between the kind of
spectrum given by the stars imprinted upon them and the mode of their
scattering, which promise to affect more and more profoundly all future
conceptions of the universe. Hints of their prevalence had, it is true,
been already gathered. Father Secchi noticed long ago that spectral
types are not indifferently distributed over the sky, a conspicuous
example of local preference being afforded by the constellation Orion,
which might be described as a colossal group of helium stars. The work
of Pickering, McClean, and others has also brought out the facts that
the Milky Way is a distinctive spectroscopic region, and that the
stellar tribes in general show aggregative tendencies not to be
mistaken. Astrophysical considerations then enter into discussions of
celestial structure; they are not wholly alien to questions as to how
the heavens move. The monumental “Draper Catalogue” of stellar spectra
served as the foundation of most of these extensive researches; and the
entire bulk of photographic and spectrographic data collected at Harvard
College with astonishing persistence and skill during the last score of
years, has incalculably promoted the larger interests of sidereal
astronomy.
Physical and descriptive catalogues of nebulæ are still a desideratum.
The materials for their compilation are, indeed, lacking. Comparatively
few such objects have been examined with the requisite care. They are
not easily dealt with. Those of a gaseous nature are strongly
characterised by rays high up in the ultra-violet, where absorption by
glass is formidably effective. Their due photographic registration is
then feasible only by means of reflecting telescopes combined with
prismatic apparatus of crystal and rock-salt, or some other materials
transparent to the shortest wave-lengths. But since these special
arrangements are rarely made, nebular spectrography makes slow progress.
Its conditions, in the case of “white” nebulæ, are still more
embarrassing. Nor are they much alleviated by substituting direct vision
for the camera. All nebulæ are intrinsically faint, and most give
continuous spectra. A scant supply of light makes, however, a much
better show, as can readily be imagined, when concentrated in a few
bright lines, than when dispersed uninterruptedly along the
colour-scale. To the eye, the resulting variegated streak is dim and
featureless; the sensitive plate takes cognisance of it only under the
compulsion of prolonged exposures, and then imperfectly. The task of
overcoming these obstacles is arduous, yet far from hopeless. It has
been taken in hand, and on the success attending its prosecution the
future of nebular physics essentially depends.
The discovery of helium as a terrestrial element marked a fresh point of
departure in the chemistry of the heavenly bodies. Its leading
chromospheric ray, D_{3}, had already been noted as an emission-line in
the Orion nebula by Dr. Copeland, as an absorption-line in Rigel by
Professor Keeler; but this was only preliminary to what was to follow
when recognition-marks were multiplied by complete experimental
acquaintance with the associates of the yellow beam. One of these, a
line in the blue (λ 4472), is much more conspicuous in stellar spectra
than D_{3}, and it was established by Vogel in 1895, with other members
of the conjoined series, as distinctive of a large class of “helium
stars.”[329] Some insight was thus gained into the extraordinary
profusion with which this strange gas, so sparingly occluded by the
earth, is dispensed to the suns in space. In order to show
spectroscopically, it must, as already stated, be voluminously present
in a glowing atmosphere. Its rays scarcely endure competition. Our sun,
for example, although surrounded by huge volumes of helium, is not a
“helium star.” Pretty sure indications, on the other hand, can be
gathered that helium is one of the principal components of all gaseous
nebulæ; and it blazes where stellar incandescence is strong—in “new”
stars, in bright-line stars, and in some “long-period” variables. The
disclosure of its great cosmical rôle is among the most important
consequences of the modern alliance between astronomy and terrestrial
physics.
The identification of oxygen and nitrogen as stellar constituents by Mr.
McClean and Sir William and Lady Huggins respectively, and the detection
of the “Pickering series” of hydrogen in certain stars, are advances of
scarcely less moment. These unusual kinds of absorption emerge, as a
rule, in stars showing helium as well, and generally assumed to be at an
early stage of growth. They apparently tend to supersede metallic
action, which becomes imperceptible when the inchoate stage is
approached. Thus the nebular spectrum includes no lines of known metals,
and they are likewise apparently missing from “Wolf-Rayet” stars.
Acquaintance with these remarkable objects has profoundly altered the
views of stellar physicists. It has introduced them to a borderland
where the prevalent conditions defy forecast. Who, for instance, could
have anticipated Campbell’s observations of mixed bright and dark
spectral series, derived from the same element, in the same object?
Nothing could well be more perplexing; but perplexities are often the
raw material of discoveries.
The application of photography to the examination of “blaze stars” began
with the apparition of Nova Aurigæ in 1892. Opportunities for its
continuance have since been frequently afforded, which would probably
have slipped by unused but for the automatic watch kept on the changes
of the heavens at Harvard College and its southern dependency. Two
generalisations have thus been authorised. One is that the spectra of
Novæ are mainly composed of bright and dark lines in pairs, emanating
from the same substances, but pushed asunder as if by the effect of
swift opposite motions. The second is that fading Novæ put on a nebular
light-vesture. Spectroscopically, they simulate minute “planetaries.”
These surprising facts are seeds of future knowledge; they need time to
germinate and yield fruit.
Stellar variability no longer remains outside the pale of successful
research. The mystery surrounding it has not, to be sure, been
dissipated; but some of its attendant circumstances have become
manifest. Eclipsing stars are now fully open to investigation; and
although not physically variable, they have indisputable connections
with stars that are. Fluctuations executed quite punctually in periods
of a few days or hours indicate a compound nature in the objects
undergoing them, even if they cannot be explained as occultation-phases.
Short-period variables, in short, are non-eclipsing spectroscopic
binaries. Yet spectroscopic binaries, indistinguishable from them as to
their orbital conditions, shine with a perfectly steady lustre. And
these are the majority. It remains to be discovered what are the special
attributes of the differentiated classes. Nor is variability found only
in conjoined objects. Stars apparently single are subject to extreme,
although more or less irregular vicissitudes. The only clue to the
nature of these vicissitudes yet found is the fact that increase of
light is ordinarily attended by the spectroscopic flashing out of
hydrogen-rays. Moreover, the resemblance of stellar light-curves to
sun-spot tracings gives a strong hint that the solar analogy should be
made a starting-point for inquiries into “long-period” variability.
The branch of stellar astronomy concerned with double and multiple
systems gains extension and importance year by year. Their evolution
under the influence of tidal friction has been studied by Dr. See. Sir
William Huggins’s device of a slit with reflective jaws having
facilitated spectroscopic observations of close stars, the analysis of
their light has at last entered upon a stadium of progress; apart from
which perennial obscurity must have hung over sidereal chromatics, and
enveloped theories of sidereal growth. Spectroscopic binaries,
meanwhile, are multiplying on our hands, and their varieties offer a
brilliant field for investigation. Their periods range from less than
one day up to two years; some revolve in subordination to larger
combinations; their orbits are variously inclined, and of various
degrees of ellipticity; and one of the circulating bodies is, more often
than not, sensibly devoid of light. The function and place in creation
of “dark stars” have thus come into the foreground of inquiry. They
occur also in telescopic systems, where their disturbing power produces
a visible _swaying_ in the movements of their bright companions; but
less commonly, it would seem, than as members of spectroscopic couples.
These are, so to speak, just out of the shell; hence dark stars can
hardly be effete suns, although suns presumably lapse with age into
obscurity. The distinction is a delicate one to draw, but should not be
lost sight of.
The scope of sidereal research is limitless—limitless because its
objects are indefinitely numerous. The difficulty is to lay hold of
them. Inferences based on partial surveys are felt to be unsatisfactory.
The star-depths beyond continually invite farther and farther advances.
Astronomical curiosity is only temporarily appeased by learning, for
instance, the radial movements of a few score of stars; they are wanted
by the hundred, and when at hand in hundreds, they will be in demand by
the thousand. Exhaustive inquiries, while remaining unattainable, must,
by the nature of the case, be perpetually aimed at. And what is
primarily needed for them is a plentiful supply of light. Ambition in
telescope-building is then justifiably insatiate, nor has it, so far,
overreached itself. Each addition to instrumental capacity has, on the
contrary, notably widened the horizon of feasible research. The erection
of the great Lick refractor made it possible to obtain legible
spectrographs of the Wolf-Rayet stars, and set the nebulæ in motion by
enabling Professor Keeler to determine their radial velocities. With the
Crossley reflector, mounted in the same superb situation, nebular
photography made a fresh start, and the law of spirality, as a
structural principle, was confirmed and generalised. The Yerkes
telescope has given access to several closed fields. By its means,
variable stars are followed through their semi-extinct phases. Professor
Barnard has performed the unique task of verifying visually the rapid
light-changes of the minute components of globular clusters; and
Professor Hale has examined spectrographically a large number of
“carbon” stars, too faint as well as too red for satisfactory treatment
under average circumstances with the camera. The completion, at the same
observatory, of a five-foot reflector will shortly afford even better
opportunities for prosecuting this work. It may be added that a special
function in nebular investigations is reserved for instruments of
abnormally short focal length, such as the Meudon reflector, in which
the rays collected by a mirror thirty-nine inches across form an image
at only thrice that distance from it.
Centuries, however, of invention and contrivance must elapse before the
_minuta plebs_ of the sky can be individualised by the peculiarities of
their spectra. Inducements to keep up this strain of toil are not
wanting. “A star’s a star for a’ that,” even though it lie beyond reach
of human questioning. To bring it virtually nearer, is the object of
perpetual efforts. And they cannot fail to be rewarded. Novelties in the
heavens are likely to prove endless and surprising. The enticements they
offer to progress are irresistible. No “anchor dropt at eve or morn” can
stay the ideal voyage. “When a man hath done, then shall he begin” his
scrutiny into the works of the Most High.
CHAPTER II.
THE CLASSIFICATION OF STELLAR SPECTRA.
Various methods of classification may be applied to the stars. The most
obvious and the most antique is that of relative brightness. From of old
the stars have been collected into ranks by “magnitude.” Or the amount
of their “proper motions” may be taken as the principle of distinction.
Within certain limits this is practicable, and for certain purposes it
is useful. But apparent lustre and projected movement alike depend in
part upon distance; they include an extraneous element; and
astrophysical science considers the heavenly bodies in themselves,
without regard to their spatial relations. Hence an absolute quality
must be made the basis of their arrangement, and it is found in the
_kind_ of light emanating from them. This system is of far more than
conventional value. It affords the only clue within reach to the
intricacies of stellar constitution. Schemes of spectral classification
may be amended and altered; but in one form or another they are
indispensable to progress.
They naturally tend to become more complex as facts multiply, and finer
shades of difference are rendered manifest; yet Father Secchi’s four
“types” continue fundamental. It is well, then, to keep their
characteristics steadily in mind. The first is marked by strong hydrogen
absorption. It consists of radiantly white stars. The second by
innumerable fine metallic rulings; the sun is an example. The third type
includes red stars with banded spectra like Antares, the bands being
sharply terminated towards the violet, diffuse towards the red. The
fourth is composed of deeply-tinted, mostly faint objects, showing wide
bands facing redward, due to carbon absorption. These four groups form
irremovable landmarks; but beside and between them many subordinate
divisions have been set up. Vogel, Huggins, McClean have all modified,
while broadly adopting Secchi’s “law of order.” They have, moreover,
regarded it, not as a mere empirical formula, but as prescribed by the
necessary conditions of development. Modes of classifying the stars have
come to be equivalent to theories of their evolution. With this aspect
of the matter, however, we are not just now concerned. The aim of the
present chapter is to establish convenient distinctions without regard
to their essential meanings. The unravelment of these will be attempted
later on.
Miss Maury’s arrangement of the stars[330] is designed for a
life-history as well. It is the most elaborate yet put forward. Based on
the examination of some 4800 spectrographs taken at Harvard College, it
embraces 681 objects, disposed in a progressive series of twenty-two
groups, most of which are further comprised within three collateral
divisions, established to meet the visible necessity for a secondary
characterisation. The work is a monument of industry and skill, and will
long hold a place of standard authority; but the fine gradations it
emphasises, although worth putting on record, are scarcely suitable for
committing to memory. Our object here is to present large outlines,
leaving minute shades of difference to be dealt with as occasion may
arise. There is danger of stellar classification degenerating into a
maze of provisional distinctions. The best remedy is to fix attention on
the summit-ranges of the landscape; when they are clearly imprinted on
the mind, mastery of detail can be safely and readily acquired.
A framework of eight compartments accommodates practically all the
stars. The separation is easy and natural, and the “notes” of the
various classes present themselves unmistakably. The constituents of the
four first show absorption spectra only; those of the four last are
marked by emission as well as by absorption. No hypothesis of growth or
affinity is implied by the order of succession given to the bright-line
objects; only the interests of clearness have been consulted in its
choice. We will now briefly describe these stellar families.
[Illustration:
PLATE IX.
Stellar Spectra photographed by Sir William and Lady Huggins.
Fig. I. Vega (α Lyræ). Fig. II. Arcturus (Fe and Ca for comparison).
Fig. III. Rigel. Fig. IV. β Cygni, Blue Star; β Cygni, Yellow Star
(solar comparison-spectrum).
]
Class i.—_Helium Stars._—In the spectra of these brilliantly white
stars, absorption by hydrogen and helium predominates. The complete
“Huggins series” is stamped upon them, from the fundamental C to its
“head” in the ultra-violet;[331] and at least twenty-six of the
strongest helium lines, culled impartially from all the six series, show
conspicuously besides. In a few helium stars, suspected of nebular
relationships, the “Pickering series” of hydrogen is represented, while
others betray the action of oxygen, nitrogen, and silicon. Metallic
lines are faint and scarce; those identified belong to sodium, iron,
calcium, and magnesium. Especially remarkable is the comparative
prominence of the magnesium line λ 4481, to the exclusion of the triplet
_b_, which takes the lead in the solar spectrum. The substitution,
according to many authorities, indicates enormous heat. Scheiner’s
criterion for high temperature is precisely the development of λ 4481,
and Professor Keeler remarked that the effacement of _b_ marked a stage
of heat beyond the possibility of artificial production.[332]
In these stars there is almost no general absorption; their photospheres
are _unveiled_. Moreover, they seemingly possess reversing layers of
very simple composition, to which circumstance their display of helium
may, with much probability, be attributed. Originally included in
Secchi’s first type, they were separated from it by Vogel in 1895, on
the identification of their distinctive lines with those of terrestrial
helium; and their importance in the sidereal scheme was accentuated by
McClean’s spectrographic researches in the southern hemisphere. The
lucid orbs of Orion and the swarming Pleiades are leading members of the
class, which are also thickly disseminated in the Southern Cross, the
Centaur, and the Greater Dog. Miss Maury’s first six groups are
subdivisions of helium stars, arranged in the assumed order of their
development from a nebulous condition.
We are indebted to Sir William and Lady Huggins for permission to
reproduce their admirable spectrograph of Rigel, the premier helium star
(Plate IX. Fig. III.), and to Sir David Gill for that of ε Canis Majoris
(Plate X. Fig. 1), taken by his assistant, Mr. Lunt, who detected many
lines of silicon and oxygen, in addition to strong helium-absorption, in
the spectrum of this star.
Class ii.—_Hydrogen Stars._—These stars are distinguished by intense
hydrogen-absorption of the ordinary kind, no Pickering lines being
present. Helium-influence on their light is null, or barely perceptible.
The “H” and “K” of calcium are thin but distinct. Feeble iron lines can
be numerously discerned. General absorption is slight; the ultra-violet
end of the spectrum lies open, imparting to hydrogen stars a
bluish-white colour. Vega is a perfect example; its spectrum,
photographed by Sir William and Lady Huggins, is shown in Plate IX. Fig.
I. The black band to the right is the fifth line of hydrogen (Hε). It
masks the calcium “H”; but “K” appears well to the left.
Hydrogen—sometimes called “Sirian” stars—abound in the heavens. They
form the main part of Secchi’s first type, and are distributed by Miss
Maury into five groups, numbered vii. to xi.
Class iii.—_Solar Stars._—The Fraunhofer spectrum sets a pattern copied,
with slight variations, by the members of this class. Its leading
feature is the powerful development of “H” and “K.” Other metallic lines
are innumerable, but mostly sharp and thin. Four hydrogen lines are
normally present, ultra-violet members of the series showing decisively
only in stars like Procyon and Canopus, which may be regarded as
intermediate between the Sirian and the solar classes. A yellow tinge
corresponds in the latter to a veiling of the blue end of the spectrum,
similar to that perceptible in the sun. Solar stars, then, resemble him,
not only in the composition of their reversing layers, but in the
possession of “smoky” envelopes. A spectrograph of Arcturus by Sir
William and Lady Huggins is shown in Plate IX. Fig. II., and one by Sir
David Gill of α_{2} Centauri, the brighter member of the southern
binary, in Plate X. Fig. 2. The precision of its correspondence with the
solar spectrum may be seen by a glance at the comparison strips in this
latter figure. Stars of the solar class constitute Secchi’s second type,
and are included in Miss Maury’s groups xii. to xvi.
[Illustration:
PLATE X.
1. Spectrum of ε Canis Majoris (central strip) compared with Lines of
Hydrogen, Helium, Oxygen, and Silicon.
2. Spectrum of α Centauri (central strip) compared with Solar Spectrum
(outer strips).
_N.B._—The relative displacement is due to a difference of temperature
at the exposure-times.
]
Class iv.—_Stars with Fluted Spectra._—Two kinds of absorption are
distinguishable in them. A linear system, somewhat reinforced from the
Fraunhofer model, has superposed upon it a set of dusky flutings, about
ten in number, of undetermined chemical origin. They suggest action by
oxides, the formation of which in stellar atmospheres would seem to
imply a considerable reduction of temperature. None of the bands occur
in the more refrangible part of the spectrum, so that the photographic
differences between solar and “fluted” stars are easily overlooked. Nor
is there an abrupt transition from one class to the other. From Capella
the line of connection passes unbroken through Arcturus and Aldebaran to
β Andromedæ and α Orionis (Betelgeux), absorption settling down more
heavily on the blue rays, and metallic lines gaining strength at the
expense of the truncated hydrogen series, until the fluted type is
definitively formed. It is equivalent to Secchi’s Type iii. Within its
compass a progression of objects with deepening bands, such as Miss
Maury has arranged in her Groups xvii., xviii., and xix., can readily be
followed out. It may be said to terminate with α Herculis, a star of the
third magnitude, displaying magnificent prismatic chiaroscuro. A drawing
of its spectrum by Mr. Espin is copied in Plate XI. Fig. 1.
Fundamentally, the same series of bands recurs in all the individuals of
this class. They vary from star to star both in relative and in absolute
intensity, but their identity remains unmistakable. The presence of
certain determinate atmospheric ingredients fixes the type, and no
others can replace them. The stars belonging to it are in diverse
degrees red or orange, their blue emissions being largely arrested in
the precincts of their photospheres. They must hence be intrinsically
brilliant far beyond the proportion of their visual, and, still more, of
their photographic magnitudes. Their apparent lustre, that is to say, is
small compared with the masses that may reasonably be assigned to them.
Their light is markedly unstable, and many are subject to periodical
variations of exceedingly wide range. These give bright-line spectra of
a very definite character, which it seems advisable to set in a class
apart.
Stars with fluted spectra (conveniently designated as “Antarian,”[333]
from their exemplar, the _lucida_ of the Scorpion), although rare
compared with Sirian and solar stars, are found plentifully in every
part of the sky. No comprehensive catalogue of them exists; the number
of those already known might, however, be roughly estimated at a couple
of thousand. They must be vastly remote. None have sensible parallaxes,
and very few show appreciable proper motions.
Class v.—_Carbon Stars._—These have also banded spectra, but of a
totally different stamp. Three shadings, particularly conspicuous in
them, testify to strong absorption by carbon vapour. Vogel found them to
reverse exactly the spectrum of an alcohol flame.[334] Scheiner
conjecturally identifies the absorbent material with acetylene.[335]
Others hold it to be pure carbon. There is much uncertainty on the
point. The carbon-bands are of different degrees of obscurity in the
various members of the class, nor do they in all preserve the same
relative strength.[336] Subordinate bands, too, of unknown origin
diversify these spectra more or less strikingly. But all are designed on
the same pattern; it is only the mode of _printing off_ that varies.
They include as well many dark lines, notably Fraunhofer’s “D” and “E,”
representing absorption by sodium and iron. A characteristic dusky
streak at λ 576 awaits chemical interpretation.
[Illustration:
PLATE XI.
1. Spectrum of α Herculis. Drawn by Mr. Espin, 30th June 1894 (red end
to the right).
2. Spectrum of 152 Schjellerup (yellow section). Photographed with the
Yerkes 40-inch refractor.
]
Carbon stars glow like rubies in the sky; they are, for the most part,
fiery red objects. To the eye they make a poor show. The brightest—19
Piscium—is of 5·5 magnitude; and only three in the northern and four in
the southern hemisphere, out of about 250 recorded, exceed the sixth.
This is not surprising when we consider that but a small percentage of
their rays can escape stoppage by enfolding vapours. They would seem,
besides, to be plunged in greater depths of space than Sirian or solar
stars; so that they ought perhaps, allowance being made for these
disadvantages, to take rank as potent light-givers. The coloured
“zones,” or luminous intervals in their spectra, are, at any rate,
curiously vivid and sparkling. Most are to some extent, some are to a
large extent, variable. Their photographic examination—rendered arduous
by the quality of their light—afforded Professor Hale in 1898 the
interesting discovery, imperfectly anticipated by Secchi, of several
unfamiliar bright lines superposed upon their dark shadings.[337]
Hydrogen lies low in carbon stars. It exerts no visible absorption, and
displays no traceable emission. Their relationships with other stellar
families are obscure; connecting spectral links are not altogether
wanting, but they are of secondary importance. The class formed by them
is coextensive with Secchi’s Type iv. and with Miss Maury’s Group xxi.
Four specimens are given in Plate XII., from spectrographs taken by
Professor Hale and Mr. Ellerman with a train of three prisms fitted to
the eye-end of the Yerkes forty-inch refractor. The wealth of detail
shown is so great as almost to obliterate the general columnar effect.
Class vi.—_Stars with Fluted Spectra showing Bright Hydrogen
Lines._—Mira is the typical star of this class. All its members (save
one doubtful case) are pronounced variables. They fluctuate in colour
too, but show in general a decided orange or ruddy hue. The flutings are
very marked, and they tend to deepen and widen as the stars lose light.
Essentially the same as in Class iv., they overlie a similar metallic
line spectrum. Vivid hydrogen rays come into view with the approach of
each maximum, and fade after it has passed. They seem, however, to
persist much longer in some stars than in others. The series is, indeed,
at all times incomplete. Its first term—the crimson C—is often missing;
the second, F, is by no means invariably present; the stress of
brilliancy is, in certain stars, laid upon the third, in most upon the
fourth line, the fifth being always concealed by the dense, distended H
of calcium. The succession of bright lines is resumed in the
ultra-violet, and continued to the limit of the spectrum, which is
curtailed by strong general absorption. A spectrographic impression of
Mira by Father Sidgreaves is shown in Plate XIII. (1). It extends from
orange to indigo, but stops short of H and K. There is no assured trace
of green hydrogen, while the two blue beams are lustrous. A
corresponding print of the spectrum of α Herculis appears on the same
plate below that of Mira. The yellow ray of helium shines in some
members of Class vi., and several dark lines in the spectrum of Mira
coincide approximately with more refrangible lines of the same
substance. Through the presence of emissive symptoms in fluted spectra,
more than a hundred new variables have been photographically discovered
by Mrs. Fleming and her staff at Harvard College. She subdivides them
into eleven families, marked by the varying relative brightness of the
hydrogen lines,[338] while Miss Maury includes them all in her twentieth
group. Secchi’s third type likewise claimed them; its limits were indeed
defined before their singularities had been noticed.
Class vii.—_Helium Stars with Bright Lines._—These objects give the
characteristic dark-line “Orion” spectrum, variously emblazoned with
rays of hydrogen, helium, and a few other substances. In some the bright
and dark lines are ranged side by side, in others they are superposed,
the system of reversal being tripled by the addition of dark _threads_
drawn across the emission rays. The historical variable “P Cygni”
exemplifies the former variety, γ Cassiopeiæ the latter. The fundamental
C is perhaps always the brightest line in these spectra, and there is a
uniform decrease in the lustre of the hydrogen-series as it progresses
upward.[339] In many cases its lower members show by emission, the rest
by absorption. The bright spectrum may indeed be reduced to a solitary
C. The same rule applies to helium. The circumstance, however, that the
lowest terms of each series are those vivified, becomes evident only
when the lines present are sorted out in their due sequential order. It
is unapparent on a collective view of them.
About fifty bright-line helium stars are known, and fresh specimens
are yearly swept up in the course of space-sounding operations at
Harvard College. A fuller acquaintance was gained with thirty-two
among the number by Professor Campbell’s scrutiny of their spectra
with the great Lick refractor in 1895. Much hesitation prevails as to
their proper place in systems of stellar classification. Most usually
they are treated as a subdivision of the dark-line helium class, but
Miss Maury and Miss Cannon leave them outside the framework of their
respective schemes, appending valuable discussions of individual
peculiarities.[340] There is much to be said for this mode of
procedure. We are enabled, by the kindness of Father Sidgreaves, to
reproduce in Plate XIV. a spectrograph of γ Cassiopeiæ taken at
Stonyhurst, 7th March 1898. Among the bright lines imprinted on it are
several due to magnesium,[341] namely, the _b_-triplet prominent in
the sun, and the blue ray at λ 4481, specialised by Scheiner as
marking a high grade of heat.
[Illustration:
PLATE XII.
Spectra of Carbon Stars. Photographed with the 40-inch Yerkes
Refractor by Professor Hale and Mr. Ellerman.
1. 280 Schjellerup (Mag. 7·8).
2. 19 Piscium (Mag. 5·5 Var.).
3. U Hydræ (Mag. 5·5 Var.).
4. 152 Schjellerup (Mag. 5·5).
]
Class viii.—_Wolf-Rayet Stars._—Acquaintance with these objects began in
1867 with the discovery, by MM. Wolf and Rayet of the Paris Observatory,
of three small stars in Cygnus, giving a spectrum composed mainly of
blue and yellow effluences. Then on 24th December 1871, Respighi[342]
observed the brilliant prismatic radiance of γ Argûs ( = γ Velorum),
which proved to be of the same quality, although vastly superior in
quantity. No other star of the kind exceeds the sixth magnitude, and
over one hundred of them have been already recognised. Their
distribution is remarkable; all are situated in or quite close to the
Milky Way, except a considerable group located in the Magellanic
Clouds;[343] and the Magellanic Clouds obviously reproduce many of the
conditions of the Milky Way.
The leading spectroscopic distinction of the Wolf-Rayet class is the
display of the Pickering series of hydrogen. Five of its constituent
lines have been recognised, but they are not all equally bright. The
upper ones may even appear dark. Emission-bands in the blue, on the
other hand, never fail to be visible. Two at least are simultaneously or
alternatively present. The more refrangible at λ 4688 is the azure beam
identified by Rydberg with the leader-line of the otherwise unknown
principal series of hydrogen. Its associate at λ 4652 may possibly owe
its origin to nitrogen, but this remains to be proved. Helium lines
show, both bright and dark, in these stars; in the same spectrum D_{3}
occasionally gleams golden beside its dusky fellow, the noted “Orion”
absorption ray at λ 4472. Similarly, in the Huggins hydrogen series, a
vivid C may have for its companions an almost neutral F (Hβ), and
obscure Ηγ and Ηδ.
The Wolf-Rayet spectrum is then triple. A band of continuous light,
fairly strong in the ultra-violet, forms its basis. Absorption lines and
bands are superposed, a few of them due to hydrogen and helium, but for
the most part unclaimed by any terrestrial substance. To these are added
hydrogen, helium, and anonymous bright rays in varying degrees of
profusion. No metallic lines, bright or dark, have been recognised.
Stars of this description are white or yellowish. They are rarely or
never variable. Pickering combined them in 1891 with planetary nebulæ
into a “Fifth Type of Spectra”;[344] yet, certain nebular affinities
notwithstanding,[345] they lie well away on the stellar side of the
dividing line between the two sidereal realms.
The eight stellar divisions just enumerated comprehend as nearly as
possible all the stars spectroscopically examined up to the present. The
few left outstanding are, in general, difficult objects, which have been
casually or defectively observed. When better known, they will probably
avow affinities not at first sight apparent. Our classification may then
fairly claim to be exhaustive; it certainly rests upon broad and
unmistakable distinctions. And it is no small achievement to have
obtained a bird’s-eye view of the celestial “maze of error.” Order is
not knowledge; _vere scire est per causas scire_; but it is an
indispensable preliminary to its attainment.
[Illustration:
PLATE XIII.
Spectra of Mira (1), December 1897, and of α Herculis (2), February
1898. Photographed by Father Sidgreaves (_Knowledge_, vol. xxi. p.
113).
1 = λ 4227. 2 = λ 4420. 3 = λ 4581. 4 = λ 4757. 5 = λ 4951. 6 = λ
5162. 7 = λ 5447. 8 = λ 5597. 9 = λ 5756.
]
CHAPTER III.
HELIUM STARS.
Helium stars are often palpably connected with nebulæ. The entire Orion
region, where they brilliantly congregate, is pervaded with cosmic fog;
cosmic fog enwraps the Pleiades; and individual instances of the same
association abound, and are likely to multiply as exploration proceeds.
It is, however, visibly closer in some stars of the class than in
others; and these nebulous gradations appear to correspond with spectral
gradations of a very interesting kind, accurately represented in the
progressive order of Miss Maury’s groups. The earliest are strongly
impressed, not only with helium and ordinary hydrogen lines, but with
the Pickering series as well, noted by Mr. McClean to characterise a
primitive stellar condition.[346] Satisfactory evidence of oxygen
absorption in them was adduced by him in 1897, and lines of nitrogen and
silicon are recognisable besides. Among metals only calcium and
magnesium make a feeble effect,[347] the one with a just discernible K,
the second with the “high-temperature” line in the indigo (λ 4481). No
sharp rays are found in these spectra. Their shadings take the form of
hazy streaks.
The chief of a triple group in Monoceros is a specimen peculiarly worthy
of consideration. It has the uncommon property, for a helium star, of
being variable in a short period, whence its catalogue title of S
Monocerotis; and it dominates a small cluster (N.G.C. 2264), measured by
Bruno Peter in 1880.[348] An involving nebula, evasive of telescopic
vision, came out fully in Professor Barnard’s photographs of 1894,[349]
and was described by him as “a very wonderful object, irregular in
outline but quite well defined, with numerous black gaps running into
it, and conforming in general with the peculiarities of the Milky Way in
that region.”[350] A picture on a larger scale was taken by Dr. Roberts
a year later.[351] That S Monocerotis will prove to be a spectroscopic
binary revolving in three days and ten hours, may be judged probable
from the analogy of its fellows in variability.
One of its contemporaries is the third magnitude star, ι Orionis (N.G.C.
1980). Sir John Herschel perceived it to be wrapt in a large feeble
nebulosity, and a divergent streak, linking it to the great “trapezium”
nebula, disclosed itself photographically to Professor W. H.
Pickering.[352] It is widely triple, but sensibly stationary. Although
its spectrum is in the main a copy of that of S Monocerotis, important
distinctions may present themselves to scrutinising inquirers. Both the
Wolf-Rayet blue bands show by absorption in these “early Orion” stars,
the upper one being especially pronounced.
The three stars forming the Belt of the Giant are slightly more
“advanced.” Helium has gained strength in them relatively to the
Pickering series; the reversal of Rydberg’s azure band verges towards
effacement, and in the middle star, ε Orionis, the spectral lines are
fairly well defined. Oxygen, nitrogen, and silicon contribute each its
quota of absorption. A great stream of nebulous matter sweeps through
the Belt, and its two lower gems, ζ and ε Orionis, claim besides shining
appurtenances of their own.[353] An analogous object, σ Scorpii, came
out on Professor Barnard’s photographs with a couple of nebulous
“prongs” attached to it,[354] and also as a focus of marked condensation
in the great nebulous field near Antares. The spectrum is perfectly
similar to that of ε Orionis.
[Illustration:
PLATE XIV.
SPECTRUM OF γ CASSIOPEIÆ, DRAWN FROM A PHOTOGRAPH TAKEN ON MARCH 7,
1898.
STONYHURST COLLEGE OBSERVATORY.
]
Among southern helium stars one deserves special mention if only for its
association with a notable discovery. This is Mr. McClean’s “oxygen
star,” β Crucis. His identification in 1897[355] of numerous lines in
its spectrum as due to the absorptive action of our vital gas was fully
confirmed two years later by Sir David Gill,[356] who employed for the
purpose the splendid apparatus bestowed by Mr. McClean upon the Cape
observatory. Silicon is also present, and the usual reversals of the
indigo line of magnesium and of the violet K of calcium appear
distinctly. Nitrogen, however, is not evident, nor the Pickering series
of hydrogen. The spectra of β and ε Canis Majoris, and of β Centauri are
of the same stamp. One of the brilliants of the Southern Cross, β Crucis
lies immersed in the Milky Way, at an unmeasured, perhaps an
immeasurable distance from the earth. We have thus no means of
estimating its actual radiance, which must, however, greatly exceed that
of the sun. A secular proper motion of fourteen seconds is ascribed to
it, and Sir David Gill finds it to be receding from the sun at the rate
of about eleven miles a second.
Bellatrix, in the shoulder of Orion, is a typical helium star, the chief
representative of Miss Maury’s fourth group. No Pickering lines have
been found in its spectrum, but Professor Keeler noticed the comparative
prominence of a subordinate nebular ray at λ 4390.[357] Nitrogen and
oxygen were identified in it by Sir William and Lady Huggins,[358] and
silicon by Sir Norman Lockyer[359] and Mr. Lunt.[360] The effacement of
iron, remarkable in nearly all helium stars, subsists also in Bellatrix,
and is explained by Sir Norman Lockyer as an effect of transcendental
temperature. It has a spectroscopic _alter ego_ in the _lucida_ of the
Southern Cross.
In the spectrum of Rigel some iron lines faintly emerge, and the sodium
D appeared conspicuously on Professor Campbell’s isochromatic plates.
The Huggins series of hydrogen is magnificently displayed from its first
term to the limit[361] (see Plate IX. Fig. 3); helium lines are strong
and numerous; those of nitrogen, oxygen, and silicon come out in
photographs, and they were found by Sir William and Lady Huggins to be
associated with certain distinctive rays of titanium.[362] The lines in
this spectrum contrast markedly by their sharpness with those of other
stars chemically similar, such as Regulus. Rigel belongs to Miss Maury’s
sixth group; it approximates to the stage where helium sinks out of
sight, and yields the sole predominance to hydrogen.
In order to form some idea of its prodigious light-power we must
remember that it has no sensible parallax (Gill), and is all but
stationary in the heavens. This implies that it is so far off as to make
almost no perspective response to the sun’s centennial advance through
space. In other words, a base line some thirty-three thousand millions
of miles in length (allowing for foreshortening) shrinks to little more
than a point as seen from Rigel. Assuming the reality of the minute
proper motion of 1·5″ a century deduced from its catalogued places, and
that it is a parallactic effect of our system’s progress towards an
“apex” on the borders of Hercules and Lyra, at the rate of twelve miles
a second, we must ascribe to the star a distance of at least 367 light
years, corresponding to an annual parallax of ¹⁄₁₁₂ of a second. It
follows that Rigel gives about 8000 times more light than the brighter
component of α Centauri, an orb considered by Sir David Gill to be the
exact match in every respect of our sun. But the sun is dimmed to about
one-third of its native lustre by effects of absorption which are
virtually absent from the star. Hence a total light emission 8000 times
greater would represent a radiating surface only 2667 times more
expansive than the solar photosphere. Rigel, moreover, is certainly not
massive in the proportion of its luminosity. Stars of the helium variety
are composed of highly rarefied materials. This has come to be known
through the study of eclipsing stars. Taking, then, the density of Rigel
to be about that of Algol, or one-fourth that of the sun, we find it
even so to be of no less than 34,000 times the solar mass, while gravity
at its surface is of just thirteen-fold power. Nevertheless its spectrum
indicates extreme tenuity in its gaseous surroundings. Calcium, for
instance, in the reversing layer of Rigel emits violet rays _only_.
There is no trace of the blue line (λ 4227). The vapour exists there in
much the same state as in the solar chromosphere and prominences—that is
to say, in a state of the utmost attenuation, which implies the
counterbalancing of gravity by a strong antagonistic influence,
presumably of an electrical nature. This merely extends an inference
already derived from solar phenomena.
The relationships of Deneb (α Cygni) have been variously assigned. There
is, however, no longer any doubt of its affinity to Rigel.[363] The
lines in its spectrum are numerous and clearly defined. Sir Norman
Lockyer measured 307 on photographs of the section above F,[364] and
there are hundreds besides. Those of helium are of subordinate
importance; they are being replaced in the supposed evolutionary
progression by metallic lines. Magnesium absorption is deeply graven in
the ultramarine (λ 4481), and begins to appear through the green triplet
(_b_). Iron lines of the kind “enhanced” in the spark are fairly
abundant; gallium shows at least one strong line,[365] and titanium
lines are prominent. Among non-metallic substances, besides helium and
hydrogen, only silicon is unquestionably present. It comes, however,
well to the front. Mr. Lunt regards α Cygni, Rigel, and Sirius as some
of “the best examples of silicon stars” yet known.[366] Nevertheless,
spectrographs of them fail to show the three silicon lines most
conspicuous in the β Crucis group, while Lockyer’s enhanced lines
imprint themselves with some emphasis.[367] These celestial
modifications of the silicon spectrum afford “valuable data,” in Mr.
Lunt’s opinion, “for the elucidation of the problem of relative stellar
temperatures.” Their interpretation on current principles would lead to
the conclusion that α Cygni, Rigel, and Sirius are hotter than the
“earlier” suns typified by the “oxygen star” in the Cross. But there is
no real certainty as to what causes the difference in kind between the
luminosity of the spark and arc. Temperature may not be the sole, or
even the chief, agent in its production.
The width and density of K in α Cygni are noted by Sir William and Lady
Huggins as anomalous, calcium absorption usually remaining feeble when
that of helium is visible. The curious thinness of the hydrogen lines
may result in part from their projection upon a photospheric background
of exceptional brilliancy.[368] The star, at any rate, is one of those
which “stand apart through a distinctive individuality,”[369] and it
invites, as such, special attention.
Regulus—an intermediate specimen—and β Centauri are perhaps the only
helium stars at determined distances from the earth. For the latter Sir
David Gill found a parallax of 0·046″, equivalent to a light-journey of
seventy-one years, so that it is by no means a near neighbour. In its
place the sun would be just perceptible to the naked eye; it would
appear of sixth magnitude, while the star is only a couple of grades
below the first rank (its photometric magnitude is 1·2). Its emissions,
in fact, surpass the solar radiance rather more than 150 times. We may
then allow that they proceed from a photospheric expanse fifty times
ampler than the sun’s, which must encompass a globe 342 times more
voluminous. Assuming further for β Centauri (as for Rigel) a density
one-fourth the solar, we obtain the result that it is of 85 times the
solar mass. Owing, however, to the comparative remoteness of its surface
from its centre, gravity has there less than twice the power which it
exercises on the sun. Putting it otherwise, the acceleration of a
falling body on β Centauri is about 750 feet a second. The value of this
“constant” is probably of essential importance in determining the
character of stellar spectra; hence attempts at its estimation, despite
the uncertainties that hamper them, are worth making.
The Pleiades are tolerably mature helium stars; Alcyone was selected by
Miss Maury as the type of her fifth group. Some of its associates show
hazy, others sharp lines. Algol approaches still closer to the boundary
of the Sirian class, its “Orion lines” being quite secondary to the
hydrogen set. Regulus is of nearly the same standing, but its spectral
markings are dim and diffuse; those of inferior intensity thus make no
appreciable impression, and the star’s rays are all but exempt from
absorptive encroachments.
They indeed tell but slightly throughout the entire class of helium
stars, and this seems to indicate the absence of any strong contrast in
temperature between their photospheres and the encompassing incandescent
vapours. That these are exceedingly tenuous is rendered almost certain
(as already pointed out) by the non-reversal of the blue line of
calcium. As to their relative temperatures, no dogmatic assertion is
possible. The effects of transcendental heat evade inquiry. We cannot
without hesitation assume that known rules apply under unknown
conditions. The “temperature” of the electric spark is a purely
conventional expression; to what state of matter it actually
corresponds, can barely be surmised. Yet it is important to remember
that the progression of helium stars—if the testimony of their silicon
lines be credible—is _towards_ this state from a lower degree of
molecular excitement; while their “high-temperature” magnesium ray,
present at the start, gains prominence by accordant gradations.
The inverse relationship between helium and metallic absorption is
extremely significant. They seem to be almost incompatible; one tends to
effacement with the incoming of the other. Yet it must be borne in mind
that both subsist together in sun-spots, where a condition of things
temporarily arises enabling helium to exert its proper stoppage upon
light. Spot-spectra thus approximate, so far, to stellar spectra of the
“Orion” stamp. Here, no doubt, we hold the clue to some profound
physical analogy, the investigation of which may help to dissolve part
of the mystery shrouding the “process of the suns.” Moreover, oxygen,
nitrogen, and “cosmic” hydrogen (if we may so call the modified gas
giving the Pickering lines) are even more sensitive than helium to the
adverse influence of metals. This statement is scarcely impugned by the
fact that a trace of oxygen-absorption survives in the sun.
The closest connections of early helium stars are with members of the
Wolf-Rayet family. But for the dusky lines of magnesium and calcium
apparent in them, their spectra might indeed be said to reverse the
Wolf-Rayet radiations. Later on, when nebular symptoms disappear, when
helium fades, and faint iron lines crowd in, they slide imperceptibly
into the Sirian stage of existence. No halt is cried. The frontier is
crossed without advertence.
Helium stars are not equably scattered over the sphere. Their
condensation towards the plane of the Milky Way, first noticed by
Pickering,[370] was strongly emphasised by McClean’s southern survey.
“In the contiguous constellations of Musca, Crux, Centaurus, and
Scorpio,” he tells us,[371] “there are twenty-seven helium stars out of
a total of thirty-six” brighter than 3·5 magnitude, and the proportion
in Perseus, Taurus, and Orion is fifteen out of nineteen. It would be
desirable to ascertain whether objects of inferior lustre are similarly
swayed by this galactic attraction. But up to the present nothing is
certainly known as to the prevalence of the type among faint stars.
Below the sixth magnitude, its distinctive marks are hardly
recognisable. The conjecture, however, is plausible that Milky Way
aggregations are composed mainly of helium suns, large and small. But
since their great intrinsic brilliancy renders them visible at distances
completely quenching the rays of solar stars of the same size, they
should preponderate in the Milky Way for this reason alone, apart from
any real numerical superiority. So that the question of their
distribution, like most others in stellar physics, has complex bearings.
Helium stars are plunged, without any known exception, in abysmal depths
of space. None have been found within a radius measured by about seventy
years of light-travel. They frequent a sidereal region different from
ours, where nebulæ linger and stars with blazing chromospheres have
their habitat.
CHAPTER IV.
HYDROGEN STARS.
Helium and hydrogen stars cannot be quite definitely set apart. The two
classes commingle. A transition specimen of uncommon interest is found
in η Leonis, which combines some of the peculiarities of α Cygni with a
powerful development of the hydrogen series.[372] Another is presented
by θ Aquilæ, placed by Mr. McClean beside such clearly characterised
helium stars as Algol and Pleione, but by Miss Maury in the group with
Sirius and Vega. Even Vega, although a perfectly normal member of the
hydrogen class, preserves a vestige of helium absorption in the typical
“Orion line,” λ 4472; and its frequent companion, λ 4026, emerges to
view in stars like ζ Aquilæ, in which hydrogen approaches a maximum of
strength. Both these lines are in the laboratory immediately associated
with D_{3}, as members of the first subordinate series of the “yellow”
helium set. None of the special Wolf-Rayet lines occur in hydrogen
stars; the Pickering series is unrepresented; the “blue bands” have no
dark counterparts; oxygen and nitrogen lines seem to have died out.
Between the broad black bars of the Huggins series, however, crowds of
ghost-like metallic rays are discernible. More than one hundred and
thirty of these nascent markings were counted by Miss Maury in the
photographed spectrum of Sirius, and her search, owing to the limited
range of the plates, could only be partial.
Sirius is perhaps a slightly “older” star than Vega. Helium has entirely
disappeared from its spectrum, and more familiar elements take its
place—sodium, iron, magnesium, calcium, silicon, with titanium,
vanadium, barium, and perhaps chromium and nickel.[373] A remarkable
group of lines high up in the ultra-violet is of unsurmised origin.
Photographed by Sir William and Lady Huggins 4th April 1890,[374] their
approximate wave-lengths are λλ 3338, 3311, 3278, 3254, 3226, 3199,
while the head of the hydrogen series stands at λ 3646. They are
accordingly more refrangible than any possible hydrogen line; nor could
impressions of them be obtained with apparatus including glass prisms or
lenses, for which reason they are to be found only on the Tulse Hill
spectrographs. The interpretation of these recondite characters offers
an alluring problem. Although absent from the light of Vega, they will
doubtless be recognised, when duly sought, in other Sirian stars; but
exposures of the requisite kind are laborious, and seldom undertaken.
Yet just such special investigations are likely to be the most fruitful.
Sirius is the best-known luminary of its class. This for two reasons.
First, because of its vicinity. Light travels from the star to the earth
in rather less than nine years. Next, because of our fairly complete
acquaintance with the nature of its binary revolutions. They have now
been closely observed during forty years, and their period is fifty-two.
Hence the mass of the system has been determined, and it has, moreover,
been apportioned with satisfactory exactness between the members. Their
disparity in gravitative power proves to be small compared with their
enormous inequality in lustre. The companion is a mere point of light
outshone 36,000 times by its radiant primary, which is, nevertheless,
more massive only in the proportion of 2·36 to 1·1. This quasi-obscurity
of the Sirian satellite is very curious; but our present concern is with
the majestic orb, in the blaze of which it is almost lost to view. From
Dr. See’s orbital elements, combined with Sir David Gill’s parallax, a
mass is deduced for it just two and a half times that of the sun. If,
then, it were a body of the same average density and surface luminosity
it should give nearly twice (1·84 times) as much light. In actual fact
Sirius is of at least twenty-one times the solar brilliancy. This
estimate is arrived at by taking α_{2}, the brighter component of α
Centauri, as an intermediary. Sir David Gill has shown that its spectrum
is a replica of the solar spectrum; its revolutions prove it to be of
equal mass with the sun, and it is hence assumed with the highest
probability to emit sensibly the same amount of light. It may
accordingly, in comparisons of stellar brightness, be substituted for
the sun, the uncertainty attending the direct confrontation of
enormously unequal light-sources being thus avoided. Now the distances
from the earth, and the photometric magnitudes both of α_{2} Centauri
and of Sirius, are well known, so that it is easy to calculate how one
star would appear in the place of the other. Sirius is twice as far off
as the southern binary; transferred to that remoteness α_{2} Centauri
would then show one-quarter its present brilliancy; it would be of 1·9
magnitude, just matching the chief star in the Plough. It would
accordingly be 3·3 magnitudes fainter than Sirius, which is as much as
to say that it gives only ¹⁄₂₁ part of its light. And the sun, similarly
located, would be of the same faintness.
Thus Sirius, while two and a half times more massive, is twenty-one
times more luminous than the sun. Or, putting it otherwise, the solar
ratio of light to englobed matter is exceeded more than eleven-fold.
Three causes may concur to produce this effect. One of them we know to
be present. It is quite certain that the Sirian beams are almost
undimmed by self-absorption, whereas those of the sun are reduced
probably to one-third their original intensity. A second contributory
cause to the brilliancy of this star may be found in its great bulk. It
is likely to be much less condensed than the sun, consequently to
possess a much larger extent of photosphere relatively to mass. A
luminous area multiplied four times would explain the outstanding
disparity of brightness, but would involve a reduction of mean density
to one-eighth the solar standard, or about one-sixth that of water.
There remains the third factor of absolute areal brilliancy; but its
value presumably depends upon temperature, and comparative stellar
temperatures must for the present be left an open question. We are only
able to conclude that _if_ Sirius and the sun be on a par as to
intrinsic shining power, then the star is probably about eight times
more tenuous.
Vega is so much more remote than Sirius that it may safely be stated to
quadruple its emissions. Its mass, however, remains undetermined, since
it sways no detected companion with a measurable force. That it is small
compared with its light can hardly be doubted. Indeed, throughout the
hydrogen class, this rule prevails to all appearance universally.
The temperature of the stars, as already remarked, is one on which
dogmatic assertions are best avoided. All authorities agree nevertheless
that the conditions governing light-production in such orbs as Sirius
and Vega approximate in many important respects to those present in a
disruptive electric discharge. One important item of evidence to this
effect is the prominence in spectra of this class of metallic lines weak
in the arc, but strong in the spark. “The general result,” Sir Norman
Lockyer says,[375] “of the investigation of the enhanced iron lines in
stellar spectra confirms the view that the absorbing regions of the
hottest stars exist at a higher temperature than is attainable in
laboratory experiments.” Concurrent testimony was derived from
variations of relative intensity in the magnesium and calcium lines
shown by particular stars. But no allowance was made for modifications
resulting from differences of pressure, which the Tulse Hill researches
had proved to be highly influential. Hence the absence in Sirian stars
of the “blue” line of calcium (λ 4227) tells nothing by itself as to
their temperature. The special value of Dr. Scheiner’s magnesium-test is
that the _opposite_ behaviour of the two lines considered (λ 4481 and λ
4352) excludes the density-factor. For increase of heat may occasion the
weakening of individual lines concurrently with the strengthening of
others; but changes of pressure must always act in the same
direction—though not necessarily to the same extent—on every element of
the spectrum affected by them.[376] Professor Keeler[377] suggested that
by means of the magnesium triplet, _b_, inferences as to temperature in
stars might be extended to grades beyond the possibility of artificial
production. This group is conspicuous alike in the flame, arc, and
spark; it cannot be experimentally abolished, yet it fails (as we have
seen) to appear in Rigel, and emerges very feebly in Sirius and Vega.
Now it belongs to a subordinate series due to a special molecular
arrangement, which could not easily persist in an extreme stage of heat.
And a break-up of the arrangement would be marked by the effacement of
the triplet in the green. “If this reasoning is correct,” Professor
Keeler wrote, “the aspect of the _b_-lines in stellar spectra gives us
an extension of the method proposed by Scheiner, and it shows that the
temperature of certain stars exceeds that of the most powerful electric
spark.”
The long range of powerful hydrogen lines in Sirian spectra, on the
other hand, cannot be regarded as a sure symptom of excessive heat. It
seems rather to indicate an approach to homogeneity in the originating
stratum. The abridgment and enfeeblement of the series, and the
development of metallic absorption, follow the same course, which is
certainly not prescribed by thermal change. This inverse relation has
not so far been satisfactorily accounted for. The most plausible
hypothesis regarding it is that of Sir William and Lady Huggins, who
connect it with the inevitable gain of effective gravity in condensing
globes.
Nor can the intensity of the higher spectral sections be taken as an
unequivocal sign that hydrogen stars are hotter than the sun. For it may
be caused not by the intrinsic emissive superiority of their
photospheres, but by their unveiled condition. We know that the sun’s
more refrangible rays would, through the removal of his absorbing
atmosphere, acquire strength enough to turn the balance of colour from
yellowish to bluish, and it is amply possible that its spectrum,
displayed to equal advantage with that of Vega, might rival or outdo its
actinic compass.
Fomalhaut (α Piscis Austrini) is a fine example of an advanced hydrogen
star. Metallic lines are considerably more developed in it than in Vega
or Sirius; the McClean spectrograms show a profoundly grooved K-line,
and its blue associate (λ 4227) is faintly reversed. The “spark ray” of
magnesium (λ 4481) is prominent. Miss Maury prints a table of
wave-lengths[378] measured from the Harvard spectrograms of this star
which deserves particular attention as a record, perhaps, of a
transition epoch in stellar growth. Lines of iron, titanium, and silicon
are readily identifiable in it, but most of the entries have no obvious
meaning. Fomalhaut is of 1·3 magnitude, and Sir David Gill has
determined for it a parallax of 0·13″ showing it to be almost six times
more remote than α Centauri. Its real brightness is hence easily found
to be fourteen and a half times that of the sun. Its mass and density,
however, remain entirely unknown.
As illustrating the physical differentiation of bodies to all appearance
chemically similar, two stars may be singled out. These are Castor and γ
Ursæ Majoris, the third of the Plough. Both are included in Miss Maury’s
Group viii., and both show deep and broad furrows of hydrogen. The
spectra, in fact, bear the same inscription, only printed from
dissimilar types. In γ Ursæ, the spectrum of Castor is viewed, as it
were, out of focus. The lines distinct in the one are hazy and diffuse
in the other; none probably are really missing, though a good many are
effaced by expansion. This peculiarity is met with in a considerable
number of helium and hydrogen stars forming Miss Maury’s “division _b_.”
In “division _c_,” on the contrary, of which α Cygni is the best
exemplar, the lines are notably sharp and narrow, while in “division
_a_” they are of normal appearance, some thin, others fringed or winged.
The cause of these variations is obscure. It would naturally be
connected with differences of pressure in the stellar reversing layers;
and this again must depend in great measure upon the locus of
absorption, which probably varies, not only from star to star, but for
each separate substance in the same star. So that the conditions to be
regarded, even from this point of view alone, are highly complex.
CHAPTER V.
SOLAR STARS.
The transition from hydrogen to solar stars is effected as gradually as
the transition from helium to hydrogen stars. Metallic absorption comes
more and more to the front in successive objects, while the Huggins
series retires into the background. There are no definite
stopping-places; the course of change flows on continuously. At a
certain stage of progress, however, the characters distinctive
respectively of the condition that has been, and of the condition that
is about to be, appear evenly balanced. The hydrogen lines, although
reduced to about one-quarter their Sirian intensity,[379] still muster
strong even in the ultra-violet, the metallic spectrum being at the same
time pronounced and crowded. This medium state can be studied to
advantage in Procyon, the lesser Dog star. So perfect is the blend of
types shown by it, that Professor Pickering found it difficult to decide
whether the spectrum was actually intermediate, or combined the Sirian
and the solar light of two separate, but closely conjoined stars.[380]
Either alternative is possible, but the former is the more probable.
Nevertheless, the presence of the full complement of Huggins lines,
together with a K-band of ten-fold the intensity possessed by it in
Castor, must be regarded as somewhat anomalous.
Procyon is one of our nearer neighbours in space. Dr. Elkin has measured
for it a parallactic shift of 0·325″, corresponding to a light-journey
of ten years. And the revolutions of a faint companion complete the data
requisite for finding the mass of the system. It comes out 2·7 times
that of the sun, and we cannot be far wrong in assigning to the
brilliant component twice the solar quantum of matter. It gives nearly
quadruple the solar light; yet the disparity between light and mass is
notably reduced from the Sirian standard. Absorption has increased as
condensation has progressed. The rays of Procyon are perceptibly tinted
with yellow.
A similar spectrum is shown by the splendid Canopus. The extreme
remoteness of this orb, which is second only to Sirius in apparent
lustre, compels us to attribute to it a prodigious real light-power. It
has no measurable parallax, and no sensible proper motion. Only a
minimum estimate then of its magnitude is practicable. Sir David Gill
attached to the zero representing its parallax a “probable error” of
0·011″. Hence the measures executed do not exclude a parallax of this
amount, although they are just as consistent with an equal negative
value. Canopus then may be no further off, but cannot be nearer than a
light-journey of 296 years. Admitting, for the sake of illustration,
that it is in fact at this distance, which is thirty times that of
Procyon, we obtain the astonishing result that it gives no less than
3600 times its radiance. And since the spectra of the two stars agree
nearly line for line, this figure must represent approximately the ratio
of their photospheric areas, that of their cubical contents being
216,000 to one. In other words, 216,000 bodies of Procyon’s size would
go to make up one such globe as the star of prehistoric Egypt. Yet
Procyon, as we have seen, is a sun constructed on a larger scale than
our own. The existence of a luminary so vast as Canopus, although
bewildering to imagination, need not appear incredible when we consider
the immense scope of creation, and the boundless resources variously
displayed throughout the ethereal spaces populous with stars.
Another interesting specimen of the Procyon variety is γ Cygni. Visually
of 2·3, it is only of 3·2 photographic magnitude. This implies blue
absorption to an extent unusual in the presence of the ultra-violet
hydrogen series. It is accompanied by a disproportionately strong K,
well brought out in Mr. McClean’s spectrograms.[381] The star resembles
α Cygni in the definite character of its lines, although their chemical
meanings are very different. They have, however, as yet been most
imperfectly deciphered.[382] The spectroscopic relations of γ Cygni
derive added importance from its apparent connection with a
far-spreading galactic nebulosity photographed by Wolf and Barnard. But
the star may be merely seen in projection upon it. The peculiarities of
its light recur with less accentuation in that of Polaris.
A very close approach is made to the solar spectrum by χ Orionis;
virtual identity is reached by Capella, η Boötis, and α_{2} Centauri. It
is scarcely compromised in Arcturus, or any of its numerous associates
in Group xv.; the same lines subsist, only drawn somewhat more heavily,
and there is an added shade of ultra-violet absorption. The steadiness
with which the solar type is maintained, all but unmodified throughout a
large collection of objects, is very remarkable. Of the 681 bright stars
investigated by Miss Maury, 19 are Capellans, 111 Arcturians; the latter
are barely distinguishable one from the other, the former only by the
finest grades of difference.[383] This seems to indicate a particularly
stable phase of stellar existence. Our sun’s constitution, we can infer,
is adjusted to a high degree of permanence; he is moving along a nearly
level tract of his evolutionary journey, and will decline with extreme
slowness from his actual state.
Solar stars are to be found of all sizes, their variety in this respect
forming an instructive commentary upon their spectral similarity.
Consider Arcturus. Dr. Elkin, from a long series of skilfully planned
observations, assigned to it in 1897 a parallax so small (0·024″) that
its light cannot reach us in less than 136 years. And since at this
abysmal remoteness it outshines the sun’s twin, α_{2} Centauri, by
one-third of a magnitude, the actual excess of its brightness must be at
the very least thirteen hundred-fold. In view, then, of its spectral
identity, Arcturus may confidently be asserted to possess a photosphere
1300 times more extensive than the sun’s. The globe it encompasses is,
accordingly, about 47,500 times more voluminous, and in the same
proportion (assuming equal mean densities) more massive. It follows that
gravity exercises over the surroundings of Arcturus thirty-six times its
solar power. Yet its spectrum bears no trace of sensibly augmented
pressure. We are confronted everywhere in sidereal physics with this
seeming inconsistency between the nominal force of gravity and its
effective action.
Pollux (β Geminorum) conforms strictly to the spectral pattern of
Arcturus. It is, however, a full magnitude fainter and at only half its
distance; it must accordingly be a much smaller body. Its superficial
area is, in fact, one-tenth that of Arcturus. Nevertheless it contains
fifteen times more matter than the sun, and gravity at the surface of
Pollux has more than eleven-fold its solar power. Planets revolving
round this star would have, at the same distances, periods about
one-quarter the length of those belonging to the earth and its sister
worlds, our year, for instance, being reduced to ninety days, so that a
whole summer would be consumed in a brief holiday excursion.
But there are small as well as large solar stars. An insignificant
object in the Great Bear, catalogued as “Groombridge 1618,” and noted
for its rapid proper motion, is, according to Sir Robert Ball’s
measures, comparatively near the earth, its light reaching us in 10·2
years. The sun, however, in that position would be four and a half
magnitudes brighter, for it radiates fifty times more powerfully. The
spectrum of 1618 Groombridge is of the Arcturian sub-class, so that the
proportion of its mass may, under reserve, be taken to follow the
proportion of its light. About 350 such stars, then, should be put into
the scale to balance one sun, and gravity at its surface has one-seventh
its value at the photospheric level. Another minor sun is “Bradley 3077”
in Cassiopeia, although the inferiority is here slighter, since
Bradley’s star emits perhaps ten times more copiously than
Groombridge’s. Further examples of the kind will certainly come to be
known when some progress has been made with the investigation of faint
spectra. But this is most baffling work, subject to the illusions that
everywhere haunt the limits of distinct visibility.
Enough has been said to make it clear that the Fraunhofer spectrum is
exactly copied in orbs of most various dimensions. This points, in Dr.
Scheiner’s opinion,[384] to the closest agreement, not only in the
percentage of the chemical elements entering into their composition, but
also in conditions of temperature and pressure. How such uniformity can
be combined with widely different gravitational constants, is extremely
hard to understand. The Tulse Hill experiments, already referred to,
showed the predominant influence of pressure in altering spectral
characters. Since, then, they are the same in Arcturus and Groombridge
1618, there is practical certainty that the calcium envelopes (for
instance) of both stars do not differ appreciably in tenuity. Yet the
compulsive force acting upon one is 252 times more powerful than that
exerted on the other. The persuasion that it is somehow neutralised is
irresistible. We might even venture tentatively to define solar stars as
bodies in which the ratio is the same between gravity and electrical
repulsion. In the course of time, doubtless, it will change; one or the
other force will gain relatively to the other, and the spectral type
will vary to correspond. Presumably the augmentation of strength will be
on the attractive side; but cosmical electricity is still an unexplored
region.
The symptoms of approach towards the fluted description of spectrum set
in gradually, and are of two kinds. General absorption of the more
refrangible rays spreads and deepens, and specific absorption becomes
intensified in certain dusky lines. Conspicuous among these is the
“blue” line of calcium (λ 4227), the stress laid upon which
unquestionably signifies increased density in the absorbing vapour. This
is just what might be expected to accompany the progress of cooling and
contraction, through which _domestic_ gravity gains advantage, as acting
in a steadily narrowing sphere. The symptoms described are visible in α
Hydræ and β Cancri; they are particularly well marked in Aldebaran. The
last is a glaringly red star, its blue emissions being mostly arrested
by its own atmosphere. Incipient flutings, too, are traceable. It is the
“type star” of Miss Maury’s sixteenth group, which includes twenty-three
objects scarcely to be discriminated as regards the quality of their
light. “From the greatly-increased width in them,” she writes[385] of
the line at λ 4227, “it would appear to be complex, and to include lines
weak or absent in the stars of the solar type.”
Aldebaran has a parallax of one-tenth of a second, and is of standard
first magnitude. Its real brightness is then certainly twenty-eight
times greater than that of the sun; and since it has suffered much more
heavily from absorptive encroachments, the emitting surface must
proportionately exceed the spread of the photosphere. Even apart from
any allowance for increased density, the Taurus luminary may be
considered by a quite moderate estimate to be of 200 times the solar
mass. On the other hand, the primary member of the famous pair, 61
Cygni, which as to spectrum is a faint duplicate of Aldebaran, ranks
very low in the hierarchy of suns, emitting, in fact, only ¹⁄₃₆₄ the
light of that great red orb. Here, again, the lesson is enforced that
the widest variety of size and mass may consist with spectral identity.
Aldebaran is encompassed by gaseous strata apparently no denser and no
hotter than the absorbing layers in 61 Cygni. This circumstance is
evidently of vital moment in stellar natural history.
CHAPTER VI.
STARS WITH FLUTED SPECTRA.
The blue light of these stars is powerfully absorbed by an
intensification of the screening effect observed in the sun. They are
accordingly rufous, or red. Their spectra are profoundly scored besides
with metallic rays, generally agreeing in position with, although
differing in relative intensity from, the Fraunhofer lines. Thus the
calcium line in the blue has gained still further upon the great pair in
the violet than in transition stars of the solar type. Finally, banded
absorption has come in. A complete system of ten or eleven flutings,
sharp towards the violet, graduated insensibly towards the red, shadows
nearly the entire visible spectrum. It is printed in stereotype. The
bands are variously impressed, but similarly located, in all members of
the class. This gives strong assurance of an identical origin. We do not
yet know how they are produced, or by what substances, but there can be
no doubt that their explanation in one star will apply to all.
With high dispersion the bands can be resolved into fine lines set very
close together.[386] The fluted effect is due to the crowding of these
lines towards a limiting wave-length prescribed, beyond question, by a
rhythmical law. For that each band represents, as it were, a condensed
series there need be no hesitation in admitting. Indeed, by mere
reduction in scale the hydrogen procession in a white star assumes the
aspect of a genuine fluting. A promising start has even been made in the
research of laws regulating the distribution of lines in bands.[387]
Each of the stellar stripes is then, so far, a separate entity, while
all may be linked into harmony by subordination to some higher unknown
principle. Whether they originate from one or many forms of matter has
still to be determined; nor is there any certainty as to whether
elements or compounds are concerned in their production.
Stars of Secchi’s third type, forming the class just now under
consideration, are divided by Miss Maury into three groups. Antares and
β Andromedæ belong to the first of these (“Group xvii.”). They show all
the characteristic bands, slightly marked, and transparent enough to
allow every detail of the linear spectrum to be clearly visible.
Betelgeux (α Orionis) is the pattern of the next group. Its prismatic
light makes a beautiful and wonderful effect. The usual multiple
absorption is exerted upon it, but with a delicately balanced power. The
blue rays retain appreciable vivacity; the flutings are not so deep as
to obscure the underlying rays; they are finely shaded, yet exquisitely
distinct. Ten were measured by Vogel and Dunér, the strongest of which
are the fifth and seventh, with their _steep_ sides at λ 5453 and λ 5169
respectively. Battalions of dark lines show through them. On the Harvard
plates Miss Maury counted 463, mostly composite, between the sodium D
and the calcium H. The latter and its associate, K, have shrunken
somewhat from their giant dimensions in Arcturus.[388] Iron absorption
predominates. The rays significant of it are more prominent than in the
sun, and some have unilateral shadings—a feature also visible in the
spectra of sun-spots and of metallic oxides, and indicative probably of
a decline in heat.[389] Professor Keeler remarked that the lines in
Antarian stars “are essentially those of the solar spectrum, but the
relative intensities are not the same, and the general aspect of the
spectrum is quite different from that of the spectrum of the sun. The
strong lines are mostly those of iron—apparently the low temperature
lines. Their relatively greater strength in the star spectrum gives to
some well-known solar groups (notably the _b_ group) quite an unfamiliar
aspect.”[390]
Hydrogen-absorption is much more effective in Betelgeux than in any
other spectrum of the fluted kind. It is, however, distinctly, though
feebly represented in all by the four lowest members of the Huggins
series. The rest are either absent or shrouded in overlying vapours.
In ρ Persei, a star capriciously variable between 3·4 and 4·2
magnitudes, the bands are perceptibly deeper than in α Orionis. On the
other hand, the metallic rays seem rather less numerous and intense. But
this diminution may be more apparent than real. The comparative
faintness of the light they interrupt would partly account for it, and
the added density of the associated flutings would help towards
effacement.
Their still greater opacity in α Herculis occasions effects of contrast
with the vividly tinted bright zones, described as “singular and
magnificent” by Father Secchi, one of their earliest observers (see
Plate XI. Fig. 1). Professor Keeler, whose study of fluted spectra was
based on photographs of high dispersion showing manifold details, found
the dark groovings, plentiful in α Orionis, to be present only as a
comparatively scanty survival in α Herculis. Miss Maury reached a
similar conclusion. Yet Dr. Vogel was struck with the richness in
absorption-lines of this spectrum and the analogous one of β
Pegasi.[391] Both these stars vary irregularly, α Herculis from 3·1 to
3·9, β Pegasi from 2·2 to 2·7 magnitude. Next to Antares, the brightest
specimens of this class in the southern hemisphere are γ Crucis and β
Gruis. Mr. McClean obtained spectrographs of all three at the Cape in
1897.[392]
Uncertainty has often been expressed as to the true nature of the
luminosity in the open spaces of fluted spectra. Are they simply
intervals of unshaded photospheric radiance, or is their brilliancy
reinforced by the addition of bright lines? Just where they meet the
black edges of the shafts of absorption, their splendour exceeds, in the
opinion of some observers, what could be produced by contrast alone. It
seems, nevertheless, unlikely that rays of emission should occur in
these positions and nowhere else in the same spectra. Moreover, illusory
impressions of the kind, both visual and photographic, are common and
pertinacious. Nothing, however, is more inimical to truth than dogmatic
denial; we must be ready to admit much that we should beforehand have
deemed impossible, even the reality of far-fetched coincidences; for
anticipation is often belied by fact. Professor Keeler, whose mind was
singularly free from prepossessions, found it “impossible to avoid the
conclusion” that in the spectrum of α Herculis “the edges of the zones
bordering on the dark bands are bright—much brighter, that is, than the
average continuous spectrum—and that they are due to a real predominance
of emission at the regions of the spectrum in which they occur. In the
case,” he wrote further, “of stars like α Orionis, of a less pure type,
such a conclusion could not be safely drawn; yet the superior brightness
of the spectrum at these places is obvious, and it can be traced even in
second-type stars. May there not, after all, be bright regions in the
solar spectrum, such as Draper supposed he had found in the places of
the bright oxygen lines? And what is the relation between the dark bands
in third-type stars and the bright zones which border on them?”[393]
Questions more easily asked than answered. They suggest doubts, not at
once to be set at rest, as to the nature of so-called “continuous”
spectra. May they not in certain cases include several maxima of
radiation? The possibility is at least not excluded of individual
differences in this respect between stellar photospheres. Yet of true
gaseous emissions there seems to be no trace in Antarian stars. The
admission of its presence could not be made for one without being
extended to all; and many spectra of the class are clearly exempt from
abrupt intensifications.
Chemical recognition has not been carried far in them. The familiar
lines of only five substances—iron, calcium, magnesium, sodium, and
hydrogen—are entirely unmistakable. The rest await more searching
scrutiny. It will be of especial interest to determine whether titanium
and its usual associate, vanadium, retain in these objects any share of
their importance in sun-spots. Helium-absorption, too, which
occasionally emerges to view in spot-spectra, might be looked for with
some prospect of success, but is likely to be inconspicuous. The
fundamental problem, however, in this connection relates to the origin
of the flutings. It ought to prove capable of a definite and, so to
speak, a simultaneous solution. For the members of the system show the
coherence of a structural design. They form a marshalled array, an
interdependent order. Their occurrence piecemeal need not then be
expected. They will be recognised together or not at all.
Stars with fluted spectra have a fixity significant of immeasurable
remoteness. Yet two—Antares and Betelgeux—are of the first order of
apparent brightness. Their real magnitude must hence be prodigious. An
approximate estimate of it can be arrived at in the case of Antares,
which has an ostensible parallax of 0·021″, corresponding to a light
journey of 155 years. This, at least, was the outcome of Mr. Finlay’s
measurements at the Cape; but it is so small as to lie but slightly
outside the margin allowed for their probable error. Its genuineness can
then only be assumed for the purpose of fixing ideas; since the star may
be indefinitely further off, while unlikely to be appreciably nearer,
than this minute annual shift asserts it to be. Under this reserve we
may compare Antares with our standard star, α_{2} Centauri, which, as we
have seen, is equivalent to comparing it with the sun. The result is to
show that four hundred suns, in its place, would barely supply the light
we receive from the _alter ego_ of Mars.[394] And this is only what
remains after a heavy absorption-toll has been levied—a toll of probably
twice the amount paid by the sifted solar beams. The photospheric extent
of Antares may then be set down as at least eight hundred times that of
the sun, while the immense sphere it covers may be held, on good
grounds, to have a comparatively high mean density. Even on the basis of
equality in this respect, its mass would exceed that of the sun more
than 22,000 times, while gravity at the surface of this unimaginable
globe must possess at least twenty-eight times its solar power. Similar
reasonings apply to Betelgeux, only with still further enlargement in
measure of the conclusions they lead to. For Betelgeux is a more
brilliant luminary than Antares, and its immobility in the sky is, if
anything, more nearly absolute.
The processes of interior circulation in such bodies are extremely
difficult to realise. The intensity of radiation must depend—other
things being equal—upon the promptitude of delivery at the photospheric
level of heated stuff from within. This must increase with the force of
gravity, which is the driving power on ascending and descending
currents; but it must, on the other hand, fall off as the sphere they
traverse grows more compact. The manner in which the balance is struck
in each individual star between these opposing influences transcends
every rational conjecture. We can only see that it must vary widely, and
that its variations necessarily affect the photospheric composition and
the radiative characteristics of the globes in which they prevail.
Antarian stars obey no special law of distribution. They are scattered
at large over the heavens, and usually in isolated positions. They show
as a rule no tendency to gather into groups. A collection of nine
specimens, located in the intervening space between the two grand
clusters in Perseus, is perhaps unique. Discovered by Mr. Espin in
1891,[395] this nest of red stars appears like a garnet clasp linking
together a pair of diamond aigrettes. They are of about the eighth
magnitude, but may nevertheless, since they are assuredly vastly remote,
be most majestic orbs. Not that we should ascribe to all stars of this
spectral type the colossal dimensions of Betelgeux and Antares. The same
degree of variety may be supposed to exist among them as among solar
stars. Yet the analogy may not hold. It is conceivable that very great
mass is a pre-requisite for the development of a fluted spectrum.
CHAPTER VII.
CARBON STARS.
Stars of Secchi’s fourth type, also known as “carbon stars,” are the
most exclusive of stellar families. They hold remarkably aloof from
every other. They have indeed traceable relationships; but the genealogy
obscurely indicated by them needs authentication.
Mr. Espin published in 1898 a catalogue of 237 carbon stars,[396] and
about a dozen objects of the kind have since been detected. None are as
bright as the fifth, and only seven exceed the sixth magnitude.[397]
Their inconspicuousness probably arises, not from any deficiency of
intrinsic light-power, but from the overwhelming absorptive action of
their atmospheric envelopes. Thus only a small part of their original
radiations attain to outer space; none of the shorter wave-lengths
escape; the spectra are cut off short a little below the place of the
blue calcium line. Intense visual redness is a consequence. These stars
glow sanguine in the field of the telescope; they are variously compared
to “drops of blood,” to carbuncles, garnets, or rubies. By a rough
estimate, 12 per cent are strikingly variable, the proportion being
nearly the same as for third-type stars. Scarcely any, however, shine
steadily; if attentively watched, they can be perceived to flicker and
fluctuate more or less extensively.[398] Some of their changes are
indeed so lasting as to suggest a permanent drop or rise (as the case
may be) in the photometric scale. The circumstance that instability of
light ordinarily accompanies redness of colour in stars is most curious
and significant.
The fundamental characteristic of fourth-type spectra is the presence of
three deep bands, degraded towards the violet, sharp towards the red.
They are a negative copy of the emission-bands displayed by comets. Five
or six additional dusky stripes, so far unidentified, are of less
distinctive construction. The general effect of these spectra differs
from that of the fluted sort chiefly in two ways. First, the _columns_
of absorption are broader and more massive; they are of Doric rather
than Ionic proportions. Secondly, they are illuminated from the opposite
direction; the chiaroscuro is inverted. Their variations of relative
intensity in different objects have been proposed as subsidiary
classification-marks, but cannot be much insisted upon. Such
individualities—as Professor Dunér pointed out—do not imply radical
distinctions; and they are so prevalent and so various that, by closely
attending to them, “one might easily get as many subdivisions as there
are stars.”[399]
The linear spectrum in carbon stars is seen with difficulty through the
cloak of the bands. It is, however, none the less important. Sodium and
iron contribute to it, but most of its constituents still lack
interpretation. Hydrogen and helium alike fail to appear. No calcium
lines are visible; indeed those in the blue and violet, which would most
naturally be looked for, could not show through the dense veil of
absorption shrouding the upper spectral reaches; so that their seeming
absence is consistent with the presence of a calcium ingredient in the
stellar atmospheres. Their carbonaceous strata nevertheless give them
their special character. No other sidereal objects, except an
imperfectly observed variable star, show a trace of the cometary analogy
prominent in the fourth spectral type.
The carbon bands, which constitute its leading feature, were identified
by Father Secchi in 1868, and he noticed besides certain bright lines,
the reality of which, long discredited, has quite recently been
confirmed. He noticed them, however, somewhat confusedly, for he at
times failed to keep them apart from the illusory effect of vivid
emission caused by the prismatic gleaming of the intercolumnar zones.
Professor Hale in 1898, effectively aided by Mr. Ellerman, applied the
photographic method with remarkable success to the investigation of
these spectra. They offer no facilities to the camera. The use of
ordinary plates is of course precluded by their deficiency in blue rays;
only those rendered “orthochromatic” by suitable dyes avail for their
delineation; and these are found practically inconvenient owing to
irregularities in sensitiveness. A series of splendid pictures was
nevertheless obtained with the aid of the Yerkes forty-inch refractor;
but the arduousness of the undertaking can be estimated from the fact
that, with a train of three prisms, exposures of nine hours were
required to secure impressions comparable with those given by the
spectrum of Betelgeux in twenty seconds.
The research embraced, to begin with, twenty-two stars, ranging from 5·4
to 8·2 magnitude.[400] Most of the spectrographs were limited to the
region D to _b_; but a few extended to λ 4450, where dark blue merges
into indigo, and one ranged far down in the crimson. This was derived
from the brightest specimen of the type, numbered 152 in Schjellerup’s
Catalogue of Red Stars. Situated in Canes Venatici, it shows a spectrum
of such rare beauty as to justify the title of “La Superba,” bestowed
upon it by Secchi in 1868. Its yellow section, photographed by Hale, is
shown in Plate XI. Fig. 2. A surprising amount of detail is imprinted in
it. On the original negatives over a hundred lines were measured where
no more than three or four had been previously recorded.[401] Most of
them are dark, but some are bright, among which, apparently, may be
reckoned two yellow rays, compared by Secchi to “exquisite threads of
gold.” They are prominent in our figure at wave-lengths λ 5593 and λ
5693, which, as Professor Hale was careful to point out, “agree very
closely with” those of “two bright lines in the spectra of the
Wolf-Rayet stars.”[402] He adds the caution that, several other similar
approximate coincidences notwithstanding, “it is too soon to conclude
that these classes of stars are related.” A fine group of vivid green
lines was also photographed, and some blue rays were suspected. Although
intensely red, 152 Schjellerup has not been observed to vary from 5·5
magnitude.
The typical star of Miss Maury’s “Group xxi.” is 19 Piscium. It shows a
splendid four-zoned spectrum, vivified by the twinkling of emission rays
(see Plate XII. Fig. 2). Secchi noted in it the shining of the same
“threads of gold” previously seen in 152 Schjellerup,[403] and they may
be considered as a feature common to all spectra of this class. The
opinion to this effect expressed by M. Dunér carries great weight. It
was in 1884 entirely adverse to the reality of the bright lines recorded
by the Roman astronomer;[404] but he changed his view on improving his
instrument. The materials for his invaluable Memoir of 1884 were
collected with the Lund ten-inch refractor; in 1893 a Steinheil of
fourteen inches aperture became available to him at Upsala. With it he
at once undertook a revision of his former work,[405] which, although
hampered by serious interruptions, progressed steadily down to 1898. M.
Dunér was not disappointed in his hopes of seeing more and better with
the larger instrument; and he chronicles as of primary importance “the
fact that he was able to detect without difficulty bright lines in
various spectra, which at Lund were either invisible, or at least could
not be discovered.” Professor Hale’s photographic registration of them
was thus visually authenticated by an observer of unrivalled experience,
and was further verified with the great Lick telescope by Professors
Keeler and Campbell under conditions so admirable as to leave little or
no room for surviving doubts.[406]
It may then be regarded as an established fact that spectra of the
fourth type include elements of direct emission. They are subsidiary,
yet distinct, and seem to be unfailingly present in all members of the
class. Three characteristics may provisionally be ascribed to these
curious bright lines. In the first place, they are of entirely unknown
origin. Hydrogen and helium are equally (so far as published measures
enable us to judge) alien to their production. Some of them may coincide
with Wolf-Rayet lines, but if so, it is with Wolf-Rayet lines which
themselves lie outside the range of terrestrial acquaintanceship. Not
even the exotic light of nebulium or coronium can be seen to glimmer in
carbon stars. Secondly, they are independent of luminous change. They do
not betoken variability. They occur indifferently in objects of steady
lustre and in those subject to wide vicissitudes. Nor has the slightest
sign of inconstancy been detected in the rays themselves. They do not
fade and flash capriciously or periodically. They shine equably—to all
appearance—from year to year, and from decade to decade. Thirdly, the
locus of their development is above the region of carbon absorption. The
tinted rays evidently overlie the dark bands; they are seen projected
upon them. The substances to which they are due must then be found at a
higher level in the stellar atmospheres than the carbon vapour. As
Professor Hale remarks,[407] the case is paralleled in the sun, where
hydrogen and calcium rise to great heights, while a shallow layer of
carbon-gas lies low at the base of the chromosphere. This arrangement of
emissive and absorptive strata does not prevail—as we shall see later—in
stars of all spectral classes. They are, on the contrary, markedly
distinguished in this respect, and the distinction implies profound
physical differences.
It was found possible at the Yerkes Observatory to form a sequence of
eleven stars,[408] in the order of growing depth of carbon absorption
(see Plate XII.). The transition from one of these objects to the next
was so gradual as to suggest that they represented actual phases of
development. This, however, is merely a convenient hypothesis. One of
the earliest of the series is unique, according to Dunér, in the
relative strength of its spectral bands. Those due to carbon are quite
feeble, while one of untraced origin in the red is broad and black. This
star, known as 280 Schjellerup, is scarcely brighter than the eighth
magnitude, so that it can be dealt with to advantage only by the aid of
powerful instruments. Passing on to 19 Piscium, we find _one_ of the
three carbon bands dim, the others—in the green and blue
respectively—very wide and dark. Their unequal prominence constitutes a
striking anomaly. It recalls the variations in relative brightness of
the hydrogen lines recorded with surprise in nebulæ and sundry species
of stars.
A spectrum intermediate between those of 19 Piscium and of 152
Schjellerup is shown by the variable star U Hydræ (132 Schjellerup.)
Dunér noticed long ago the wonderful chromatic effect of its four
brilliant zones,[409] set off by deep bays of absorption; and Secchi
perceived in it a green, as well as a yellow pair of fine rays, the
genuineness of which is more than probable. The star fluctuates
irregularly from 4·5 to 6·3 magnitude, but is very rarely seen at its
maximum brightness. Professor Hale’s spectrographs afforded evidence of
a partial but very interesting resemblance between its spectrum and that
of μ Geminorum, a fine example of the fluted description, and the
agreement—as can be seen by inspecting Plate XV.—extends to the sun.
“Further toward the red,” he tells us,[410] speaking of the banded
varieties, “the spectra become very unlike, though even here there are
certain important points of resemblance which must be carefully
investigated.” Only their linear elements are naturally in question; the
shadowing bands are totally unlike in the two classes. The coincidences
detected, however, are of real importance as forging a link, even if a
slight one, between stellar families that stood previously entirely
apart.
The invisibility (up to the present) of hydrogen in carbon stars is not
easily accounted for. The substance must enter into their composition;
its diffusion is seemingly universal and profuse; why, then, is its
manifestation, whether by emission or by absorption, suppressed in this
particular class of objects? The same query may be put in regard to
comets, and the same obvious, although perhaps insufficient answer
presents itself, namely, that their stock of hydrogen has been consumed
in the fabrication of hydrocarbons. It is worth noting besides that the
only metals yet identified in these stars—sodium and iron—are precisely
those perceived to glow in one or two exceptional comets. But this may
be only a chance concurrence.
[Illustration:
PLATE XV.
Spectra of Stars of Types II., III., and IV. (Hale and Ellerman).
1. The Sun (Type II.). 2. μ Geminorum (Type III.). 3. 132 Schjellerup
(Type IV.).
]
The rule of colour in carbon stars long seemed inviolable; yet there are
exceptions to it. Two spectra of the kind well extended in the blue were
photographed at Harvard College in 1891, and they belong, in fact, to
white stars.[411] They are situated, one in Aquila, the other about
three degrees north of ε Ceti, and are of the seventh and eighth
magnitudes respectively. Their investigation ought to prove peculiarly
instructive, for in them the type has developed under most unusual
conditions. It is besides more completely exhibited. Sections of these
spectra can be registered and examined which in other analogous objects
are concealed by dense general absorption. It should, for instance, be
possible to determine whether calcium lines are really or only
apparently absent from fourth-type spectra.
Carbon stars, there is little doubt, are inordinately distant from the
earth. None, we believe, have any measurable proper motion, and
experiments on their annual parallaxes would certainly prove a waste of
time and trouble. We have, then, no means of estimating their real
brilliancy, but it _must_ in some cases, and it _may_ in all cases, be
exceedingly great. These objects show a marked preference for the Milky
Way.[412] They occur, however, in other parts of the sky as well. They
are condensed towards the galactic plane, but not limited to it. They
are unmistakably, yet far from exclusively, swayed by its attraction.
CHAPTER VIII.
STARS WITH FLUTED SPECTRA SHOWING BRIGHT LINES.
Although the display of bright lines in fluted spectra is a sure sign of
extensive variability, it is not the light-changes of the stars thus
characterised that here concern us. The two classes of phenomena are
beyond question intimately related; but as a matter of pure convenience
they have to be treated of apart.
Mira Ceti has the advantage over its fellows of rising to more brilliant
maxima, and of having received longer attention and more careful study.
Since, however, all the members of Class vi. are not copied from one
pattern, investigations conducted too exclusively can only lead to
partial knowledge; and, indeed, the varieties distinguishing the
different specimens are precisely their most instructive feature. Thus
they agree in showing _some_ brightened hydrogen lines, but not in the
selection of those to be brightened. Then helium rays are vivid in
certain of these stars, dark in others, and there are further, less
assured diversities. These cannot yet be explained by any single
consistent theory, but they may be definitely ascertained and brought
into some kind of orderly relationship.
The spectrum of Mira has a prescriptive right to be considered first. It
is the model, deviations from which count as exceptional. The bands are
profound, the radiations of hydrogen intense during fully one-third of
the light-period of eleven months. Detected photographically at Harvard
College in 1886, they are now looked for, and rarely missed, in every
analogous object as it rises from quasi-extinction. The hydrogen stratum
in Mira seems to be in a peculiar condition. It emits only the higher
members of the Huggins series, the red and the green lines (C and F)
being alike invisible. The blue and the indigo lines, on the other hand,
shine with extraordinary brilliancy—a brilliancy “too great to be shown
on a drawing or to be safely expressed by a number representing relative
intensity”[413] (see Plate XIII. Fig. 1). The fifth hydrogen line, by a
rule without exception in Mira variables, is hidden; but eight of its
associates in the ultra-violet have been recorded. Very singular,
indeed, is the partial presentation in this star of a closely-linked
sequence of vibrations. It can hardly result from an extraordinary
elevation of temperature; we can better conceive it as due to some
subtle form of electrical action not yet evoked in the laboratory. The
state of things as regards the hydrogen spectrum is the opposite of that
prevailing in the reversing stratum of the sun. Here the upper
radiations are suppressed; in Mira the series starts from its third
term.
The star has been spectrographically investigated by Vogel, Sidgreaves,
and Campbell. The Potsdam plates were exposed during the low maximum of
January-February 1896,[414] when the variable scarcely exceeded fourth
magnitude; and this is a circumstance to be borne in mind, since there
is reason to suspect that emission may differ, not only in degree but in
kind, at light crises of different intensities. However this be, _only_
the hydrogen lines were perceptibly vivified in 1896. Of the dark lines
measured by Dr. Vogel, many coincided with Fraunhofer rulings, but a
goodly proportion seemed unfamiliar.
The maximum of 1897–98 was studied by Father Sidgreaves.[415] It was an
improvement upon that observed by Vogel. Mira attained 3·2 magnitude on
30th November. The Stonyhurst plates were isochromatic; their range of
sensitiveness extended from high up in the violet to near D in the
yellow, and they continued to be exposed until 5th February, when the
variable had sunk to the sixth magnitude. But the light remained
essentially unchanged in quality, although reduced to one-thirteenth its
original amount. Only the continuous spectrum in the blue had faded,
relatively as well as absolutely, showing that the star grew redder in
its decline.
Professor Campbell’s[416] observations at the maximum of October 1898
had the twofold advantage of being made with a magnificent apparatus and
at an exceptionally bright phase. In more ways than one they mark a
beginning. They both suggest relations and establish facts. The plates
exposed with the Mills spectrograph attached to the great Lick refractor
show only the region near the third hydrogen line (Hγ) in the fine
detail needed for measures of precision. These were designed primarily
for the determination of the star’s radial movement, which proved to be
one of recession at a speed of 62 kilometres (38½ miles) per second. It
may be regarded as constant. No part of it seems to be due to orbital
motion round an invisible companion. It was, however, derived
exclusively from the _dark_ lines in the spectrum. The _bright_ lines
told a different tale. Four were compared—a hydrogen pair and a pair
ascribed to iron—and all showed a much smaller displacement redward than
the dark lines. The amount of the discrepancy, moreover, proved subject
to fluctuations; but to fluctuations obviously depending upon intrinsic,
not upon extrinsic causes. No attempt has been made to explain them on
the hypothesis of variable motion. It may be accepted, on the evidence
of lines physically in a normal state, that Mira—so far as appears
yet—is a solitary body in course of withdrawal from the earth at a
uniform rate of 38 miles a second.
Early in October 1898 the star reached 2·6 magnitude, and during the few
weeks of its greatest brightness the blue and indigo hydrogen bands were
perceived to be broken up each into three unequal components. This
remarkable appearance falls into line with symptoms of disturbance in
stellar spectra of other types, but had not previously been observed in
a Mira variable. It is of very curious interest. In studying the
“intensity curves” of the tripled line (see Fig. 19) the conviction
becomes almost irresistible that here a “Zeeman effect” is in question.
The polarisation test might decide. If the lines are distended and
shattered by powerful magnetic action, then the lateral components and
the central component must be polarised in planes at right angles to one
another, and the rotating of a Nicol’s prism in the field should produce
alternating extinction. Professor Campbell was prepared to make the
experiment at the maximum of 1899, but the star unfortunately failed to
replenish its due measure of light, and gave an imperfectly legible
spectrum. Favourable opportunities, however, for applying this simple
criterion must frequently recur, and they are well worth watching and
waiting for. Positive results of the kind indicated would be of
revolutionary importance; obscure phenomena would be illuminated;
anomalies would be removed; a boundless region would be thrown open to
investigation. The issue, it is scarcely too much to say, is vital to
the progress of astrophysics.
[Illustration:
FIG. 19.—Intensity-Curves of Hγ in Spectrum of Mira (Campbell).
]
The production of multiple hydrogen-lines in the spectrum of Mira may
quite possibly be restricted to brilliant phases. Metallic emission
almost certainly is. Thus the iron rays (λλ 4376, 4308) registered as
bright in 1898 were, the one strongly dark, the other either dark or
invisible in 1896 and 1897. This gives a hint of the diversities to be
looked for in the future, and lends enhanced interest to minutiæ of
observation which in themselves might seem trivial.
The spectrum of Mira includes a good many dark lines closely adjacent
to, if not actually coincident with, rays of helium. None have been seen
bright. Calcium absorption is very prominent. The line in the blue,
which develops with increase of pressure, comes out as a black grooving;
the giant pair in the violet are of surprising intensity. The less
refrangible, as in all such stars, smothers the hydrogen emission of
nearly its own wave-length; and this circumstance demonstrates some
unexpected relations. The calcium-layer, plainly under considerable
pressure, must be located, as in the sun, quite close to the
photosphere. But the glowing hydrogen necessarily lies lower still,
stoppage of light implying superincumbence of the arresting vapour, and
there seems no room for it except in the very interstices of the
photosphere itself. The overlaying of a light by a heavy substance is
indeed anomalous, yet no other arrangement is consistent with the
spectral phenomena of Mira and its congeners. Besides calcium, iron,
magnesium, strontium, titanium, manganese, and chromium are easily
recognised as absorptive constituents of its atmosphere.
The mode of hydrogen-radiation characterising Mira does not recur in all
stars of its type. Some show the two lowest lines conspicuously bright,
and they are often accompanied by the glimmering of the yellow helium
ray. R Aquilæ is an example.[417] Other members of the class have F for
their chief bright line, C being invisible, as in R Andromedæ and S
Cassiopeiæ, or dim as in V Boötis. Hydrogen in these stars appears to
exist in its nebular condition.[418] Analogous to them in some respects,
R Cygni may in others be divergent. Its chief bright lines are F and
D;[419] but the nature of the accompanying banded absorption appears
somewhat indeterminate.[420] It might be definitely ascertained by a few
well-timed observations. An important spectrographic investigation of χ
Cygni was carried out by M. Eberhard at Potsdam in 1901.[421] It
disclosed phenomena closely analogous to those detected by Campbell in
Mira. Thus the maximum of lustre was attained, in the hydrogen series,
by its fourth member (Hδ); iron lines, both bright and dark, were
abundantly visible; above all, the absorption and emission-spectra were
relatively displaced, just as in Mira in 1898. The bright lines, that is
to say, were pushed towards the blue, while the dark lines deviated in
an opposite sense, though very slightly, from their normal places. This
surprising feature may then prove common to the whole of this class of
stars, and doubtless depends upon some essential peculiarity of their
constitution.
About two hundred variable stars with fluted spectra are known to emit
bright lines, and this kind of spectrum is a distinctive badge of
variability. Mrs. Fleming’s classification of them[422] is based mainly
upon differences of hydrogen-emission. For the typical star of the first
of her eleven groups she chose R Lyncis, in which Hβ and Ηγ are
brilliant, while Ηδ—sometimes the brightest line in Mira—is scarcely
visible. She then traced a continuous sequence of change to R Leonis,
the exemplar of her last group, in which Hβ is imperceptible, Ηγ
excessively faint, Ηδ conspicuous. But this order of relative lustre is
not in R Leonis permanently maintained. The effacement of the green ray
is only transient. In April 1895 Dr. Krüger, observing with the
eleven-inch Bamberg refractor, found it to dominate the spectrum; and
MM. Gruss and Laska saw in the same star, 6th May 1894, Hα doubtfully,
Hβ and D_{3} unmistakably, although two nights later Hα shone alone,
while on 28th May Hβ was similarly isolated. Such changes, inexplicable
as they are, cannot be set aside as incredible. Their further
investigation is most desirable. Meanwhile, the relative brilliancy of
the hydrogen lines in variables evidently supplies a highly insecure
basis for their arrangement.
Mira is the only member of its family which has been at all adequately
studied. A good beginning has been made with χ Cygni; but about most of
the remaining couple of hundred, particulars are wholly lacking. The
great majority, having been registered in sweeping spectrographic
surveys, were pigeon-holed for future reference, after brief inquiry
into the history of their recorded light-changes; and in their
pigeon-holes they have been mostly allowed to rest. Enough is known,
however, to whet curiosity as to what remains unknown. Spectral changes
of a remarkable kind affect these stars; their thorough verification and
the unravelment of their tangled relationships are essential to
progress. The work may be difficult, but it is of profound interest. The
elucidation of the hydrogen-spectrum in one variable star may indeed
open the door to unexpected and far-reaching discoveries.
The helium-spectrum is equally significant, but more evasive. The
emergence of the yellow ray seems to accompany the brightening of the
two lower hydrogen rays; but its shining may be comparatively transient.
The important point, however, is that it does not seem to occur at all
in stars showing, like Mira, a mutilated hydrogen series. Then there is
the further question whether D_{3}, when it does shine out, shines
alone. Are all its numerous associates invisible, or are they dark, as
some of them appear to be in Mira? Finally, we know very little as yet
about the lighting-up of metallic rays in such spectra. It is
nevertheless certain that some regulating principle governs the
selection of those brightened; and only by detailed study can the nature
of that principle be ascertained. All this, and much more, needs
prolonged and extended inquiry; but in a field that will yield ample
return for the expended labour.
CHAPTER IX.
HELIUM STARS WITH BRIGHT LINES.
Temporary stars, and a few stars variable in short periods, belong,
properly speaking, to this class; but for the sake of clearness and
convenience, they are reserved for separate treatment. The question of
light-change will demand later on our undivided attention; it bristles
with difficulties, which we are not at present prepared to encounter.
Alcyone, the chief Pleiad, long passed for an ordinary Orion star. Dark
lines of hydrogen and helium were prominent in its spectrum; there
seemed no reason to suspect the slightest deviation from normality.
Nevertheless, Campbell perceived in 1893[423] the red radiance of C set
off by a narrow dark line on its more refrangible side; and this state
of incipient emission appears to be permanent.[424] Alcyone might then
be counted a linking instance between Classes i. and vii. The discovery
was startling that a single substance could show certain of its rays
bright, the remainder dark, in one and the same star. But the fact,
although highly perplexing,[425] has become so common to experience as
to have ceased to be surprising. Examples of its occurrence have been
registered by the score as regards both hydrogen and helium. They are
all found in Classes vii. and viii. (bright-line helium and Wolf-Rayet
stars); Mira-variables seem never to have their hydrogen spectrum thus
conditioned. Professor Campbell[426] noticed it as an invariable rule
that the bright lines in Orion stars “are those of greater wave-length,”
while “the dark lines are those of shorter wave-length.”[427] This
applies also to helium, but in a qualified sense. If all the lines in
its spectrum are taken indiscriminately, the bright and the dark appear
to succeed each other without method; but their consideration by series
makes it at once evident that, within the limits of each set of
vibrations, the bright members are invariably fundamental. This is
important, not only for the better ordering of stellar phenomena, but as
regards the theory of spectral series in general.
In Alcyone, then, emission is at a minimum; it could scarcely diminish
and remain existent. And it is quite possible that it may be on the
wane; after the lapse of some hundreds, or thousands of years, the chief
Atlantid will perhaps have lost the distinctive note of its spectrum,
and will have sunk to the level of unrelieved absorption. One of its
companions at present stands out from the crowd in the same way, but
more decidedly. Pleione shows three hydrogen lines pretty strongly
bright, and they are inevitably C, F, and Hγ. Centrally superposed upon
wide dark bands,[428] they assert by this fact alone the
non-correspondence in position of their originating stratum with the
glowing hydrogen in stars like Mira. And this teaches us the important
lesson that there is no stereotyped recipe for the production of stellar
bright lines, but that they may originate diversely in the various
spectral classes.
The Pleiades are nebulous collectively, and in many cases individually
as well; but they are less closely folded in nebular swaddling-bands
than the group of stars forming the nucleus of the great Orion nebula.
It consists of four leaders, of about the fourth, fifth, sixth, and
seventh magnitudes respectively, two of which have faint companions;
and, scattered promiscuously, there are to be found besides four minute
stellar points, detected at Lick by Professor Barnard and Mr. Alvan G.
Clark. For spectroscopic purposes the “trapezium,” or quartette of
bright stars, may be treated as one, since they shine with sensibly the
same quality of light, while their scarcely visible associates give
radiations negligible in amount. Most difficult questions arise in
attempting to decide upon the true nature of the trapezium-spectrum. The
prevalent view at first was that the stars were of the ordinary
dark-line helium type, bright lines coming in here and there simply as
projections from the enormous volume of gaseous stuff interposed between
the eye and the stellar nucleus of the formation. But the opinion was
grounded on superficial evidence, and has not held its ground. Some
lines, bright in the nebula, _refuse_ to cross the thin strip of
continuous light due to the star; they stop short on one side of it, and
reform on the other,[429] the two sections being divided by a narrow gap
of absorption. This proves that the nebular rays do not in all cases
show bright against the background of continuous stellar light. The very
strongest may do so; but it is just possible that in them the appearance
is illusory, and due to a kind of irradiation.
There can, on the other hand, be no reasonable doubt that the
trapezium-stars have bright lines of their own. But they are peculiar,
and peculiarly conditioned. A spectrograph taken by Sir William and Lady
Huggins, 5th February 1888,[430] proved to be crossed in the
ultra-violet by at least four groups of fine, faint, bright lines,
derived primarily from two stars of the trapezium, but extending,
through their influence, as it were, some little way into the adjacent
nebula. Their origin is problematical; they have not been recorded
elsewhere;[431] they have been only partially verified on later Tulse
Hill plates. Yet the original negative survives, and its examination has
convinced several experts of the reality of the curious script read from
it. Conviction, however, on such a point is apt to share the dim
character of gloaming phenomena—phenomena on the border between the seen
and the not seen.
But there is more. With refined apparatus the same observers succeeded,
in 1894 and subsequently,[432] in separately photographing three of
these remarkable spectra, and they were now perceived to be rich
throughout in bright and dark lines, “with the special character
strongly marked of bright bands associated with corresponding dark
absorption lines.” Most singular of all, the relative positions of these
bright and dark lines were found subject to change. Hydrogen radiations,
for instance, which in 1894 lay on the blue sides of the absorption
stripes, lay in 1897 on their redward margins. This might be explained
on the hypothesis of orbital movement by supposing each star of the
trapezium composed of a dark-line and a bright-line member, the spectra
of which are periodically shifted through the alternations of their
velocities in the line of sight. It remains to be seen, however, whether
or not the shiftings are periodical; for by this one condition the
explanation stands or falls.
So far as their absorption-elements are concerned, the stars of the
trapezium belong to the earliest variety of the Orion type. All the
lines are wide and diffuse, and the strongest are members of the
Pickering series of hydrogen. Rydberg’s series—if we may call it so
under reserve—is represented by the prominent reversal of its solitary
ray at λ 4689. Mr. McClean recognised oxygen absorption in these stars;
Sir William and Lady Huggins identified nitrogen, silicium, and
titanium, and the calcium K shows both bright and dark. Few sidereal
objects combine so many points of interest as the multiple star at the
heart of the great nebula. The origin and meaning of the throngs of
delicate rays, here just tantalising vision, pressingly invite research;
nor less the manner of relative displacement exhibited by the bright and
dark coupled lines. Do they betray a circulatory period? And if so, is
it the same for each member of the group? Or do they rather form
independent systems, in subordination to a higher scheme, completing
itself in the long leisure of many millenniums? Other problems suggest
themselves in immediate connection with these stars; nor is it
impossible that they may be proposed over again, perhaps in a modified
form, by the multiple stellar nucleus of the Trifid Nebula in
Sagittarius. But instruments of no insignificant light-power will be
needed for the satisfactory examination of its spectrum.
In one other star besides θ Orionis, the shifting of bright hydrogen
lines occurs irrespectively, to all appearance, of binary revolution.
Spectroscopic duplicity was at first naturally attributed to 11
Monocerotis when its peculiar character disclosed itself on the Harvard
plates. Thus in the years 1888–90 the dark F of hydrogen had an
illuminated border lying redward; it was on the blue side in
1891–92.[433] We are not informed whether it has since changed its
position; but any attempt to impose a period upon alterations so
spasmodical would evidently be hopeless. Like θ Orionis, 11 Monocerotis
( = Σ 919) is compound. It consists of three stars of about fifth and
sixth magnitudes, which have maintained a strict relative immobility
since Herschel divided them in 1781. Their spectra, photographed as one
by Pickering, were separately examined by Campbell in 1894.[434] He
found two of them to include the brilliant red ray of hydrogen, while it
was absent from the third. Presumably, then, only two of the trio are
bright-line stars, and it may be that in these two, significant
differences in the mode of emission will be brought to light by detailed
and systematic investigation.
The swing of the bright lines observed in θ Orionis and 11 Monocerotis
is extremely uncommon. In general, a fixed arrangement prevails, and it
is of two alternative varieties. Either the bright lines centrally
divide broader dark bands, as in γ Cassiopeiæ, or the bright and dark
lines are bracketed in pairs, the bright below, the dark above, as in P
Cygni.
Father Secchi’s notice of γ Cassiopeiæ as a gaseous star goes back to
1866. He noticed the vividness in its spectrum of C, F, and D_{3}, but
the helium line has not since held its own with the others. It is
subject to prolonged extinctions; nor is it certain that even the
hydrogen rays always keep up the same standard of brightness. The
variability of the spectrum will, however, be discussed later in
connection with other similar instances; here we have to do with its
fundamental characteristics. The hydrogen lines in γ Cassiopeiæ are
doubly reversed.[435] Wide absorption bands are divided by narrower
emission bands, and these again by hair-lines of darkness. Their
structure is analogous to that of H and K in the solar spectrum. The
radiations fall off in intensity—as Campbell’s rule prescribes—with
diminishing wave-length, while the absorptions gain in the same
proportion. F is “superlatively bright”;[436] Hε is neutral; no bright
lines have been photographed in the ultra-violet. The helium lines are
dark, with occasional exceptions; but the green and blue magnesium lines
shine by direct emission, and Father Sidgreaves recognises as a probable
vanadium line a strong dark-blue ray (λ 4586), which seems to fluctuate
in brightness. Another remarkable circumstance relating to this star is
the recent effacement from its spectrum of the signs of sodium
absorption formerly visible in it. They have, at any rate, escaped
notice since Von Konkoly’s record of 15th September 1884.[437] But the
immense vogue and value of spectrography have tended to reduce to a
minimum the attention bestowed upon the lower spectral sections, and
thus unduly to incline the balance of observation. The study of γ
Cassiopeiæ might alone furnish a not inadequate task for a well-equipped
observer. Only individual enthusiasm is likely to deal successfully with
the baffling problems it presents. Spectral variability is, in its case,
accentuated by perfect photometric constancy. The star is steadily of
2·3 magnitude. It is purely white in colour, lies immersed in the Milky
Way, and has no measurable parallax. Its real size and splendour are
then inestimably great.
The spectrum of P Cygni is not known to vary, although the star itself
was reckoned a “Nova” on its discovery by Janson in 1600, and by its
capricious emergences earned from Huygens, half a century later, the
title of the “_revenante_ of the Swan.”[438] Finally, it settled down
to fifth-magnitude brightness, which it seems disposed indefinitely to
retain. Its spectrum shows an approximately complete set of bright and
dark hydrogen and helium rays; but in their arrangement into couples
_juxtaposition_ replaces _superposition_—that is to say, the bright
lines are in their normal places,[439] while the corresponding dark
ones are shifted upward, as if by rapid motion, towards the eye. But
there can be no real question of motion, since the relation persists
without change year after year. Nor can it be explained on the
pressure-principle of altered refrangibility. The action, if exerted
at all, would be of the opposite kind to that observed. The
displacements in the spectrum of P Cygni are towards the blue; if due
to pressure, they should be towards the red. The phenomenon of the
relative displacement of bright and dark lines in the same spectrum is
one of the most interesting in stellar physics, and has received, up
to the present, no adequate explanation.
The absorption lines in P Cygni are much sharper and narrower than in γ
Cassiopeiæ. Those of calcium, magnesium, and sodium are at once
apparent, and Bélopolsky ascribes many of the remainder to nitrogen.
The spectrum of the great southern variable, η Carinæ, resembles that of
P Cygni by its inclusion of many bright lines shadowed by dark ones on
their blue sides. It has been photographically studied by Sir David
Gill, Mr. McClean, and Miss A. J. Cannon.[440]
Among stars nearly related to γ Cassiopeiæ may be mentioned φ Persei
(4·2 magnitude), υ Cygni (4·4 magnitude), α Columbæ (2·7 magnitude), δ
and μ Centauri (2·8 and 3·4 magnitude). Bright F was detected by Mr.
Espin[441] in the spectrum of the star in Perseus, and was found by
Campbell[442] to be accompanied by a much brighter C. The total absence
of K is surprising, but may not be permanent if the spectral variability
suspected at Potsdam[443] be substantiated. In υ Cygni there appear to
be double reversals of helium as well as of hydrogen.[444] Of α Columbæ
it is only known that F is a broad dark line bisected by a narrow bright
one. The spectra of the two stars in Centaur are thought to be almost
identical.[445] Hydrogen emissions in them are strong and numerous, but
none others have been recognised. The helium lines are all dark;
metallic lines are inconspicuous.
Bright hydrogen and helium lines seem like relics of past
conflagrations. In a few cases we know them to be such, and it is
possible that in all they have the same implications. For any of these
stars may have undergone prehistoric vicissitudes of lustre, after which
they would have settled down into stability; although the recurrence of
such incidents in the future can alone afford secure grounds for
inferring that they diversified stellar biographies in earlier times.
Bright-line helium stars are for the most part situated in the Milky
Way. They are subject to the influences exercised by that strange
aggregation.
CHAPTER X.
WOLF-RAYET STARS.
A very remarkable star was described by Professor Pickering in
1896.[446] Through the measurement of its absorption lines a companion
hydrogen series to that already known was, for the first time,
recognised; while both the Wolf-Rayet bands in the azure showed in it by
direct emission. Only the presence of a dark K obliges us to separate ζ
Puppis from Wolf-Rayet stars proper, and to consider it as a linking
instance between them and helium stars of the earliest variety. Just
this trace of calcium-absorption differentiates the hybrid spectrum of
the star in the Poop from spectra of Class viii., which include no
legible metallic impressions. Their absence is of especial importance as
extending to nebulæ. Nebular chemistry is entirely non-metallic.
The Pickering series in Wolf-Rayet stars is not infrequently bright in
its lower members, submerged by absorption higher up. The bands in the
blue, on the other hand, are always bright. They form a multiple group,
the mutual relations of which await more complete disentanglement. The
lowest member is, or may be, the “fundamental” of Rydberg’s hydrogen
series at λ 469. Yet the circumstance that it is not really solitary
tends to discountenance this identification.[447] The “lazulite” ray at
λ 465 is never entirely effaced, and Campbell has provided it with
several associates of still shorter wave-lengths.[448] Some of these
fall suspiciously near nitrogen lines. The possibility is not then
excluded that all these enigmatical blue effluences may be the joint
products of glowing hydrogen and nitrogen, although this view, like
every other that can be proposed, is beset by serious difficulties.
Among the less refrangible of the Wolf-Rayet bright lines there are two
(at about λ 569 and 559) which appear to coincide with rays photographed
by Professor Hale in carbon stars. Neither has been chemically
interpreted. One of them, the greenish or “citron” line (λ 569), seems
in some way correlated with the blue band at λ 465. A more definite
connection can be traced between the latter and the yellow line at λ
581. They are emphasised in the same stars, while the brilliancy of the
Rydberg beam at λ 469 is accompanied by a special vivifying of the green
Pickering line at λ 541. This rule is quasi-universal; it may stand, at
least temporarily, as a useful principle of order amid half-intelligible
phenomena.
Hydrogen is most diversely imprinted on the Wolf-Rayet spectra. Its
lines, according to Professor Campbell, “have nearly every known
character. In many of the stars they are dark. Again, they are dark with
bright borders, and suggest strongly that they are doubly reversed. The
bright hydrogen lines vary from faint to very bright, from monochromatic
lines to very broad bands, and from those clearly single to those
apparently multiple.” This splitting-up of widened lines is evidently
analogous to the tripling of the brilliant hydrogen rays observed in
Mira, and once more recalls the possibility that certain peculiar
stellar spectra may be produced in powerful magnetic fields. Helium is
not very prominent in spectra of this type. It frequently shows by
emission in D_{3}, and occasionally, even in the same star, by
absorption at λ 4472; but its display is subordinate to that of other
known and unknown elements.
The grand exemplar of the Wolf-Rayet class is γ Argûs (_alias_ γ
Velorum), a star of 2·4 magnitude, giving a resplendent spectrum ablaze
with yellow and blue lines. It was first effectively studied by
Professor Campbell in 1893–94,[449] although at the Lick Observatory the
star barely attains an altitude of six degrees, and can be observed for
only a few minutes on any one night. The main facts that struck him were
the brilliancy of C, the transitional character of F, and the
unmitigated darkness of all the upper hydrogen lines. Similarly, the
initial term of the Pickering series at λ 541 shows by emission, the
rest by absorption, while a bright D_{3} contrasts with a number of more
refrangible dark associates. As usual, both blue bands are visible, but
the weight of radiation falls upon that of shorter wave-length, the
Rydberg line being comparatively inconspicuous. Miss Cannon found
several of the hydrogen lines in both series to be dark with illuminated
borders, an arrangement, as she remarked, the inverse of that prevailing
in γ Cassiopeiæ and its allies.[450] Mr. McClean recognised oxygen
absorption in this star,[451] and the feature is not unlikely to prove,
on fuller inquiry, common to all the members of its class. The absence
of H and K is more than probable, and suggests comparisons and
reflections.
A 7·5 magnitude star in Cygnus (D.M. + 43° 3571) shows, like γ Argûs, a
mixed succession of hydrogen lines, but modified, perhaps, by double
reversals.[452] With the Lick thirty-six inch Professor Keeler perceived
its spectrum as “an extremely complicated range of absorption bands and
faint bright lines,”[453] the unusual width of which struck both him and
Professor Campbell. Thus the azure bands actually overlap, forming a
single indistinct glow one hundred tenth-metres broad. A spectrograph of
this object, taken by Mr. Ellerman with the forty-inch Yerkes refractor,
is described[454] as totally unlike any spectrum of the fourth type.
Whether the dissimilarity is of a nature to be generalised so as to
exclude all idea of kinship between these stellar families is more than
we can tell at present.
A star of about the same brightness, distinguished as
“Argelander-Oeltzen, 17,681,” was swept up in Sagittarius by Pickering
in 1881. In its spectrum the golden ray at λ 581 and the lazulite beam
at λ 465 predominate almost exclusively. Vogel could see no others with
the great Vienna refractor in 1883;[455] nevertheless, Campbell
succeeded, ten years later, in measuring twenty bright lines in this
wonderful spectrum.[456] Only one among them, and that of secondary
importance, can be attributed to helium; but many due to that substance
may be included in the unexplored absorption spectrum of “A.O. 17,681.”
A star of 6·4 magnitude in Canis Major, catalogued as “Lalande 13,412,”
shows, instead of the unknown blue and yellow rays at λ 465 and λ 581,
the “new” hydrogen lines at λ 469 and λ 541.[457] The spectrum includes,
besides, a more refrangible blue band, centred about λ 461, but diffuse
and divided. These multiple azure effulgences in the Wolf-Rayet stars
offer a problem of singular interest. They possess none of the structure
of genuine flutings; they seem apt to spread unsymmetrically. Is this an
effect of pressure on the emitting vapour? Or does it arise from some
property inherent in it, or some mode of action exerted upon it? The
answers may be long postponed, but cannot fail to prove interesting.
A unique specimen of this class was photographically detected in Cygnus
in the course of the “Draper Memorial” surveys.[458] It is extremely
faint—below the ninth magnitude—and was enrolled in the Bonn
Durchmusterung as D.M. + 30° 3639. Nevertheless, its spectrum offers
more than common facilities for exact observation, owing to the
sharpness of its component rays. Thirty were measured by Campbell in
1893,[459] and they include, with many common to the type, several that
appear to be individual to the star. The two brightest lines, however,
are F and λ 569, the “citron” line strong in γ Argûs. The Pickering and
Rydberg series are faint, while the alternative blue band at λ 465 glows
intensely. But the distinctive feature of the star is that it is
_spectroscopically nebulous_. Observed on the F-line like a solar
prominence, Professor Campbell found it to present a very appreciable
disc,[460] which, on narrowing the slit, became reduced to a line, as
shown in Figure 20. The length of this line is about 5″ of arc, and it
measures the apparent diameter of the incandescent envelope of hydrogen
which surrounds the body of the star. Only the hydrogen lines behave
thus exceptionally; all the other spectral rays show as mere bright
points upon the continuous background, which they do not transcend by a
hair’s-breadth. That is to say, hydrogen is the sole glowing constituent
of the enormous appendage revealed by the powerful appliances available
at the Lick Observatory. It has been seen nowhere else, but Runge[461]
and Keeler[462] separately verified its existence.
[Illustration:
FIG. 20.—Hydrogen-Envelope of the Wolf-Rayet Star, D.M. + 30° 3639
(Campbell).
]
There are a few circumstances worth noting in connection with this
extraordinary appearance. First, the vast spread of incandescent
hydrogen round the star has no effect in thickening the representative
lines of that substance in the star’s spectrum. They are, on the
contrary, particularly fine and narrow. The envelope, we can hence
infer, is not an atmosphere; there is no appreciable downward pressure
of its strata. Again, it must be—in the ordinary sense—hotter than the
photosphere it surrounds; for the bright lines emanating from it are not
reversed where they cross the prismatic thread due to the nucleus, as
they should be if the nucleus were at a higher temperature than its
envelope. Finally, it may be possible, by researches into the parallax
and proper motion of this star, to form some estimate of its actual
distance, and consequently of the real extent of its gaseous
surroundings. Thus might be opened a novel line of inquiry destined to
lead future students of the skies far afield.
The chemistry of the Wolf-Rayet stars—judging from partial
interpretations of the disclosing script—is of the simplest. They have
as unfailing constituents hydrogen and helium; oxygen is at least
occasionally present, and the detection of nitrogen may be expected with
some confidence. Sir William and Lady Huggins showed in 1890[463] the
inadmissibility of a carbon origin for any of the blue bands, and
defined their positions by exact measurements. Professor Campbell’s
observations, visual and photographic, of thirty-two members of the
class contributed materially to promote acquaintance with their
peculiarities; yet they stimulated, rather than satisfied curiosity.
The distribution of these objects is strongly selective. They are
virtually confined to the Milky Way. The rule is emphasised by its
apparent exceptions, for the single specimen of the type deviating
considerably in position from the galactic plane has proved, on closer
inquiry, to be situated in a galactic offset; while the twenty-two
grouped in the Magellanic Clouds belong to aggregations of the galactic
order, and subject presumably to galactic conditions. The Milky Way,
then, and the Nubeculæ afford analogous and exclusive facilities for the
development of such bodies. They seem, moreover, to be provided more
freely in some regions than in others, since the objects in question
tend to collect into knots or groups, the finding of one Wolf-Rayet star
being generally the prelude to additional detections in the same
neighbourhood. Yet they rarely or never form binary combinations. They
are loosely associated without any suggestion of mutual circulation. Nor
has any of them, so far, given signs of spectroscopic duplicity. They
are singularly steady light-givers. No Wolf-Rayet star is under the
slightest suspicion of variability. Atmospheric incandescence on the
largest scale is compatible in them with the perfectly uniform working
of arrangements for the transference of heated matter from the interior
to the surface of the radiating bodies. Considering the frequent and
extreme instability of many bright-line stars of other varieties, this
fact can hardly be too strenuously insisted upon.
CHAPTER XI.
THE GENERAL QUESTION OF BRIGHT LINES IN STELLAR SPECTRA.
The more closely we study the phenomena of bright lines in stellar
spectra, the more fully convinced we become that no single or simple
principle avails to explain them. They are evidently produced under
varied circumstances, at different elevations above the stellar
photospheres, and in manifold forms of connection with the adjacent
absorbent layers. Reviewing rapidly the chief types of emissive spectra,
we can, to some extent, gather their implications.
The _sine quâ non_ for the display of bright lines is the presence of a
stratum in the star’s atmosphere outshining the photosphere. The
difficulty is indeed very great of attributing this superiority in
brightness to a superiority in temperature; but it can be evaded by the
use of Wiedemann’s convenient term “luminescence,” signifying a state of
glow unconditioned in the strict sense by heat. The rationale of
“luminescence” is still uncertain, but the introduction of the new idea
it represents marks an important departure from the old groove of
thought. Now we can trace two modes of emission in the sun, faintly
indicated, it is true, but instructive as being within reach of
comparatively immediate study. In the first place, some of the
Fraunhofer lines seem to be relieved against vague effusions of
light,[464] originating, almost certainly, beneath the reversing layer,
among the interstices of the photospheric clouds. In the second place,
the violet calcium lines, and occasionally one or two lines of hydrogen,
are doubly reversed in the chromosphere. Both kinds of effect are
reproduced in stellar spectra.
Carbon stars show bright lines, which may be described as chromospheric;
the gases emitting them surmount the vapours generating the noted dark
bands. Moreover, these rays appear to be simple and uncompounded of
bright and dark elements; they are not perceptibly affected by reversals
or absorptions. The same may be said of the bright lines in
Mira-variables. Yet the locality of their origin is widely different.
They are essentially photospheric and deep-seated, shining from beneath
the dusky flutings they diversify. The leading characteristic of
bright-line helium stars is the duplication of their spectra. The vivid
rays have dark companions. And this, not through the optical conjunction
of two distinct bodies, but as the result of physical conditions
prevailing in a single globe. In such stars, then, there are complex
stratifications of emitting and reversing vapours diversely affected, we
cannot tell how, by heat, pressure, magnetism, or electricity. The
gradual penetration into the secrets of nature that must accompany their
study offers an enticing, and a far from hopeless prospect to the rising
generation of astrophysicists. But it will involve an indefinite
expenditure of time and labour. The conditions of bright-line production
in Wolf-Rayet stars are extremely hard to define. They probably vary
greatly in individual specimens. The state of some probably resembles
that of helium stars showing analogous symptoms of gaseous
incandescence. The corresponding reversals, however, are less emphatic,
and have indeed been rather suspected than perceived. Other members of
the Wolf-Rayet class (for Campbell’s star presumably has fellows)
possess vast gaseous envelopes, uniformly glowing, and scarcely
arresting light.
As to the theory of bright lines in stellar spectra, it is only certain
that they testify to a real excess of incandescence in certain layers of
the stellar atmospheres. They are not optically created by the
concentration, through distance, of far-reaching, cool, gaseous
appurtenances. This is proved by the example of the sun, by the study of
Campbell’s star in Cygnus, in which not a cool, but a strongly glowing
appurtenance is actually visible, and by the phenomena of spectral
variability, totally inexplicable on the view that mere extent of
gaseous surroundings is competent to produce bright lines. It is much
easier, however, to deny than to affirm—to perceive incongruities with
fact than to trace the lines of a true hypothesis. This cannot be done
off-hand; much preliminary toil must be undertaken. Of prime necessity
is the continuous study of the sun’s facular rays, of their originating
conditions, their displacements, their periodical changes. Laboratory
inquiries will proceed simultaneously—inquiries into the nature of
“luminescence,” into the temperature of radiating gases, into the
spectral effects of varied modes of electrical illumination, all which
topics may demand subterfuges of treatment not yet easily imaginable.
But need will stimulate invention, and knowledge will advance along the
arduous ways by which alone future progress is possible.
CHAPTER XII.
ANOMALOUS AND VARIABLE SPECTRA.
A small proportion of stellar spectra show marked individualities; they
differ notably from the recognised types, and may help, by their very
diversity, to elucidate them. Some belong to stars variable in light;
most are probably inconstant in their peculiarities; none have as yet
received the persistent attention that they deserve.
The “comet variable,” R Geminorum, was last observed by Vogel in
1874,[465] and then not to the best advantage. A maximum, at which the
star rose somewhat above the seventh magnitude, occurred 7th April, and
during a month previously dark and bright bands were distinguishable in
its spectrum. Repeated measurements proved the latter to coincide in
position with the dusky colonnade in fourth-type stars. They were due,
quite unmistakably, to the direct radiations of carbon. The variable, in
other words, glowed with a light fundamentally the same in quality as
that of a comet approaching perihelion. No hydrogen or helium lines seem
to have been present; but the strong yellow ray located at λ 581 must be
identified with the Wolf-Rayet line,[466] specially associated, as we
have seen, with the lazulite band at λ 465. It will be interesting to
learn whether the same connection subsists in the variable; but some
years must elapse before any kind of new information regarding it can be
obtained. Just at present the star, in its bright phases, is too near
the sun for purposes of useful research, and its period of 370 days
differs so little from a year that the conjunction passes off with
extreme slowness. The minima of this curious variable carry it to the
very verge of extinction, below the range of ordinary telescopes. The
attempt has not yet been made to keep it in view with those of large
aperture.
A star capriciously variable both in the kind and in the quantity of its
light was discovered by Pigott in 1795. Usually almost constant at the
sixth magnitude, R Coronæ is liable, at intervals counted by years, to
lapses into obscurity,[467] varied by spasmodic efforts towards
recovery, each crisis of instability lasting many months. The spectral
fluctuations of the star seem unrelated to its changes of brightness.
They were first noticed by Mr. Espin in 1890, but so far have not been
very clearly defined. Migrations from type to type are somewhat vaguely
indicated; their reality would involve so much that is novel and
surprising that it can only be admitted on irrefragable evidence. On
14th September 1890[468] the spectrum appeared of Type iv.; it was
interrupted by obscure bands held to be those of carbon absorption. In
April 1893[469] bright lines and nondescript dark bands alternately
stood out to view and became effaced; and on 4th May 1899[470] a
spectrum resembling that of the sun had established itself. Mr. Espin is
convinced that a double light-source is in question, and that the
observed changes are explicable by the conjunctions and elongations of
revolving stars, giving contrasted spectra; but unless they can be
brought into conformity with some time-regimen, such an hypothesis is
evidently inapplicable.
The spectrum of a star in the Shield of Sobieski is probably analogous
to that of R Coronæ, but the suspected variation is not from the fourth,
but from the third to the second type. In August 1890 Mr. Espin derived
from R Scuti a set of faint bands, modelled to all appearance on those
of Mira or α Herculis;[471] a glimmer of bright lines in the blue and
violet, however, betrayed unusual characteristics, completely drowned,
like the concomitant absorption, in the flood of continuous light
accompanying and occasioning the maximum in October. Less than two years
later, 25th June 1892, Dr. Krüger found the spectrum definitely solar,
the _b_-group of magnesium being particularly distinct.[472] R Scuti
never sinks below the ninth magnitude. It is then at all times fairly
accessible to exact investigations, which should serve, without much
difficulty, to fix the true nature of this still enigmatical object. The
mode of spectral variability which it exemplifies is not generally
accounted as possible by those who undertake to discuss the intricate
subject of stellar development; this could hardly, indeed, be dealt with
on the old lines if the suspected phenomenon proved to be an
unquestionable fact.
Spectral and luminous variability are, in certain stars, inseparably
connected. With increasing light the hydrogen lines brighten; with
ebbing light they fade. A common cause evidently governs the two kinds
of change. This close correspondence is observed only in periodical
objects of the Mira-class. Variables of a less regular type, such as R
Coronæ, fluctuate spectrally in apparent disregard of their
light-vicissitudes; while yet a third description of spectroscopic
variables are exempt from the least suspicion of instability in
brightness. A conspicuous example of the last kind is γ Cassiopeiæ.
Normally, the red hydrogen line is the brightest in its spectrum; yet it
seemed extinct when looked for by Dr. Vogel, 18th June 1872.[473] To the
Dunecht observers, however, it showed “superbly” bright 20th December
1879,[474] then lapsed into invisibility, until rekindled under the eyes
of Von Gothard at Herény, 13th August 1883.[475] Since then it has not
been missed, but has rarely been sought; for it can scarcely be called
accessible to spectrographic observation, and direct inspection of
stellar spectra has unfortunately fallen out of vogue. On 13th September
1885 Dr. Copeland found C very bright in γ Cassiopeiæ, F just
measurable;[476] on fifty-two plates taken at Stonyhurst during the
years 1891–99[477] F was “superlatively bright,” C being of course below
their range. No evidence was elicited from them of alteration in the
hydrogen spectrum, but they gave strong grounds for suspicion of change
in lines due to other substances, notably in a strong ray at λ 4586,
doubtfully associated with vanadium. Suspicion might have been raised to
certainty if the yellow helium line had fallen within the scope of
inquiry, for the variability of D_{3} in this star may safely be taken
as proved. Although observed as bright by Secchi in 1867,[478] measured
by Von Gothard 20th August 1883,[479] and just recognised at the O
Gyalla Observatory in 1891, and at South Kensington late in 1889 and
1894,[480] the line is commonly invisible under the most favourable
conditions. Keeler could not find it with the great Lick refractor in
the summer of 1889;[481] three spectrograms, taken on orthochromatic
plates with the Pulkowa thirty-inch by Bélopolsky in 1892, bore no trace
of it;[482] its emergences, in brief, are so transient and uncertain as
to be altogether exceptional. They seem quite casual—that is to say,
their law of causation is to us inscrutable. The remaining chief lines
of helium in γ Cassiopeiæ are dark, with indications of fine bright
reversals at their centres.[483]
The metallic spectrum, too, is unmistakably variable. Von Konkoly noted
sodium absorption at D, 15th September 1884.[484] Yet Keeler’s
examination, five years later, showed the spectrum in that vicinity to
be absolutely continuous. No subsequent record of dark D has probably
been made. The magnesium group _b_, on the other hand, seen dark by
Keeler, appeared bright on the Stonyhurst photographs, and it was
accompanied by the blue emission from “high temperature” magnesium at λ
4482. No iron lines are brightened in this spectrum. A clue to its
intricacies may be found by the employment of combined visual and
photographic methods. Neither by itself is altogether satisfactory.
Changes in one part of the spectrum lose half their significance unless
correlated with changes in the remaining parts. The assurance of their
reality, besides, would be immensely strengthened by the demonstration
of their not being isolated. Sympathetic variations, independently
recorded, could hardly be the creation of instrumental or other
extraneous causes.
The chromospheric instability of γ Cassiopeiæ is shared by a good many
other stars. Among them is J Velorum (A.G.C. 14,145), spectrographically
registered at various dates in the course of the Harvard College
southern surveys of the heavens. Miss A. J. Cannon’s examination[485]
showed that on the plate taken 2nd June 1893 the green and blue hydrogen
lines (Hβ and Hγ) were bright, superposed upon broad absorption bands.
Nevertheless on 19th April 1895 and 19th March 1896 they were simply
dark, with no trace of vivid reversals. And so they remained in 1899.
Whether the red line, usually in such stars the most brilliant of the
series, responded to the chromospheric extinction attested by its more
refrangible associates, can never now be known; but something may, in
the future, be discovered about its behaviour should the spectrum of J
Velorum ever be rekindled. The star, which is of the fifth magnitude,
does not vary appreciably in lustre.
An inverse change to that observed in J Velorum was detected by similar
means in a seventh-magnitude star in the southern constellation
Chamæleon (A.G.C. 14,686). On 20th May 1892 it appeared to be of the
Sirian type; hydrogen showed only by absorption; but on 3rd April 1895
Hβ was bright, eighteen days later Hγ had followed suit, and fringes of
light seemed attached to the lower edges of two of the ultra-violet
hydrogen lines.[486] The progressive incandescence, betokened by the
creeping upward of the illumination, was a complete novelty; its
probable initial symptom in the blaze of C necessarily remained
unnoticed. It is earnestly to be hoped that this unique object may not
slip out of view amid so many conflicting claims for attention advanced
by the denizens of the southern heavens.
A star in Canis Major (A.G.C. 9181)[487] is distinguished by frequent
fluctuations from brilliancy to obscurity of the green and blue hydrogen
lines. Miss Cannon has traced their spectrographic history since 1892.
The more refrangible members of the series are present—it can be
inferred—as absorption lines, and C is explicitly stated by Campbell to
have been glowing in 1894.[488] Variations of its green companion (Hβ),
analogous to those just described, have recently been detected in the
spectra of η Centauri and κ^2 Apodis.[489]
The percentage of abnormal stars found on the Draper Memorial plates is
exceedingly small; yet they sum up to a not insignificant total. A few
may be named as worthy of sustained attention. A sixth-magnitude star in
Libra (A.G.C. 20,937) was announced in 1895[490] to show a spectrum
resembling that of the great Looped Nebula (30 Doradûs), which differs
in light-quality from other gaseous nebulæ; and a stellar apparition in
the Centaur, then visible, was added to the spectral group. “Nova
Centauri” promptly disappeared, but the star in Libra is permanently
present, and available year by year for prismatic comparisons. The
peculiarities of the following objects have not been expressly defined:—
1. S.B.D. −22° 1070; R.A. 5^h 14·5^m; Dec. −22° 19′. Mag. 8·7.
Photographed at Arequipa.[491]
2. Z.C. 17^h 734; R.A. 17^h 13·2^m; Dec. −66° 15′. Mag. 8·5.
Photographed at Arequipa.
3. S.D.M. −8° 1467; R.A. 6^h 28·1^m; Dec. −8° 48′. Mag. 8·5, but
slightly variable.[492]
4. S.D.M. −11° 1941; R.A. 7^h 22·4^m; Dec. −11° 31′. Mag. 8·9
variable. Banded spectrum of uncertain type.
5. S.D.M. −10° 5057; R.A. 19^h 7·7^m; Dec. −10° 54′. Mag. 7. Unique
spectrum of bands.[493]
Possibly the original records secured of these stars may never be
duplicated, since they are not unlikely to be spectrally variable as
well as peculiar. In any case their characteristics, which are precisely
of the kind to prove specially instructive, need to be more fully
ascertained.
The spectra of φ and ψ Persei—both bright-line helium stars—have been
suspected of fluctuations; but those of the first may depend upon
varying radial velocity,[494] while those of the second, although slight
in amount, are of a particularly interesting description. They consist
in the occasional shifting of the brilliant narrow F of hydrogen to one
or other side of the absorption-band upon which, in general, it is
centrally placed.[495] These displacements are probably of the same
nature as the alternate marginal illuminations of Hβ in θ Orionis and 11
Monocerotis; they are certainly unconnected with revolving movements in
a system of bodies. Many more instances might be given of stellar
spectra apparently abnormal, and at least ostensibly variable, but the
above may suffice as specimens.
CHAPTER XIII.
COLOUR VARIABILITY.
Colour variation in stars is a somewhat elusive phenomenon. It cannot be
measured; no “colorimeter” yet constructed has given satisfactory
results. Then it is subject to adventitious modifications depending upon
the state of the atmosphere, the fluctuating sensitiveness of the
retina, the nature and aperture of the telescope employed. The same
observer after a prolonged vigil will often receive quite different
chromatic impressions from those derived with unfatigued sight; nay, his
right and left eyes may sometimes pronounce incongruous judgments upon a
colour-harmony or a colour-contrast. Such counterfeit changes, however,
are slight and evanescent; with due care they can always be separated
from intrinsic variations. The endless individualities of colour-vision
have, indeed, also to be taken into account. There is no branch in which
personal equation tells so heavily, yet so intangibly. Hence casual
anomalies of description hardly raise a presumption of actual change.
Evidence that it has occurred can only be admitted with extreme caution.
The difficulty is to disengage what really deserves consideration from
the multitude of floating statements tending only to bewilderment.
Three kinds of colour-variation may be discriminated. They severally
affect periodical stars, red stars fairly constant in light, and tinted
star couples.
(1) Stars with a light-cycle of less than a hundred days are usually of
an unchanging yellowish hue; but “long-period” variables are
characteristically red, and redness in stars appears to be rarely a
fixed or stable property. It might be compared to an external covering
capable of alteration in opacity, or even of entire removal, and
connected in its effective action with complex, more or less unsettled
conditions. Very commonly, the rises and descents in magnitude of such
stars are associated with fadings and flushings of colour, a deeper tint
generally accompanying a low light-phase. For this there is a double
cause, in the diminution of brightness, and in the increase of
absorption. The first acts physiologically. A faint ray strikes the eye
as redder than one more brilliant, although both be of the same
refrangibility. The second works objectively. Absorption in stellar
atmospheres tells mainly on the blue end of the spectrum. Hence, as
darkening closes in upon the shorter wave-lengths, the stars redden more
and more. Mira, which is not properly a colour-variable, shows this
effect markedly. Certain objects of the same class, however, change more
radically, and less consistently. Their fluctuations in hue correspond
very partially to their fluctuations in light. Colour-change seems to
progress independently, and from a superficial point of view quite
capriciously.
At Sir Cuthbert Peek’s observatory near Lyme Regis, a score of variable
stars have been kept under watch since 1887. The data regarding them
collected by Mr. Grover are remarkable and suggestive in several
particulars, especially as regards the correlation of colour with
magnitude. An important example is afforded by S Herculis, a star
varying from the seventh to the thirteenth magnitude in about ten
months. It is strongly red with a fine fluted spectrum, yet has, at
sundry times, been seen completely blanched. We extract from the
_Rousdon Observations_ some notes of its colour, with the corresponding
dates and magnitudes.
_S Herculis_
┌──────────────┬──────────┬──────────────────────────────┐
│ Date. │Magnitude.│ Remarks. │
├──────────────┼──────────┼──────────────────────────────┤
│1886, Nov. 12 │ 9·4│White, sharp, and distinct. │
│ „ Nov. 29 │ 8·5│Decided red. │
│1887, May 16 │ 10·9│White; little, if any, colour.│
│ „ Dec. 20 │ 7·3│Deep ruddy. │
│1888, Oct. 15 │ 6·5│Fiery red. │
│1889, June 29 │ 9·0│Dull greyish. │
│ „ Sept. 30│ 8·3│Deep coppery red. │
│ „ Oct. 22 │ 9·1│Blood red, well defined. │
│1890, May 23 │ 9·1│Grey or ashen colour. │
│1891, May 12 │ 6·8│Brilliant scarlet. │
│1893, Aug. 17 │ 9·3│Dull white, well defined. │
│1894, Sept. 8 │ 7·5│Nearly white, sharp. │
└──────────────┴──────────┴──────────────────────────────┘
No trace of regularity is perceptible in these changes. The mean
magnitude of the star when white was 9·0, when at its reddest, 8·5. They
are then obviously unrelated to its light-phases.
An analogous object is T Ursæ Majoris, which alternates between deep red
and “creamy,” or pure white, though with a decided tendency to assume
paler tints as brightness increases. A typical pair of observations were
made on 5th and 14th February 1893. On the first evening T Ursæ was
estimated as of 10·1 magnitude, and of a “deep, dull, ruddy hue”; on the
second, it had risen to 9·3 magnitude, and become a “dull leaden
colour,” showing “no trace of orange or red—a very curious change,” and
one altogether unaccountable. Spectroscopic information, simultaneously
procured, would have been likely to prove instructive, but none,
unfortunately, is available.
A counter-example to T Ursæ is S Cephei, which, observed under the same
conditions, and undergoing similar variations of brightness, was
nevertheless recorded as at all times conspicuously red. On the other
hand, χ Cygni, a flagrantly red variable of the Mira type, occasionally
divests itself of colour as it brightens, although “scarlet” maxima are
more common than “white.” Espin’s observations confirm the striking
variability in hue of χ Cygni. On the whole, it cannot be doubted that
temporary whiteness is a frequent feature of this class of ruddy stars,
and the fact implies a great deal.
(2) In the second class of colour variables, light-change supervenes
incidentally or not at all. It includes two historic examples—Sirius and
Algol, both exceedingly unlikely, yet both attested on good authority to
have been red within the scientific memory of man. The Sirian question
has been exhaustively discussed by Dr. See[496] and by M.
Schiaparelli;[497] their arguments are of most curious interest, but we
can here only attempt to give what appears, on a fair view, to be their
upshot. Two facts are incontestable; Seneca compared the colours of Mars
and Sirius, and pronounced the star to be more intensely red than the
planet, and Ptolemy applied to it his current epithet for “glowing
ruddy” objects (ὑπόκιρρος), a piece of evidence vainly sought to be
explained away as a transcriber’s error. Many other ancient authors
imply, or are held to imply, what Seneca and Ptolemy definitely state;
but even apart from these confirmatory hints, the simplest and perhaps
the safest course appears to be to accept such definite statements.
Their improbability does not in itself warrant their rejection. It has
been suggested that the rapid scintillation of the Dog-star may have
lent to it a fictitious redness, but it does not do so now. “Sirius is
glancing blue-bright like a spirit,” Carlyle wrote from Templands in
April 1842. And certainly the atmospheric disguise of colour cannot have
been less effective in Dumfriesshire than at Rome or Alexandria. In the
_Iliad_, a fiery nature and aspect are ascribed to Sirius; but Homeric
indications are often loose or figurative. They, however, lend in this
case countenance to the plausible surmise that the redness of the star
was of antique standing. As to the date of its vanishing, nothing
positive can be asserted; but the negative testimony of Al-Sûfi places
it almost conclusively before the tenth century.
The same Persian astronomer supplies the only extant notice of Algol’s
early redness. Perhaps a merely temporary phase, it seems nevertheless
to have recurred after nine centuries. This was in 1841, when Schmidt at
Athens perceived the star as yellowish red,[498] although its subsequent
whiteness was patent to him as to all other observers. Was Schmidt
deluded? It is very difficult to determine. Only the star itself can
authenticate, by renewing, its evanescent glows of colour.
The pronounced redness of a seventh-magnitude star, No. 8 in
Schjellerup’s “Red” Catalogue,[499] was recorded by Oeltzen during his
revision of Argelander’s northern zones. Copeland, nevertheless, found
it white, 1st January 1876; Espin, yellow, with a continuous spectrum,
14th November 1887; while Krüger registered on 6th October 1891
well-developed bands of the third type corresponding to an orange tint.
Again, a ninth-magnitude star in Taurus[500] appeared to Hind “very
red,” 3rd September 1848, but “bluish” 14th November 1850. Lost sight of
for a quarter of a century, it was next observed by Copeland in January
and February 1876 as pale yellow, and by Doberck, three years later, as
reddish orange. Finally, on 10th January 1888, Espin saw it white, with
a seemingly continuous spectrum; since when no attention—that the
present writer is aware of—has been paid to it. A much brighter star in
Aquila[501] (seventh magnitude) showed red to Schjellerup in 1863, but
to Birmingham colourless in 1872 and 1874, and _blue_ 18th May and 20th
July 1873. These changes were in a manner verified by subsequent
spectroscopic observations; for the object, which had then recovered a
ruddy tinge, was classed by Espin as of the fluted type, 20th September
1889, but by Krüger, 25th June 1892, as a solar star with a pale yellow
cast. The colour-phases of an eighth-magnitude star, “63
Schjellerup,”[502] are attested by the best authorities; it is
impossible to doubt their reality. Picked out for its redness at
Copenhagen in 1863, the object, after numerous alternations, was
described by Franks in 1885 as white. No later observations appear to be
extant.
The following short list of the best-authenticated colour-variables may
be useful to observers:—
┌───────────────────┬──────────┬──────────────────────────────────────┐
│ Designation. │Magnitude.│ Remarks. │
├───────────────────┼──────────┼──────────────────────────────────────┤
│ 5 Schjellerup │ 7·0 │“Full garnet,” J. Herschel; red, │
│ = Krüger 75 │variable? │ Schjellerup, 1863; white, Dreyer, │
│ │ │ 1876. │
│ 8 Schjellerup │ 7·0 │Deep red about 1850; white, 1st │
│ = Krüger 102 │ │ January 1876. │
│ 63 Schjellerup │ 7·8 │Rubra, Schjellerup, 1863; blue, │
│ = Krüger 504 │ │ Birmingham frequently in 1873; │
│ │ │ decided red, Gould; colourless, │
│ │ │ Dreyer, 1880. │
│ 90 Schjellerup │ 7·7 │Rubra, Struve; bluish white, │
│ = Krüger 687 │ │ Birmingham, 1874; orange, Dreyer, │
│ │ │ 1879; white, Espin, 1888. │
│ 93 Schjellerup │ 9·0 │Blood red, Schjellerup, 1863; orange, │
│ = Krüger 698 │ │ Copeland, February 1876; colourless,│
│ │ │ Espin, 10th February 1888. │
│ γ Circini[503] │ 3·4–5·2 │Very red, Gould, about 1875; white, │
│ │ │ Stanley Williams, 1886. │
│64 _b_ Schjellerup │ 8·8 │Very red, Hind, 1848; bluish white, │
│ = Krüger 513 │ │ Hind, 1850; red, Dreyer, 1879; │
│ │ │ white, Espin, 1888. │
│ 148 Schjellerup │ 8·5–9·5 │Scarlet, Rosse, 1861; dark red, │
│ = Krüger 983 │ │ d’Arrest, 1866; colourless, │
│ │ │ Birmingham, 1874; red, intense │
│ │ │ bands, Dunér, 1878. │
│ 214 Schjellerup │ 7·0 │Red, Schjellerup, 1863; not red, │
│ = Krüger 1436 │ │ Birmingham, 1872, 1874; blue, │
│ │ │ Birmingham, 1873; orange, fluted │
│ │ │ spectrum, Espin, 1889; yellowish, │
│ │ │ solar spectrum, Krüger, 1892. │
│ _r_ Velorum │ 5·0 │Red, Gould, 1870·73; leaden white, │
│ │ │ 1888, A. M. Clerke; slight red │
│ │ │ tinge, Tebbutt, 1891. │
│222 _b_ Schjellerup│ 7·8 │Red, Lamont; yellow, Dreyer, 21st July│
│ = Krüger 1512 │ │ 1875; white, Dreyer, 18th August │
│ │ │ 1875; yellow, Espin, 1889; white, │
│ │ │ Krüger, 1891. │
└───────────────────┴──────────┴──────────────────────────────────────┘
Two stars[504] have been mentioned in an earlier chapter as anomalously
white, considering that their spectra are of the fourth type. The
possibility should not be overlooked that their paleness is only
temporary. They are perhaps colour-variables, and will, at some future
time, show the ruddy hue appropriate to the quality of their light.
(3) The colour changes of double stars are a peculiarly baffling subject
of inquiry. Many have been recorded that can safely be dismissed as
illusory; some that are unquestionably real. Yet in most cases there is
a large element of doubt. Personal idiosyncrasies come strongly into
play; meteorological influences, instrumental diversities, and all the
chances and changes of existence swell the reckoning of uncertainty. To
say nothing of the indeterminateness of language. Star tints are often
so delicate as to defy verbal definition. Distinctions between rose-pink
and amethyst, sea-green and apple-green, ashen, lilac, and grey, have
only a nominal meaning. These tender shades, moreover, while escaping
some eyes altogether, are enhanced by others into vivid contrasts; and
hence observers, expecting to see star-couples glowing like fruits of
the Hesperides, are apt to carry away the impression that the subtle
coloration actually presented to them implies a marked change. To
separate the kernel of fact from the husk of opinion or illusion is then
no easy matter. Yet an inadequate attempt to banish confusion is almost
always better than none, and may here be worth making.
The more closely the chromatics of double stars are studied, the more
clearly emerges an irreducible minimum of change. A satisfactory example
is afforded by one of the most carefully watched binaries in the
heavens. The primary in 70 Ophiuchi is of 4·5, the satellite of 6·5
magnitude, and it is unquestionably the satellite which conspicuously
varies in hue. Sir William Herschel in 1779 perceived in it a very
slight reddish suffusion, and J. Herschel and South described the pair
in 1824 as “white and livid.” Yet the elder Struve, an incomparable
authority, considered their “yellow and purple” tints remarkable enough
to warrant their inclusion in a restricted list of objects showing
_colores insignes_,[505] and they were still “topaz and violet” when
observed successively by Smyth and Webb.[506] “Gold and purple” again
they appeared in July 1883 to Perrotin at Nice, although less than a
month previously he had noted them “greenish yellow and reddish yellow,”
while a year later he recorded them as “golden and orange.”[507] This
vesture they continue to wear. They are ordinary yellowish stars with an
ordinary solar spectrum. Sooner or later, however, the companion may be
expected to put off its crocus-veil and shine Tyrian-hued.
The stars of γ Delphini are now finely contrasted in orange and green.
They appeared, nevertheless, white to the elder Herschel in 1779; white
and yellowish to Herschel and South in 1824; “reddish yellow and greyish
lilac” to Gore in 1874;[508] pale rose and light green to Dembowski in
1876–77; orange and green to Flammarion in 1877. Moreover, the companion
showed “light emerald” during the years 1831–39, but “flushed grey” in
1850. Doberck found it bluish in 1882, and the primary yellow;[509]
Vogel in 1883 recorded both stars as creamy white; while in
1895—according to Mr. Franks—the colours were “very pronounced, the
chief star being a strong yellow and the companion greenish.”[510] They
are of fourth and fifth magnitudes respectively, and a sky-gap of 11″
divides them. Their mutual revolutions have made little sensible
progress during a century and a quarter, but their common drift through
space certifies their systematic connection.
The case of 95 Herculis is somewhat perplexing.[511] This is an equal
pair of fifth-magnitude stars, rigidly fixed during the last twelve
decades at an apparent distance of 6″. Their “magnificent tints of
orange and green” excited Father Secchi’s admiration in 1855; and Piazzi
Smyth was accustomed to see them “apple green and cherry red” until 29th
July 1856, when he perceived with stupefaction, from his point of
vantage on the Peak of Teneriffe, that both were of the undistinguished
white attributed to them by Herschel in 1780. Fitful and partial
displays of their original chromatic brilliancy appear to be vouched for
by Dunér’s and Flammarion’s[512] observations of the stars as “bright
green and yellow,” and “gold and azure”; but their pale primrose is now
unrelieved by a shade of difference. There is no good reason to doubt
that, in the earlier part of the century, they were marked by vivid
complementary colours. Obvious to Webb, they were remarked by Admiral
Smyth as an unusual instance of diversity in tint “between components so
nearly equal in brightness.”
Instances are not infrequent of the small star in pairs of disparate
brightness varying in colour; but the relation is never inverted; no
primary is exclusively subject to change of tint. The satellite of δ
Herculis, a greenish star of the fourth magnitude, appeared to Struve
alternately grape red and ashen white; to Dembowski, blue; to Knott,
bluish green in 1850, ruddy purple in 1871; to Fletcher, in 1851, red;
to Flammarion, violet. The conjunction of these stars is thought to be
merely fortuitous. They are moving along divergent straight lines, and
hence seem destined to definitive separation. Yet colour-changes of the
kind affecting the satellite do not occur in isolated objects, and would
rather imply a physical connection with a dominating orb. It will then
be of particular interest to determine quite certainly whether δ
Herculis is a truly gravitational, or simply an optical couple.
The companion of δ Cygni shows analogous variations. “Ashen grey” to
Struve’s perception during the years 1826–33, it surprised him with a
strong red glow in 1836; three years later, Dawes found it blue; Secchi,
by turns red, blue, and violet in 1856–57; Dembowski, grey in 1862–63;
Engelmann, red in 1865. Of late its blue aspect has predominated; yet
Perrotin recorded it as yellow or orange with the great Nice refractor
both in 1883 and in 1886. These stars make a very much closer pair than
δ Herculis, and are in slow orbital movement.
Two at least of the four stars grouped in σ Orionis may be admitted to
fluctuate in hue.[513] One of 7·5 magnitude appeared ashen grey in 1837,
ruddy in 1851 and 1869, bluish in 1883. A more distant, somewhat
brighter component, usually dust-coloured, was marked “grape red” by
Smyth in 1832. Even the chief star is not of the perennial whiteness
that should match its helium spectrum. Webb found it yellow in 1851, and
Gould entered it as “red” in the Argentine Uranometry. It was divided by
Burnham in 1888 into an excessively close pair (fourth and sixth
magnitudes at 0·26″), which, already in 1891, gave indications of
circulatory movement.[514]
The following is an enumeration of some double stars reputed, on good
grounds, to be colour-variables:—
┌────────────┬───────────┬─────────┬──────────────────────────────────┐
│Designation.│Magnitudes.│Distance.│ Remarks. │
├────────────┼───────────┼─────────┼──────────────────────────────────┤
│70 Ophiuchi │ 4·5, 6·5 │ 1·6″ │Primary white or yellow, satellite│
│ = Σ 2272 │ │ │ alternately purple, rosy, and │
│ │ │ │ yellow. Spectrum, solar. │
│ γ Delphini │ 4, 5 │ 11″ │Primary cowslip to orange; │
│ = Σ 2727 │ │ │ companion emerald to blue, │
│ │ │ │ lilac, and topaz. Spectra, solar│
│ │ │ │ and Sirian. │
│95 Herculis │ 5·3, 5·3 │ 6″ │Contrasted green and red to │
│ = Σ 2264 │ │ │ uniform yellow. Spectra, solar │
│ │ │ │ and Sirian. │
│ δ Herculis │ 4·0, 8·5 │ 26″ │Companion by turns ashen, red, │
│ = Σ 3127 │ │ │ violet. Chief star gives a │
│ │ │ │ helium spectrum. │
│δ Cygni = Σ │ 3, 8 │ 1·5″ │Satellite grey to red, blue, or │
│ 2579 │ │ │ green. Slow binary. Large star │
│ │ │ │ gives a Sirian spectrum. │
│ σ Orionis │ 4·1, 7·5, │13″, 41″ │Chief star white to reddish; │
│ = Σ 762 │ 7·0 │ │ helium spectrum. Companions grey│
│ │ │ │ to ruddy. Fixed. │
│38 Geminorum│ 5·5, 8·0 │ 6·8″ │Companion varies in magnitude, 7·5│
│ = Σ 982 │ │ │ to 10; in colour, from bluish │
│ │ │ │ (1829) to red (1856, 1863), and │
│ │ │ │ azure (1872). │
│ γ Leporis │ 4·0, 6·5 │ 93″ │Companion pale green, 1832; │
│ │ │ │ garnet, 1851 and 1874 (Webb). │
│ │ │ │ Chief star gives a solar │
│ │ │ │ spectrum. │
│γ Serpentis │ 4·5, 9·0 │ 51″ │Small star lilac, 1832; “native │
│ │ │ │ copper,” 1851 (Webb). │
└────────────┴───────────┴─────────┴──────────────────────────────────┘
Colour-variability has hitherto been only observed, as it were, in
passing. And the casual study of a subject is seldom effectual. Here
much more is required if any progress is to be made towards discovering
the laws and cause of the phenomenon. What is essential to ascertain is
the nature of the spectroscopic response to colour-change. On this side
the problem can be attacked with some hope of getting nearer to a
solution. If visual alterations of hue can satisfactorily be brought to
the test of prismatic analysis, the way will be thrown open for an
important gain of knowledge; while it is hard to see by what other means
ignorance on the curious topic we have been discussing can be
dissipated. It is not, indeed, always easy to combine work in different
branches; yet the correlation of results is a vital need of astronomy,
and scarcely ever fails to prove especially and widely illuminative.
CHAPTER XIV.
THE SPECTRA OF DOUBLE STARS.
The spectra of double stars stand in the closest relations to their
colours. This, indeed, is almost a truism, since spectra merely show in
detail what is summarised in mixed tints. Yet the two forms of statement
are not tautological. The result of prismatic analysis cannot be wholly
anticipated from the visual impression. The eye makes no attempt to
reduce its sensations to their elements. Totally different rays may be
blended and balanced so as to produce an identical sum-total to the
perception of the optic nerve. Nor would it be in all cases easy to
pronounce upon the colour of a star from the simple inspection of its
spectrum. One cannot tell beforehand, so to speak, how the eye will take
things. Some scarcely measurable reinforcement of selective stoppage, a
few rays of absorption added or removed, may make the difference between
rosy and golden, or purple and pink. Thus neither direct nor prismatic
observations are superfluous; but the latter, as affording
scientifically accurate and—through photographic means—permanent
records, are by far the more important.
The spectra of double stars unlike in colour are usually of different
types; and here a remarkable rule applies. Contrasted pairs are, with
few and doubtful exceptions, notably unequal in brightness, and the
warmer tint invariably belongs to the larger component. Blue, green, or
violet stars are always the satellites of red or yellow primaries; and,
in accordance with these indications, they give first-type spectra,
while their brighter and more ruddy neighbours show Antarian flutings or
solar lines. We are thus led to the unexpected conclusion that, of two
globes simultaneously contracting, the larger, which should naturally
cool more gradually, and therefore run through its evolutionary stages
at a more leisurely pace, attains solar standing while its companion
still remains a “white star.” This relation is the very crux of cosmic
growth-theories; something more will be said about it in the next
chapter.
The separate spectral examination of coupled stars is far from easy, and
has indeed rarely been attempted. Only by Sir William and Lady Huggins
has the subject been prosecuted systematically and with success. Their
application to it of photography, rendered possible by the completion,
in 1897, of an ingeniously devised reflecting slit, constituted in
itself an immense advance. Previously, only the superposed spectra of
double stars had been chemically recorded, and these, for discriminatory
purposes, were of no more than provisional use. One coloured pair,
however, presents less difficulty in this respect than the rest. The
components of β Cygni lie far enough apart to give distinct
spectrographic images, formed by an objective prism, on the Draper
Memorial plates.[515] Specially inviting as well to direct scrutiny,
they were among the earliest objects subjected to Sir William Huggins’s
light-analysis.
The pair consists of a third-magnitude “topaz” star and a
fifth-magnitude “sapphire” at 34″. The unaltered value of this interval
since Bradley’s measurement of it in 1755 almost assures us that they
drift together through space under the stress of a physical bond. For
their proper motion, though very small, would have sufficed, in the
course of a century and a half, to produce unmistakable relative
displacement. Blue stars, besides, are never solitary; and the
companionship upon which their uncommon hue depends must evidently be
real, not simply optical. Plate IX. Fig. iv. shows the spectra of these
stars as photographed by Sir William and Lady Huggins.
Their complete diversity is apparent at a glance.[516] The hydrogen
series is writ large and strong on that of the minor luminary; helium
absorption is not apparent; the Sirian type is pronounced. Of solar
quality, no less decidedly, is the golden light of the primary. Yet it
cannot escape notice that the photographed spectra do not explain the
vivid colouring of β Cygni; they might have been taken (speaking
broadly) from any two stars of the types represented. This, indeed, was
just what should have been expected, since the special absorption
differentiating them from the common run of stars was known to lie
outside the range of sensitiveness of ordinary plates. As regards the
blue member of the pair, at any rate, there could be no doubt of the
fact. A set of dark bands, cutting out a goodly proportion of its yellow
and orange rays, were observed by Sir William Huggins in 1864,[517] and
again by Dr. Vogel in 1872,[518] and they correspond with, and fully
explain, its chromatic peculiarity. The topaz hue of the primary cannot
be so directly associated with the subtraction of particular qualities
of light; it is more probably due to an enhancement of that veiling of
the higher spectral reaches to which sunshine owes its primrose tinge.
Further investigation is, however, desirable; above all, the
photographic registration on isochromatic plates of the unfamiliar
absorption-bands from which the companion of β Cygni derives its
distinction as an azure star.
The theory of “composite stellar spectra” was proposed by Professor
Pickering in 1891.[519] Spectrographic impressions showing a mixture of
types should, he explained, result from the superposition of dissimilar
spectra derived from close or telescopically indivisible stars. The
forecast was verified by Miss Maury’s detection of eighteen
self-imprinted images of the compound sort.[520] “In spectra of this
class,” she writes, “the K-line appears either unduly narrow or
overspread with a peculiar haziness. This appears to be due to the
presence of an additional star, having a spectrum which belongs to some
group earlier in the series. It is also significant that in such spectra
the first-type characteristics predominate in the ultra-violet, the
second or third-type features in the green and blue.” These duplex
effects demonstrably own, in certain of the instances enumerated, a
duplex cause; for they include γ Andromedæ, ε Boötis, and α Piscium, all
three remarkable pairs. The presumption is accordingly strong that
spectra appearing hybrid in small-scale delineations really emanate from
a double source, although visual evidence of duplicity be wanting.
Indeed, several of Miss Maury’s _crypto-doubles_, ο Leonis, ο Andromedæ,
and α Equulei among the number, have been spectroscopically resolved by
Professor Campbell into unlike pairs. And even should the motion-test
fail, it need not be inferred that the star recalcitrant to it is
single; for a negative result may signify merely that the method is
inapplicable owing to the high inclination of the plane in which coupled
stars revolve.
One of the show-objects of the heavens is γ Andromedæ, composed of a
chrome-yellow star of 2·2 magnitude, and a sea-green fifth-magnitude
attendant at 11″. The attendant itself can be divided with a good
telescope into a blue and a green star, considerably advanced along an
elliptic track since their first observation by Otto Struve in 1842;
while the wide pair, discovered by Christian Mayer in 1777, remains
relatively fixed, although their systematic union is attested by an
identical secular progress of about 7″. Their spectra, photographed at
Tulse Hill, closely resemble those of the components of β Cygni, the
different patterns of absorption stamped on them forming almost as
striking a contrast in the negatives as the colours of the original
objects do in the sky. A similar combination is offered by ε Boötis, but
on a reduced scale. The ultramarine satellite is here poised at a
distance of only 3″ from its golden primary. Their spectra have, indeed,
been no more than inferentially distinguished. Miss Maury’s scrutiny of
the joint impression left by them upon the Harvard plates made it,
however, fairly certain that, as usual, the blue star is of Sirian, the
yellow star of solar quality; so that a relation of development is again
indicated just the converse of that held, on _à priori_ grounds, to be
probable.
The theoretical incongruity is, in some cases, heightened by the
substitution for the sun-like primary of a red star giving a fluted
spectrum. Such a pair is α Herculis. An “emerald” star of the sixth
magnitude, at a distance of 5″ from its glowing leader-orb, yielded to
Sir William Huggins’s early examination a spectrum of precisely the same
character as that of the satellite to β Cygni. Antares, too, is quite
similarly coupled with a green star, the spectrum of which, judging by
the duplex impressions obtained at Harvard College, resembles that of
Sirius, with, it may be, some approximation to that of Procyon.[521]
Mr. Burnham performed in 1881 the unprecedented feat of dividing a
third-type star into a very close pair. He detected a satellite of the
ninth magnitude situated within just one second of arc of η Geminorum, a
fine red star, variable in a period of 229 days, although its maxima are
unmarked by any signs of gaseous emission, doubtless because of the
comparatively slight extent of the light-change. The spectrum of the
small star cannot, of course, be directly observed, but its nature may
be indicated by colour-observations. Should a glint of blue or green be
caught under favourable circumstances, the inference that it proceeds
from a source of the Sirian quality can be unhesitatingly drawn. A
particular interest attaches to η Geminorum as the only Antarian star
with a companion likely to prove sensibly mobile within a reasonable
lapse of time.
The spectra of double stars that are unstable in colour have an
importance both evidential and explanatory. They illustrate and tend to
expound chromatic phenomena. The diversity in light-quality of 95
Herculis is then of extreme significance. These stars, as our readers
will remember, are now perfectly matched. They are of equal brightness,
and of the same yellow shade. But half a century ago they displayed
brilliant complementary radiance in red and green. And their spectra
correspond, not to their present uniformity, but to their historic
contrast. Vogel in 1899 recorded for one component—presumably the star
formerly green—a Sirian, for its twin a solar spectrum. Additional
weight is thus lent to the old observations; and a hint, not to be
neglected, is given as to the probability of future change.
It is less surprising to meet with spectral dissimilarity in the
components of γ Delphini. For they differ in magnitude, and very
markedly in colour, notwithstanding past intervals of agreement. And it
was just during one of their periods of agreement, in 1883, that Vogel
found the larger star to be of solar type, while its companion, now
green, but then colourless, gave a Sirian spectrum. Here again, as in 95
Herculis, spectral distinctions seem to persist while chromatic
distinctions are alternately effaced and restored.
A good many yellow stars have purplish attendants of dubious
spectroscopic standing. Their quality remains untried, and is difficult
to conjecture. One of the best examples is η Cassiopeiæ, a revolving
pair consisting of a 3·5 and a 7·5 star 5″ apart. The primary emits
golden light marked with the solar stamp of absorption; its satellite
has been variously described as violet, rosy, and garnet. These notes of
colour, indeed, supply no hint as to the nature of the corresponding
spectrum; but some indication that it is more “advanced” than that of
the large star may be gathered from the mass-relations of the pair.
Their gravitational disparity, as determined by Otto Struve, is 3·7 to
1, while their light disparity is 40 to 1. In other words, the satellite
is nearly seventeen times less luminous than it would be if of the same
mean density with its primary, and of equal areal lustre. In reality it
is probably both more compressed and less brilliant. But these
properties belong to a comparatively late stage of growth, and should be
associated with a strongly absorptive atmosphere. The precise type of
absorption characterising dependent stars of a violet hue it would be
rash to predict, but it is very desirable to ascertain.
A pair closely resembling η Cassiopeiæ is ξ Boötis. Again in this case a
yellow primary of solar type has a rose-purple attendant actively
circulating round it. With it may be classed a couple in Pisces (Σ
3036), coloured “very little yellow and dusky red,”[522] and probably β
Cephei, composed of a sulphur-tinted helium star of 3·4 magnitude,[523]
and an eighth-magnitude violet attendant at 14″.
The great majority of double stars present, however, the same or similar
tints; they are white and creamy, or sulphur-coloured and golden, and
the spectra derived from them accord entirely with these indications.
They are almost always variants of one type. But the rule observed in
contrasted pairs that the smaller is the earlier star is here inverted.
The subordinate members of systems undistinguished for colour often show
signs of having progressed further on the road towards extinction than
the larger orbs. This principle is finely illustrated by the grand
southern binary, α Centauri. Now these stars are almost exactly equal in
mass, yet one gives only a quarter of the other’s light. It is also more
deeply tinted with yellow; we may, indeed, safely infer that it is
dimmer mainly because of the additional absorption to which its colour
testifies. The spectra of the pair, splendidly delineated in Sir David
Gill’s photographs, are both of the solar class, but with a difference.
That of the brilliant component is an exact copy of the Fraunhofer
spectrum (see Plate X. Fig. 2), while that of the inferior star might be
called post-Arcturian, manifesting traces of affinity with the fluted
type of Betelgeux.[524] The spectral relations of α Centauri doubtless
prevail in many other systems, but they do not arise inevitably, even
under quite similar conditions. Thus the unequal stars of γ Leonis give
virtually identical spectra of the Arcturian or post-Arcturian
species;[525] and the equal stars of γ Virginis, though of Sirian type,
are unmarked by the smallest difference in the mode of absorption. It
would then appear that, while two globes cast, as it were, in the same
mould, like those united in α Centauri and γ Virginis, frequently
proceed _pari passu_ along the life-course of suns, one may outrun the
other under the influence of unknown circumstances. Couples unassorted
in size comport themselves differently; but here, too, allowance has to
be made for original diversities of constitution, or supervening
incidents of development.
To resume. The colours of double stars afford preliminary indications
as to the nature of their spectra, but cannot, in all cases, be
interpreted with much confidence. Blue and green stars are, as a
nearly invariable rule, the satellites of red or yellow primaries.
They belong to the Sirian type, modified, probably, by special
absorption serving to lend predominance to the more refrangible rays,
and so produce their unusual tints. “Purple” attendant stars have also
been observed; the quality of light, however, corresponding to this
designation remains unknown. It may prove to be stamped with strong
absorption, such as would be symptomatic of advanced condensation; and
if so, purplish or violet satellites are a radically distinct class of
bodies from azure stars; for they might be inferred to be
proportionately more massive and less luminous than their primaries,
while the inverse relation doubtless holds good in gold and green as
well as in topaz and turquoise combinations. Stellar pairs of equal
magnitudes are, with the rarest exceptions, alike in colour and
spectrum. They are primrose-tinted—scarcely ever pure white—and of
solar or Sirian type.
The spectra of couples no more than two seconds apart can be separately
photographed with the Tulse Hill apparatus; and indications of duplicity
are often obtained from the composite nature of the spectral impressions
given by apparently single stars. Only dissimilar components, however,
are capable of being thus distinguished; superposed spectra disclose
themselves as such by their differences, among which opposite
motion-displacements are occasionally met with. The discrimination of
mixed qualities of light in single spectrographic records is a branch of
research promising further developments.
CHAPTER XV.
THE EVOLUTION OF THE STARS.
The suns of space are subject to the _sic transit_ of mortality. The
time has been when they were not, and in the time to come they will
surely cease to be. The “incorruptibility of the heavens” is no longer a
postulate of science; it has been in a measure superseded by the still
more antique notion of the “perpetual flux” of things. Creation is a
process; it has a history; and the records of its history are not wholly
illegible to science.
Those inscribed in the heavens more particularly invite attempts at
decipherment. Inquiries into the physical constitution of the stars
inevitably lead to them, nay, insensibly merge into them. In the
celestial regions, more than elsewhere, we are impelled to read the past
and future between the lines of the present. There, by a wonderful
course of development, the designs of the Maker are being unfolded, but
with such majestic leisureliness that each step represents the lapse of
millions of years. To trace even its broad features is, then, a task to
be undertaken only with extreme diffidence; yet some few safe principles
are available, guided by which we hope not to wander far from the truth.
Long ago it became evident to observation that nebulæ were the matrices
of stars. Stars visibly nebulous are then in the earliest stage of
growth. So much, at any rate, may be assumed without sensible risk of
error. Again, radiating globes necessarily condense with the efflux of
time. As heat, the source of their expansive vigour, is dissipated,
their particles succumb to gravity, which suffers no waste. They
contract; the same quantity of matter occupies in them a continually
diminishing space, and acquires a proportionately more substantial
consistence.
The application of these tests gives a concordant result. Both point to
helium stars as being at the start of the cosmical procession. They have
often nebulous appurtenances; they congregate in nebulous regions; they
show with nebulæ spectral relationships of a subordinate, but
significant kind. Their mean density, moreover, is known to be extremely
small. The conditions of their eclipses, where they form occulting
couples, gives the means of assigning to it a fairly definite value; and
it appears to be about one-seventh that of the sun. This result is, of
course, only preliminary, and cannot legitimately be generalised. It
serves, however, to confirm what evidence of a different kind more
vaguely indicates.
Helium stars are, then, the most primitive class of suns; and the point
of outset being once established, the advance takes a prescribed and
inevitable line. We have seen that helium stars pass by the finest
gradations into Sirian, Sirian into solar stars, and these again into
stars giving fluted spectra. So far there is no breach of continuity.
Individual varieties must unquestionably arise, varieties due to minor
diversities of chemical constitution, to systemic conditions, to
physical influences exerted, possibly, in certain tracts of space; but
the great wave of change sweeps on independently of these ripples on its
surface.
The order of succession of the four chief stellar families leaves,
accordingly, little room for doubt. Our next inquiry relates to the
causes of their progressive transformation. We know of two which must be
operative—dissipation of heat and augmentation of gravity. The function
of a sun is to dispense energy; its distinctive organ, the photosphere,
is precisely an apparatus for discharging this function rapidly and
effectively; every year of a star’s radiation involves, then, a
corresponding subtraction from its not unlimited thermal store. Yet this
is not necessarily accompanied by a fall in temperature. Gaseous bodies,
on the contrary, grow hotter as they cool. This seeming paradox was
enunciated by Homer Lane of Washington in 1870. It is now a universally
admitted principle of science. What is signified by it is that the
contraction of masses in the gaseous state more than supplies their loss
of heat by radiation. It ceases to apply when liquefaction sets in, but
we are entirely unable to fix the stage of evolution at which this point
is reached. We are only certain that the youngest stars, being
unquestionably gaseous to the core, are rising in temperature; their
acme is still to come.
But average temperature is not the same thing as surface temperature.
The former, in two radiating globes, may be the same, while the latter
is very different. For it depends essentially upon the rapidity with
which heat can be conveyed outward and upward, and this again is
prescribed by interior conditions varying with mass, density, and
radiative facilities. Now Lane’s law has to do only with average
temperature, while spectral indications relate to purely superficial
heat-conditions. If we can learn something definite even as to these it
will be much; but at present the utmost uncertainty prevails as to how
the recorded facts should be interpreted. It seems indeed pretty clear,
from the frequent occurrence of “enhanced” lines in the spectra of white
stars, both of the Orion and the Sirian kinds, that the state of things
in their reversing strata approximates to that in the electric spark,
while vapours glowing in the arc represent better the layers absorbing
sunbeams and the rays of Antarian stars. But in regard to the essential
nature of the difference, authorities are not unanimous. According to
Sir Norman Lockyer, Dr. Scheiner, and others, temperature alone is
concerned; the spark is hotter than the arc. Intense molecular
excitement, due to the disruptive discharge, gives rise to altered modes
of vibration, betrayed by substitutions of new spectral lines for those
previously visible; and these substitutions, reiterated in the stars,
tell emphatically of their enormous temperatures. On the other hand, Sir
William and Lady Huggins relegate temperature to a position of secondary
importance, and count density the main factor in spectral change, their
contention being supported by impressive experimental arguments. Their
photographs, too, show some unexpected signs of superior strength of
ultra-violet radiation in solar as compared with white stars; and this,
if substantiated, would assuredly imply their higher temperature. For
the radiative _centre of gravity_ shifts upward with increase of heat, a
relation familiarly illustrated by the whitening of red-hot iron before
the melting-point is reached.
All of which is exceedingly perplexing; and there is more behind.
Gravity is of potent influence in determining temperature. The physical
condition of bodies cannot be compared without reference to the scale of
their construction, and their spectra vary to correspond. A score of
years ago Ritter enunciated the theorem that “the surface temperatures
of two stars of equal densities are to each other nearly as the square
roots of their masses.”[526] And Professor Perry reached quite lately
the analogous conclusion that the temperature of a star varies as the
product of its age and mass so long as it behaves after the manner of a
body gaseous throughout.[527] Further, the superficial heat of stars
obviously depends upon the activity of convection-currents in their
interiors, and these of course slacken as viscosity increases. This adds
greatly to the complexity of the problem, since the transcendental
temperature and pressure reigning in the depth of stellar globes must
affect in unforeseen ways the viscosity of the materials placed under
circumstances outside experience.
Clearly, then, the stars can be arranged in order of temperature only
with hesitation and tentatively. If we might accept Ritter’s inference
that the sun’s radiating layer was never in the past, and can never be
in the future, at a much higher temperature than that now belonging to
it, some difficulties would be removed. For it involves the consequences
that the solar type of spectrum marks the culminating point of
superficial heat, and that no star can be hotter than the sun unless it
contains a larger quantity of matter; and these, if valid, would provide
solid ground for classification. But they are highly disputable, and we
can only conclude that it is safest not to dogmatise about relative
stellar temperatures.
Sir William and Lady Huggins regard as of primary importance in the
development of stars the gain of surface-gravity which inevitably
accompanies their contraction. They are unquestionably right.
Atmospheric pressure varies with gravity, and the spectral
characteristics of incandescent vapours are affected to an incalculable
degree by their density. Every addition to gravitational power,
moreover, serves to quicken atmospheric circulation. The tendency to
sorting out by the formation of concentric shells of substances
differing in atomic weight, is overborne by the uprushing of
convection-currents. The strata become mixed, and the heat-gradient
becomes steep. These atmospheric modifications are reasonably numbered
among the concurrent causes of development from the Sirian to the solar
spectral type. They must, at any rate, be concomitants of stellar
condensation, unless the path of progress is deflected by unknown
agencies. It is well to remember that electro-magnetic forces play a
part in cosmical evolution—a part deprived of none of its importance by
our inability to define its nature. We can only see that they may not be
excluded, and await patiently the outcome of future research.
All this refers to the individual history of cooling globes. How, we may
ask, does it apply to the relative histories of various globes differing
very greatly in mass? The customary answer is that massiveness retards
development. That it retards cooling is quite certain, since the larger
of two unequal spheres has, relatively, the smaller radiating surface.
Hence the old view that change of temperature and spectrum proceeds
evenly together had as a corollary that the quicker pace belonged to the
lesser star. Spectra of the Orion and Sirian patterns should,
accordingly, distinguish orbs on the whole of far more imposing
proportions than those giving light of the solar and Antarian qualities.
Just the reverse, however, appears to be the case. All practicable modes
of comparison agree to indicate that the “mean” solar star sends out a
larger sum-total of light from a considerably smaller luminous surface
than the “mean” Sirian star.[528] The solar star is, moreover, the
denser body, and therefore the more massive in a ratio very much beyond
that of its superiority in luminous power.
But the most cogent proof that giant suns develop quickly is derived
from the spectra of double stars. The members of binary systems may
fairly be regarded as contemporaneous. Their origin was in common; their
destinies are indissoluble; they are identically circumstanced; they
must be similarly composed. They should then be exceptionally
trustworthy guides to the unravelment of evolutionary time-relations.
Now they inform us, in distinct terms, that in contrasted pairs the
earlier type of spectrum characterises the minor body. The primary being
solar or Antarian, the satellite is of the Sirian class. Further, the
inequality of mass in such cases is certainly greater than the
inequality of light. The small blue star is more tenuous than the
reddish luminary it attends. These phenomena enforce the conclusion—the
inverse of Ritter’s—that stars of the first type are greatly less
massive than coeval stars of the second.
Here resides the crux of the evolutionary problem. We have no choice but
to believe that the four ages of stellar life succeed each other with
relative promptitude in globes built on a great scale. But what looks
like an insurmountable difficulty may, on closer inspection, prove a
most valuable help towards the establishment of sound doctrine. Sir
William and Lady Huggins threw out the suggestion in 1897[529] that “the
effect of great mass on surface density, together with the working of
Lane’s law, will favour the coming in of a solar type of spectrum at a
somewhat earlier relative time.” They indeed finally rejected the
idea;[530] yet it is strongly confirmatory of their own views as to the
importance of the gravitational factor in the unfolding of stellar
life-history. Rapid atmospheric circulation, indispensable, as they
hold, to the production of a solar spectrum, would be set up earlier in
_heavy_ than in _light_ globes; and the requisite adjustment between
temperature and pressure should be similarly anticipated. That this is
what really happens, we are assured by the prismatic observation of
jewel-tinted star couples.
It does not, however, follow that large stars are short-lived. The
explanation of the facts just offered involves no such paradox. For it
is amply possible that the lesser order of stars may not survive to
reach the Antarian stage. They may perish on the way. Extinction perhaps
overtakes them while still in mid-career. They may lapse into the ranks
of “dark stars” before time has been allowed them to put on any
recognisable badge of decadence. If this be so, stars with fluted
spectra are the outcome of a kind of natural selection. They are bodies
endowed with sufficient heat to keep them luminous to the end, while
others, having squandered less ample supplies by quicker cooling, sink
prematurely into invisibility. This is no idle speculation. The sidereal
system is known to include countless non-luminous globes, the origin of
which is largely enigmatical. Their obscurity, most likely, dates from
various epochs in stellar life. And the smallest masses should, under
similar circumstances, cease first from sun-like existence.
So far, account has been taken of only four stellar families, selected
as the basis of the evolutionary argument because their mutual relations
seem unmistakable. Helium stars are the direct progeny of nebulæ. The
formation of a photosphere definitely marks the transition. By the
gradual effacement of “Orion” lines they merge into hydrogen suns,
these, through the creeping into prominence of innumerable metallic
absorption rays, into solar orbs, which finally pass, by successive
minute changes, into the fluted stage. But what, we cannot refrain from
asking ourselves, lies beyond? Through what phases of decline do great
red stars of the Antarian order subside into extinction? No confident
pronouncement on the subject is possible, but the conjecture may be
hazarded that a stadium of variability precedes the end. Periodic
light-spasms perhaps indicate failing vitality. They may eventually die
out, and be succeeded by a permanent minimum. Already one such example
seems to be afforded by T Ophiuchi, which has for some time ceased from
its annual brightenings. Recurrent maxima may, after all, be only
flickerings in the socket. This possibility lends a particular interest
to research into the causes of these extraordinary outbursts.
Now about the same proportion of carbon stars as of Antarians are
markedly variable. Hence, if radiative instability betoken decrepitude
in one class, it must do so in the other. It would, indeed, on many
grounds, be unreasonable to set the two families far apart in the
chronology of the heavens. Professor Vogel regards them as collaterals.
They represent, in his scheme, alternative lines of descent towards the
final quenching, there being a total absence of evidence that either has
sprung from the other. The pedigree of carbon stars is, in truth, highly
obscure. Besides them only one celestial body shows recognisable traces
of carbon absorption, and that body is our sun. As effete suns,
accordingly, Sir Norman Lockyer ranks these remarkable objects. But the
transitional spectra we should expect to meet with, if this were the
case, are missing. Solar stars with incipient carbon flutings are
unknown. No road runs between the designated stations. A line of
communication is wanting. Nor is the development of Antarian into carbon
stars easy to admit. A few instances of nondescript banded spectra have,
to be sure, been recorded, and might conceivably serve to bridge the
gap; nothing, however, resembling an intermediary series can be made
out. Now stars without obvious relationships presumably developed
quickly under abnormal conditions. And carbon stars seem to be in this
case. They must indeed have had progenitors, although none openly claim
them. With three stocks, nevertheless, they show distant affinities, and
from one or other they must have sprung. Their banded spectrum can be
traced in embryo in the sun; their dark-line spectrum is analogous to
that associated with Antarian flutings; their bright-line spectrum
partially matches Wolf-Rayet emissions. But these are no more than hints
towards a genealogy, of which nature still keeps the secret.
There can be no hesitation in placing the Wolf-Rayet and the bright-line
helium groups at an early stage of cosmic growth. The Pickering and
Rydberg hydrogen lines, which commonly go together, are, for some
unknown reason, characteristic of a primitive condition, and they are
essential elements of the Wolf-Rayet spectrum. The absence from it of
metallic rays is an indication of the same purport; for they are
similarly suppressed in nebulæ, while gaining strength and depth in the
successive stellar generations. Yet Wolf-Rayet stars are not visibly
nebulous. Must we then suppose that they have sprung from stars that
are? This is scarcely possible, in view of the peculiarities just
adverted to; nor is the admission necessary. Small, compact nebulæ,
without hazy appendages, are quite likely, by their condensation, to
have given rise to this class of stars. If so, their telescopic
sharpness is a necessary consequence of their mode of origin. But the
connecting links have still to be detected. Until they are, the
suggested parentage remains an unverified conjecture.
Nebulous attachments, on the other hand, plainly seen or photographed,
not unfrequently declare the affinities of bright-line helium stars.
They are accordingly at the outset of their careers as suns—that is to
say, they have given since the time when they were first formed into
powerfully radiating globes the same kind of spectrum now exhibited by
them. It will, however, eventually become modified; and the most
probable modification to which, so far as our limited view extends, it
can be subject, is by the disappearance of its specific rays of
emission. Their progressive effacement might plausibly be represented by
a series of objects, in which linear radiation grows less and less, from
γ Cassiopeiæ, with its full complement of bright lines, down to Alcyone,
showing a solitary C. Yet this would not compel the admission that every
dark-line helium star has traversed a bright-line phase. Such an
episode, on the contrary, can be inferred from many indications to occur
by exception in stellar history as a consequence, perhaps of
peculiarities of internal constitution, perhaps of unusual influences
exerted from without, possibly of the mutually reactive effects of both
classes of cause.
We must be prepared to meet with side-tracks in evolution. Nature does
not run in a groove. Her operations are free and various; they defy the
restrictions of feasibility which a rigid methodism of thought would
seek to impose. The order of the universe has a wider scope than is
imaginable by us. Creative Wisdom disposes of superabundant resources,
and, if we may dare say so, takes delight in bringing them into play.
Our best attitude of mind, then, in attempting to speculate on the
course of things, is that of the utmost possible flexibility to the
teaching of well-ascertained facts.
CHAPTER XVI.
ROTATION OF THE STARS.
Sir William Abney adverted, in 1877,[531] to the theoretical effects of
rotation on stellar spectra. Quite obviously, they must tend to make the
component lines wide and diffuse. For each line integrates the
displacements and counter-displacements occasioned by the opposite
radial movements of the limbs; while the central and polar sections of
the disc, having their velocities directed across the line of sight,
send out rays in their normal positions, fringed on either side through
the juxtaposition of the shifted rays. The amount of broadening in each
particular star depends, first, upon the linear speed of rotation,
secondly, upon the position of its axis. If this be erect as viewed from
the earth, the motion-shifts will tell to their full extent in spreading
the bright or dark spectral lines; they will become less and less
effective as the axis is less inclined, and will disappear wholly on its
coincidence with the visual ray. Now there can be no doubt that every
star has a movement of gyration as well as a movement of translation;
and it is no less certain that stellar spectra are modified in
accordance with its rapidity and direction. Only the question of degree
has to be considered. Are the effects produced likely to be appreciable?
And if so, have they been perceived?
The spectrum of α Aquilæ (Altair), noticed for some time back as
peculiar, has sometimes been thought to intimate a composite origin. It
is of the Sirian type, but with a reinforced contingent of metallic
lines; and these run together into hazy bands, the general aspect of
which was imitated at Potsdam in 1895 in spectrographs of the sun taken
out of focus.[532] The defective nature of the agreement, however,
discredited the hypothesis of a double spectrum, marked by diffuse
hydrogen absorption proceeding from one source, and by metallic lines
_fused_ into bands, proceeding from another. Yet M. Deslandres
considered that his measures of the star’s radial motion lent it
support. They seemed to indicate velocity variable in a period of
forty-two days, with minor fluctuations superadded.[533] But the
supposed multiple system is, according to Dr. Vogel, a mere creation of
accidental errors,[534] and α Aquilæ must for the present, at any rate,
be counted a solitary star.
Its spectrum was, in 1893, commented upon by Professor Pickering.[535]
He had recourse, for the explanation of its ill-defined character, to
the rotational principle, adding a caveat based on the improbable
greatness of the required velocity of about 100 miles per second.
Adopted, nevertheless, five years later by Dr. Vogel, it was rendered
more plausible by his reduction to 27 kilometres (16·8 miles) of the
equatorial speed needed to widen the lines to the observed extent. This
rate of movement, which is just double that of a point on Jupiter’s
equator, might reasonably be admitted as subsisting in a star. But the
view encounters other, and more fundamental objections. If it were true,
_all_ the lines in the affected spectrum should be similarly diffuse.
Movement acts indiscriminately. Every ray emanating from the advancing
or receding surface is, in due measure, displaced. None can be exempt
from change of refrangibility. The occurrence, then, of a single sharp
line in a stellar spectrum suffices to show that the haziness of its
associates must be due to some other cause than rotation. And there are
many sharp lines in the spectrum of α Aquilæ. They are faintly
discernible, as Sir Norman Lockyer pointed out in 1894,[536] on the
South Kensington plates, and are unmistakably apparent in Sir William
and Lady Huggins’s spectrographs.[537] Those taken at Potsdam are so
limited in range of wave-length that negative conclusions cannot safely
be founded on them. The hypothesis of rotation must, accordingly, be
regarded as inapplicable to the case of α Aquilæ.
Now α Aquilæ is not without analogues. It belongs to a pretty numerous
stellar group, differing in chemical constitution, but agreeing in the
diffuseness of the absorption traits significant of it. They form one of
Miss Maury’s three collateral series—her “Division _b_.” It embraces no
“advanced” stars; only those of the helium and hydrogen types, with a
few verging towards the intermediate stage of Procyon, are represented
in it. Hazy spectra are thus a sign of cosmic youth. They characterise,
without exception, the stars of Miss Maury’s “Group i.,” in which the
Pickering series of hydrogen is prominent; they cease to appear, or
appear by imperfect indications, soon after the Sirian stage is passed.
Their explanation by opposite displacements through axial movement would
then involve the consequence that stars, as they develop, lose much of
their rotational speed. There is, however, but one recognised agency by
which it can be retarded—the agency of tidal friction; and it acts
sensibly only on bodies attended by closely-revolving satellites of
considerable relative mass. Solitary suns like our own can have spent
but little of their energy of rotation. Actual velocity in spinning
becomes, in fact, accelerated as contraction proceeds, so that ageing
stars should have their spectral lines more broadened by motion than
those in a primitive condition. And since the effect is imperceptible in
the former, we may feel assured that it has not been observed in the
latter.
Confirmatory evidence is not wanting. There is a certain class of stars
which, we have the strongest reason to believe, rotate in very short
periods, and on axes almost perpendicular to the line of sight. They
combine, accordingly, both the conditions needed for the display of
spectra rendered diffuse by motion-shifts. These are occulting variables
like Algol. Since they revolve in planes passing very nearly through the
earth, and their equators cannot deviate materially from the same level,
it is certain that virtually the whole speed of their advancing and
receding limbs is radially directed; no considerable part of it is
spectroscopically ineffective. Further, although they may rotate faster,
they cannot rotate more slowly than they revolve, and their orbital
periods are extraordinarily short. The system of Algol, which is by no
means one of the quickest eclipsing pairs, circulates in sixty-nine
hours. Its equatorial rate of rotation, by a minimum estimate, is
thirteen miles a second, or just eleven times the solar. The absorption
rays in the light from one limb are accordingly displaced towards the
blue, and those from the opposite limb towards the red, eleven times
more than the Fraunhofer lines measured by Young and Dunér; and their
compounded effect in the stellar spectrum is to widen the lines by an
amount corresponding to a speed of twenty-six miles. In other words,
each should spread over nearly one quarter the interval between the
D-lines in the sun. The alteration is, nevertheless, inconspicuous. The
spectrum of Algol does not strike the eye as hazy. The hydrogen series
shows the distension proper to the type, no more; the rays of helium,
magnesium, and calcium are of the average sharpness. Now some eclipsing
stars must rotate much more rapidly than Algol. U Ophiuchi, for
instance, has a period of only twenty hours. Yet in none of them have
blurred spectra been noticed. Enormous velocities—velocities most
probably non-existent—would evidently be indispensable for their
production.
Such spectra as that of α Aquilæ must then be accounted for otherwise
than by rotation. For the suggested geometrical cause, which proves
inadequate, a physical cause has to be substituted. One may be found in
excessive pressure. The diffuse lines possibly originate at unusual
depths in the stellar atmospheres. Sir William and Lady Huggins
advert[538] to the probability of great differences in this respect
between various stellar classes. In early stars they say—and none of the
members of “Division _b_” are mature—“we may see deep down into the
star, and the continuous spectrum may come from a thick region of dense
gas, throughout which little, or possibly no condensation to the liquid
or the solid state takes place. Under these conditions, the absorbing
gases in front of it will not be, as in the sun, of very limited
thickness, but will occupy a region of vast extent.”
The solar H and K illustrate the character of lines generated in dense
vapours at a high temperature; their “wings,” as we may remind our
readers, being added in the immediate vicinity of the photosphere to the
comparatively definite lines produced in the upper reversing strata. Now
it is a curious fact that distended lines, such as H and K, are apt to
be doubly reversed. Dr. Scheiner has noticed symptoms of incipient
illumination at the centres of the broad hydrogen bands distinctive of
first-type stars, and they are similarly manifest in Wolf-Rayet stars
showing mixed series of emission and absorption. Hence the particular
significance of M. Deslandres’ detection in α Aquilæ of fine
“chromospheric” lines of hydrogen, and occasionally of calcium and
iron,[539] superposed upon the dim, dusky bands indicative of the state
of those substances in the reversing layer. Diffuse spectra may thus,
with some probability, be assigned to abortive bright-line stars. Or
they perhaps mark objects just losing the faculty of specific emission.
If so, the mode of its departure is different from that exemplified by
Alcyone, in which the dark lines have their normal aspect, while one red
ray survives as the sole remnant of what was perhaps once a blazing
spectrum. The future course of stars resembling α Aquilæ can be traced
only by conjecture. But what hints are at hand lead to the supposition
that they will proceed by insensible gradations to the solar stage,
their absorption rays becoming narrower, more numerous, and better
defined with the slow advance of condensation.
The upshot of our inquiry is to bring the conviction that no approach
has yet been made towards determining the rotation of any star. Spectra
are not rare composed mainly of blurred lines, and so suggesting at
first sight diffusion on the principle of movement; the intermixture of
sharp lines, visible on closer scrutiny, nevertheless peremptorily
negatives the suggestion. Again, rotational velocity in the line of
sight should be at a maximum in Algol variables; yet they do not possess
specially diffuse spectra. Theory, however, need not be at fault because
it fails to be verified by observation. The failure merely informs us
that its consequences, by their smallness, elude our means of discovery.
CHAPTER XVII.
SPECTROSCOPIC BINARIES.
Between the old and the new astronomy lies a region claimed by both, yet
belonging by exclusive right to neither. This is the department of
spectroscopically determined, or radial movements. Now the distinction
between radial and transversal movement is purely artificial; it is made
in the interests of our imperfect faculties; nature ignores it. Motion,
although by a geometrical fiction resolvable _ad infinitum_, is
essentially simple. At any given instant it takes place in one direction
only. But if that direction be oblique to our view, we see the line of
travel foreshortened; nor can we tell by direct vision how much
foreshortened. That is to say, the eye perceives one component of
velocity—the tangential component, the component lying square before
it—while of the other component along the visual ray it takes no heed.
Here the spectroscope comes to the rescue. Helpless to deal with
tangential speed, it can measure radial speed by its effects upon the
refrangibility of light. The first method gives one side of the
parallelogram of velocities, the second, the other; combining their
results, we get the diagonal actually traversed by the observed luminous
body. Data obtained by visual means formed the sole materials of the
elder gravitational science; none others were indeed available at the
period of its growth and elaboration; nor, even if they had been, would
they have been of essential service in constructing planetary theories.
In sidereal astronomy they occupy quite a different position. Without
them its progress is crippled, and the possibility of procuring them
fortunately developed just when they began to be urgently needed.
As grist for the mathematical mill, motions determined spectroscopically
serve equally well with motions determined telescopically. The calculus
deals indifferently with either kind. The calculus, however, is an
instrument of precision, and demands minute accuracy in the materials
operated upon. And it is just the effort to satisfy this demand which
has broken down the barrier between celestial mechanics and celestial
physics. For radial velocities cannot be unadvisedly accepted; the
line-displacements significant of them may be otherwise occasioned.
Cases have to be discriminated; conditions scrutinised; recondite
problems attacked. The information within reach is, nevertheless, too
important to be neglected; at any cost of pains it must be extricated
from uncertainty and used for all that it is worth; and so it has come
about that traditional astronomers of the most abstract type find
themselves involved in experimental difficulties, and confronted by
questions answerable only in the laboratory.
Astrophysicists, on their side, have no choice but to cultivate the
spectroscopic branch of dynamical astronomy. Not merely because it
springs from the stem of physical principles and physical experience,
but because the study of its subject-matter is essential to the
furtherance of knowledge respecting stellar constitution and
development. Thus sidereal physics merges into sidereal mechanics; yet
somewhere between them a line of demarcation must, for the purposes of
the present book, be drawn. Arbitrarily drawn in many places it will be;
but there are cases in which it is better to lack logic than limit; and
this is one of them. We shall then regard spectroscopic but not
telescopic binaries as making part of our subject, although fully aware
that the two classes are fundamentally one. Stellar and nebular proper
motions, transversal or radial, must also as such be excluded. Only when
they give evidence, by periodic variability, of the progress of orbital
revolutions, do they fall within our scope. Uniform velocities in space,
in whatever direction, or however determined, do not concern us. Foreign
to our theme as well, is the grand topic of sidereal construction,
prescribed as it is and conditioned by the flittings of the stars.
Something, however, will be said about the physical peculiarities of the
Milky Way, since apart from them the spectroscopic relations of the
various families of stars and nebulæ would remain imperfectly
intelligible.
Spectroscopic binaries are stars telescopically single, but inferred to
be compound from the evidence of a regular flow of spectral change. The
change is of a perfectly definite nature. It consists in the swinging of
all the bright or dark lines to and fro in a fixed period across their
average positions. These, indeed, are not precisely their normal
terrestrial places. They are affected by the motion of the system, as a
whole, towards or from the eye. This constant element of shifting is,
however, easily eliminated, and the circulating pair can then be studied
as if their centre of gravity were at rest relatively to the sun.
Ascribing to them, to begin with, a circular orbit, it is easy to see
that, at two diametrically opposite points—the extremities of the line
of conjunction—radial motion vanishes, the entire measurable velocity
being across the direction of view; while at the corresponding points of
greatest elongation, the radial component represents full speed. In an
elliptic orbit these four points of zero and maximum line-of-sight
movement may be slightly displaced according to the situation of the
major axis; they can, however, be located with the help of exact
observational data, and their positions then serve as an index to the
shape and orientation of the ellipse. One of its elements, nevertheless,
remains incalculable. Unless the revolving stars eclipse one another,
there is no means of determining its plane. It cannot be perpendicular
to the line of sight, for in that case all the motion would be
tangential; no part of it could be spectroscopically apparent. But it
may be inclined at any angle short of a right angle; and the larger the
angle, the smaller the proportion of the orbital velocity directed along
the visual ray. In other words, the measured rates are the true rates
multiplied by the cosine of an unknown angle. They are, accordingly,
minimum values; the actual speed indefinitely exceeds them.
Nevertheless, it can rarely be more than double the seeming amount,
since the chances of discovery manifestly fall off for systems with
highly inclined planes. Thus the scale of orbits computed from
spectroscopic data is indeterminate; but the uncertainty does not extend
to their form, which is as strictly derivable as that of the paths of
visible binary stars.[540] The mass of the revolving bodies, however,
escapes us. It depends upon the compass of their rounds; we can only
assign to it a value less than which it cannot be, although it may be
considerably greater.
Spectroscopic binaries are broadly divisible into three classes: those
that are constant in brightness, those that vary in light through
eclipses, and those that vary in light otherwise than by eclipse. In the
present chapter we shall confine our attention to the first category.
And here again we have to distinguish between stars with bright, and
stars with dark companions. In the spectra of bright pairs the lines
split into doublets twice in the course of each revolution; in those of
bright and dark pairs they execute a complete vibration simultaneously
with the description of a circuit, but remain always single. But these
different conditions are not sharply separated. As usual, the rule of
continuity is observed. Satellites are met with in all gradations of
luminosity. Some shine no less brilliantly than their primaries, in
which case the duplicated lines are exact pairs; others are large but
dim, and show very faint rays periodically appended to the comparatively
intense ones of the greater orb; while the majority, remarkably enough,
are quite obscure, so far at least as spectral indications enable us to
judge.
These marvellous systems have, one and all, been discovered by
spectrographic means. The required consistency and certainty of
measurement are unattainable visually. They have been realised only by
the employment of chemical impressions comparable night by night and
year after year. The first result of the kind was achieved at Harvard
University by the detection as a close double of ζ Ursæ Majoris, the
middle star in the Plough handle. The components are Sirian suns of the
same stamp, and of nearly equal brightness; but they may be to some
extent relatively variable.[541] The nature of their movements was long
an enigma. The period of 104 days at first assigned to them implied
duplication of the spectral lines once in 52 days. Yet this consequence
did not ensue in fact with the inevitableness exacted by theory. Grave
discrepancies became manifest; the anticipated separation of the two
spectra, often non-apparent, was always curtailed in duration, and
repeated attempts were baffled to get rid of these anomalies by adding a
third body to the system, by heightening the eccentricity of the ellipse
traversed, or by shortening the period of its description.[542] At last,
in 1901,[543] by a discussion of a fresh series of Potsdam
spectrographs, Dr. Vogel reached a satisfactory conclusion. He now
assigned to the revolving couple a period of only 20 days 14 hours; the
eccentricity of their orbit came out = 0·5, so that their
periastron-approach is one-half their mean distance; while their least
possible mass proved to be four times that of the sun. Up to the present
they seem inclined to conform to these rules of the road.
The parallax of 0·045″ found by Klinkerfues for Mizar (to give ζ Ursæ
its Arabic name) is likely to be largely erroneous, but erroneous by
excess rather than by defect. If it be correct, the star sends abroad
thirty-eight times more light than our sun. If it be too large, the
star is proportionately brighter; and, judging by Höffler’s
estimate[544]—certainly a most precarious one—it is greatly too large.
Since the components are spectroscopically unlike the sun, the ratio
of the quantity of light they emit to the quantity of matter they
contain must be widely different. Were it the same, a 38-fold
brilliancy would imply their possession of about 160 times the solar
mass; whereas Vogel’s data almost enforce the belief that this value
at least decuples the truth. This may serve to exemplify the
difficulty of deducing massiveness from luminosity. Only in
exceptional cases can any fixed proportionality be safely assumed as
the basis of calculation.
Mizar is attended visibly as well as invisibly. A star of the fourth
magnitude circulates round it at a distance of 14″ with an almost
imperceptible progression. Unless it mends its pace, 10,000 years will
have elapsed before a revolution is completed. It may be worth recalling
that on 18th April 1841, Mädler was unable to find this object,
ordinarily plain to be seen; and the failure seemed so strange that he
endeavoured to account for it by supposing the star subject to sudden
obscurations like those of Algol.[545] No second disappearance is on
record; but phases of the kind are extremely evasive when no time-bill
of their recurrences is at hand. The connection into one system of two
close pairs is not in itself improbable, although no such arrangement
has yet been anywhere verified.
The detection of a second spectroscopic binary followed immediately upon
that of the first. Miss Maury announced late in 1889, as a result of her
scrutiny of the Harvard plates, that the absorption lines in the
spectrum of β Aurigæ appear alternately single and double once in
forty-eight hours. Hence the period of revolution is four days, while
the relative velocity of the components, given by the extent to which
their spectral rays separate, is 150 miles a second. Their joint orbit
has been computed by Rambaut,[546] Lehmann-Filhés,[547] and
Schwarzschild[548] so accordantly as to leave little room for
improvement. They find its eccentricity to be somewhat less than that of
the orbit of Mercury; the major axis makes an angle of 32° with the line
of sight, and the periastron is at the end farthest from the earth. The
plane being unknown, its dimensions, as already explained, cannot be
ascertained. If indeed the observed velocity were the true
velocity,—that is, if it lay in a level passing through the eye—the
corresponding orbital radius would be 7,500,000 miles, and the mass of
the system would rather exceed that of 4½ suns;[549] but these are only
the least possible values; they may be greatly surpassed. The components
of β Aurigæ shine with the Sirian quality of light; they are almost
perfectly matched, reciprocal variability, according to Miss Maury,[550]
causing each in turn to appear the brighter. A parallax of 0·062″ was
photographically determined for them by Professor Pritchard. At the
distance indicated, the sun would seem twenty-eight times less bright;
it would sink to the paltry status of a 5·6 magnitude star. If β Aurigæ
were a single globe, constituted like the sun, and giving out a
twenty-eight-fold supply of light, it should be of 147-fold mass.
Neither of these conditions is fulfilled; and allowance for their
realisation might possibly bring into fair agreement calculations of
mass from gravitational and from photometric data. In which case the
plane of revolution would deviate but slightly from that traversed by
the line of sight.
The preceding star of two named μ Scorpii was disclosed as compound in
1896 by Professor Bailey’s notice of the doubling of its spectral lines
on many of the Draper Memorial negatives.[551] The period is only 34
hours 42 minutes; the velocity is correspondingly high. By their wide
separation the lines tell of relative movement at the rate of 286 miles
a second, this being the sum of the opposite velocities of the conjoined
bodies. Their orbit, apart from the foreshortening effect of its
inclination, has a radius of nearly 6,000,000 miles, and they possess at
least fifteen times the gravitating power of the sun. They show a helium
spectrum; and one member of the pair is not only fainter than the other,
but fainter, apparently, in a variable degree.
The spectrographic method employed at Harvard supplies evidence of
line-duplications, but none of the shiftings of solitary lines. This is
easily understood. The dispersing apparatus is placed in front of the
object glass; the stellar images formed at its focus are then ready-made
spectra. Hence no slit is required, and without a slit no
comparison-spectrum can be availed of. Displacements therefore betray
themselves only when the rays furnish standards of mutual reference by
splitting into pairs. The binary nature of stars both bright is,
accordingly, discoverable in this way, but not the association of a
bright with a dark body.
The spectroscopic discovery of these remarkable combinations was
initiated at Potsdam. Dr. Vogel[552] in 1890 found the brilliant Spica
(α Virginis) to have its spectrum shifted to and fro once in four days,
the revolutions thereby intimated proceeding at the rate—apart from
perspective abridgment—of fifty-seven English miles a second. This
implies, if the components be of equal mass, a distance for each from
their common centre of gravity of 3,100,000 miles, and a joint mass 2·6
times that of the sun. The spectrum is of the helium type. Sir David
Gill obtained a null result for the parallax of the star, which must
accordingly be of amazing actual splendour. The attendant of Spica is
not wholly obscure. Traces of its spectrum can be perceived, although
too faintly for purposes of exact measurement. The couple might then be
termed a linking instance between systems composed of twin suns, and
those consisting of a luminous and a non-luminous member. The conditions
of observation are not only more arduous in the latter case, but the
data within reach prove less adequate. They are one-sided; they relate
exclusively to the bright star. All that can be learned about its dark
companion is that it describes, in the same period, a similar orbit. The
size of the orbit is left vague. It is large if the mass of the body
traversing it is small, and _vice versâ_; and the mass of the primary is
involved in the same uncertainty.
A peculiarly interesting bright and dark couple was brought to notice in
1896 through M. Bélopolsky’s work with the thirty-inch Pulkowa
refractor.[553] Castor, the lucida of the constellation Gemini, was the
object of his researches; and it dominates a system no less wonderful
than that of ζ Ursæ Majoris. Two lustrous Sirian stars, of second and
third magnitudes respectively, wheel slowly at an interval of 6″.
Although they have been wards of science since Bradley’s measure of them
in 1719, their orbit is not yet, in any strict sense, calculable. It is,
however, certainly very eccentric, and takes many centuries to describe.
A small, distant star shares the proper motion of this majestic pair,
and is hence known to form with them a ternary combination, which the
spectroscope has rendered quaternary by assigning an invisible satellite
to the minor component of the original binary. Its revolutions are
rapid; they have a period of not quite three days, and its gravitative
power suffices to impart a radial velocity of 22 miles a second to the
luminous orb in its vicinity. The corresponding distance from the centre
is 1,800,000 miles; but this, as in all such cases, is a minimum value.
The ellipse traversed is slightly more eccentric than the path of
Mercury round the sun. And here arose a curious complication.[554]
Obviously, the interval between two periastron passages should, if the
major axis remained fixed in space, be identical with the time occupied
in making the circuit from node to node. It proved, nevertheless, to be
notably longer, and the hypothesis was, so to speak, compulsorily
adopted that the major axis shifts forward as the stars revolve. The
period of the inequality, as determined by Bélopolsky in 1899, is 2100
days. In four years and forty days the orbital axis makes a complete
gyration, and is once more directed as at first. The cause assigned for
the disturbance is the spheroidal form impressed upon the conjoined
bodies by their reciprocal tide-raising power. A flattening of
one-seventh would suffice to produce the observed effect, admitting that
the system has the same dimensions as that of Algol. This special kind
of perturbation, long theoretically recognised, has been found
practically operative only within the narrow precincts of close stellar
systems. Its continued study in them may lead to important developments.
The disclosure of a duplex character in Capella was eminently
unexpected. Its reputation as an exemplary solar star, irreproachably
regular in its movements, had been established at Potsdam, where the
spectrographic investigation of radial velocities was set on foot in
1888.[555] But the instrumental resources then at Professor Vogel’s
command were inadequate to bring out certain deviations from the
Fraunhofer pattern lately noticed. The spectrum is indeed both
oscillatory and compound; but a powerful and perfect apparatus is
required for the manifestation of these singularities. The discovery
was, nevertheless, made in duplicate by Professor Campbell at Lick,[556]
and by Mr. Newall at Cambridge.[557] It is of unique interest.
Capella is one of the very few spectroscopic binaries at a determined
distance from the earth. The carefully revised parallax of 0·081″,
implying a light-journey of forty years, assigned to it by Dr. Elkin in
1897, has every mark of authenticity. Located thus remotely, our sun
would appear as a star below the fifth magnitude; 102 orbs like it
should be combined to give the light of Capella. Hence the joint volume
of the components, assuming them to be equal globes of solar intrinsic
brilliancy, must be just 730 times that of the sun. And their joint mass
should be even proportionately greater, unless the tremendous force of
internal compression proper to bodies so gigantic were counterbalanced
by exorbitant heat. There is strong, if not convincing evidence that
this is actually the case.
The members of the Capellan system are unequally luminous, and differ in
their absorptive qualities. One is spectroscopically indistinguishable
from the sun, the other, of about half its brightness, is more akin to
Procyon. It has, accordingly, been found possible to measure separately
the swing of each set of lines, and they have proved to be of much the
same amplitude. This means that the two bodies are animated by
equivalent movements, and are hence nearly equal in mass. Their
revolutions proceed in a slightly eccentric orbit, the mean radius of
which, if seen edgewise, would measure 52,000,000 miles. It is not,
however, seen edgewise; very far from it. Capella possibly makes an
exception to the rule that the circulatory plane of non-eclipsing
spectroscopic pairs is unascertainable; and for this reason, that it may
be, to some extent, a visual pair as well. Mr. Newall adverted to the
probability, based upon the star’s parallax and conditions of movement,
that the distance between its components would subtend from the earth an
angle of nearly one-tenth of a second. Telescopic observations might
then have a successful issue; and the opportunity for testing the powers
of great instruments was, at any rate, too tempting to be neglected.
Those of the Lick refractor, strange to say, though employed with all
Professor Hussey’s skill, failed to answer the demand made upon them.
The star was seen persistently round.[558] Yet Messrs. Dyson and Lewis
were persuaded of its genuine elongation with the Greenwich
twenty-eight-inch equatorial. The components were never seen clearly
divided; no thread of dark space was perceptible between them; but they
formed together an oval image, and the direction of lengthening changed
consistently with the period of their mutual circulation.[559] The
visual data thus procured even served for the calculation of a
provisional orbit, and the missing element of its inclination proved to
have a value of 30°. The stars, if this be so, travel in a plane making
an angle of 60° with the line of sight, and the actual radius of their
path is double that derived from spectroscopic velocities. It measures
104,000,000 miles; these immense bodies are not much farther apart than
the earth is from the sun. Their mass can hence be arrived at; together
they contain somewhat more matter than seventeen suns. This result,
though in many ways plausible, is extraordinarily discrepant from what
we may call the light-value of the same element. If they gravitated in
the proportion of their luminosity, these orbs, as we have seen, should
outweigh the sun 730 times; in point of fact, they seem to possess no
more than seventeen times the solar mass. Even apart from the Greenwich
results, we seem here to find evidence that stars differing enormously
in density may show spectra of the same stamp, and that large bodies
traverse the various stages of development more rapidly than small. The
members of the Capellan pair, admitting the alleged facts, are some
forty times more rarefied than the sun; liquid hydrogen is a heavy
substance compared with them. They are, accordingly, at an early epoch
of their career as radiant globes, while their spectra indicate full
maturity. Confirmation is thus afforded to the inference, already drawn
from the spectral diversities of double stars, that mass accelerates
evolution.
Late in 1896 Professor Campbell undertook at the Lick Observatory a
comprehensive investigation of stellar radial motion. The adjustment of
the fine apparatus known as the “Mills spectrograph” to the great
refractor made this feasible; and among its “bye-products,” the analysis
of visually single stars into revolving pairs is particularly important,
first, because of the individual interest of the facts collected, next,
on account of their significance in the scheme of the Cosmos. The
abundance with which such systems occur is quite unexpected, and very
remarkable. Professor Campbell finds that, of 285 stars observed by him,
more than one in nine is a spectroscopic binary.[560] And this excludes
several objects suspected, but not demonstrated to belong to the same
class, and makes no allowance for those cases in which the orbital plane
is so nearly vertical that movement in it makes no appreciable
spectroscopic effect. He has then good reason for holding it probable
“that at least one star in five or six will be found to be a
spectroscopic binary,” and he is quite prepared “to see a still larger
ratio established. The proven existence,” he adds, “of so large a number
of stellar systems differing widely in structure from the solar system
gives rise to a suspicion, at least, that our system is not of the
prevailing type of stellar systems. The new field of astronomical
research thus opened up is of great richness, and may well occupy the
attention, for an indefinite period, of the large number of observers
and institutions now engaging in its development. It is perhaps
unnecessary to say that the measure of success attainable is dependent
upon the degree of accuracy realised in the observed velocities.”
The Lick results in this branch are in several respects noteworthy.[561]
They show the wide diffusion of arrangements strange to experience, if
admissible in speculation. They serve, by the variety of periods which
they indicate, to narrow the lacuna between stars directly seen, and
stars inferred from Doppler’s principle to be revolving. They have
rendered it obvious that no generic distinction separates the two
classes. A pair revolving in 2½ years, like η Pegasi, cannot be set
apart fundamentally from δ Equudei, with its period of 5·7 years. It is,
indeed, clear that spectroscopic and telescopic binaries differ only in
the mode of their observation; not what they are in themselves, but the
aspect under which we regard them, has caused them to be artificially
disconnected. Already, however, a tentative beginning has been made with
Capella in the telescopic observation of spectroscopic couples; and
spectroscopic determinations of orbital speed, long a desideratum for
visual pairs, have been tried with good promise of success.
Individually, too, the detections announced from Mount Hamilton are
frequently curious and suggestive. Thus Polaris claims the attendance of
two dark companions; while the bluer component of κ Pegasi, besides
circulating visibly in a period of 11·4 years, circulates invisibly in
six days. Again, the primary of the classic pair, ξ Ursæ Majoris, proves
to have a variable radial movement, the conditions of which invite
investigation. The duplicity of υ Sagittarii is significant for a
different reason. The spectrum of this star includes bright hydrogen
lines; it was noted by Miss Maury[562] as compound, and perhaps mutable.
Motion-displacements, giving a velocity-range of about twenty-five miles
a second, were measured in it by Professor Campbell in 1899; but their
period remains undetermined, as well as the extent to which they are
shared by the responsive lines of the companion-spectrum.[563] For in
this case both the stars are luminous, and both show absorption of the
helium type. The exceptional constitution of υ Sagittarii, indicated by
its emissive symptoms, accentuates the interest of its systemic
relations. It should accordingly be an object of particular and
persevering attention. Indeed, each of these wonderful pairs may be
expected to develop peculiarities of its own, demanding study by varied,
and perhaps recondite methods.
CHAPTER XVIII.
ECLIPSING STARS.
Stellar eclipses are necessarily included among the phenomena of
spectroscopic binaries. For the planes of a proportion of these systems
must pass through the earth, with the result that the circulating bodies
occult one another when they cross the line of conjunction. The
circumstance is particularly valuable as supplying a datum unattainable
with the spectroscope for star couples differently conditioned. The
rapid loss and recovery of light which tell us that one body is passing
in front of the other, tell us, at the same time, that they revolve in
an orbit seen edge on—that is to say, making an angle of 90° with the
“tangent plane” of the sphere. Radial velocities are, accordingly,
measured in their true proportions, and the masses of stars giving
double spectra become strictly determinable. The duration of eclipse,
moreover, indicates the density of the obscured and obscuring globes;
and where the dimensions of even one of their orbits is known, it
supplies a measure for their actual diameters. Thus precise evaluations
of the light-changes and radial velocities of these singular objects go
hand in hand; both kinds of research are equally necessary to the
advancement of knowledge concerning systems which are peculiarly
interesting because they are, more than any others in the sidereal
world, accessible to investigation.
The circumstances of stellar eclipses are endlessly varied, and their
differences are full of meaning. They are central when the orbit is
directed straight towards us; they are partial when it deviates from
coincidence with the line of vision. Then the interposing body may be
dark or bright; it may be larger or smaller than the globe behind it.
Supposing it to be sensibly obscure and the transit central, the eclipse
will be either total or annular. The first case is not known, although
quite likely to occur; no complete periodical disappearances have been
witnessed. The second has not been definitely attested. Its inevitable
indication would obviously be a stationary minimum. While the two discs
are superposed, the light must remain steadily at its lowest level; and
if the eclipse be annular, the discs are, _ipso facto_, superposed
during an appreciable interval, the length of which bears an inverse
ratio to the depth of the obscuration.
While a bright and dark pair can undergo but one eclipse in each
revolution, two are the portion of a couple radiant in both its members.
If they are alike in size and brilliancy and pass one another centrally,
the eclipses will be equal and the loss of light one-half. And it is
noteworthy that in nature this seems to be a somewhat prevalent
arrangement. Several systems are known in which it may subsist;
spectroscopic observations can peremptorily decide whether it does, in
fact, subsist or not. The principle of their decision lies on the
surface. Eclipses necessarily take place at points of the orbit where
radial motion approximates to zero. If they are duplicated in each
period, the cessation of movement marks at one, the transition from
speed of approach to speed of recession; at the next, the turning-point
where speed of recession changes to speed of approach. A minimum of
light, that is to say, precedes each reversal of movement. If, on the
other hand, the eclipses are single, occurring only once in a
revolution, the cycle of motion to and from the eye is completed in the
interval between them. The criterion is thus absolute and unmistakable;
only the faintness of the stars impedes its general application. Where
unequal eclipses alternate, however, the spectroscope is not needed to
inform us that they take place two and two in each circuit. They are
then evidently due to the mutual occultations of disparate stars; and
the species of combination they indicate is frequently met with. It is
varied to the utmost, as might be expected, by gradations of disparity,
in the production of which deficient luminosity may concur with
inferiority of size, or be partially neutralised by its superiority.
Thus as each globe in its turn comes in front of the other, there
results a double series of eclipses, the odd ones (dating from a fixed
epoch) being perfectly similar each to each, but differing in depth and
duration from those of the even or intermediate series.
Further diversities arise through slight tilting of the orbits. The
corresponding eclipses are partial, even to the limit of evanescence,
when the star discs just escape contact. And it may be noted that the
second eclipse in each revolution, properly belonging to a luminous
pair, may be suppressed where the path traversed is at the same time
sensibly inclined and considerably eccentric. Partial eclipses are
doubtless the rule, central ones the exception; but their discrimination
is often a matter of some delicacy. Indeed, the photometric study of
such phases is an art in itself, the practice of which demands skill,
vigilance, and patience beyond the common. The aid of photography,
lately enlisted for it, is likely to enhance its security and precision.
Eclipsing stars, once more, are close binaries circulating nearly in the
line of sight. Not any intrinsic peculiarity, but our situation in
space, determines their special character. Relatively to us, they are
periodically variable, as Spica or Castor would become were our place
suitably shifted. Their changes are of a distinctive kind; they are
short, sharp, and decisive. When well developed, that is to say; for
photographic photometry may ere long afford the means of detecting
occultations barely adumbrated by a drop not perhaps exceeding one-tenth
of a magnitude. All members of the class so far have been recognised by
their variation in brightness; their accompanying circulatory motion,
inferred in all cases, has been verified in only a few. The following
list gives the designations, periods, and phases of the eclipsing stars
with which astronomers had made acquaintance down to the end of 1902.
They are enumerated in the order of their discovery.
┌──────────┬─────────────────┬───────────┬─────────────┬─────────┐
│Chandler’s│ Name. │ Period. │Light Range. │Duration │
│ No. │ │ │ │of Phase.│
├──────────┼─────────────────┼───────────┼─────────────┼─────────┤
│ │ │d. h. m. s.│ m. m.│ h. m.│
│ 1090│ Algol │ 2 20 48 55│ 2·3 to 3·4│ 9 20│
│ 3109│ S Cancri │ 9 11 37 45│ 8·2 „ 9·8│ 21 30│
│ 1411│ λ Tauri │ 3 22 52 12│ 3·4 „ 4·2│ 10 0│
│ 5374│ δ Libræ │ 2 7 51 23│ 5·0 „ 6·2│ 12 0│
│ 5484│ U Coronæ │ 3 10 51 12│ 7·5 „ 8·9│ 9 42│
│ 6546│ RS Sagittarii │ 2 9 58 36│ 6·4 „ 7·6│ 10 40│
│ 320│ U Cephei │ 2 11 49 38│ 7·1 „ 9·4│ 11 0│
│ 6189│ U Ophiuchi │ 0 20 7 43│ 6·0 „ 6·7│ 5 20│
│ 7488│ Y Cygni │ 1 11 57 26│ 7·1 „ 7·9│ 9 0│
│ 2610│ R Canis Majoris│ 1 3 15 46│ 5·9 „ 6·7│ 5 0│
│ 5949│ R Aræ │ 4 10 12 42│ 6·9 „ 8·0│ 9 30│
│ 3055│ X Carinæ │ 0 12 59 30│ 7·9 „ 8·6│ 6 39│
│ 5144│ Y Boötis │ 2 14 24 0│ 8·0 „ 8·6│ 4 0│
│ 3416│ S Velorum │ 5 22 24 35│ 7·8 „ 9·3│ 15 11│
│ 6442│ Z Herculis │ 3 23 50 0│ 6·9 „ 8·0│ 6 36│
│ 7399│ W Delphini │ 4 19 21 12│ 9·3 „ 12·1│ 14 0│
│ 6636a│ RX Herculis │ 0 21 20 33│ 7·0 „ 7·8│ 4 8│
│ 2781│R^2 Puppis │ 6 10 19 36│ 9·1 „ 10·8│ 17 0│
│ │U^3 Cygni │ 4 13 45 2│ 8·7 „ 11·4│ 13 0│
│ │V^2 Cygni │ 6 0 8 48│10·8 „ 12·8│ │
│ 3707│R^2 Velorum │ 1 20 30 2│10·0 „ 10·9│ 3 20│
│ 6773│ U Scuti │ 0 22 54 0│ 9·1 „ 9·6│ 5 0│
│ 7318│ UW Cygni │ 3 10 49 12│10·5 „ 12·0│ 8 30│
│ 6927│ U Sagittæ │ 3 9 19 12│ 6·5 „ 9·1│ 12 0│
│ 7891│ UZ Cygni │31 7 17 46│ 8·9 „ 11·85│ 48 0│
│ │ RV Lyræ │ 3 14 22 23│11·0 „ 12·8│ │
│ │14, 1902 Persei │ 3 1 21 32│ 9·4 „ 12·0│ │
└──────────┴─────────────────┴───────────┴─────────────┴─────────┘
Algol (β Persei) is the model eclipsing star. Goodricke’s sagacious
conjecture that such was its nature, adopted and developed by Pickering,
obtained experimental confirmation after a lapse of 107 years. Vogel’s
spectrographs showed in 1889 that, previously to each obscuration, the
star was swiftly receding from the earth, while recovered brightness was
attended by a somewhat greater velocity of approach. The excess is
simply due to the fact that the system is travelling towards the sun at
the leisurely pace of 2⅓ miles a second. The true orbital speed is
sensibly uniform; accelerations, pointing to ellipticity in the track
pursued, have not been detected. One eclipse takes place in each
revolution; the eclipsing body is to all intents and purposes a gigantic
planet; it gives no perceptible light. The sun round which it circulates
is, on the contrary, peculiarly brilliant for its size, partly because
its absorption, being of the helium type, produces little or no
mellowing effect upon its keen white rays.
Much has been learned about the system formed by these contrasted
globes. Its visible member travels at the rate of 26⅓ miles a second;
and since the period of revolution comprises 247,735 seconds, the
distance of its centre from the centre of gravity slightly exceeds
1,000,000 miles. The length of the eclipse, moreover, gives the actual
size of the star. It has a diameter of slightly more than 1,000,000
miles, or about five-fourths that of the sun. The dimensions of the
satellite, too, are approximately known. From the loss of light through
its interposition, Professor Pickering in 1880[564] calculated the ratio
of its diameter to that of its primary to be as 764 to 1000. The ratio
should be increased if the transit were not central, which it does not
appear to be. For the eclipse is not annular, since there is no pause at
minimum; the flow of change is continuous; the turning of the luminous
tide is not appreciably delayed; decline is immediately succeeded by
restoration. The orbit is not then level with the eye; and Mr. Yendell
considers that an inclination of 7° would agree best with the
light-curve.[565] In Vogel’s opinion it is such as to imply for the dark
body a diameter of 830,000 miles, an estimate which can hardly be far
from the truth.[566] Admitting with him that the two stars are of equal
density—possibly a hazardous assumption—we can infer their masses,
knowing their respective volumes, and the circulatory speed of one of
them. They are in the proportion of two to one, and both spheres
together contain two-thirds as much matter as our sun. They are,
accordingly, at least four times more tenuous; but the consequence
cannot be said to discredit the postulate in view of the extreme
rarefaction characterising white stars in general and helium stars in
particular. The distance of Algol from its satellite (always on Vogel’s
hypothesis of equal densities) is 3,230,000 miles, leaving an interval
between their surfaces of scarcely more than 2,250,000 miles. But we
shall find that even closer degrees of contiguity are compatible with
stability in the mechanism of the stellar heavens.
The period of Algol has long been known to vary minutely but
continuously. But as to the nature, law, or cause of these inequalities
nothing had been ascertained, and little had even been surmised, prior
to Dr. Chandler’s discussion of them in 1888.[567] He proved them to be
slowly compensatory, not indefinitely progressive. Consistently in
advance of their due time down to about the year 1804, the obscurations
of the star then began to fall behind it, and the delay had in 1843
accumulated to 156 minutes. A gradual process of restoration thereupon
set in, and the normal epoch was reached near the beginning of 1873. It
was, however, quickly transcended, for acceleration was still going
forward, and may not attain its term for some years yet to come.[568]
These irregularities are evidently comprised in a cycle of considerably
more than a century; they can scarcely, for that very reason, be
accounted for on gravitational principles; since a third body, revolving
in so long a period, would be too distant to perturb markedly the
movements of the close pair traversing an inner circuit. Dr. Chandler
hence resorted to another mode of explanation.[569] He proposed to
account for the alternate anticipations and retardations of Algol’s
eclipses on the principle of the equation of light. They might result,
he pointed out, from the description, by the occulting pair, of an orbit
so wide that the transmission of light across it takes close upon 300
minutes. The star’s phases would then be observed too soon or too late
according as they occurred on the hither or the farther side of the
great ellipse. They would be shifted by turns backward and forward in
time just as are the eclipses of Jupiter’s satellites while the earth
performs its annual circuits. The system of Algol is, on this view,
triple. Two dark masses and a vividly shining one unite to form it. The
revolutions of the eclipsing pair round the common centre of gravity,
which is at a distance from it just equal to that of Uranus from the
sun, are accomplished in about 130 years, at the rate of 2·7 miles a
second. Its members are at present nearer to us than their mean place,
and their occultations consequently forerun the mean times; this will
continue until towards the year 1934, when, on the passage of the
ascending node, a coincidence of epochs should be observed. Two other
criteria are applicable to Chandler’s theory. If it be true, Algol’s
approaching systemic movement of 2·3 miles a second should disappear
within the next decade, neutralised by orbital velocity at that time
directed away from the sun. Again, the wide circuits performed in a
plane supposed to make an angle of 20° with the line of sight, might be
directly traceable as undulations impressed upon the straight track of
the star’s proper motion. Minute fluctuations of position simulating the
looked-for effects have indeed been observed; but whether they are
merely casual, or represent an actual, though almost evanescent
phenomenon, is too delicate a question to be decided off-hand.[570]
Twenty years hence the waves of disturbance may have defined
themselves;[571] scarcely sooner.
An alternative hypothesis to Chandler’s was put forward by M. Tisserand
early in 1895.[572] Rejected by the former investigator as insufficient,
it assumed in the hands of the latter an extremely plausible form. No
third body is demanded by it; a slight flattening of the globe of Algol,
together with a moderate degree of ellipticity in the orbit of its
satellite, meet the needs of explanation. The combined effect would be
to produce a slow revolution of the orbital major axis, occasioning just
such an inequality in the times of conjunction as that discussed by Dr.
Chandler; and the fundamental postulated cause is likely to be present.
Algol must have a rapid rotation; otherwise its system could not long
subsist. That is to say, the maximum length of its axial period is 2^d
21^h, the period of its revolution. This implies an equatorial speed of
13½ miles a second, and a consequent equatorial bulging of very
considerable amount. One of the conditions stipulated by Tisserand may
then be granted, and the second can scarcely be absent, since the
eccentricity of stellar orbits rarely falls short of the degree
required. Verification may be procured by the spectroscopic detection of
variations of velocity in different sections of Algol’s path. But the
crucial test of the theory is of the photometric kind. At intervals of
120 years, if it correspond with fact, the shortest and longest radii of
the ellipse traversed would, owing to the progression of the apsides,
alternately point towards the earth. In the first case, the eclipses
would be abbreviated by periastral acceleration; in the second, they
would be long, because the movement at apastron should be slow. Their
duration at present approaches to being the longest possible; if they
shorten notably from 1910 onward, Tisserand’s hypothesis will be amply
confirmed, while their failure to do so will compel its final rejection.
Chandler’s, on the other hand, will remain in possession of the field
should the alleged periodical disturbance of Algol’s proper motion be
definitively established. Thus the rival theories alike wait on the
future, and invite the award of events.
As the upshot of a careful series of measurements with the Yale
heliometer, Dr. Chase[573] ascribes to Algol a parallax of 0·035″,
equivalent to a light-journey of ninety-three years. If actually so
remote, it gives just eighty times as much light as the sun from a
surface not very greatly larger, but fifty-two times more brilliant—an
inference surprising indeed, but not incredible.
A second Algol-variable was recognised by Hind in 1848.[574] Usually of
8·2 magnitude, S Cancri loses and regains more than three-fourths of its
light in 21½ hours, divided between 8½ of decline and thirteen of
restoration. The dissymmetry of the phases is increased by a remarkable
pause in the brightening after minimum, as if a secondary cause of
obscuration had supervened. Nor should it be forgotten that Schmidt
observed on 14th April 1882 an excessive darkening of the star, which
remained for a whole hour sunk nearly to the twelfth magnitude. The
period which, until lately, was the longest ascertained for any member
of its class, is subject to a cyclical disturbance embracing at least
300 light-cycles.[575] The deviations of the computed minima sometimes
run up to forty minutes. It will be of great interest to determine
whether they imply the presence of a third attractive body, or whether
spheroidal deformation will suffice to account for them. The dimness of
S Cancri places it for the present beyond the reach of useful
spectrographic research. Its density has, however, been calculated by
Mr. H. N. Russell of Princeton University,[576] from the ratio between
its period of revolution and the duration of the eclipses suffered by
it, with the result of showing that the star is composed of materials
forty times more attenuated than those of the sun! And this is an upper
limit.
Shortly after Hind’s detection of S Cancri, Baxendell found that its
peculiarities were shared by λ Tauri, a radiantly white star of 3·4
magnitude. Its eclipses, as in several other cases, deepen more quickly
than they lighten. They occur at intervals of 3^d 23^h, and last ten
hours. They do not, indeed, come off quite punctually. An oscillatory
disturbance of unknown law affects them, which occasions “errors” from
the computed epochs, amounting at times to three hours.[577] Plassmann
regards λ Tauri as continuously variable.[578] He noticed in 1891 a
secondary dip in brightness fifty hours after the chief minimum, besides
two intermediate maxima; and the Pulkowa photographs lent in 1897[579]
some partial countenance to his views. They showed the spectrum to be
occasionally and unequally double, the fainter rays, by their relatively
large displacements, betraying their origin from a mass greatly inferior
to that of the star characterised by the less mobile, and more intense
absorption-lines they accompany. Thus M. Plassmann’s second eclipse[580]
is real, though inconspicuous. No orbital elements have yet been
assigned to this star. M. Bélopolsky regarded his materials as
inadequate for purposes of computation; and indeed the movements derived
from his plates were of a somewhat problematic nature. They greatly need
elucidation, which it ought not to be very difficult to supply. The
spectrum of λ Tauri is of pure helium type. The calculated density of
the pair is about one-tenth that of the sun.
The fourth Algol variable is an all but perfect timekeeper. The phases
of δ Libræ have been watched since 1858 without the detection of any
assured irregularity. They last twelve hours, of which 5½ are spent in a
decline from 5·0 to 6·2 magnitude, and 6½ in the reversal of the
process. The eclipsing body appears to be wholly obscure, but the
spectrographic method has not been applied to the system. The limit of
density found for it by Mr. H. N. Russell is one-twenty-fifth that of
the sun.
The phases of U Coronæ are very similar to those of Algol, but the
intervening time is longer—eighty-three in lieu of sixty-nine hours.
Each pair, too, is similarly composed of a bright and dark member, and
their mean density comes out nearly the same. The analogy is completed
by the presence of a variation in the period, evidently akin to the
disturbances of Algol,[581] and explicable, doubtless, on an identical
principle. U Coronæ, however, is a comparatively faint object; at high
light it is of only 7·5 magnitude, and consequently offers scant
facilities for research.
The eclipsing system designated RS Sagittarii, discovered by Gould in
1874, was subjected in 1896 to exact inquiries by Alexander W. Roberts
of Lovedale, South Africa.[582] From them it appears that the coupled
stars are alike in size, but so unlike in lustre that one gives more
than twice as much light as the other. There result two unequal minima
in each revolution; at the first, the combined magnitude of 6·6 drops to
7·6, at the second, to 6·9. The former has, besides, a duration of 10^h
40^m, the latter of only seven hours; whence the orbit is found to have
an eccentricity of 0·25, the long, deep eclipse occurring at apastron,
the slighter phase coinciding with the rapid sweep through periastron.
The intervals of time from each to the next are of 2^d 10^h very nearly;
and their equality implies that the major axis of the path pursued is
directed towards the earth. The plane of the ellipse, however, must be
somewhat inclined, since the mutual transits of the globes circulating
in it are not central; and the amount of its inclination may be
determined when the course of light-change is more accurately known. The
gravitational period (as it may be called) is, of course, double the
eclipse-period, or 4^d 20^h; but the mass of the system can be
ascertained only by spectroscopic means. Its brighter member proves to
be of about one-sixth, the dim component of one-fifth, the solar
density.[583] All these particulars have been gathered from the
photometric relations of these intimately conjoined bodies. From their
dynamical relations, truths no less remarkable will perhaps before long
be elicited.
[Illustration:
FIG. 21.—Photometric Curves of Algol Variables (Pickering).
]
At Moscow, in 1880, U Cephei was added by Ceraski to the Algol family.
This object is distinguished by the abruptness and extent of its
changes. In four and a half hours it descends from 7·1 to 9·4 magnitude,
this profound obscuration lasting for about two hours, after which
brightness returns, almost, if not quite as quickly as it departed. The
light-curve, determined photometrically by Professor Pickering, is shown
in Fig. 21.
He explains its singularities on the hypothesis of a total eclipse by a
large semi-obscure body,[584] and alleges confirmatory evidence in a
barely perceptible secondary minimum corresponding to the transit of the
brilliant over the dusky globe. Chandler and Yendell, nevertheless, deny
the reality of the minor eclipse;[585] and Wilsing vainly endeavoured in
1890[586] to bring the observed phases of U Cephei into harmony with any
conceivable form of the occultation-theory. And the star, by its
faintness, evades spectrographic tests for motion. Some increase of blue
absorption is, however, stated to occur at its minima; and this is
noteworthy as an instance, unique among Algol variables, of alteration
in the quality of their diminished rays. Irregularities of the same type
as those of Algol affect the period of U Cephei. If dependent on the
light-equation principle, they signify the description, by the eclipsing
couple, of an orbit larger than the Saturnian, in a period of between
thirty and forty years.[587] Changes in radial motion, due to deflection
in this wide path, would, if measurable, lend authenticity to an
ingenious speculation, which may otherwise be superseded by Tisserand’s
hypothesis[588] of a revolving major axis. Mr. Russell finds U Cephei to
be a considerably more tenuous body than Algol.
The exceedingly short period of U Ophiuchi—20^h 8^m—and the halving of
its light at minimum, suggest that it is composed of twin suns,
alternately occulting one another. The period of revolution would, in
that case, be twice the period of variation, and the globes would have
more room to circulate than if one of them were dark. For then the brief
intervals between the eclipses would represent each a complete round of
the orbit, and the duration of the phases would imply such close
proximity of the bright and the dark stars that the gap dividing their
surfaces would scarcely exceed three-tenths of their joint radii. Such
an arrangement is possible; a single spectrographic impression, taken
five hours before or after a minimum, would show whether it actually
subsists. If it do, the spectral lines will be single, though shifted;
if not, they should appear incipiently double, and the movements
indicated might unhesitatingly be taken to be included in a cycle twice
the length of the eclipse-interval.
Discovered by Sawyer in 1881, U Ophiuchi showed to Chandler’s patient
scrutiny individualities that should not be passed over.[589] Thus the
return of light after minimum is interrupted by a pause similar to that
observed in S Cancri, but which tends to become obliterated in a “mean
curve.” Its reality has not been established photometrically (see Fig.
21), but may emerge with the application of finer methods. The
circumstance, too, is worth remark that—again like S Cancri—U Ophiuchi
was once caught sight of during an abnormally obscure phase.[590] An
inequality of its period, comprised within thirty-seven to forty years,
further noticed by Chandler, is perhaps visibly reflected in the
disturbance of the star’s proper motion.
A still more curiously interesting object of the same class is met with
in Y Cygni. Here, at least, as M. Dunér virtually demonstrated in
1892,[591] eclipses are duplicated; two occur in the course of each
revolution. Its phases, first recognised by Chandler, 9th December 1886,
range from 7·1 to 7·9 magnitude, and are completed in nine hours. But
they were soon perceived to recur with conspicuous irregularity.[592]
Towards the middle of 1888 they were no less than seven hours behind
their calculated times, which shortly afterwards began to be largely
anticipated. Perturbations on such a scale had never previously been
betrayed by the occultations of a binary; and the task of accounting for
them by inequalities of light-transmission was evidently a formidable
one. The problem they offer was, however, destined to receive a
different, and, we may add, a definitive solution.
When Dunér came to discuss the results of his own observations at Upsala
in 1891–92, and to compare them with those made elsewhere, he was at
once struck with a persistent discrepancy between the odd and the even
sets of minima. The first, third, fifth, and so on, from an assigned
epoch, obeyed a law of recurrence quite distinct from that conformed to
by the intervening obscurations. Thus in November 1891, the intervals
from an even to an odd minimum, and from an odd to an even minimum,
differed by no less than nine hours, forty-three minutes. Clearly, the
constant sum of these discrepant periods gives the true time-measure of
systemic circulation. Two bright bodies, then, eclipse each other, and
they seem to be matched “to a hair”; their eclipses, moreover, must be
central, since the loss of light amounts to just one-half. The disparity
of their intervals depends primarily upon the eccentricity of the orbit;
secondarily, upon the situation of the line of apsides. It vanishes when
the line in question points directly towards the earth; it attains a
maximum when it is viewed at right angles, for then the right and left
sections of the eclipse being traversed respectively with the least and
greatest possible velocities, the succession of alternate occultations
reaches the limit of time-inequality. Dunér’s final conclusions
regarding the system were expressed as follows:—[593]
“The variable star Y Cygni consists of two stars of equal size and equal
brightness, which move about their common centre of gravity in an
elliptical orbit whose major axis is eight times the radius of the
stars. The period of an anomalistic revolution[594] is 2·996933 days,
and the eccentricity is 0·145. A minimum occurred while the stars were
at periastron, on 8th December 1885. The line of apsides of the orbit,
which then coincided with the line of sight, completes one revolution in
the plane of the orbit in 41·1 tropical years.”
The next coincidence of the kind, if the above elements are correct,
should take place in 1906. The compelling cause of the orbit’s gyration
remains to be investigated. It may be found in disturbance exercised by
an unseen, exteriorly revolving mass upon the conjoined suns; or their
own spheroidal shape may be solely concerned in producing it; the
question has an important bearing upon the construction of all such
systems. The linear dimensions of the orbit of Y Cygni will
unquestionably be determined ere long with the spectroscope, whence the
mass of the bodies travelling in it will at once follow. A mean density
less than one-sixth the solar is ascribed to them with some
confidence.[595]
The variations of R Canis Majoris, detected by Sawyer in 1887, have been
traced, as it were, only in outline. Their period is 28^h 16^m, five
hours of which are occupied by the phases, and since the star fades to
half its normal lustre, they are likely to be conditioned much as are
those of Y Cygni. Two similar globes presumably undergo them in turn,
mutually revolving in double the period of their occultations.
The character of R Aræ was noticed by Mr. A. W. Roberts in 1891.[596] He
considers the eclipses, which recur once in 4^d 10^h, to be not always
of the same depth; but this symptom of intrinsic variability in one or
both of the transiting stars needs to be verified. The curve at minimum
is symmetrical.
The nature of X Carinæ—another southern variable discovered by Roberts
in 1892—is still dubious. It changes from 7·9 to 8·6 magnitude in 6^h
39^m, and remains constant only during 6^h 20^m. So that the phases
extend over more than half the period of variation, which must evidently
be doubled to give the period of revolution, since no eclipse can
possibly have a duration of more than one-half the occulted body’s
orbital circuit. The alternate minima of X Carinæ are thought by
Roberts[597] to be, to a very small extent, unequal, and to succeed each
other at slightly different intervals. If this be so, the system is
composed of two stars, one a little brighter than the other, pursuing a
nearly circular track in a period of twenty-six hours, and in such close
contiguity that the times during which their discs overlap are longer
than the intervals of their apparent separation. The actual subsistence,
however, of this, or some analogous arrangement has yet to be proved.
The variations of Y Boötis are also more or less enigmatical.[598] They
are limited to six-tenths of a magnitude, and have a period rather
shorter than that of Algol. The eclipsing character of this
eighth-magnitude star, suggested by Parkhurst in 1893,[599] was
confirmed, on the strength of a year’s observations, by Yendell.
Chandler, nevertheless, expresses doubts as to its genuineness, which is
compromised by extraordinary anomalies, hardly amenable to explanatory
efforts. The predicted minima do not always occur, and their failures
seem capricious and inconsequential. But if they depended, as Parkhurst
thinks they must,[600] upon a certain critical inclination of the orbit,
causing transits to be occasionally missed, a law of periodicity should
be traceable in the lapsed phenomena.
The obscurations of S Velorum recorded themselves on the plates of the
Cape “Durchmusterung,” and the record was duly interpreted by Mr. Ray
Woods in 1894.[601] They are marked by the same peculiarities as those
of U Cephei, and probably indicate total effacements of a radiant sun by
the prolonged transits across it of a voluminous, but dimly shining
companion sphere, the diameter of which, according to Roberts,[602]
cannot fall short of half the distance between the revolving bodies.
Their respective densities, as estimated by him, are 0·61 and 0·03 that
of the sun,[603] the dusky mass proving, on the assumed data, to be
twenty times more rarefied than the brilliant one. A disparity so
extreme cannot readily be admitted as real. If only the star could be
elevated on the photometric scale,[604] the taking of a few
spectrographs would at once acquaint us with the true plan of its
system; but its faintness—7·8 magnitude—must long continue to baffle
experiments of this kind.
A modified specimen of the Y Cygni sub-class is met with in Z Herculis.
Its eclipsing character was announced by Chandler in 1894;[605] about a
month later, Hartwig and Dunér independently detected in it a double
sequence of disparate minima, with periods respectively of forty-seven
and forty-nine hours. Hence the revolution of a pair of unequally bright
stars in a period of just four days was inferred with virtual
certainty.[606] In M. Dunér’s words, “Z Herculis consists of two stars
of equal size, one of which is twice as bright as the other. These stars
revolve round their common centre of gravity in an elliptical orbit, the
semi-axis major of which is six times the diameter of the stars. The
plane of the orbit passes through the sun, the eccentricity is 0·2475,
and the line of apsides is inclined at an angle of 4° to the line of
sight.” The chief minimum lasts 6·6 hours, and occurs not far from
apastron. The secondary phase is hurried through in four hours, when the
stars are moving with nearly their greatest speed. It is, however,
unlikely that this relation will continue unchanged;[607] since it may
be taken almost as an axiom that orbits so conditioned pivot round in
space, turning their longest axes successively in every direction.
The first Algol variable photographically discovered was W Delphini. On
18th July 1895, Miss Louisa D. Wells missed a 9·3 magnitude star from a
Harvard plate exposed 26th September 1891,[608] while upon seventy-one
earlier and subsequent ones it was normally imprinted. The one tell-tale
photograph had been taken during eclipse, when it sinks to 12·1
magnitude—that is to say, eleven-twelfths of its light are cut off by
the interposing body. Pickering[609] believes the latter to be partially
luminous and very large, affording prolonged totalities, but the
photometric curve (see Figure 21) hardly warrants this assumption. It is
fairly sharp at minimum, not flat, like that of U Cephei, and
corresponds better with a partial occultation by a wholly dark satellite
than with the central transit of one dimly radiative. The period is not
constant.[610] Deviations from regularity amounting to one hour had
become manifest early in 1898.
The phases of a seventh-magnitude star (DM + 12° 3557) named RX Herculis
were discovered by Sawyer in 1898.[611] They range over eight-tenths of
a magnitude, and recur at intervals of 21^h 21^m. Like those of U
Ophiuchi, they probably indicate the revolution, in double that period,
of two equal stars; and since the minimum brightness is just one-half
the maximum, their mutual occultations may be total.
The variability of R^2 Puppis (CPD − 41° 1681), noticed by Professor
Kapteyn during his inspection of the Cape Durchmusterung negatives, was
verified and defined by Mr. Innes in 1899.[612] The period at first
assigned of nearly thirteen days was abridged to one-half that length by
Mr. Roberts’s investigations.[613] The light fades at minimum to
one-third its full amount, through the intervention of a dimly luminous
mass.
U^3 Cygni and V^2 Cygni were both detected by Madame Ceraski in studying
photographs of the sky taken at Moscow.[614] They undergo analogous
changes, investigated at Harvard College in 1899–1900.[615] Those of U^3
Cygni are remarkable for their extent, the greatest known in an
eclipsing star, unless (which is doubtful) W Delphini should be
bracketed with it. V^2 Cygni, at full brightness, ranks little higher
than the eleventh magnitude, and descends, once in six days, nearly to
the thirteenth. It is thus an object at the limit of detailed
observation. Innumerable systems of the same kind must lie beyond that
limit. The twenty-first star on our list—R^2 Velorum—suspected as an
Algol variable by Innes in 1901, was verified and investigated by
Roberts.[616] Of the remaining six objects enumerated, U Sagittæ was
found by M. Schwab of Ilmenau to vary after the manner of U Cephei;[617]
and UZ Cygni, detected by Mrs. Fleming in 1902, is remarkable for a
period more than thrice as long as that of S Cancri. UW Cygni, RV Lyræ,
and the still unnamed star in Perseus have been recently discovered by
Mr. Stanley Williams.
Algol variables, without any recognised exception, show first-type
spectra. They are either helium or Sirian stars. This specialty is
unaccountable, and may perhaps vanish with the widening of experience;
for many close binaries exempt from eclipses belong to the solar class,
and no reason is apparent why those happening to revolve in planes
coincident with the visual ray should differ in quality of light from
those revolving in orbits variously inclined to it. Nor is it yet quite
certain that the eclipse-theory accounts for certain minor phenomena in
the stars to which it applies. Thus some of their light-curves, as drawn
visually, are marked by peculiarities incapable of being explained as
the outcome of purely dynamical relations. They may, however, turn out
to be illusory or subjective; their reality is not incontrovertible.
Again, the exceptionally low minima recorded for S Cancri and U Ophiuchi
need confirmation. The possibility of mistake is not excluded so long as
each remains an isolated event.
Three varieties of eclipsing stars may be distinguished. The first
includes bright and dark pairs, like Algol and its companion, revolving
in slightly oblique orbits. One partial occultation takes place in each
revolution. The intimate association which they present of bodies at
opposite extremes of luminosity is not a little remarkable. In the
second variety, exemplified by U Cephei, a brilliant star circulates
round a larger, but far less lustrous globe. One prolonged totality
marks the orbital period. The secondary minima, theoretically inevitable
in such cases, have not been certainly observed. Vanishing stars, could
they be discovered, would appropriately illustrate this mode of
construction where the contrast in light-power had reached its limit.
Finally, the third species of occulting systems consists of stars
undergoing nearly equal double eclipses, the period of revolution
comprising two periods of variation. Y Cygni is a typical example. If
the loss of light amount to one-half, or eight-tenths of a magnitude,
and the alternate minima be of the same intensity, the eclipses are
total; for two similar stars, one is temporarily substituted. If, owing
to the inclination of their path, they only partially conceal one
another, the phases will be slighter, yet still equal. Their disparity,
in odd and even series, shows at once that the balance of luminosity is
tilted; and indications are not wanting that its level is disturbed
rather by inequalities of intrinsic lustre than of shining area.
The time-keeping of eclipse-stars is a subject demanding profound and
persistent study. The minutest irregularities traceable in it may be of
far-reaching significance.[618] On what principle they should be
explained, is still largely an open question. Possibly several forms of
action conspire, even in the same system, to produce the sum-total of
their deviations. In no case has the presence of a third body been
proved; in no case have perturbations of the ordinary gravitational type
been suspected. On the other hand, the occulting and the occulted globes
must be deformed through rotation; hence one true cause for the observed
inequalities falls within our ken; whether it is a _sufficient_ cause
alone remains doubtful. Essentially, however, increase of knowledge
regarding these marvellous combinations depends upon the development of
spectrographic methods. Surely, although perhaps in slow succession,
they will yield the secrets of their construction to a mode of inquiry
that continually gains power and accuracy, and is capable of dealing
directly with the most recondite springs of celestial mechanics.
CHAPTER XIX.
SHORT-PERIOD VARIABLES.
The limit of length for “short periods” of stellar variation is
conventionally fixed at thirty days; but it is seldom reached by objects
of typical character. Rapid fluctuations are almost always accomplished
with extreme exactitude both as to time and amount. To this rule there
are very few exceptions, and a reason for it is not difficult to find.
Variability of the kind in question is precise because it originates
extrinsically. It might be called a “forced vibration” of change. Its
course is, in some way, prescribed by the revolutions of a satellite. No
more curious spectroscopic discovery has been made than that of the
binary nature of short-period variables. By it a breach has been made in
the wall of mystery surrounding stellar light-change. The breach has not
yet been mounted, nor is it quite practicable; but by persevering
efforts it can be gradually widened and levelled.
In dealing with variable stars we must proceed tentatively. The subject
is so complex that no intelligible view of it can be gained all at once.
A unifying principle is still lacking. We can only take things as they
present themselves, noting differences, tracing partial analogies, and
arranging into some fashion of order a multitude of heterogeneous
examples. Thus stars fluctuating in short periods may be separated into
three families. The first has δ Cephei, the second ζ Geminorum for its
head, and their members may conveniently be designated Cepheid and
Geminid variables. The third is represented profusely, but almost
exclusively, in globular clusters. To begin with the Cepheids.
They are numerous and well known. Their changes are continuous and of
moderate amplitude, but proceed unsymmetrically as regards time. The
rise to maximum occupies on an average about a third of the period, or
half the time allowed for the decline from it to minimum. This
retardation is accompanied by an inherent tendency to a second maximum,
sometimes barely indicated as a pause in descent, but in several cases
giving rise to a pronounced “hump” on the downward slope of the
light-curve. The variations of δ Cephei range from 3·7 to 4·9 magnitude
in 5^d 8^h 47^m 39^s, of which 1^d 14^h 36^m suffice for the phases of
increase. The spectrum is of the solar type, and does not change with
the brightness. Oscillatory movements, however, of its constituent
lines, detected by Bélopolsky in 1894,[619] betray the presence of an
obscure companion revolving in the light-period. The ellipse described
is so eccentric that the companion-bodies when at apastron are three
times further apart than at their nearest approach; and its major axis
deviates only by two degrees from a vertical plane passing through the
earth. There is nothing indeed to show that it may not be highly
inclined to the corresponding horizontal plane. The orbital level is
undetermined, being evidently such as to exclude eclipses. This was
unexpected, but it is certain. The criterion is simple. Radial velocity
should vanish at minima if a transiting globe were concerned in their
production; in point of fact, the epochs of least brightness precede the
epochs of conjunction by a full day. The system of δ Cephei is not then
an eclipsing system. The star’s variability must be otherwise accounted
for. Nor would it be easy, without abusing the licence of hypothesis, to
expound it as the result of occultations. The inducement to make the
attempt is, at any rate, removed by the ascertainment of their
non-occurrence. Nevertheless, the coincidence of periods assures us that
orbital revolution, in one mode or another, prescribes the flow of
change. But the ideas so far entertained on the subject scarcely bear
examination. Obviously untenable, for instance, is Mr. Roberts’s view
that the companion of δ Cephei is raised, by the heat received at
periastron, from sensible obscurity to a nearly equal grade of lustre
with its primary.[620] If this were so a double spectrum should be
observed at quadratures. Again, the maximum should, on the hypothesis,
fall short of twice the minimum brightness; actually, it exceeds it
three times. Mr. Roberts himself adverts to these objections, but holds
them not insuperable. Mr. Eddie’s suggestion[621] of luminous increase
through intensified tidal action at periastral approach, is more
plausible. Bodily strain due to mutual gravitation would, in so
eccentric an orbit, gain twenty-seven-fold efficacy as the bodies moving
in it come together from its farthest point; and the processes of
internal circulation might possibly be sufficiently quickened by the
disturbance to yield a largely augmented output of light. But commotions
of the requisite violence could not subside with perfect regularity
every five days, and they should inevitably be accompanied by gaseous
outbursts spectroscopically evident. That they do not occur may be
securely inferred from the one circumstance that no trace of emissive
symptoms is met with in the light of this star. As to the scale of its
system we are, moreover, completely ignorant, and are hence unable to
estimate the absolute power over its members of tidal influences. The
line-displacements of the bright star acquaint us merely with its rate
of motion as projected upon the visual plane; they correspond to a mean
orbital radius of 620,000 miles, the real path being perhaps six or
eight times wider than that spectroscopically indicated, while the
companion-ellipse traversed by the dark satellite may be of any
imaginable size. A sapphire-blue star of the sixth magnitude forms with
δ Cephei, which has a golden sheen, a combination resembling that so
beautifully exhibited in β Cygni.
The variations of η Aquilæ are, in every respect, analogous to those of
δ Cephei. They have the same range of 1·2 magnitudes, and a somewhat
longer period of 7^d 4^h 14^m, which they divide, in about the same
proportions, between a rapid increase and a leisurely decay of
brightness. The abortive secondary maximum of δ Cephei, however,
develops in η Aquilæ into a pronounced recovery of lustre. The
light-curve shown in Fig. 22, from Dr. Schur’s delineation, renders the
duplex phase conspicuous. The binary character of the star was detected
by Bélopolsky in 1895,[622] and he computed its orbit from improved
spectrographic data in 1897.[623] It proved to be of nearly the same
_apparent_ size as that of δ Cephei, and the conditions of revolution
were, in this case again, manifestly inconsistent with the occurrence of
eclipses. An interval of two days was found to separate each minimum
from the ensuing conjunction of the bright and dark spheres. Their
conjunctions, accordingly, are not transits, since they are
unaccompanied by any diminution of light. “Very remarkably,” Bélopolsky
writes, “the same state of things is present in the variable star δ
Cephei. Hence some other cause must be sought by which the variations in
lustre and the spectral displacements of these two stars may be brought
into harmony.”
[Illustration:
FIG. 22.—Light-Curve of η Aquilæ (Schur).
]
Mr. W. H. Wright of the Lick Observatory renewed in 1899,[624] and
substantially confirmed the Pulkowa investigations. His orbit, however,
came out considerably more eccentric than that computed by his
predecessor, and he located its major axis somewhat differently. But on
the essential point for the theory of the star’s variability—the
impossibility of eclipses—they agreed. Fig. 23 reproduces Mr. Wright’s
drawing of the ellipse described by η Aquilæ round its invisible
companion, supposed immovable at the focus (O). Its situation at minimum
epochs (marked _Min._ in the figure) obviously corresponds to nearly the
highest rate of speed in the line of sight, and to a comparatively wide
visual separation of the coupled bodies. The periastron is at P; the
points where the secondary phases take place are indicated respectively
as _Min.__{2} and _Max.__{2}. They are unaccompanied by irregularities
of movement. In fact, neither in this object nor in the analogous one
decorating the crown of Cepheus is there any traceable connection,
except their significant agreement in period, between the flow of
spectroscopic velocity and the rise and fall of brightness. The spectra
of these two variables are quite similar, only the lines of η Aquilæ are
more diffuse. Both, too, are approaching the sun at uniform rates of
about fourteen and nine miles a second respectively. Finally, both are
subject to some slight disturbances in time.[625] Those of η Aquilæ,
discussed by Dr. Lockyer,[626] have an amplitude of five hours, and are
self-compensatory in a cycle of 400 light periods.
[Illustration:
FIG. 23.—Orbit of η Aquilæ (Wright).
]
The generalisation is a tolerably safe one that all Cepheid stars are
binaries; but its establishment must be a work of time. Those of the
shortest periods are the most promising for purposes of spectrographic
inquiry, since they are likely to be in swift circulation, and hence to
show conspicuous line-displacements. Negative results, however, as
already said, may simply imply an unfavourable situation of the orbits;
they do not necessarily indicate the solitary condition of the stars. A
few examples of Cepheid variables are given below in the order of
increasing length of period. Some brief description of the peculiarities
of each follows.
┌──────────────┬──────────────────┬───────────┐
│ Name. │Limits of Change. │ Period. │
├──────────────┼──────────────────┼───────────┤
│ │ │ d. h. m.│
│ R Muscæ │6·6 to 7·4 mag. │ 0 21 10│
│ R Trianguli │6·6 „ 8·0 „ │ 3 9 20│
│ Australis │ │ │
│ST Cygni │6·6 „ 7·4 „ │ 3 20 10│
│ T Vulpeculæ │5·5 „ 6·5 „ │ 4 10 28│
│ Y Sagittarii │5·8 „ 6·6 „ │ 5 18 33│
│ U Sagittarii │7·0 „ 8·3 „ │ 6 17 46│
│ X Sagittarii │4·0 „ 6·0 „ │ 7 2 50│
│ W Sagittarii │4·8 „ 5·8 „ │ 7 14 16│
│ S Sagittæ │5·6 „ 6·4 „ │ 8 9 7│
│ X Cygni │6·4 „ 7·7 „ │ 16 9 15│
│ W Virginis │8·7 „ 10·4 „ │ 17 6 30│
│ T Monocerotis│5·8 „ 8·2 „ │ 27 0 18│
└──────────────┴──────────────────┴───────────┘
R Muscæ, being circumpolar at the Cape, can be observed to advantage
only in southern latitudes. Once in twenty-one hours it doubles its
light, and so emerges into naked-eye visibility, then sinks back again
out of sight. The rise occupies just seven hours, or one-third of the
period—the normal proportion for stars of this class. Evidence of rapid
circulatory motion is pretty sure to be elicited by spectrographic
means.
The phases of R Trianguli Australis were noticed by Gould in 1871, and
have of late engaged the attention of Roberts. They show the
peculiarity, surprising in a Cepheid star, of being uncertain in extent.
The full measure of change is from 6·6 to 8·0 magnitude, but at certain
maxima it mounts no higher than 6·8; at certain minima it descends no
lower than 7·5 magnitude. It would be of interest to learn whether these
oppositely incomplete phases occur together or disconnectedly. No
methodical account of them has, we believe, been published. The increase
of brightness in R Trianguli occupies little more than one quarter of
the period.
The light-curve of ST Cygni is “humped” like that of δ Cephei. About
forty-six hours after maximum the decline is stayed, then, a brief pause
ended, resumes its course. The interval from minimum to maximum is 21½
hours, that from maximum to minimum seventy.[627] The variability of
this star[628] was detected at Potsdam in 1896 by G. Müller and P.
Kempf.
The spectrum of T Vulpeculæ is similar to that of δ Cephei.[629] Its
changes, discovered by Sawyer in 1885, consist in a rise of one
magnitude in 1^d 7^h, followed by subsidence in 4^d 3^h. The comparative
brightness of the star brings its movements well within range of
spectroscopic investigation.
The four stars in Sagittarius, which come next on our list, form a
singular group, discovered by Schmidt in 1866.[630] They are included in
a space of about ninety square degrees, and all vary after the fashion
of δ Cephei in periods comprised between 5·8 and 7·6 days. Among them X
Sagittarii, which attains once a week fourth-magnitude rank, is the most
conspicuous. Its spectrum is of the solar type. One of its associates, U
Sagittarii, distinguished by a strong orange tint, is the centre of a
little cluster. Its fluctuations, upward in 2½, downward in 4¼ days,
proceed with the regularity of clockwork.
S Sagittæ shows the retarded decrease and inflected curve distinctive of
its class.[631] Yendell considered the maximum to be double (see Fig.
24). The change at minimum is unusually slow. In quality of light the
star was found by Sir Norman Lockyer to be an exact match for δ Cephei.
X Cygni was discovered by Chandler in 1886. It rises without fail in 6^d
19^h to 6·4 magnitude, but shows an inconstant minimum brightness,
sometimes descending to 7·7, at others stopping short at 7·2 magnitude.
The period of 16^d 9¼^h is not known to vary. The star is quite
colourless; information as to the character of its spectrum is not at
present forthcoming.
W Virginis is uncertain both at maximum and minimum. Its highest rise is
to 8·7, its lowest descent to 10·4 magnitude. But these limits are far
from being always reached; phases deficient by quite half a magnitude
either way are often observed, and seem to intervene casually. The
period, on the other hand, of 17^d 6½^h is strictly conformed to. The
time of increase, 8^d 5^h, is proportionately long for a Cepheid
variable.
[Illustration:
FIG. 24.—Light-Curve of S Sagittæ (Yendell).
]
That of T Monocerotis is, on the contrary, relatively short, although
the entire cycle of twenty-seven days is the most protracted yet
ascertained for a star of its class. And here again the amplitude of
change is inconstant. Maxima occur as high as 5·8, as low as 6·4
magnitude, while the minimum brightness ranges between 7·4 and 8·3
magnitude. There are then spring and neap tides of stellar fluctuation;
yet no clue can be found to the cause of their difference. The four
Cepheid variables in which they appear—namely, R Trianguli, X Cygni, W
Virginis, and T Monocerotis—have no other features visibly in common.
Perhaps the spectroscope may reveal unlooked-for analogies connecting
their physical qualities or their systematic relations; but its dicta
have still to be pronounced.
About three dozen stars had been registered at the close of last century
as variable on the model of δ Cephei. Most generalisations regarding
them are liable to lose validity by future experience; but there are two
properties in the absence of which they should be otherwise classified.
In all, light-change progresses unceasingly; in all, it advances more
rapidly in the direction of increase than of decrease. An inclination,
more or less accentuated, to pause in descent is probably connected with
this kind of dissymmetry. Among their other distinctive qualities the
following may be provisionally enumerated:—(1) Their spectra are of the
solar type. (2) They are binaries revolving in the periods of variation.
(3) They are not eclipsing pairs; their orbits may be inclined at any
angles to the visual plane. (4) Their fluctuations in lustre are
unaccompanied by spectral change. (5) Being nearly devoid of proper
motion, they are presumably at vast distances from the earth. They are
then giant suns.
Future research will decide whether Cepheid stars are marked off from
ordinary spectroscopic binaries by any peculiarities in their manner of
circulation. Are their orbits, for instance, in all cases highly
eccentric? Is there a fixed relation between the situation of the
periastron and the point of lowest brightness? Do the major axes
revolve? Above all, what differentiates short-period variables from
revolving pairs constant in light? Why do δ Cephei and η Aquilæ show an
incessantly changing lustre, while Polaris and Θ Ursæ shine steadily?
The four appear to be of analogous constitution, and to suffer no
diminution by eclipse. What form of influence, then, is it which acts so
strikingly upon the two former objects, while leaving the two latter
unaffected? Undoubtedly the close attendance of a satellite is
instrumental to the production of the observed changes of luminosity;
but other conditions also come into play—conditions that are absent in
Polaris, in β Aurigæ, in α Virginis, in Θ Ursæ, in Θ Draconis. What is
their nature? Here is the main issue as regards short-period
variability.
CHAPTER XX.
SHORT-PERIOD VARIABLES—_Continued_.
The second family of short-period variables resemble the first in the
continuous nature of their fluctuations. They are distinguished from
them by the symmetrical apportionment in time of those fluctuations. The
intervals from minimum to maximum, and from maximum to minimum, are
almost exactly equal. Not very many such stars are known. First singled
out as rarities by Dr. Chandler in 1896,[632] they have for their
exemplar ζ Geminorum.
[Illustration:
FIG. 25.—Curves representing the Variations (A) in Light, (B) in
Radial Velocity of ζ Geminorum (Campbell).
]
The variations of this star, discovered by Schmidt in 1847, carry it
from 4·5 to 3·7 magnitude in five days and twenty-two minutes, and back
again to its former level in a space of time just three hours longer.
The difference may be called negligible. Fig. 25, A, shows the
representative light-curve. Its undulations at present succeed each
other without sensible alteration; but observers of an earlier
generation regarded them as subject to disturbance. Their period,
according to Argelander, was ten minutes longer in 1869 than it had been
in 1847;[633] and Schmidt pronounced the fluctuations in brightness to
be nearly suppressed in 1868, and irregular in 1881.[634] Fresh interest
was imparted to the history of the star by Bélopolsky’s detection, early
in 1898, of its composite nature;[635] and the discovery, which had not
been published, was repeated a year later by Campbell at Lick. He
remarked besides unaccountable deviations from the even pace of
elliptical progression, established as genuine by critical
test-observations. Their nature will be seen at a glance by a reference
to Fig. 25; the lower curve in which (B) represents the radial
velocities of ζ Geminorum. The unit of time is one day, the unit of
speed five kilometers per second. A comparison with the upper curve (A)
shows that the period of motion agrees with the period of light, but
that its rate varies oppositely to the brightness—that is to say, the
star is moving rapidly in the line of sight just when its minima take
place. They are, accordingly, not due to eclipses, which should coincide
absolutely, or very approximately with zero radial velocity. The
relations of the pair are made still clearer by Professor Campbell’s
drawing of the orbit deduced from the velocity-curve (see Fig. 26). It
represents that of the bright component round the centre of gravity,
while the similarly shaped path followed by the dark star must be larger
or smaller in the inverse ratio of its mass. The ellipse depicted in
Fig. 26 has an eccentricity of 0·22; PA is the line of apsides, OE the
line of sight. Minima occur 1^d 7^h after periastron at the point
_Min._; and since the companion is then situated somewhere in the
direction _Om_, its interposition is evidently out of the question.
Recurring now to Fig. 25, B, we perceive that the heavy black line
connecting the points determined by actual measurement, pursues an
undulating course. In Professor Campbell’s words, “The observed
velocity-curve is alternately above and below the elliptic curve, and
the intersections of the two occur at approximately equal intervals of
time. There are six of these intersections, corresponding to three
complete periods or cycles in one period of the light-curve.” The
oscillations showed no signs of intermission during fifteen months, and
assuredly indicate an inherent peculiarity of the system. They might be
formally explained by assuming it to be triple, the bright star
revolving in 3^d 9¼^h round one invisible attractive mass, and the two
together in 10^d 3½^h round another more distant. But the arrangement,
as Professor Campbell points out,[636] could scarcely be stable, owing
to the commensurability of periods, and the consequent subversive
piling-up of disturbances. Nor is it easy to accept the idea that the
digressions of ζ Geminorum from a mean rate of travel are “minor tidal
effects.” Prolonged and diversified researches are, in fact, needed
before any promising theory of them can be formed.
[Illustration:
FIG. 26.—Orbit of ζ Geminorum (Campbell).
]
The star is of a golden yellow colour. Its spectrum is a replica of that
of δ Cephei. An annual proper motion is attributed to it of 0·0165″, but
the value is too small to be altogether reliable. The remoteness of this
problematic system is hence unimaginably great.
The following brief list of Geminid variables, with some ensuing
comments, will serve to widen the reader’s acquaintance with the
characteristics of the species.
┌─────────────┬──────────────────┬────────┐
│ Name. │Limits of Change. │Period. │
├─────────────┼──────────────────┼────────┤
│ │ │d. h. m.│
│R^2 Centauri │ 7·4 to 7·8 mag.│ 0 7 16│
│ S Antliæ │ 6·7 „ 7·3 „ │ 0 7 47│
│ U Pegasi │ 9·3 „ 9·9 „ │ 0 9 0│
│ 24 Centauri │13·4 „ 14·0 „ │ 0 11 5│
│ ω │ │ │
│ V Puppis │ 4·1 „ 4·9 „ │ 0 17 27│
│ U Vulpeculæ│ 6·9 „ 7·6 „ │ 8 0 4│
│_d_ Serpentis│ 5·0 „ 5·7 „ │ 8 17 17│
│ β Lyræ │ 3·4 „ 4·5 „ │12 21 47│
└─────────────┴──────────────────┴────────┘
The variability of R^2 Centauri, discovered by Mr. A. W. Roberts in
1896,[637] is slight but sure, and proceeds by evenly measured steps of
increase and decrease. The period is the shortest found for any star
outside the precincts of a cluster. With its companion, should it prove
to be spectroscopically double, R^2 Centauri must form an exceedingly
close, or an enormously massive pair.
S Antliæ bore for eight years the reputation of being an Algol
variable.[638] The shortness of the period and the flatness of the
curve, conveying the impression of a stationary maximum, produced a
deception removed in 1896 by the Harvard photometric results. They
showed the light-change to advance continuously along a smooth curve,
unbroken by the sudden drop indicative of an eclipse. It is, however,
marked by the “interesting feature” (in Professor Pickering’s
words[639]) “that the time of increase occupies 0·62 of the entire time
of variation.” Here then is an ostensible case of dissymmetry opposite
to the usual kind. But the relation was stated by Mr. Sperra of
Randolph, Ohio, to be inconstant, and he deduced from his observations
an average equality of the intervals between opposite phases.[640] This
is perhaps the essential fact. S Antliæ is a white star with a
transition-spectrum[641] resembling that of Procyon. The lines are never
seen double, so that two bright components cannot be present unless they
revolve in a plane nearly at right angles to the visual ray. But
line-displacements due to motion round an obscure body, may possibly be
detected by the application to this curious object of a
slit-spectroscope.
[Illustration:
FIG. 27.—Photometric Curve of U Pegasi (Pickering).
]
A small star in Pegasus was noticed by Dr. Chandler in 1894 as
apparently subject to eclipses; but its waxings and wanings proved, on
fuller inquiry, to be without pause. A controversy as to their
nature[642] was practically terminated by Mr. Wendell’s measures with
the polarising photometer at Harvard College in December 1897.[643]
Their upshot is graphically exhibited in Fig. 27. Each of the closely
set dots through which the curve was drawn represented eighty settings
made on eight nights, none being rejected for discordance. The crosses,
each of which gives the average of sixteen settings, embody single
nights’ results. The divisions in time (abscissæ) represent intervals of
thirty minutes; the divisions in brightness (ordinates), tenths of a
magnitude. The complete period of the star is nine hours less nine
seconds, but it is nearly cut in two by a secondary decline falling
short of full minimum by only 0·15 of a magnitude. The reality of this
trifling difference, contested by Chandler, appears to be substantiated
by the Harvard data, and stamps U Pegasi as of the kindred of ß Lyræ,
the premier short-period variable. A mathematical discussion of the
conditions of U Pegasi, based upon the eclipse-theory, was published in
1898 by Mr. G. W. Myers of the Yerkes Observatory.[644] It is valuable
as an authoritative proposal of the terms demanded by that theory under
the given circumstances. They are not wholly unacceptable. They involve,
at least, no contradiction of any known law of nature; yet they are
difficult to imagine realised. The system must obviously be composed, if
its luminous variations be geometrically explicable, of two bright
bodies revolving almost in contact. Indeed, they are probably _more than
in contact_, if the assumed data are correct; they interpenetrate and
together form an “apioid,” which, according to Poincaré, is a figure of
equilibrium for rotating masses. One of the conjoined globes proved,
moreover, to have a radius about four-fifths that of its primary, and to
be less intrinsically brilliant in sensibly the same proportion. Their
revolutions in a period of nine hours afford eclipses, alternately total
and partial, at intervals of four and a half hours, the orbit being
viewed by us edgewise. Now such an arrangement as that indicated for U
Pegasi might conceivably prevail in an embryonic binary system. It
would, in fact, accord well with Dr. See’s views as to the evolution of
double stars; but evidence of its actual existence is still a
desideratum. Until affirmed by the spectroscope, it must be treated as
only a speculative possibility; and the example of ζ Geminorum is not
encouraging to the occultation-rationale of continuous variability. The
time, however, is not far distant when the decisive motion-test will be
applied, if not to this faint object, at any rate to others that are
analogous and more accessible.
Some few of the variable stars in clusters belong to the Geminid class.
An example is afforded by No. 24 in the great southern star globe ω
Centauri. One of five or six thousand silvery specks crowded together
into the “span of a man’s hand,” it yet preserves individuality. Once in
11^h 5^m it gains twofold brightness, then fades even more rapidly than
it increased.[645] The changes are said to resemble those of S Antliæ.
If occasioned by the uninterrupted mutual eclipses of equally luminous
bodies, a mean density would be implied for them of one-fifth the solar.
And this irrespectively of their mass. In a system composed of a pair of
globes revolving just in contact, density depends solely upon
period.[646] The reason is easily seen. With a constant period the mass
of a system varies as the cube of the distance, and in the same
proportion the component spheres must, if they remain contiguous, vary
in bulk. Thus, since volume and mass preserve under these circumstances
a fixed ratio, density is the same for any assignable value of their
absolute amounts.
A variable star, more than commonly enigmatical in its procedure, comes
next on our list. Discovered by Mr. Stanley Williams in 1886,[647] V
Puppis fluctuates between 4·1 and 4·9 magnitude in a period fixed by Mr.
Roberts, with vigilant care, at 17^h 27^m 13^s. The alternate minima are
slightly unequal, and he assumes them to correspond to a trifling
disparity in brightness between the members of a mutually occulting
“dumb-bell” combination revolving in double the light-period, or 1^d
10^h 54½^m.[648] But here the spectroscope intervenes. From an
examination of spectrographs taken by Bailey at Arequipa, Professor
Pickering inferred in 1896[649] the binary character of the star. During
thirty-seven hours at a stretch it shows double absorption lines, which
then close up, and after a brief interval open out again, this time with
the fainter component in the reversed position. These shiftings to and
fro take place in a cycle of 3^d 2^h 46^m, and indicate a relative
velocity of 385 miles a second, giving a minimum value for the radius of
the orbit of 16,500,000 miles, and a combined mass seventy-seven times
that of the sun. Have we then, in V Puppis, a genuine instance of
discrepancy between the motion and light-periods? Or is their eventual
reconcilement probable? Mr. Roberts has spoken his last word on the
subject, and the spectrographic data seem sure. Yet if any compromise
were possible, it should be, one would suppose, by subdividing the
longer period, not by extending the shorter one. The anomaly of their
discordance is too flagrant to be admitted without cogent proof.[650]
The variations of U Vulpeculæ proceed equably, according to their
discoverers, MM. Müller and Kempf,[651] in a period of eight days. M.
Luizet of Lyons agrees, and regards them as strictly conformable to
those of ζ Geminorum.[652] The Harvard measures, nevertheless, indicated
an accelerated increase.[653] If it be substantiated, the star should
rank as intermediate between the Cepheid and the Geminid families.
The instability of _d_ Serpentis, suspected at Potsdam in 1891, was
verified by Yendell in 1894.[654] Its phases, as determined by him,
resemble those of β Lyræ. They include a secondary minimum,
symmetrically placed between two equal maxima. The spectroscopic
investigation of this star, which never descends so low as the sixth
magnitude, should present few difficulties, and will be of special
interest from the side-lights it may throw on the problem of the Lyre
variable. This latter subject is so complex as to demand treatment in a
separate chapter.
Another inviting object to the possessors of spectrographic apparatus is
the southern variable κ Pavonis. It ranges from 3·8 to 5·2 magnitude in
a period of nine days two hours, but by gradations lacking distinctive
character. Their correlation with spectral line-shiftings might serve
more clearly to define their nature. Physically, the star belongs to the
solar family.
One of the many singularities connected with stellar variation is that
it takes a special form in condensed clusters.[655] Even in them this
form does not prevail universally; sporadic cases of many kinds are met
with; but in general the light change of aggregated stars has the
following characteristics. The periods are extremely short. They average
half a day in “Messier 5,” and 90 out of 132 determined for the
components of ω Centauri fall below twenty-four hours.[656] The rise to
maximum is wonderfully swift. One-tenth part of the cycle is about the
proportion claimed by it, and No. 45 ω Centauri increases by two
magnitudes in the space of one hour. The minima are prolonged dead-level
tracts. In other words, the variation is discontinuous. It might be
described as a sudden leap upward into comparative brightness from a
habitually low state. The maxima are episodes, foreign, as it were, to
the internal economy of the stars. They recur, nevertheless, with the
utmost precision. Hundreds, nay, thousands of successive periods have
been watched without the detection of the smallest irregularity. The
light-curves of two stars in Messier 5 are given in Fig. 28 from
Professor Bailey’s drawings. Each has a range exceeding one magnitude,
and a period of approximately twelve hours.
[Illustration:
FIG. 28.—Light-Curves of Cluster-Variables (Bailey).
]
The discovery of “cluster variables” as a class apart was made by
Professor S. I. Bailey in the course of his photographic work at the
equatorial station of Harvard College. They literally swarm in certain
groups, while in others they occur scantily or not at all. An isolated
southern star, S Aræ, the character of which was detected by Innes, and
has been investigated by Roberts,[657] appears to be of their type.
Ordinarily hibernating near the eleventh, it springs up to 9·5
magnitude, at the rate of a magnitude in twenty minutes, once in eleven
hours. U Leporis, over which Mr. Innes has kept watch, approximates to
the same type. This mode of variation is peculiarly difficult to
explain. Eclipses will not here serve our turn; however modified, they
evidently fail to meet the requirements of the situation.[658] The
phenomena, indeed, to a certain extent, invert those with which eclipses
are associated. Instead of an abrupt failure, a sudden access of light
has to be accounted for. The question whether such stars are binaries is
of great interest. Cluster-components, which are rarely brighter than
the thirteenth magnitude, can indeed scarcely be expected to furnish a
reply to it; but something definite on the point may be learned by a
spectrographic appeal to S Aræ. The direction that should be given to
further inquiries will then become apparent.
CHAPTER XXI.
THE PROBLEM OF BETA LYRÆ.
[Illustration:
FIG. 29.—Light-Curve of β Lyræ (Argelander).
]
On the 10th of September 1784, John Goodricke of York, a deaf-mute
scarcely twenty years of age, perceived the second brightest star in the
Lyre to be variable. He ascertained, further, the main features of its
light-change. They are very peculiar.[659] Four phases of approximately
equal duration are comprised in a period of twelve days and nearly
twenty-two hours. They are portrayed in the symmetrical curve drawn from
Argelander’s observations in Fig. 29. The twin maxima, situated midway
between the principal and secondary minima, are of absolutely constant
brilliancy. Constant, too, is the chief phase of obscurity, so that the
compass of variation, from 3·4 to 4·4 magnitude, is a fixed quantity.
The intermediate minimum, however, is not so immutable. Defect and
excess are occasionally observed in it. But the flow of change is always
smooth and uninterrupted. Nor is there any pause in a slow lengthening
of the period, which has progressed, during the last hundred years, at
the average rate of about one-third of a second at each recurrence. That
the disturbance will prove compensatory can scarcely be doubted; but the
law of restoration is not yet apparent.
The spectrum of β Lyræ is dominated by helium. It includes members of
all the six series emanating from that substance, and they are mostly
composite aggregations of bright and dark rays.[660] The Huggins series
of hydrogen is similarly represented; among metals, calcium and
magnesium are prominent, and ten dark oxygen lines in the ultra-violet
were photographed at Tulse Hill in 1899.[661] But the special
characteristic of this spectrum is its variability. The coupled lines
are neither fixed in position nor constant in structure. They shift,
they split, they flash and fade; they spread into diffuse bands or
contract to definite filaments; and this in obvious, though disturbed,
subordination to the light-period of the star. The two kinds of
variation are, to some extent, mutually dependent; yet they are far from
showing a strict concurrence. The loose and indeterminate nature of
their relations places formidable obstacles in the way of investigating
either.
Already, in 1866, Father Secchi noticed bright lines in the dispersed
light of β Lyræ, and Von Gothard was struck in 1883 with their
unaccountable fluctuations of visibility.[662] But the complexities they
presented wholly baffled direct observation; their unravelment only
began to be possible when spectrographic methods became fully developed.
Through Mrs. Fleming’s examination of the Harvard plates, it was made
evident in 1891 that the emission-rays had dark companions, and were not
stationary with regard to them; and Professor Pickering[663] gathered
from their displacements the probability that the two sets belonged
severally to the unlike components of a close binary, revolving
synchronously with the ebb and flow of total brightness. He estimated
their relative velocity at 300 miles a second in a circular orbit, with
a radius of 50,000,000 miles. This hypothesis is beyond question founded
in fact. The star is composite, and the emissive and absorptive elements
of its spectrum shift, on the whole, oppositely; each battalion, as Mr.
McClean has indicated, moves as a unit, and in a contrary sense to the
other. To distinguish them ought then to be a simple matter. The
differently affected lines ought of themselves, one might expect, to
declare their separate origin. Difficulties well-nigh insuperable,
nevertheless, beset the interpretation of this spectrum. Their main
source is this. The constituent lines do indeed oscillate through
motion, but they are subject to further influences of a more complex
kind, and of a barely conjecturable manner of working. The various
species of change are hence entangled and disguised to a bewildering
extent; and totally divergent views have been expressed as to the proper
apportionment of the spectrum between the bodies jointly originating it.
Sir Norman Lockyer attributes the absorption lines to a pair of “Orion”
stars, unequally advanced in development,[664] with a relative velocity
of 156 miles a second; and the addition of a bright-line companion is an
implied necessity of his scheme. Mr. McClean[665] demands a dark-line
and a bright-line component, mutually circling at a speed of 400 miles a
second. Miss Maury considers that three stars must be engaged.[666] Dr.
Vogel[667] and Father Sidgreaves,[668] although they have investigated
the spectrum in detail, make no attempts at its analysis. M. Bélopolsky,
by minimising the scope of attack, made a substantial advance towards
the solution of the problem.[669] He dealt with only two lines—the
absorption ray of magnesium at λ 4482 and the brilliant F of hydrogen;
but succeeded in establishing, it might be said, incontrovertibly, their
separate production from conjoined bodies dissimilarly constituted. The
magnesium line is better adapted for measurement than most of the
spectral elements of β Lyræ; it is subject to only moderate alterations
in width and definition, and determinations of its motion-shifts afford,
accordingly, consistent results. From them Bélopolsky has calculated the
orbit of the originating globe, which may be identified with Lockyer’s
Rigel star. He found it to be but slightly eccentric (_e_ = 0·04); the
mean radius (supposing the plane to coincide with the line of sight) is
15,000,000 miles; the system is advancing towards the sun at the rate of
seven miles a second, and the times of zero radial velocity agree so
nearly with the epochs of minimum as to lend countenance to the
eclipse-rationale of light-failure. A second orbit was then
computed—though far less securely—for the component showing bright F,
each being described round the common centre of gravity. It proved to be
about half the size of the former, which implied that the body
travelling in it (designated A) was twice as massive as the companion
(B). It possesses, in fact, the gravitating power of eighteen, the
latter of nine suns. Nevertheless, the principal minimum corresponds to
the obscuration of the minor globe, while at the secondary phase, the
primary star is the one partially occulted. The bright-line star, A,
must then be much less luminous in proportion to the quantity of matter
it contains than the dark-line star, B. This does not appear probable,
but it cannot be pronounced impossible.
On the whole, Bélopolsky’s results are plausible, and the basis they
rest upon is solid, if narrow. Yet the development of their consequences
leads to a network of perplexities. The star A, characterised by
hydrogen-emission, can be no other than Lockyer’s second dark-line
star—that resembling Bellatrix; but if so, “the bright bands,” as Miss
Maury says, “have a residual motion of their own, which places them
sometimes towards the red, and sometimes towards the violet end of their
own system of dark lines, and at other times upon the lines of one, or
both spectra.” Yet the suggested triple combination is inadmissible. The
presence of a third body would require the introduction of a second
period, and of this no trace is discernible. The spectral phenomena are
in many ways abnormal and unaccountable, but in the long run they
conform to the single and nearly uniform time-measure of the system, and
preserve a modified fidelity to the course of its light-change.
Gravitational disturbances, too, might be expected to betray the
influence of an extra member, and none have been detected; for the
slight retardation now going forward is otherwise explicable. We seem
prohibited from carrying the subdivision of β Lyræ any further than into
a pair of globes, exemplifying distinct varieties of the Orion spectral
pattern, one or both vivified by a range of bright lines.
[Illustration:
FIG. 30.—System of β Lyræ (Myers).
]
An effort was made by Mr. G. W. Myers in 1897[670] to bring this star’s
variations within the explanatory scope of the “satellite-theory.” By
suitably combining effects of occultation with effects of tidal
deformation, he showed that the observed periodicity could be
represented with the satisfactory exactitude conveyed in the upper
section of Fig. 30, the lower section of which exhibits his plan of the
supposed orbit and its egg-shaped occupants. Evidently, when they are
seen _broadside on_ there is full light, while a minimum attends an
_end-on_ view of them. And this altogether apart from possible eclipses.
If these occur as well, the effects reinforce each other; while those
due to the gradual turning of the discs soften off the abruptness of
occultation-phases, and thus serve to give the light-curve its smooth
character. The mutually eclipsing spheroids must, however, be extremely
close together, if they do not actually coalesce. Combining, on the
questionable assumption of their congruity, the displacements of F
measured by Bélopolsky with those attributed by Lockyer to three dark
lines, Mr. Myers found the masses of his two stars to be respectively
twenty-one and ten (nearly) in terms of that of the sun, and determined
the radius of their joint orbit at 31,000,000 miles. Their mean density
proved to be lower than that of air at sea-level, and suggested a
“nebulous condition.” Indications were even discerned of a process of
separation between the components, scarcely yet, or just recently
accomplished. “In either case,” Mr. Myers adds, “we seem to have here
the first concrete example of a world in the act of being born.” And it
cannot but be noted with profound interest that “an attempt at a formal
representation of the condition of things prevailing in the system of β
Lyræ leads to the assumption of a single body, such as Poincaré’s or
Darwin’s figures of equilibrium.”
Yet the “formal representation” in question is difficult to accept as a
physical actuality. The extreme tenuity attributed by it to a star
shining with vivid lustre almost defies credence, yet is an inevitable
consequence of the satellite-hypothesis of variability. Where there is
no halt in change, there can be only a transient cessation of eclipse,
and the revolving globes must be virtually in contact. But under these
circumstances, their density, as we have seen, is a function of the
period alone; and thirteen days is long compared with the nine hours of
U Pegasi, for which star the upshot of a similar experimental
investigation has been recorded. This theory, moreover, takes account
only of the optical changes in β Lyræ. Occultation-effects,
distortion-effects, and motion-displacements of spectral lines are of
this kind. They imply no intrinsic alteration. They are compatible with
an absolute constancy in the state of the system; they depend merely
upon the visual relations to ourselves of the bodies forming it. They
are accordingly calculable and measurable. Exactly what sort and amount
of fluctuations they are capable of producing, can be ascertained from
given data. But with the physical influences of close duplicity upon
radiation we have only a speculative acquaintance. And in the present
case, those that might be due to unequal tidal disturbances are excluded
by the circular shape necessarily ascribed to the path of a star noted
for the equal duration of all its phases. Intrinsic variations in its
spectrum are, nevertheless, glaringly apparent, and they tend to recur
cyclically in just thirteen days. We spare our readers the bewilderment
of their minute description, asking them instead to fix their attention
on a few salient points.
Let us consider, for instance, the spectral symptoms at the critical
epoch of chief minimum. Almost as a matter of course, the continuous
radiance has faded; sixty per cent of it is intercepted or otherwise
suppressed. This is, in fact, the essential cause of the falling-off in
brightness. What is distinctive is that the emission-lines have become
narrower, sharper, and fainter than usual; they are considerably shifted
towards the red, and strongly developed dark companions, in their normal
places, are attached to their more refrangible sides. Now the downward
shove of the whole range of bright lines is either due to motion, or it
is not. If it is, the emitting body is travelling rapidly away from the
earth at the time of the supposed eclipse, which must accordingly be
dismissed as fictitious. If, on the other hand, the alteration of
wave-length denotes physical action of some kind in the atmosphere of
the star, then inferences as to its orbital revolution, since they have
only a spectroscopic warrant, are highly precarious. The possibility, to
be sure, may be admitted that the dark lines shift optically, the bright
lines physically _and_ optically; but the distinction has an air of
arbitrariness which does not recommend its confident adoption.
Another significant circumstance is that the spectral appearances at the
secondary minimum and at the ensuing maximum are much alike. The most
characteristic among them is the projection of a black line centrally
upon a wide bright band. Dr. Vogel’s drawings of the first ultra-violet
hydrogen line (Η ζ) at these successive phases are reproduced in Plate
XVI., Figs. 1 and 1a. Here, at any rate, a single light-source is
concerned. A moment’s consideration suffices to show that a dark line in
the spectrum of one star cannot cut a slice out of a brilliant band
proceeding from another. Absorption implies real superposition of the
arresting and absorbing layers. The effect observed is then one of
reversal. It arises through the stoppage by a cooler stratum of hydrogen
of the emissions from a denser and hotter underlying stratum in the same
stellar atmosphere. Fig. 2 in the same Plate represents, from a drawing
by Professor Keeler, the “D lines” in β Lyræ at principal minimum. It
was made with the great Lick refractor, 14th and 15th November 1889, and
shows the sodium pair to the right merged into a dark, hazy band, with
above it D_{3} brilliant and unsymmetrically reversed. Moreover, the
thin dark line constituting the reversal seemed to be nearly, or exactly
in its proper place;[671] the obvious relative shift measured the
lessened refrangibility of the emissive beam. In the gaseous envelope,
then, of one and the same star we find a helium line originating at a
low level moved towards the red, while its repetition by absorption
higher up preserves its wave-length unchanged. The indicated difference
in conditions can here scarcely be anything else than a decrease of
pressure upward from the photospheric surface. The significance of such
an inference hardly needs to be pointed out, and it seems impossible to
avoid drawing it.
The red ray of hydrogen is particularly brilliant in β Lyræ; but since
it lies beyond the ordinary spectrographic range it has of late received
little attention. Yet the history of the modifications which it
assuredly undergoes, and of the modes of their correlation with those of
its associates in the spectrum, must be learned, unless knowledge of
this wonderful star is to remain essentially incomplete.
[Illustration:
PLATE XVI.
THE ALTERNATING SPECTRUM OF NOVA PERSEI STONYHURST COLLEGE
OBSERVATORY.
1. Hζ Line in Spectrum of β Lyræ at Secondary Minimum.
1a. Hζ Line in Spectrum of β Lyræ at Second Maximum.
2. D-Lines in β Lyræ (Keeler).
3. D-Lines in Nova Persei (Hale).
4. Spectra of Nova Sagittarii.
5. Alternating Spectrum of Nova Persei (Sidgreaves).
]
We may now endeavour to sum up our conclusions regarding its nature,
tentative and fragmentary though they be. A finished theory on the
subject cannot at present be formulated; but the ground may be prepared
for it by the removal of inadmissible hypotheses and by the
clarification of thought.
(1) The system of β Lyræ is binary. Two stars, and no more, are
concerned in producing the observed changes in the quantity and quality
of its light. Both show strong hydrogen and helium absorption; one is
distinguished besides by oxygen absorption, and it is this latter which,
in all probability, emits the more conspicuous set of bright lines.
There are indications, too, that a second set is occasionally
sub-apparent, and that the spectrum really consists of two separate
ranges of dark, and two accompanying ranges of vivid rays.
(2) The dark lines are in their normal positions at minima; they are
shifted from them at maxima, when some of their number open out into
doublets. The conjunctions at times of least light, and elongations at
intervening epochs, of two bodies giving absorption spectra are thus
presumably signified.
(3) At chief minimum, the more prominent bright lines are shifted
towards the red, so as to lie beside the corresponding dark lines. The
spectrum has then the coupled aspect distinctive of “Novæ” and of
certain other emission-stars. The change of refrangibility during this
phase cannot be due to motion; it may be due to pressure.
(4) During the second half of the period, reversals are a leading
feature of the spectrum, which thus affords evidence, not only of
orbital revolution, but also of a course of physical vicissitudes
comprised in the same cycle.
(5) Finally, the cause of variability has to be considered. Is it to be
found in mutual occultations? The geometrical conditions are such as to
admit of an affirmative reply; the physical conditions are adverse. They
involve a rarefaction of the circling bodies so extreme as to repel
assent unless under the stress of rigid demonstration. And that is by no
means at hand. Evidence on the subject could perhaps more easily be
collected from objects with analogous light-curves, than from β Lyræ
itself. The endless complications which embarrass research in the
“problem star” would not, for instance, be likely to present themselves
in _d_ Serpentis. Another variable, highly desirable to be included in
such a comparative study, is R Sagittæ. This remarkable object has a
period of seventy days, symmetrically divided by two unequal minima, and
two slightly disparate maxima. The light-curve, however, underwent a
curious change in 1874. A reversal of the minima was perceived.[672]
Equalisation first took place. Then the secondary minimum gradually
gained emphasis at the expense of the primary; and the exchange of
relative values was not redressed until 1883, when the pristine state
was restored. An arrested tendency towards such a transformation is
sometimes shown by β Lyræ in the fluctuating accentuation of its
subordinate phase, but it has never reached so far as a bisection of the
period. A suggestion is, nevertheless, irresistibly conveyed that the
two stars form similarly constructed systems. When the spectra of R
Sagittæ, _d_ Serpentis, and U Pegasi have been examined, and their
changes tabulated and collated, we shall be in a better position to
interpret those manifest in the Lyre variable.
CHAPTER XXII.
STARS VARIABLE IN LONG PERIODS.
[Illustration:
FIG. 31.—Distribution of the Periods of 208 Variable Stars.
]
“Long periods” of variability range from 120 to 610 days. None more
protracted have been definitely ascertained, and those that are shorter
belong, with rare exceptions, to stars differently characterised. The
distribution of the periods we are at present concerned with is
exhibited graphically in Fig. 31. They number 208, and are taken from
Chandler’s _Third Catalogue_, only three, which exceed 500 days, being
for convenience omitted. Periods of several years have besides been
ascribed to a few stars, but on insufficient grounds. No true conformity
to them is maintained. Phases that are unusually slow are also extremely
uncertain in development. So far, the 610-day cycle of S Cassiopeiæ is
the longest that can be depended upon to recur. A cursory reference to
our diagram will show how largely, among long periods, those between 280
and 300 days preponderate. Accidents of discovery connected with the
length of the year cannot well have produced this preference, which
seems to be genuine, and not merely apparent. On the other hand, the
indentations of the curve are assuredly casual, and will be smoothed
down with the multiplication of objects.
The typical long-period variable is Mira Ceti. It was the first
detected; it rises to the brightest maxima; it presents the most vivid
and distinctive spectrum. More than 300 of its cycles have been watched,
more or less attentively; yet familiarity has not diminished wonder at
the “Wonderful” star.[673] Its modes of procedure are as much an enigma
to the spectroscopists of Lick and Potsdam as they were to Fabricius and
Holwarda. An instructive comment upon them is the omission, from
Chandler’s _Third Catalogue_, of the modifying terms appended to the
mean period of 332 days in his _Second_. It amounts to an abandonment of
the attempt to predict, with even approximate accuracy, the capricious
changes of the Frisian pastor’s _Stella insolita_. Argelander’s
laborious efforts for their regulation have thus proved futile. He
considered an oscillation extending over 80 years, and comprising 88
periods, to be fairly well established, and found indications of another
of 160 years;[674] but their supposed effects have ceased to be
apparent. Guthnick’s “long inequality,” covering 200 cycles, will
doubtless prove equally illusory. No method is indeed securely traceable
in the accelerations or retardations of the maxima, and they digress to
the extent of fully two months. Long and short periods can neither be
perceived to alternate nor to occur in series; still opposite deviations
balance each other; there is no progressive alteration in the length of
the cycle.
The highest maximum and the lowest minimum recorded for Mira were both
observed by Sir William Herschel. He found the variable nearly equal to
Aldebaran on 6th November 1779,[675] while four years later it was
invisible with a telescope showing stars of the tenth magnitude. Of late
it has not been known to descend below 9·5, and it sometimes stops short
at 8·0 magnitude. Its greatest brightness is even more inconstant. No
more than 5·6 magnitude was attained in November 1868,[676] or
one-fortieth the lustre of the phase viewed by the Bath organist, and
maxima higher than the third magnitude are uncommon. The course of
change likewise fluctuates, but in general the rise is considerably more
rapid than the decline, and the high-level status is maintained for
about two months, the low-level for at least three, without striking
alterations. Yet change is always in progress. The light-curve has no
flat stretches.[677] No connection is apparent between the acuteness of
the light crises in this star and the times of their occurrence.[678]
They do not tend to become abortive when hurried, nor is delay
accompanied by intensification. Argelander entirely failed to correlate
irregularities of period with discrepancies in the amplitude of change.
As the fruit of tercentennial experience it has, however, been learned
that long-period variables are no transitory phenomena. Mira, at any
rate, exhibits no symptoms of decadence since the maximum which
surprised Fabricius in August 1596.
Its spectrum gives evidence of powerful disturbance, but none of
duplicity. Motion-shifts depending upon orbital revolution are
imperceptible. The periodicity of the star must be explained otherwise
than by attributing to it a binary character. The task of doing so is
indeed most arduous. Once in eleven months the brightness augments some
hundreds of times, and concomitant spectral modifications afford
assurance that these annual outbursts are accompanied by atmospheric
ignition. What occasions them? We are ignorant; yet the issue may be
narrowed by the following consideration. If external action of any kind
were concerned in their production we should expect the incandescence to
be coronal or chromospheric—to affect primarily the outer layers of the
gaseous envelope. But in fact the innermost strata are those set aglow,
while the overlying vapours remain comparatively cool. The masking of a
bright hydrogen line by calcium absorption places the subsistence of
this relation beyond question. So far, then, the evidence favours the
view that variability of the Mira type arises spontaneously, rather than
through outside influence.
On the 13th of December 1885 Mr. J. E. Gore was struck with an
unfamiliar reddish star of the sixth magnitude in the Club of Orion. No
map included it, and until it reappeared a year later after an interval
of quasi-extinction, there was no telling whether it should be reckoned
as a Nova or a variable. Dr. Copeland recorded for it “a very beautiful
banded spectrum of the third type, seven dark bands being readily
distinguished with the prism.” The intervening spaces appeared “full of
bright lines, especially in the green and blue.” Two of these were
certainly emitted by hydrogen, and others probably by helium, since a
vivid D_{3} was observed by Von Konkoly ten days later, the adjacent
sodium pair being, as usual, dark. Fig. 32 exhibits the light-curve of U
Orionis during the maximum half of its period, as delineated by M. Porro
at Turin 1889–90. Its form is by no means invariable. In general, the
brightest phase is reached much more abruptly than it was in 1890. The
main rise, however, is always prompt, and the decline gradual, although
the minima appear to be well defined. They have not, indeed, owing to
their faintness, been much observed. The widest amplitude of the star’s
change is from 5·3 to about 12·5 magnitude; but these limits are seldom
attained. Nor is the assigned period of 375 days conformed to with any
exactitude. As in the case of Mira, prediction has to be qualified with
a large allowance for unexplained disturbance.
[Illustration:
FIG. 32.—Light-Curve of U Orionis (Porro).
]
At high maxima χ Cygni radiates about 6500 times more powerfully than at
low minima. The star, that is to say, has a range of fully nine and a
half magnitudes, from the fourth to near the fourteenth. But in some of
its cycles it fails to ascend beyond 6·5 magnitude; for it resembles the
other stars of this class in having no fixed measure of change.[679] Its
period is now 406 days; it has lengthened, on an average, by a quarter
of an hour at each recurrence since Kirch, in July 1686, missed from its
wonted place the star located by Bayer _in collo Cygni_;[680] nor is
there yet any sure sign of a compensatory reversal. The nature of the
secular perturbation thus betrayed can scarcely be imagined. Argelander
noticed besides deviations from the mean period up to forty days, and
sought, with imperfect success, to analyse and regularise the
inequalities upon which they depended.[681] The increase of light in
this variable occupies 171 days, or considerably less than half the
period. Its most brilliant phases are brief, while fainter maxima are
sometimes prolonged for a couple of months. In 1847 the star remained
visible to the naked eye during 97 days, although the usual time of
“lucidity” is, by Argelander’s estimate, only 52 days.[682] The scarlet
blaze of its light is often very striking.
R Hydræ is an accelerating variable. In 1708 the interval from one
maximum to the next was 500 days; it had shortened to 437 in 1870, and
to 425 in 1891. The highest maxima are of 3·5 magnitude, the lowest more
than six times less bright. The minima, on the other hand, occur with
fair uniformity at 9·7 magnitude. Strongly red in all its phases, R
Hydræ displays a gorgeous colonnaded spectrum lit up with bright
hydrogen lines.
The variability of L_{2} Puppis was discovered by Gould at Cordoba in
1872. The range in magnitude—3·5 to 6·3—is moderate, the period—137
days—comparatively short. Wide departures from it, however, are not
infrequent, and it is almost equally divided between the ebb and the
flow of luminosity.[683] The colour of this star suggests a
conflagration, and its spectrum resembles and is no less effective than
that of Mira.[684] An exceptionally large proper motion, for a member of
its class, has been determined for it by Professor Porter of Cincinnati.
An analogous object is met with in W Puppis. Here the rise occupies 62
days, the decline only 58, the visual limits of variation being the
eighth and eleventh,[685] the photographic, the ninth and twelfth
magnitudes. The discrepancy is naturally accounted for by the
non-actinic quality of light conspicuously red to the eye. The detection
of bright hydrogen lines in a third-type spectrum gave Mrs. Fleming in
1895[686] the clue to the character of this star. The fluctuations
inferred to take place from that unfailing symptom were looked for and
quickly found. The light-curve of W Puppis is very regular, and takes a
much sharper bend at minimum than at maximum.
In S Ursæ Majoris we meet a much older acquaintance. Its periodicity,
discovered by Pogson in 1853, was established by a record of its
magnitude made by Lalande in 1790. It is of a highly perturbed nature.
The maximum brightness varies between 6·7 and 8·2 magnitude; the minima
are uncertain to the extent of perhaps three magnitudes, some unusually
faint at 13·3 magnitude having been watched by Baxendell. The actual
length of the cycle is about 226 days; it is modified by a recurring
inequality with a range of nearly eight days, but much more extensively
by irregular deviations. These seem to be connected with two curious
inflections of the light-curve. About six weeks before maximum the rise
is arrested, sometimes for a few days, sometimes for as many weeks. A
corresponding stay in the decrease of light usually precedes each
minimum.[687] Upon the duration of these halts evidently depends the
retarded or hurried accomplishment of the phases. Noteworthy besides is
the occasional equalisation of the times occupied in waxing and waning.
This is apparently a consequence of the partial abolition in certain
cycles of the pause before minimum. That of 1875 was distinguished by a
steady maximum lasting from 23rd February to 13th April, and followed by
a decrease quicker than the preceding increase.[688] Similarly, the late
Sir Cuthbert Peek’s diagram for 1894 (copied in Fig. 33) shows a flat
maximum antecedent to a precipitate decline. Again, four years later,
the light during two months scarcely varied from the eighth
magnitude;[689] and it may be remarked that, as in the sun, long maxima
are low maxima. The mean light-curve of S Ursæ, from observations made
at Harvard College, 1889–99, is depicted in Fig. 34. Only the general
course of change can be followed by its means; the effects of temporary
obstructions or disturbances are necessarily eliminated.
Deep red, and often hazy when faint, this star has been seen nearly
white at maximum—a not uncommon kind of colour-fluctuation. It gives a
poorly-developed spectrum of the third type.
T Ursæ Majoris has a total range of nearly six magnitudes, and a period
of 302 days; but its variations make no approach to uniformity. The
maximum brightness is largely uncertain; the minima are sometimes,
though rarely, as low as 13·5 magnitude. The luminous tide flows, at
certain epochs, with extreme rapidity. Between 5th January and 22nd
February 1901, for instance, the star increased from 12·4 to 7·7
magnitude. In forty-six days it acquired a seventy-six-fold brilliancy,
and the augmentation was accompanied by a blanching of its rays. Of
their dull ruddy hue scarcely a tinge survived at full light. Fig. 35
reproduces the mean curve drawn at Harvard, which is, of course, much
more symmetrical than any of the individual curves serving as its basis.
The maxima and the minima appear from it to be about equally sharp.
[Illustration:
FIG. 33.—Light-Curve of S Ursæ Majoris in 1894 (Peek).
]
The curve of T Cassiopeiæ, on the other hand, cannot be smoothed into
shapeliness. It is represented in Fig. 36. The secondary maximum
occasioning the hump on the upward branch is never absent, and protracts
the cycle to 445 days, considerably more than half of which (240 days)
are occupied by the abnormally impeded phase of increase. The period is
affected by a compensatory inequality.
[Illustration:
FIG. 34.—Mean Light-Curve of S Ursæ Majoris (Pickering).
]
[Illustration:
FIG. 35.—Mean Light-Curve of T Ursæ Majoris (Pickering).
]
[Illustration:
FIG. 36.—Mean Light-Curve of T Cassiopeiæ (Pickering).
]
The variability of R Leonis was detected by Koch in 1782; yet six score
years of scrutiny have only sufficed to render more manifest the almost
hopeless intricacy of the laws to which it is subject. Since 1890 the
maxima have been persistently accelerated, computations, founded on a
nominal period of 312 days, being left in the lurch in November 1896 by
forty-three days. That there is a large periodic inequality admits of no
doubt; “but the observations of the last few years,” Dr. Chandler
remarked in 1896, “show that it is complicated with other unknown
terms,” the neglect of which, pending the development of their nature,
seemed to him safer than the attempt to use them for purposes of
prediction in ignorance of their value. That is to say, the phases can
be registered as they occur, but defy accurate anticipation. The glowing
colour and brilliant spectrum of the star make it an object of singular
beauty and interest. Its total range is from 5·2 to 10 magnitude, but
the oscillations are often of less amplitude.
The variability of V Delphini was discovered by Mrs. Fleming in 1891 by
the shining of bright hydrogen lines amid the flutings of its
spectrum.[690] It is of enormous extent. Between maximum and
minimum-light there is a difference of close upon ten magnitudes.
Observed as of 7·5 magnitude on 1st October 1899 with the forty-inch
Yerkes refractor, the star had on the ensuing 20th July sunk to
invisibility, and must therefore have been below the seventeenth
magnitude.[691] The period is 540 days.
A strange anomaly in the light-change of R Lyncis was placed on record
at Sir Cuthbert Peek’s observatory in 1898.[692] From a smouldering
minimum the star had risen by 2nd March to 10·6 magnitude; when,
suddenly reversing its course, it dropped in eighteen days to the
thirteenth magnitude, but finally resumed the interrupted process of
brightening, and mounted at the customary rate to a maximum of 7·5
magnitude on 11th August. Such apparent caprices constitute indeed a
baffling enigma, but should, for that reason, be the more steadily kept
in view in dealing with the general question of stellar variability. The
high and low phases of R Lyncis are alike definitely marked. The period
assigned to them is 380 days.
The variation of R Cygni exceeds eight magnitudes. In rising from one
extreme to the opposite it gains a 2500-fold increase of light. The
maxima, however, as usual in this class of variables, are very unequal,
some being seven times more brilliant than others. Mr. Espin believed in
1888 that a regular alternation of high and low phases might be counted
upon;[693] but their subsequent disordered succession belied the
inference. The star is among those in which dimness is occasionally
attended by a curious diffuseness of aspect. Thus in February and March
1894, having previously sunk out of sight with a 6⅓-inch refractor, it
came again into view, at the Rousdon Observatory, in the shape of a
small bluish nebula, resembling a faint comet.[694] This object, which
had contracted on 24th March into a needle-point of light of the twelfth
magnitude, resumed its nebular appearance in February 1895 and March
1896. The mean period of R Cygni is 426 days; but the interval between
the maxima of November 1890 and February 1892 amounted to 457.[695] No
more than 150 days are, as a rule, spent in the rise.
[Illustration:
FIG. 37.—Light-Curve of T Andromedæ (Pickering).
]
The unique form of light-curve represented in Fig. 37 is assigned by
Professor Pickering to T Andromedæ. Derived from photographic data, it
awaits visual confirmation, yet can scarcely be widely erroneous.[696]
The uniform progression it depicts need not, indeed, be accepted as a
rigid reality; it must be encroached upon by sundry kinds of
disturbance, and the maxima, however brief, cannot be instantaneous; at
every full tide there is an interval of “slack water.” Nevertheless, if
even the mean curve prove to be linear, the circumstance will be of
great interest, and the star, meanwhile, merits close attention. Owing
to its redness the photographic curve is transposed downward on the
scale to the extent of one and a half magnitudes,[697] so that the
variable shows four times brighter to the eye than to the sensitive
plate. Hence changes of colour, should they at any time supervene, would
necessarily produce large distortions in the automatically registered
course of light-fluctuation. The systematic comparison of visual and
photographic determinations of magnitude might, indeed, be used as an
effectual means of testing the permanence in hue of long-period
variables. While it remained constant, the curves of light-change,
obtained retinally and chemically, should flow parallel to one another;
they would merge together if the star blanched, and diverge still
farther if it reddened. The variability of T Andromedæ was discovered by
Dr. Anderson of Edinburgh in 1893. The period seems to have suddenly
shortened from 281 to 265 days in 1895.
Variables of the fourth spectral type are mostly crimson-tinted, and
have protracted periods. Indeed, these two characteristics show some
kind of mutual dependence,[698] Chandler’s rule, “the redder the star
the longer the period,” being, on the whole, conformed to. Apposite
examples are furnished by S Cephei, U and V Cygni. The period of S
Cephei averages 484 days, but is subject to an alternate lengthening and
shortening.[699] More than half of it, or about 257 days, is occupied in
the ascent from minimum to maximum, and this exceptional arrangement is
consistently maintained. The light-curve is highly irregular. Smoothed
out by striking a decennial balance, it took the form shown in Fig. 38
from the Harvard College representation. But its dissymmetry is greatly
modified from that of the tracing given by the observations of any
single period. Usually there is a rather swift increase and decrease,
followed by intervals of approximate constancy, at maximum, of about a
hundred, at minimum, of fifty days.[700] Not unfrequently, however, the
curve has a sharp apex, and its downward flow is interrupted by a
secondary rise. Such “stand-stills” (as Mr. Maxwell Reed calls them) are
a familiar feature of long-period variability. Seventh-magnitude
brightness is never fully attained by S Cephei, and it occasionally
drops below the thirteenth magnitude. Like many very red stars, it has
accesses of bad definition. They occur, very remarkably, not at low
light, but near maxima, when “a ruddy haze” seems to envelop a definite
disc.[701]
[Illustration:
FIG. 38.—Mean Light-Curve of S Cephei (Pickering).
]
In 463 days, almost equally divided between the gain and the loss of
brightness, U Cygni varies from 7·0 to 11·6 magnitude; yet with no
approach to regularity. The maxima are sometimes fainter than the eighth
magnitude; the minima fluctuate, it is thought cyclically,[702] from 9·4
to 11·6 magnitude. The periodicity, too, is complicated by an
outstanding disturbance.
The period of V Cygni is 418 days, and the rise, which occupies 220
days, is disproportionately slow. Lindemann registered a steady decline
in the maximum-brightness of the star, from 6·8 magnitude in 1882 to 8·4
in 1891;[703] in some cycles it does not exceed 9·5 magnitude; while the
minima occur, with tolerable uniformity, at the low level of 13·5
magnitude. The maxima are succeeded, at intervals of two months, by
subordinate phases of recovery. The colour of V Cygni is especially
intense. The carmine of its beams corresponds to a powerful stoppage of
the complementary blue and violet radiations, by which a splendid
preponderance is secured to the red end of the spectrum. No bright lines
have been certainly recognised in the dispersed light of either U or Y
Cygni; they are, as we have seen, prominent in U Hydræ, a fourth-type
variable of no settled periodicity.
A fundamental distinction is apparent between the two chief kinds of
stellar variability. Stars with “short” periods are—in a few cases
demonstrably, in the rest presumably—close pairs, their mode of
circulation prescribing, in some unknown way, their laws of
light-change. The strict accuracy of its fulfilment hence results as if
by mechanical constraint. Mira-variables, on the contrary, give no signs
of duplicity; and the marked irregularity of their phases affirms their
origin through a complex interaction of physical disturbances. That
these are internal and constitutional, there is the best reason to
believe. Spectroscopic symptoms are fairly decisive on the point. They
have as yet, however, been very partially observed. A bare gleaning of
facts has been gathered, and we want a full harvest as a foundation for
safe inferences. A spectroscopic study throughout their cycles of
variable carbon-stars would, for instance, be most valuable. The
behaviour of the bright lines shown by them might even prove crucial,
first, as regards the position in their atmospheres of the emitting
strata, next, as to the seat of the recurring commotions. The spectra of
U and V Cygni and of R Leporis may be cited as among those claiming
systematic and prolonged observation.
The reality of the diffuse aspect intermittently presented by certain
variables could readily be tested by examining them at such times with a
reflector. Refractors, owing to their imperfect colour-correction, often
produce abnormal images of objects peculiarly tinted. Nevertheless, if
this were the true and only explanation of the effects in question, we
should expect to find them develop under uniform conditions, and they
appear instead incalculably, and as if by caprice. An instructive
example is furnished by V Cygni. On 19th July 1882, six weeks after a
maximum, Lindemann[704] saw the variable at Pulkowa as an indistinct
coppery disc. But at the same interval, _before_ the high maximum of
31st August 1882, it showed not a trace of nebulosity, although
intensely red. On 8th October 1883 it appeared almost blood-coloured and
very diffuse, while nine days later its image was point-like, stellar,
and precise. Analogous observations have been made by Mr. Grover, Sir
Cuthbert Peek’s assistant, on R, S, and T Cassiopeiæ, R and S Ursæ
Majoris, and several other objects of their class; and by Mr. Knott on U
Geminorum, which, as a white star giving a continuous spectrum, ought to
come regularly to focus. For not a few stars, such as R Coronæ and S
Herculis, dim, bluish nebulosities are substituted at low minima; and
not uncommonly, even on nights of excellent definition, variables in
moderately high phases appear sharp, though very red, and as if
projected on a background of glowing haze. Then again, they present a
clearly outlined disc, or a “large, woolly, ill-defined image resembling
a small but bright planetary nebula.”[705] Most of these diversities
defy anticipation; they can be associated with no particular stages of
variation, and some of the reddest stars, R Leonis and R Leporis among
the number, appear to be exempt from them. Yet their literal
interpretation as indicative of physical alterations in the bodies
affected by them would lead to consequences of outrageous improbability.
Provisionally, at any rate, the wiser course is to refer them to a
combination of atmospheric and instrumental causes. With these, no
doubt, a genuine change of luminous quality concurs, whereby
semi-extinct stars, being thrown out of focus, assume a nebular
disguise. It is noteworthy that the records of a series of observations
on the minima of twenty-two long-period variables, executed with the
Yerkes forty-inch refractor in 1900,[706] include no mention of unusual
phenomena. Attention, however, seems to have been directed entirely to
the determination of magnitudes, nor had any of the stars on the list (U
Geminorum excepted) been previously remarked for optical peculiarities.
A clue to the labyrinth of stellar variations is likely to be afforded
by the continued investigation of solar periodicity. Comparisons of the
spot-curve with the light-curves of Mira, χ Cygni, T Ursæ, or almost any
of their congeners, bring a conviction that the phenomena differ in
degree rather than in kind. The plottings of solar and stellar
disturbances show the same character of dissymmetry, and the same order
of irregularity. In both classes of representation, high summits are
usually sharp, low summits blunt. In both, the course of change is now
halting, now hurried. Hesitations, subordinate ascents, and subordinate
subsidences before completing the phase, are common features. There is,
indeed, no mode of departure from uniformity traceable in the solar
cycle that cannot be strictly paralleled in the caprices of stellar
emission. The analogy has been rounded out by the discovery that the
sun, at spot-maxima, is essentially a bright-line star. Its spectrum
then shares, in a just perceptible degree, the blazing quality that
distinguishes the spectra of Mira-variables. This is a further and an
irrefragable proof of the correspondence of the epochs. Light-maxima in
the stars match spot-maxima in the sun. In each case a development of
internal energy gives rise to enhanced incandescence, accompanied, in
the single specimen of a sub-variable star within reach of detailed
observation, by rendings of the photospheric envelope, and outbursts of
chromospheric flames. Looking a little closer, we can discern the
probability that the cyclical variations of all these bodies depend
essentially upon a rise and fall of activity in the vertical circulation
by which radiation is maintained. The rate of conveyance of heated
matter from within outward must be a determining factor of photospheric
brilliancy, transcendent lustre implying unusual celerity of transport.
This is the vital process of suns, the checking of which must
immediately become sensible in their diminished output of light. Here,
if anywhere, will be found the secret of stellar variability.
CHAPTER XXIII.
PECULIAR AND IRREGULAR VARIABLES.
The stars varying in periods comprised between thirty and a hundred days
are not numerous, and they are often peculiar. Among them are to be
found such remarkable objects as R Sagittæ, R Scuti, U Geminorum, and
S^2 Cygni. Of R Sagittæ, with its double period and reversing minima,
something has already been said. The possibility that it is in reality a
“short-period variable” on a magnified time-scale is suggested by its
resemblance to β Lyræ, and emphasises the question as to its
spectroscopic duplicity. Its irregularities, though considerable, do not
appear to transcend the limit of what might be explicable in a
gravitational system.
[Illustration:
FIG. 39.—Light-Curve of R Scuti (Flanery).
]
This, however, cannot be said of R Scuti; and since the two stars are
analogous in their mode of variation, a rationale clearly impossible for
one must be regarded, for that reason alone, as highly improbable for
the other. The fluctuations of R Scuti, first observed by Edward Pigott
in 1795, extend from 4·4 to about 9·7 magnitude, and have a nominal
period of seventy-one days. But they cannot be even empirically embraced
in any formula. As Mr. Flanery remarked in 1896, “No set of elements yet
devised will fit this star long.”[707] Each in turn has to be rejected
as unserviceable. Thus, on 6th May 1897, the star showed a complete
inversion of phase;[708] it was at a low minimum instead of at the
computed maximum. It then rose to an unforeseen brightness of 5·4 on
11th June, diminished to half-light during eighteen days, and finally,
remounting the slope it had just descended, shone duly at the predicted
maximum of 17th July. Again, it varied less than a magnitude for two
months after the maximum of 29th August 1896, but underwent a
precipitate decline at the end of the stationary spell. The light-curve,
from Mr. Flanery’s observations July to October 1895, is given in Fig.
39. It is that of a star which refuses to be bound by the shackles of
any definite theory. Faint and brighter minima alternate, as Argelander
long ago perceived; and they perhaps, now and again, exchange relative
values, like those of R Sagittæ. If, then, we double the period, and
call it 142 instead of 71 days, the star might rank, despite its
vagaries, as an analogue of β Lyræ and R Sagittæ. For a subordinate
minimum, placed midway between two maxima, is a feature common to all
three, though the other circumstances of variation are in each star
widely different. Such resemblances in the midst of diversity are
extremely perplexing to students of stellar light-change. The similarity
of some of the phenomena suggests a uniform principle of explanation;
but the attempt to extend its application serves only to undermine the
credit it originally possessed. The eclipse rationale, for example,
suits β Lyræ passably well, and might be accommodated to the less
equable phases of R Sagittæ, but is wholly incompatible with the
disordered fluctuations of R Scuti. This, however, in view of their
fundamental resemblance to those of the accurately variable star in the
Lyre, raises the question whether eclipses can be regarded as occurring
in the one case, when they assuredly do not occur in the other. Over and
over again this difficulty presents itself. No theory seems elastic
enough to bear the strain put upon it by the variety of the facts. Each
member of a group of related stars adds its quota to the burthen of
explanation to be borne; until finally the breaking point is reached,
and a collapse ensues, leaving the ground encumbered with the débris of
the original speculation.
[Illustration:
FIG. 40.—Long and Short Maxima of U Geminorum.
]
R Scuti might usefully be made the subject of detailed spectrographic
investigation. Bright lines shine in the blue and violet sections of its
light; but they have not been identified, and the flutings associated
with them appear ill-pronounced, or even subject to effacement.
As a curiosity of the skies, R Scuti is much outdone by a small star in
Gemini, the abnormal behaviour of which was noticed by Hind in 1855.
Habitually tranquil at 13·1 magnitude, U Geminorum rises with amazing
celerity to near the ninth once in two, three, or four months. A leap
upward of nearly four magnitudes is often accomplished in a single day,
and that without preliminary fluctuations. The descent is always much
slower, but along a very changeable curve. Two types of maximum are
shown in Fig. 40. In one, the episode of brightening occupies fifteen to
twenty days, in the other it is terminated in nine or ten. And, as a
rule, they alternate one with the other. Nothing, indeed, is certain
about this star except its uncertainty. “Predictions in regard to it,”
Mr. Parkhurst concludes from his experience, “can be better made after
the fact.”[709] The greatest light varies from 8·9 to 9·7
magnitude;[710] the least to a rather larger extent. Thus on 28th
February and 26th March 1897 the star must have been below fourteenth
magnitude, since Father Hagen lost sight of it with the twelve-inch
refractor of the Georgetown College Observatory.[711] No relation is
perceptible between the amount and the duration of change; long and
short maxima are indifferently high and low. They are fickle, too, in
their time-connections. The period—if it can be called a period—may be
as short as 71, or as long as 126 days. Their unpunctuality apart, the
changes undergone by U Geminorum bear a strong resemblance to those of
cluster-variables. There are the same relatively prolonged intervals of
repose, followed by vehement spasms of activity, beginning abruptly,
dying out gradually. It will be of much interest to inquire whether the
rays of objects so singularly and so similarly affected approximate to
uniformity in quality. Those of U Geminorum are in colour dull bluish
white; they give, according to Pickering and Copeland, an ordinary
continuous spectrum. Still it is possible that peculiarities might be
revealed by special scrutiny with powerful instruments.
[Illustration:
FIG. 41.—Light-Curve of S^2 Cygni.
]
U Geminorum ranked as a unique object until S^2 Cygni was discovered.
Miss Louisa D. Wells in 1896 traced the fluctuations of the latter on
the Harvard plates from 7·2 to below 11·2 magnitude; and the shortness
of their apparent period of forty days combined so unusually with their
wide range,[712] that they immediately became a cynosure for observers
in that branch. The more closely they were watched, the more nearly they
were found to conform to those of Hind’s variable. In both stars,
stationary minima are interrupted, at intervals not wholly irregular, by
sudden ascents of three or four magnitudes; and as in U Geminorum, so in
S^2 Cygni, long and short maxima are coupled together in pairs, yet by
no invariable law. Breaks in the alternate succession have been in each
case recorded. In March and April 1897, for instance, S^2 Cygni rose to
consecutive short maxima,[713] and a pair of long maxima again marred
the rhythmical flow early in 1900. The curve for a double period is
reproduced, from a drawing by Messrs. Parkhurst and Daniel, in Fig. 41.
They comment on “the sharp turns in the curve at the beginning and end
of maximum” as being “peculiar to this type of variable,”[714] which
they judiciously assimilate to that prevalent in clusters. The mean
period of about fifty-seven days attributed to S^2 Cygni has a wide
margin of uncertainty. The interval from one maximum to the next may be
curtailed to thirty-six, or expanded to sixty-three days, and that
without traceable plan or method. Revolution in an orbit is hence
absolutely excluded from among the possible causes of light-change. An
hypothesis proposed by M. Hartwig of Bamberg[715] merely illustrates the
baffling nature of the problem set by this star. He suggests that the
maxima are occasioned by the brief kindling, at periastron passage, of a
satellite pursuing a highly eccentric path. The irregularities of the
period, he thinks, may be accounted for by a rapid gyration of the line
of apsides. The blazing of “new” stars is, in his view, an analogous
phenomenon;[716] but new stars exhaust their energy in a single display,
while the variables in Gemini and Cygnus seem to have a limitless power
of recuperation. The light of S^2 Cygni is tinged with blue; nothing has
been published regarding its spectrum.
Although narrow in range (4·6 to 5·4 magnitude), the variations of υ
Herculis are unsurpassed in singularity. Recognised by Schmidt in
1872,[717] they have proved to be “irregularly periodic”; their tendency
to preserve definite time-relations appears to be continually resisted
and sometimes overborne by countervailing influences. Occasionally they
conform approximately to a forty-day cycle, then break loose, and become
for a time utterly lawless. The minima are attended by extraordinary
fluctuations; the maxima are normally tranquil. The star was marked
“red” in the Copenhagen Catalogue, but is now pale yellow, and shows a
helium spectrum. Scant attention has of late been bestowed upon it.
A southern star noticed by Dr. A. W. Roberts[718] in 1891 to vary from
6·8 to 8·0 magnitude in a period of 38½ days, is remarkable for its
phenomenally quick rise. Only 5½ days are needed for the tripling of its
light, while thirty-three elapse during the corresponding subsidence.
Hence, if the average ratio for Cepheid variables of the times of
increase and decrease held good for U Carinæ, its period would at once
be abridged to eighteen days, and there could be no mistake about its
membership of a class to which it is affiliated by the type of its
variations.
The periodicity of R Lyræ, detected by Baxendell in 1856, often much
perturbed, is never wholly effaced. Argelander found it to be comprised
within forty-eight days, which Schönfeld reduced to forty-six; yet in
1872 Schmidt considered it uncertain between the limits of thirty and
sixty days.[719] Pannekoek’s introduction of a periodic term for its
regularisation[720] can be reckoned only a temporary expedient. The
oscillation is of small amplitude, from 4·0 to 4·7 magnitude, but two
striking outbursts of light, witnessed by Sawyer in November 1884,[721]
imply essential instability. The simultaneous development of
emission-rays might possibly have been observed had a prismatic
eye-piece been at hand; but no spectroscopic examination was feasible at
the critical moments. Ordinarily, the star, which is deeply tinted with
orange, gives a superb colonnaded spectrum unmarked by bright lines.
Periodic cannot be sharply distinguished from irregular variables. Stars
of an intermediate character are quite common. Some degree of precision
in change may even be temporarily maintained by objects eventually found
to be eminently unmethodical in their modes of procedure. Such are α
Herculis, α Orionis, and β Pegasi. Each assumed on first acquaintance a
false air of submissiveness to a time-law, which each very soon laid
aside. Their fluctuations rather exceed half a magnitude, and are
included in an indeterminate number of months; their progress can in no
wise be anticipated. Similarly, stars credited on historical grounds
with extremely long periods have of late paid not the smallest regard to
them. An instructive example is met with in R Cephei. Catalogued by
Hevelius in the seventeenth century, and by Groombridge in 1807, as of
the fifth magnitude,[722] it thereafter lost light, and in 1840 had sunk
to the tenth magnitude. Collating all the available data, Pogson in 1856
assigned to the diminished star a period of seventy-three years, and
predicted its restoration to naked-eye visibility in 1880.[723] But the
prediction remains unfulfilled; the obscurity of R Cephei seems likely
to be permanent.
At Potsdam in 1898 MM. Müller and Kempf noticed a star in Perseus[724]
as variable in an unprecedented fashion.[725] After an indefinite term
of constancy at 6·3 magnitude, it began in 1892 to decline at the very
slow rate of one-eighth of a magnitude yearly, and continued to do so
for six years. The counter-process was comparatively rapid. In twenty
months the object had regained its former status, so that the complete
oscillation occupied 7⅔ years. This time, however, there was no long
stationary maximum. Already by the end of 1899 fading had made some
progress; but it remains to be seen whether any true periodicity can be
established.
A period of five years, ascribed by Mr. Espin to 63 Cygni, has been
rejected on further experience of fluctuations distinctively capricious.
Many, perhaps most red stars, are unstable to the extent of half a
magnitude; and 63 Cygni is a very red star. The fact that it is one and
a half magnitudes fainter chemically than visually, supplies a kind of
measure for the intensity of its colour.
The empirical rule that irregularity gains more and more the upper hand
with increasing length of period[726] is illustrated by S Persei. Indeed
the order of succession in the changes of this object is by no means
satisfactorily ascertained. Safarik[727] and Hagen[728] hold them to be
rudely periodical in about two and a third years; but most other
observers prefer to consider them as entirely irregular.[729] Their
range though wide, from near the seventh to the thirteenth magnitude, is
seldom completely traversed. Experience alone can decide whether the
rudimentary method traceable in these variations during fourteen years
previous to 1894 will continue to regulate them in the future. Phases so
unpunctual are liable to effacement. Quite possibly, the actual
instability of S Persei represents a more or less transitory state,
which may be succeeded by one of approximate constancy in shining.
The most illustrious of casually variable stars is η Carinæ, formerly
designated η Argûs. Futile attempts have been made to accommodate it
with periods. It has none. It is, in the full sense of the term,
irregular. Its changes are perhaps modified by influences of an
unimaginable nature connected with the vast surrounding nebula. But
those influences undoubtedly act upon a body of inherently peculiar
constitution. The spectrum of η Carinæ is of a kind associated in every
other known instance with absolute whiteness. It resembles that of P
Cygni;[730] many hydrogen and helium lines are brightened in it, yet the
star shows the colour of a Mira-variable. Whether this was always so or
not, we have no means of deciding. The first note of a distinctive hue
in the southern wonder-star was made by Piazzi Smyth, 1st January 1845,
when he announced from the Cape a fresh increase in its light. For a
month back, he wrote, it had been brighter than Canopus, and very
red.[731] Then in 1850 Gilliss found it to outmatch Mars in depth of
tint; and Thome described it in 1887 as of a “dull scarlet,” passing
into “bright orange” during a slight temporary rise. The history of this
star is familiar to most of our readers.[732] It need not here be
repeated. One fact in addition to those currently stated may, however,
be mentioned. In his star-maps of 1603, Bayer marked η Argûs as of the
second magnitude, probably on the authority of Petrus Theodorus of
Embden, who navigated the Indian seas 1594 to 1596. The variable was
then equally bright in the sixteenth and in the eighteenth centuries,
and its comparative insignificance when Halley placed it in the fourth
rank was due to a merely transient decline. Whether the splendour of its
beams has ever before been so completely shorn away as it is now, might
be questioned. Excesses entail exhaustion, and the flaring maximum which
culminated in 1843 was followed by a reactive sinking towards the ashes
of extinction. Since 1886, as the observations of Finlay, Innes, and
Roberts testify, the star has wavered between 7·0 and 7·7 magnitude, and
no sign of its speedy restitution to brilliancy is perceptible. Its
future is beyond divination. The present minimum may be indefinitely
prolonged, but further change is more likely in the case of so ruddy an
object. Another great outburst cannot indeed be reckoned upon even for a
remote age. A “temporary” character may so far belong to η Carinæ that
its biography will include but one absolute maximum. The star is
sensibly devoid of proper motion.[733] Its distance from the earth must
accordingly be prodigious.
The capricious disappearances of R Coronæ surprised Pigott in 1795.
Usually of about the sixth magnitude, the star is liable at any moment,
without note of warning, to drop to the thirteenth. The intervals of
maximum lustre sometimes last for years. One extended from 1817 to 1824,
another from 1843 to 1845.[734] But in the last-named year, and again in
1852, R Coronæ vanished from view with Argelander’s comet-seeker,
regaining brightness on each occasion slowly, and, as it were, with
difficulty. Of late its descents have been less profound. Schmidt
observed a minimum at twelfth magnitude in August 1883;[735] Sawyer
recorded on 13th October 1885 one arrested at 7·4 magnitude.[736] Having
been visible to the naked eye nearly throughout 1893, the star sank to
the ninth magnitude about 7th March 1894, and after an intermediate
partial recovery, to 10·25 on 1st August.[737] By the end of the year
the phase of instability seemed to have terminated. These lawless
fluctuations, taken in connection with the extraordinary spectral
changes ascribed to it, render this object one exceptionally inviting to
careful study.
An analogue to it, but with a much narrower range of mutability, is the
lucid white star ε Aurigæ. Ordinarily of the third magnitude, it fades
at long and uncertain intervals to one-quarter of this brightness. One
such diminution was observed by Heis in 1847; another by Schmidt in
1875.[738] There is nothing in the quality of the light to account for
them. The spectrum is modelled on that of Procyon, only with an increase
of definiteness, and marked differences of relative intensity in the
lines.[739]
The vicissitudes of T Tauri derive special interest from their
presumable connection with those of a group of nebulæ. Discovered by
Hind 11th October 1852, it dwindled during fifteen years from tenth to
twelfth magnitude _pari passu_ with the fading of the adjacent
“temporary” nebula, but attained in March 1868 a second and higher
maximum, coincidently with the brightening of “Struve’s nebula,” another
member of the collection. Again it declined, and was left unnoticed from
1877 until Burnham and Barnard, directing the Lick thirty-six inch to
its place in October 1890, perceived it as the faint nucleus of a small
condensed nebula,[740] which four and a half years later survived only
as a feeble glow round the almost extinct variable. The glow was
resolved by the Yerkes refractor into a little wisp of nebulosity,
attached brush-wise to the star;[741] and partial impressions of it came
out on plates exposed by Professor Keeler for four hours with the
Crossley reflector, 6th and 29th December 1899.[742] “Can it be,”
Professor Barnard asks in surprise, “that the star becomes essentially a
nebula as it sinks in light?” The question goes to the root of cosmic
relations, and it is raised under more than one aspect by investigations
of stellar variability. The associations and transformations of T Tauri
are hence of profound significance, and should be diligently supervised
until they can be linked together by some rational principle of
causation.
Irregular variability has a wide and indefinite reach. It includes
changes almost instantaneous, and changes well-nigh millennial in their
development. The light of certain stars has undergone a slow secular
decline. A noteworthy instance is that of Θ Eridani, identical, as Dr.
Anderson has conclusively shown,[743] with Ptolemy’s “Last in the River”
(the Arabic _Achernar_), the title and honour of which have been usurped
by the more southerly, and now far brighter α Eridani. Al-Sûfi in the
tenth century reckoned Θ among the thirteen brightest stars visible in
Irak; and it was still of the first magnitude in 1437, the epoch of
Ulugh Beigh’s Catalogue. Yet it had sunk to the third when Halley
visited St. Helena in 1677, and of the third it still remains. Two other
stars which have undeniably faded with the lapse of centuries are β
Leonis and δ Ursæ Majoris,[744] and their fading may even now be
imperceptibly progressing. Nor is their eventual restoration by any
means assured. Accessions of lustre are rarer, and often transitory. On
6th August 1868, 83 Ursæ Majoris, a sixth-magnitude star near Mizar, was
seen by Birmingham to be the equal of δ Ursæ; though for that night
only. The next, it had visibly gone off, and before long the whole of
its added splendour had departed. Its amount was very considerable. The
star attained, during its unexplained rise, to threefold its customary
brilliancy. One of Burnham’s close pairs, z Virginis, underwent in 1866
a similar phase. This was before it was known to be double, so that our
curiosity as to whether both the nearly equal components shared in the
brightening remains ungratified. Red stars not infrequently drop
abruptly to a lower rank. Thus the Danish astronomer Torwald Köhl had
for years observed B.D. + 20° 1083, in the constellation Taurus, as of
7·7 magnitude, when on 22nd January 1898 he was taken aback to find it
not much above the ninth.[745] Sixty per cent of its rays had been, as a
consequence of some inexplicable collapse of energy, subtracted or
suppressed. A step towards the bourne from which there is no returning
may be taken in such cases. Stars vanish, but they seldom or never
reappear. Yet renovation plays its part no less than decay. Waning stars
have their correlatives in waxing stars. Alcor, the Rider, and
Benetnasch, the third Horse of the Wain, are among these. Pollux, too,
seems to have bettered its position, and Alcyone is far more predominant
than of yore in the Atlantid family. Night’s robe will still be
profusely spangled, even though a few of its gems grow dim.
CHAPTER XXIV.
TEMPORARY STARS.
A temporary star is a variable that rises sheer from profound obscurity
to a single maximum. The maximum may be prolonged or multiple, but it
must be essentially one. The occurrence of a second independent outburst
would at once relegate the object to the category of irregular
variables. The distinction is perhaps arbitrary, but we can only
investigate by dividing. It will be best to plunge at once _in medias
res_ with some account of an apparition which attained to epochal
importance through the efficacy of photographic methods of research.
Nova Aurigæ sprang into conspicuousness with the stealthy speed of
Jack’s Beanstalk. On 8th December 1891 Dr. Max Wolf took, at Heidelberg,
a photograph of the sky round χ Aurigæ, showing stars to the ninth
magnitude. The Nova was not among them; the spot destined for its
occupation was vacant. Forty-eight hours later, as a Harvard negative
attested, a fifth magnitude star filled the blank. By 20th January 1892,
twelve records of this stranger’s presence were included in the same
series; but inadvertently, for the documents were stored up unread, and
it was only through their subsequent examination that a maximum of 4·4
magnitude was inferred to have taken place on 20th December. During
nearly two months, then, a new star, readily visible to the naked eye,
shone unnoticed in the heavens. It was finally perceived by an amateur,
Dr. Thomas D. Anderson of Edinburgh, and an anonymous post-card, by
which, on 1st February 1892, he conveyed the news to Dr. Copeland,
formed the starting-point of widespread astronomical activity. The
spectrum of the Nova was promptly photographed at Tulse Hill, South
Kensington, Stonyhurst, Potsdam, and Mount Hamilton; eager study was
devoted to its implications; and they were of a most unexpected kind.
For no previous stellar apparition had been _analytically_ recorded; and
the similar spectral phenomena, doubtless present in Nova Coronæ and
Nova Cygni, eluded definite determination with the eye. But when the
turn of Nova Aurigæ came, the spectrographic method was effectively at
hand, and the peculiarities of its light could be rendered obvious,
salient, and permanent. Some of them were indeed visually manifest. The
spectrum was at once seen to blaze with bright lines, many of them
greatly widened (see Plate XVII.); and nearly all came out
photographically as attended by strong dark companions on their more
refrangible sides. The entire hydrogen series, from crimson C to the
last of its ultra-violet associates, was thus doubled, no less than the
pre-eminent calcium pair, the sodium D, and a considerable number of
lines since identified as originating from helium. An exceptional
feature was the predominance of “green” helium; D_{3} and the rest of
the lines belonging to the “yellow” set were comparatively faint; while
λ 4922, λ 5016, and their fundamental, λ 6678, shone lustrously. Their
superiority, never before observed in the spectrum of a heavenly body,
results in the laboratory from heightening the exhaustion of the
emitting gas; but this condition seemed to be excluded in Nova Aurigæ by
the distended aspect of rays unmistakably proceeding from a more than
ordinarily condensed atmospheric stratum. Unprecedented, likewise, was
the simultaneous brightening of the _three_ D-lines. No stellar spectrum
previously observed had shown the action of sodium otherwise than by
absorption. Both characteristics, it is true, may have been present in
earlier Novæ, but they first arrested attention in Anderson’s star.
[Illustration:
PLATE XVII.
Visible Spectrum of Nova Aurigæ, 28th February 1892. Intensity-Curve
below (Campbell).
]
These novelties, curious though they were, sank nevertheless into
insignificance compared with one dominant trait. This was the large
opposite displacements of the bright and dark sets of lines. Contrary
motions of prodigious velocity appeared to be indicated, and for a time
their prevalence was taken to be incontrovertibly attested. The
outburst, by a consensus of opinion, resulted from the approach, and
integrated the light of two components, one a bright-line star receding
from the earth at the rate of about 230 miles a second, the other a
dark-line star hurrying towards it with a speed of 320 miles. Soon,
however, incongruities began to develop, and they grew and multiplied as
time went on. To begin with, the spectral shifts underwent no
alteration; the movements indicated by them—if they did truly indicate
movements—persisted without abatement as the bodies animated by them
withdrew from each other’s vicinity to a distance greatly exceeding that
of Neptune from the sun. Now velocities, to continue uniform, must be
inherent; that is to say, they cannot represent the merely temporary
effects of gravitational pull, since orbital acceleration is strictly
balanced by retardation. Evidently, then, the components of Nova Aurigæ
did not simply fall together; they should have been fabulously massive
to have produced, by their mutual attraction from infinity, a speed
which continued at the rate of 550 miles a second three months after the
periastral rush-past, the date of which presumably coincided with the
first rise to brilliancy on 10th December. Professor Seeliger of
Munich[746] calculated that 15,000 times the sun’s gravitative power, at
the very least, must have been at work if the orbits traversed were
parabolic; and the extravagance of the estimate sufficed, and was
designed to compel its rejection. Hyperbolic motion was accordingly
resorted to; the encountering bodies brought, it was supposed, their own
velocities with them from the farthest bounds of space; and they were of
so high an order that the increments due to mutual gravity left them
sensibly unaltered. A pair of “runaway” stars, one moving towards, the
other from the earth, must, it seemed, have accidentally passed each
other almost within grazing distance. They were primitively unrelated;
their quasi-collision could never be repeated; they were as unlikely as
any two stars in the heavens to be similar in constitution. Yet their
spectra affirmed their close affinity; both were of pure helium type;
one might be called the _negative copy_ of the other. Nor was this all.
Anomalies still more glaring presently disclosed themselves. Too
obviously, on the adopted hypothesis, one pair of meeting stars could
not suffice to explain the phenomena. Vogel stipulated for a triple
encounter;[747] Campbell found evidence of the interaction of four
luminous masses.[748] The more attentively, in fact, the spectrum was
examined, the more complex it appeared. The bright lines were not simple
emanations, but groups of differently refrangible rays; the dark lines
were intersected by bright threads, variable in number and position.
Several distinct sets of lines, each with its separate amount of shift,
and each hence associated with a differently moving mass, thus stood out
independently. “On the hypothesis of four bodies,” Professor Campbell
wrote, “the principal system of bright lines was not displaced
appreciably, and the star yielding it was practically at rest with
reference to the solar system. Another system was displaced towards the
red, a distance corresponding to a velocity of recession of about 315
miles a second. The system of fine bright lines, and likewise the system
of dark lines, were displaced towards the violet, a distance
corresponding to a velocity of approach of about 400 miles a second.”
The analysed light of the Nova, on this showing, consisted of four
superposed spectra disconnected in their origin. The case, however, was
presented with diffidence; no conclusive force was claimed for it. The
spectrum, indeed, if multiple at all, was more than quadruplicate.
Victor Schumann pointed out[749] that not less than six, if as many as
two stars were engaged in the outburst, and planets _ad libitum_ were
thrown into the _mêlée_ by Vogel.[750] The collision-theory, in short,
collapsed under the weight of the facts it had to carry; speculation was
plainly off the track; a new principle of explanation had to be sought.
It was difficult to find; yet some probable though partial truths had
been laid hold of. Sir William Huggins, for instance, pointed out that
the complexities of the spectrum might be in part due to “reversals” in
the atmosphere of a single star.[751] Such effects of the stratification
of glowing vapours are common in the sun, and may be artificially
produced in a diversity of forms. Their presence in Nova Aurigæ was
clearly recognisable. The opinion, too, expressed by Father Sidgreaves
might safely be adopted that the spectrum of the new star “was, on the
whole, what the solar chromospheric spectrum might be expected to show
on a grander scale of disturbance.”[752] Finally, M. Seeliger’s general
theory of stellar conflagrations,[753] as arising from the passage of
compact bodies through “cosmical clouds,” or nebulæ, though strained to
meet superfluous requirements, had fundamentally much to recommend it.
Entire originality could not be claimed for it; Mr. Monck, and possibly
others besides, had earlier proposed a similar view; but it was by
Seeliger independently developed, and independently applied to the
circumstances of the latest event. These were indeed particularly
embarrassing to theorists. They seemed to imply the continuous progress
of opposite radial movements of enormous velocity, and the problem of
bringing them into play, whether by the rushing of inflamed gases or by
the bodily transport of luminous globes, was, when considered in all its
complicated bearings, formidable, if not desperate. Yet it had to be
faced, for at that time spectral displacements were explicable only as
effects of motion; they should perforce be interpreted on the radial
velocity principle. This is no longer absolutely prescribed; the same
phenomena have been found to bear other meanings—meanings not yet
thoroughly intelligible, but promising, when they become so, to provide
the keys to many enigmas.
[Illustration:
FIG. 42.—Light-Curve of Nova Aurigæ.
]
We may now trace the further course of the apparition. During nearly
three months it retained most of its brightness, despite wide
fluctuations; then on 6th March a precipitate decline set in, bringing
the object on 26th April to the limit of visibility with the great Lick
refractor. In Fig. 42 the course of change so far is graphically
portrayed. It was naturally believed to have reached its term; the Nova
seemed definitively extinct; but in this, as in other respects, its
behaviour defied anticipation. Observations resumed 17th August 1892,
after its conjunction with the sun, showed, in the place of the vanished
star, a stellar nebula of the tenth magnitude.[754] The recovered light
was entirely altered in quality. No stranger disclosure has been made by
the prismatic method than that of the spectral transformation of Nova
Aurigæ between March and August. The dark lines of the former spectrum
had become effaced; the bright lines were no longer chromospheric but
nebular. Their character, moreover, denoted disturbance of a very
peculiar kind. They were wide and hazy, and some at least could be
resolved into groups projected upon a feeble luminous background.[755]
Plainly the state of things producing the complex “reversals” noted in
the spring had become intensified in the reappearance of the autumn. Yet
the multiple bands composing the new spectrum corresponded most
faithfully, in number and mean position, with the rays of a planetary
nebula. The representation was essentially perfect. Out of nineteen
Nova-lines measured by Professor Campbell, only one of subordinate
importance appeared foreign to nebular light.[756] Both the Wolf-Rayet
blue bands were present, as in certain planetaries; while the
distinctive, though enigmatical, nebular line at λ 373, noticed as
absent from the Tulse Hill photographs of the original spectrum, emerged
as it altered, and was recorded by Von Gothard at Herény in November
1892.[757] All the metallic lines, all the helium lines (except a trace
of λ 4472) had died out; only those of hydrogen survived; and hydrogen
glows universally. The metamorphosis could hardly have been more
complete. Professor Campbell might well say that the relation of the
later to the earlier spectrum was “not apparent.”[758] It was certainly
most obscure. At least, however, the conviction was acquired that the
light “emanated from one source;” although subsequent experience obliges
us to regard as fictitious the rapid approaching movement attributed to
that source on what then seemed incontrovertible evidence. The presumed
velocity reached a maximum of 190 miles a second in September, but in
November 1892 had decreased to about half that value. And the slackening
continued until, at the end of two years, the shifted lines occupied
almost normal positions. Subsidence of physical agitation might safely
be associated with the change. It was attended by spectral modifications
of a different kind. In the transformed Nova bright lines at λ 436 and λ
575 were at first prominent. They also occur in nebulæ, but by exception
and inconspicuously. Eventually they faded out, and the spectrum
remained typically nebular save for the anomalous breadth of the
lines.[759]
As a star, Nova Aurigæ had been decidedly yellow; in its nebular stage
it assumed a greenish tint. Its chromatic peculiarities accounted for
the haziness of its aspect with many refractors. Outstanding rays, left
unfocussed when the rest were united, created a spurious disc of which
no trace could be perceived with a reflector.[760] The revived Nova
maintained tenth-magnitude lustre with trifling fluctuations for upwards
of four years; then it began once more slowly to decline, and in the
latter part of 1897 looked “more like a most minute and faint planetary
nebula than a star.”[761] In 1897 it had sunk below the thirteenth
magnitude; in 1900 it touched the fourteenth.[762] A partial revival was
noted at the Lick and Yerkes observatories in 1901, the peculiarity of
its focus continuing to indicate the nebular quality of its light.[763]
Nova Aurigæ was neither the first nor the last temporary star to don, in
fading, a garb of nebular light. Nova Cygni passed through an identical
phase, and the example has since been frequently imitated. Chiefly,
indeed, the methods of investigation brought to bear upon it, and the
novel considerations to which, in consequence, it gave rise, rendered
the apparition of 1892 memorable. Its dominant features were those of a
model “blaze star”; but they were for the first time decisively
ascertained. An evanescent phenomenon was rendered virtually permanent,
and for dubious visual impressions, definite transcripts of fact were
substituted. They are likely to be multiplied in the future; and it has
meantime become abundantly clear that no special theory, fitting only
the circumstances of Nova Aurigæ, can be true.[764] Whatever rationale
of them may be ultimately adopted, it must be one capable of general
application.
The photographic era of investigation began with Nova Aurigæ, that of
photographic discovery with Nova Normæ. The former object was still
visible when the latter came into notice. Virtually a replica of the
earlier spectrum imprinted itself upon a Draper Memorial plate exposed
at Arequipa by Professor Bailey 10th July, and examined by Mrs. Fleming
at Harvard College 26th October 1893.[765] It included about a dozen
bright lines, each shadowed by a dark one on its more refrangible side.
In position, in relative shift, in character, the system of duplicate
rays matched with singular fidelity that scrutinised and wondered at in
the previous year. There was then nothing casual about its production.
The spectral displacements and the augmented refrangibility of the dark
lines belonged to the essence of the phenomenon. Portentous velocities,
specially directed, and inherent in specially constituted masses, could
have had nothing to do with them. The appearance of Nova Normæ, then,
disposed of what was left of the encountering star-theory.
The conflagration was of a strictly temporary nature. Absence from a
chart-plate proved the star to have been fainter than the fourteenth
magnitude, 27th May 1893; and a spectrographic plate, taken 21st June,
added the information that it had not at the later date reached
tenth-magnitude brightness. Its rise to the seventh, on or before 10th
July, was probably effected in a few hours; but though fortunately
registered at an early stage in its career, it was not recognised for
nearly four months, by which time it had sunk to one-sixth of its
primitive lustre. The same marvellous change witnessed in Nova Aurigæ
attended its decline. On 13th February 1894 Professor Campbell[766]
succeeded in observing the star with the Lick thirty-six inch, in spite
of its low meridian altitude of barely 2½ degrees. The spectrum was
nebular. It consisted, to the eye, of the well-known trio of green rays,
together with the golden line (λ 575), also emitted by the transformed
Nova of August 1892. Measurements of them afforded no evidence of
displacement; their wave-lengths were as usual. Towards the middle of
1895 Nova Normæ became telescopically invisible. It was already provided
with a successor. Just as it glimmered out, a star in the constellation
Argo lit up. Nova Carinæ was detected by the same means as, though with
somewhat more delay than, Nova Normæ. Two impressions of its spectrum,
which chanced to be secured, on 14th April and 15th June 1895
respectively, disclosed precisely the same arrangement of coupled lines,
the dark set above the bright, so remarkable in earlier examples.[767]
During those two months this had not appreciably varied, but a
significant change of a different kind had come about. A vivid ray at λ
4700 (approximately), scarcely visible 14th April, was as bright as the
hydrogen lines on 15th June. Its identity with the Wolf-Rayet azure
band—the leader line of the Rydberg hydrogen series—may safely be
assumed, and its kindling was most likely the prelude to a complete
nebular transformation. But of this we have no certain knowledge, since
the later history of Nova Carinæ remained unwritten. The conflagration
was brief; it had burnt itself out before Mrs. Fleming’s examination of
the plates transmitted from Arequipa gave the alarm of its occurrence.
Hence no express observations were feasible.
The year 1895 was prolific of temporary stars. One was retrospectively
announced to have appeared in the constellation Perseus in 1887.[768] A
bright-line spectrum, dimly self-imprinted upon a Draper Memorial plate
exposed at that period, was at first thought to signify the rise to
maximum of an ordinary Mira-variable. But the originating object soon
vanished, to all seeming, definitively, and its brief incandescence
asserted itself, by its non-recurrence, as that of a new star. There was
a further note of distinction. The fifth line of hydrogen (Hε), never
apparent in an ordinary variable, glimmered in the peculiar spectrum of
the star of 1887, which included besides an unknown line at λ 4060.
The last record obtained of Nova Carinæ preceded the first of Nova
Centauri by just a fortnight. They differed, however, widely in
character. The spectrum singled out by Mrs. Fleming, 12th December 1895,
from a crowded spectrographic picture taken at Arequipa 18th July in the
same year, resembled that of an exceptional nebula, 30 Doradûs. It
seemed abortively or imperfectly stellar.[769] The source, too, from
which it emanated was situated in a nebulous environment. With the
fading of its brightness—which probably never greatly exceeded the
seventh magnitude—an outlying portion of a known nebula (N.G.C. 5253),
momentarily effaced by the blaze, as a fire is “put out” by the sun,
shimmered into view, like a halo round the dying star.[770] It still
survives, while not a trace can be seen of its quondam inmate. By the
time that Nova Centauri came to be recognised, it was unfortunately
already far advanced on its return to obscurity. It had sunk below the
eleventh magnitude on 22nd December, when Professor Campbell secured the
first of three observations of its spectrum. He described it as
continuous, though peculiar, for the blue section was of
disproportionate strength, and the yellow showed inequalities, as if
through the superposition of bright lines.[771] A parting glimpse of the
strange star was caught by Professor Hussey at Lick on 16th July 1896.
It was then excessively faint, and immersed in nebulosity. The analogy
between this apparition and that which illuminated the great Andromeda
nebula in 1885 was unmistakable, and both diverged significantly from
the type of Nova Aurigæ.
It was reverted to by Nova Sagittarii. This star was photographed at
Arequipa 8th March 1898, and identified by Mrs. Fleming as “new” a year
later.[772] Yet it was at maximum fully the equal of Nova Aurigæ, and
might have been seen at a glance by any one familiar with sky scenery.
It waned, however, very speedily. It was of only 8·2 magnitude on 19th
April, when a spectrographic record chanced to be secured, and had
declined to the eleventh before its deliberate investigation became
possible. Plate XVI. Fig. 4, reproduces two spectral photographs of Nova
Sagittarii, obtained within forty-eight hours of each other. Six or
seven hydrogen lines are shown in them, broad and bright, but without
the dark companions usually visible in such spectra. The second band
from the right is the more refrangible of the Wolf-Rayet blue radiations
(λ 4643); the K of calcium is absent, but six sharp rays, identified
with chemically unclaimed lines in the spectrum of Nova Aurigæ, came out
distinctly on the negatives. Although taken at so brief an interval, the
records disagreed in some particulars. Thus the unknown line at λ 4060,
emitted by the first Nova Persei (1887), was strongly absorbed in Nova
Sagittarii on 19th April, but had vanished on 21st April; while the
chief nebular line, invisible at the earlier, shone at the later date.
The initiated change did not stop here. In March and April 1899 a purely
nebular spectrum was derived from the semi-extinct object by Professors
Campbell and Wright.[773] The regular cycle had been run through; a
planetary nebula replaced the faded star. In one important respect,
nevertheless, Nova Sagittarii had departed, so far as the extant data
could inform us, from the pattern set by Nova Aurigæ. The spectrum, as
already mentioned, was single, not duplicated by absorption. But there
is good reason to believe that this difference did not originally exist;
it supervened with the declension of light. In “temporary” spectra the
bright lines always seem to survive the dark; the chiaroscuro effect is
produced only near the period of maximum, and this period had terminated
before the star of 1898 was spectrographically registered. Earlier
impressions, we may be sure, would have displayed the coupled lines
symptomatic of the enormous disturbances attending these extraordinary
outbursts.
Their frequency, established by the camera, is a fact of subversive
import, but has already ceased to excite surprise. The recognition of
the autographs of new stars has become a regular part of the business
conducted at such a “Solomon’s House” as the Harvard College
Observatory. In the _Annual Reports_ thence issued the mention of new
stars has grown to be habitual and familiar. Of Nova Sagittarii the last
had only just been heard, when Nova Aquilæ was announced to have
appeared.[774] This was about fifteen months after date. The star made
its _début_ on a Draper Memorial chart-plate 21st April 1899 as an
object of the seventh magnitude, and displayed the quality of its light
in a spectrograph taken 3rd July. It was characteristically that of a
Nova, although nebular lines had begun to come in, and the
transformation was completed in the course of September. During a year,
Nova Aquilæ wore the aspect of a planetary nebula, sinking gradually
from the tenth to the twelfth magnitude. It was last observed by
Professors Campbell and Wright with the Lick thirty-six-inch refractor
and a 60° prism spectroscope. “The visible spectrum,” the former
wrote,[775] “consisted of extremely faint continuous light in the green,
and of three bright bands in the positions of the three principal
nebular lines. The relative intensities of the three bands agreed
approximately with the corresponding intensities in the well-known
nebular spectra. The bands were not monochromatic, but on the contrary
were very broad, perhaps fully twice as broad as the bands in the
nebular spectrum of Nova Aurigæ in August 1892.”
It may be useful to tabulate the results, in the discovery of new stars,
of the photographic surveys executed at Harvard College and at Arequipa,
its southern dependency.
(i.) Nova Persei. Appeared 1887; ninth magnitude; hydrogen lines and λ
4060 bright. Temporary character recognised 1895.
(ii.) Nova Normæ. Spectrum photographed 10th July 1893; character
detected 26th October 1893. Seventh magnitude; showed twelve bright
lines coupled with more refrangible dark ones; spectrum nebular 13th
February 1894.
(iii.) Nova Carinæ. Spectrum photographed 21st April 1895; examination
followed in October. Hydrogen lines bright with more refrangible dark
companions. Faded from eighth to eleventh magnitude between April and
July.
(iv.) Nova Centauri. Spectrum photographed 18th July 1895; irregularly
continuous; resembled that of 30 Doradûs. Character detected 12th
December 1895. Maximum magnitude 7·2. Situated within a nebula.
(v.) Nova Sagittarii. Photographed 8th March 1898, when of 4·7
magnitude; detected March 1899. Spectrum photographed 19th and 21st
April 1898. Hydrogen lines bright; no dark companions; λ 4060 dark.
Spectrum nebular, 13th March 1899.
(vi.) Nova Aquilæ. Photographed 21st April 1899, when of seventh
magnitude. Bright-line spectrum photographed 3rd July 1899; recognition
ensued in July 1900. Spectrum nebular, October 1898.
In the course of seven years, then, five temporary stars were
photographically discovered, and it is safe to assert that all would
have escaped direct visual notice. Apparitions of the kind are, we hence
learn, hardly to be counted as exceptional; their occurrence cannot
depend upon rare contingencies, but must enter, to some extent, into the
regular economy of nature. The conditions precedent are doubtless widely
prevalent, and they are the same for outbursts of all magnitudes
indifferently. The famous Novæ of history—the stars of Hipparchus, of
Tycho Brahe, of Kepler—were due to just such prepared accidents as
result, more frequently and less obtrusively, in the appearance of an
extra dark dot on a chart-plate, or in the emergence, on a prismatic
negative, of a “peculiar” spectrum among a crowd of normal ones.
[Illustration:
FIG. 43.—Photographs of the same field taken at Harvard College before
and after the outburst of Nova Persei. No. 1, 19th February 1901.
No. 2, 26th February 1901.
]
But this obscure kind of manifestation was far transcended by the “New
Star of the New Century” (to quote Father Sidgreaves’s designation of
Nova Persei, No. 2). This object worthily commemorated the turning of a
leaf in the book of ages. The sidereal heavens had harboured no such
brilliant “guest” since Kepler’s star shone in Ophiuchus. Dr. Anderson,
the discoverer of Nova Aurigæ, was still more lucky with Nova Persei,
for he caught it on the rise. It was, however, already nearly the equal
of Algol when he sighted it in the early morning of 22nd February 1901;
while twenty-eight hours previously, as a photograph taken by Mr.
Stanley Williams attested, it must have been fainter than the twelfth
magnitude. Confirmatory evidence was derived from the Harvard College
series, two specimens of which are reproduced in Fig. 43 by the kind
permission of Professor Pickering. In the earlier photograph, taken with
an exposure of sixty-six minutes 19th February, no trace of anything
unusual is perceptible; in the second, to which, seven days later, a
shorter exposure was given, the Nova is of dominating importance. It
continued to gain light rapidly for about a day and a half after its
detection at Edinburgh. On the evening of 23rd February, it was observed
through drifting clouds at Harvard College to be brighter and bluer than
Capella.[776] In thirty hours it had increased by two and a half
magnitudes; during that one night it took rank as the premier star of
the northern hemisphere. But its supremacy was quite transient. Increase
was at once followed by decrease; there can have been no appreciable
pause at maximum. Already on 24th February, the star had lost fully
one-third of its light; the inevitable downward course was entered upon,
and was pursued, although with singular intermittences. But the most
remarkable characteristic of Nova Persei was its spectral variability.
It showed, to begin with, a spectrum of the ordinary Orion type—a
continuous prismatic strip, scarcely encroached upon by narrow lines of
hydrogen and helium absorption. Some of these, it is true, proved on
close examination to have their lower edges slightly brightened, but
there was no other sign of disturbance. On the following night it was
noticed that the K of calcium, previously imperceptible, stood out
markedly obscure, after which an extraordinary change took place. During
the twenty-four hours that followed the maximum of 23rd February, the
character of the spectrum was completely transformed. Hydrogen now
blazed in it; and the range of broad bright lines representing its
emissions was duplicated, just as in Nova Aurigæ, by a range of more
refrangible shadow-bands. These were strongly displaced towards the
violet, and their displacement appears to have increased progressively
for some days. It finally corresponded, if interpreted on the Doppler
principle, to an approaching velocity of about 1000 miles a second,
while the bright lines, so far as their distended condition allowed a
judgment on the point to be formed, retained pretty nearly their usual
places. The Doppler principle, however, had plainly a very restricted
application to the case of Nova Persei. Dr. Vogel[777] used it, with
excellent judgment, to determine the star’s true radial motion from
measurements of fine dark reversals of H and K, which seemed entirely
exempt from alterations of a physical kind. He found it to be about
twelve miles a second in a direction away from the sun. He was less
successful in his endeavour to explain the conspicuous shifting of the
dark hydrogen lines on the pressure-theory as adapted by Wilsing. For
pressure does indeed alter the refrangibility of the rays emitted by
vaporous strata submitted to it, but the alteration is always in the
same sense. The refrangibility of light is diminished by condensation;
the resulting spectral displacements are towards the red. Nevertheless,
those of the absorption lines in Nova Persei were towards the blue. Nor
was the masking arrangement devised for the purpose of smoothing away
this fundamental contradiction, one that would work. The star itself,
through the gradual unfolding of its peculiarities, emphatically
disavowed it. A Stonyhurst spectrograph of 7th March showed the dark
hydrogen lines well separated from the bright;[778] they were unmasked,
yet none the less shifted. Their altered positions were thus seen to be
those of the rays in their entirety, and not merely of outlying wings
left visible, while their central parts were concealed. Subsequently to
7th March these lines thinned off, and in about a fortnight disappeared
finally. The spectrum then remained in much the same condition as that
of Nova Sagittarii at the date of the earliest record of it; the
distinctive chiaroscuro effect was gone; the bright lines no longer
_cast shadows_.
[Illustration:
FIG. 44.—Light-Curve of Nova Persei, 22nd February to 6th March 1901.
]
[Illustration:
FIG. 45.—Light-Curve of Nova Persei, 17th March to 4th May 1901.
]
Sections of the light-curve of Nova Persei are delineated in Figs. 44
and 45. They seem to tell of a long succession of efforts towards
recovery, constantly defeated by the invincible advance of exhaustion.
These variations in magnitude were associated with remarkable spectral
modifications. The light of this wonderful object was, from the first,
of extremely unstable quality. The emitting and absorbing vaporous
layers were evidently in a state of continual flux. Their turmoil was
betrayed by changes of intensity, width, and wave-length in both dark
and bright lines, most of them taking place unconnectedly, so to speak,
and capriciously. By degrees, however, their confusion reduced itself to
some kind of partial order. During the month of March, Nova Persei
assumed the character of a variable fluctuating extensively in a period
of three days, and the spectrum was found to vary quite decidedly in
correspondence with the light. The phenomena of its alternations are
illustrated in the subjoined reproductions of photographs taken by
Father Sidgreaves (see Plate XVI. Fig. 5). The second and fourth are
distinctive of minimum epochs; the first and third characterise much
higher grades of brightness. Among the features of the minimum spectrum
are the fading of the continuous radiance, the displacement upward of
the first ultra-violet member of the hydrogen series (Hζ), and the
development of a blue band at λ 463. The two types of spectrum continued
to succeed each other with approximate regularity from 19th March to 3rd
May. Numerous anomalies, indeed, presented themselves. They were
described in detail by Fathers Sidgreaves[779] and Cortie,[780] by
Lockyer, Pickering[781] and Hale,[782] but no clue to their physical
interpretation has been found. Only those affecting one narrow region of
the spectrum can here be further adverted to. The appearance of the
D-lines in Nova Persei was most remarkable. Special attention was paid
to them at the Yerkes Observatory, and they are delineated in Plate XVI.
Fig. 3, from a photograph taken on an “Erythro” plate with the
forty-inch refractor of that establishment. The relations of the various
lines shown in it need some brief explanation. The broad bright band is
due to the radiation of sodium; the fine dark lines projected upon it
are obvious reversals by a cooler and rarer overlying stratum of the
same metallic vapour. These threads of absorption are slightly displaced
towards the red; they indicate about the same rate of recession from the
earth that was derived by Vogel from the similar reversals of H and K.
The fact, then, admits of no doubt that the kindled mass was travelling
away from us with a small velocity.[783] This remained constant—so far
as our information goes—from first to last; but the position of bright D
underwent a marked alteration. The broad bright band, traversed almost
centrally in the early days of March by hair-like reversals, shifted in
the course of a month so notably towards the blue that the fiducial
lines (as they might be called) lay in April close to its less
refrangible edge; while the absorption line concealing D_{3} had
meantime become diffused towards the violet. We have no inkling of a
possible cause for these changes. They were steadily progressive, and so
disclaimed any immediate connection with the periodical variations of
the Hζ line. According to Father Sidgreaves’s view, indeed, these last
originated from no change of refrangibility, but from the brightening of
a cyanogen band situated just above the hydrogen line it illusorily
modified. The sodium band, however, actually moved upward; the emitting
molecules quickened their vibrations through some unknown kind of
influence. Only the less refrangible members of the helium-series glowed
in this marvellous spectrum. None higher up than the blue “Orion” line
at λ 4472 could be seen; while the blood-red λ 6678, and D_{3} (when
sodium-absorption thinned off) shone intensely.
The colour of Nova Persei changed from white to red a few days after its
maximum on 23rd February, and red it remained for some months. The
tinge, indeed, lightened to clear orange in its spasms of recovery, but
deepened to a purplish glow at each subsequent decline. It faded
completely in July 1901, when the spectrum of the Nova resembled that of
a planetary nebula.[784] Plate XVIII. Fig. 2 illustrates, from the Lick
observations, the predominance in it, on 11th August, of the leading
nebulium-radiation. This was in accordance with precedent; but the
wildest flights of imagination were outrun by what ensued.
On 22nd and 23rd August 1901, Professor Max Wolf took long-exposure
photographs of the Nova with the sixteen-inch Bruce objectives lately
mounted as a twin-telescope at Königstuhl. Both plates showed the
presence of detached nebulous masses to the south-east of the star;[785]
and these proved to be only the brightest parts of a vast spiral
formation photographed by Mr. Ritchey at the Yerkes Observatory on 20th
September, under the form represented in Plate XVIII. Fig. 1.[786] Its
intimate structural relationship to the star is patent; but there was
more to come. Renewed impressions obtained 7th and 13th November with
the same instrument—a twenty-four-inch reflector of his own
construction—showed Mr. Ritchey that the nebula was expanding with
portentous speed;[787] and an identical discovery was made by Mr.
Perrine from a comparison of the Yerkes photograph of 20th September
with a Crossley picture secured by a duplicated exposure 7th and 8th
November.[788] The movements indicated were at the rate of about one
minute of arc in five weeks; and they were maintained until the dimly
shining spires affected by them faded into chemical invisibility.
Moreover, the spinning of the nebulous web was found to have been an
initial accompaniment of the stellar outburst. Two close coils of it
were discerned by Mr. Perrine on scrutiny of a negative exposed for ten
minutes 29th March 1901.[789] And since they were closer in the ratio of
their expansion between September and November, it became evident that
the process had gone on unchecked during seven or eight months; while
the tracing of it backward showed it to have actually commenced almost
simultaneously with the kindling of the conflagration. That is to say,
the rate of flow of the nebulous streams indicated their issue from the
Nova about 17th February, or five days previously to its visible
manifestation.
[Illustration:
PLATE XVIII.
1. Nebulosity round Nova Persei (Ritchey).
2. Spectrum of Nova Persei, 11th August 1901, with corresponding
Intensity-Curve (Campbell and Wright).
]
Speculation regarding this unique phenomenon has naturally been active;
and an explanatory hypothesis of considerable plausibility was hit off
independently by Professor Kapteyn[790] and Mr. W. E. Wilson.[791] It
affirms the nebula to have been pre-existent, and to remain unchanged.
But since we see it by the reflected light of the Nova, its various
spires and condensations have come successively into view as the flare
of the explosion travelled outward in widening circles. Hence an
illusory effect of radial expansion was produced, while, in point of
fact, the temporarily illuminated cosmic folds were as immovable as
aligned snow-peaks, in turn set aglow by the rising sun. The parallax
corresponding to this rationale is 0″·011; it implies that the Nova
actually blazed in the third year of James I. (1605), and rose to a
culminating splendour eight thousand times that of our sun. In this, at
least, there is no improbability. The star is devoid of sensible
parallax or proper motion; and its distance from the earth cannot,
accordingly, be expressed in “numbers that have name.”
A strictly temporary character may safely be ascribed to its nebulous
appurtenances; they were either evolved from its mass, or rendered
luminous through its influence. Professor Max Wolf suggested the
propagation of electric waves of the Hertzian type as the cause of the
far-spreading excitement attested by the gleaming annuli. Professor
Very[792] inclined to regard them as constituted by material corpuscles,
such as give rise to cathode-rays, ejected from the star under the
stress of light-pressure or electrical repulsion. There was, however, as
Mr. Louis Bell[793] remarked, no evidence of acceleration, consequently
none of the continued action of a repulsive force; while the supposition
that the nebula round Nova Persei had been photographed by reflected
light was, in his opinion, discredited by the absence of
polarisation-effects, as well as by the long persistence of strongly
nebulous patches close to the star, which, unless they had been
self-luminous, should have vanished with its fading. Their light,
accordingly, is surmised by Mr. Bell to have been of an auroral nature.
It developed as a secondary consequence of electro-magnetic strains,
propagated through space with the velocity of light, from a sun-like
centre of disturbance.
What cannot be gainsaid is that the apparition, with its strange
attendant circumstances, ranks as the most interesting on record. Even
the classic Nova Aurigæ was far outdone by the surprising diversity of
the phases, both spectral and luminous, exhibited by its brilliant
successor. Their study afforded much new knowledge, and opened wide
tracts for future experiment and research. A critical point of
difference between the observations made on the two occasions was the
timeliness of those on the latter star. They were in full swing while it
was still mounting towards the summit of its splendour. As a
consequence, the order of its spectral phenomena was clearly ascertained
and proved wholly unexpected. At the outset, the light was of ordinary
quality; waning had already set in before the displacements and
distension of its linear ingredients announced the prevalence of
disturbance. This fact is full of meaning; let us weigh it well. We
learn from it that the sudden development of luminous energy was
essentially of photospheric origin. Gaseous eruptions and explosions
followed, and produced the duplicated spectrum characteristic of
temporary stars. But they were merely incidental to the primary event,
which occurred independently of them. This disclosure narrows the field
of speculation as to the nature of that primary event, and is so far
enlightening. Further, we now know that the symptoms associated with
prodigious radial velocities were not inherent, but consequential. The
obscure body abruptly kindled to vivid incandescence gave no evidence of
rapid motion; it seemed a leisurely traveller through space.
We may now attempt to arrange and generalise the items of information
lately gained about Novæ. Three have appeared in nebulæ or clusters—Nova
Scorpii in 1860, Nova Andromedæ in 1885, and Nova Centauri in 1895. Set
apart by their nature no less than by their situation, they were
evidently transient adjuncts to the formations in which they were
immersed, and _probably_ imitated their luminous peculiarities. Our
knowledge of them, however, is partial and unsatisfactory. Passing on to
“blaze stars” proper, we note the following circumstances as common to
the class.
1. They have their habitat in the Milky Way. Nova Coronæ alone had any
considerable galactic latitude.
2. None have any sensible parallax or proper motion. They must then be
vastly remote.
3. They rise from and relapse into approximate obscurity. The one
_known_ star distinguished by a temporary flare was Nova Coronæ.
Several, nevertheless, continue perceptible in their effete state.
4. The bright lines of Novæ are coupled with dark lines of shorter
wave-lengths. This mode of juxtaposition is invariable.
5. The spectra of Novæ resemble, in their early stages, that of the
solar chromosphere, later that of nebulæ, the emissions of hydrogen and
helium ultimately yielding their predominance to the green rays of
nebulium. The Wolf-Rayet blue bands generally make an intermediate
appearance.
Now what may we legitimately conclude from these varied phenomena? Very
little, unfortunately, of a positive nature; we must be content, in the
main, with negative inferences. Yet it is no small advantage to clear
the ways of thought by abolishing untenable hypotheses. It may then
safely be stated that the remarkable spectral shiftings in temporary
stars are not effects of translatory motion; they supply no argument for
the duplicity of the light-source. Neither do they originate through
pressure, which tends to damp down vibrations, not to accelerate them;
and it is chiefly a shortening of wave-lengths that has to be explained.
Staggered by this difficulty, Dr. Wilsing suggested[794] the alternative
view that Novæ are not incandescent, but “luminescent” bodies. The
distinction, first made by E. Wiedemann,[795] is valid and valuable,
although next to nothing is known about the essential conditions upon
which it rests. All that can be said is that they involve the production
of light to a great extent without heat. But the supposition that they
are found in temporary stars is an extremely hazardous one. It is
countenanced only by the one fact that emissions due to luminescence
are, or may be, accompanied by corresponding absorptions of greater
refrangibility, and so present a colourable imitation of the perplexing
chiaroscuro spectrum displayed during stellar outbursts.
On the whole, the most promising theory of their occurrence is that
stars in the Milky Way occasionally get entangled in the diffused
nebulosities with which that region abounds, and blaze through the
resistance offered to their motion, just as meteors kindle to brief
splendour in shooting athwart our cloud of “circumfluous air.” We must,
it is true, be content for the present to accept it in principle;
attempts to elaborate it in detail can only, until much fresh knowledge
has been acquired, result in failure. Even M. Seeliger’s[796] ingenuity
did not avail to conduct him to a successful issue. He demanded too much
from the star-and-nebula hypothesis—demanded, indeed, more than the
conditions (as we now know) actually required; and it hence incurred
unmerited discredit. Admitted provisionally, it will perhaps serve as a
guide to ultimate truth. An important discussion of the possibilities
connected with it, and of the manner in which they might serve the
purposes of spectroscopic interpretation, was published by Mr. J. Halm
of the Edinburgh Observatory in July 1901.[797] Proceeding from the
hypothesis that a Nova becomes visible when “a dark body impinges upon,
and penetrates into a mass of nebular material,” he constructed a
vorticose system of radiating and absorbing gases travelling with the
great intruded globe, and skilfully adapted in all its parts to give
rise to the observed phenomena. His arguments are not, scarcely indeed
could be, in all respects convincing; yet they form a contribution of
stimulating quality to the general doctrine of temporary stars. This has
wide bearings. It should include, not only a rationale of the actual
conflagrations, but also some definite and consistent view as to the
previous state and history of the bodies subjected to them. They are
glibly designated “dark stars,” but embarrassments supervene when we
attempt to give precision to our conceptions of what constitutes a “dark
star.” We will, nevertheless, essay the task in the next chapter.
CHAPTER XXV.
DARK STARS.
Dark stars are of sun-like dimensions, but of planetary obscurity. They
constitute a realm scarcely penetrable by direct investigation, and
therefore eminently inviting to speculative thought. They are
indefinitely numerous; they may be, in many cases, enormously massive
and bulky; they are probably endowed, in general, with very high
velocities: some are perhaps among our nearest neighbours in space. They
exist both in a solitary and in an associated condition. The dynamical
relations of unattached dark stars, although of the deepest interest,
from their bearing upon large questions of sidereal construction, are
unknown, and seem likely long to remain so; but in mixed systems of
shining and obscure bodies they are readily open to investigation. Such
systems occur quite freely, as we learn from observations of three
distinct kinds. Their unseen members variously betray their presence:
(i.) by perturbing the orbital movements of visually double stars; (ii.)
by occasioning periodic changes of velocity in visually single stars;
(iii.) by occulting their bright primaries, and so causing them to be
affected with a special type of variability. In the first case, the
effects can be discerned telescopically, in the second they come out
spectrographically, in the third they are both photometric and
spectrographic. Solitary dark globes, on the other hand, are brought
within our ken in only one fashion. Given a certain combination of
circumstances of a barely conjecturable nature, they leap into light as
Novæ, and in this capacity attain various degrees of notoriety.
The existence of an “invisible” department in astronomy was divined by
Laplace; it was demonstrated by Bessel through his discovery, with the
mind’s eye, of the semi-obscure satellites of Sirius and Procyon. Its
importance cannot well be exaggerated. Unseen bodies may, for aught we
can tell, predominate in mass over the sum-total of those that shine;
they supply possibly the chief part of the motive power of the universe.
They appear, at any rate, to be profusely distributed throughout its
compass. One star in six, by a fair estimate, is a spectroscopic binary;
and spectroscopic binaries commonly disclose their nature, not by the
doubling, but by the shifting of their spectral lines. This is as much
as to say that one of the revolving masses radiates imperfectly or not
at all. Again, temporary stars presumably rise from crowded ranks. For
every one that blazes into view, multitudes must remain undistinguished.
The raw material of such outbursts is doubtless lavishly provided, while
the accidents actually occasioning them must be few. Stellar apparitions
thus derive extraordinary importance from their implication of an
unfathomable background stored with effete or undeveloped suns, the
presence of which evades immediate cognisance.
“Dark stars” need not be absolutely obscure. Some, at any rate, may be
feebly luminous, though powerfully attractive, like the companions of
the Dog stars. These, if situated much nearer to their primaries, would
escape telescopic recognition, and could challenge spectrographic notice
only indirectly, through the motion-displacements due to their influence
on the brilliant spectra superposed upon, and effacing their own. In
other cases, stars are swayed from their computed paths by the power of
entirely rayless satellites. The fourth member of the system of ζ
Cancri, and the hypothetical third component of 70 Ophiuchi appear to be
dark in the strictest sense. Most probably an indefinite number of
gradations connect the planetary and sun-like states. Thus in some Algol
pairs the occulting body is to our appreciation obscure; in others it
gives some, though very little light; while in a third variety the
companions stand on the same level of lustre. Algol itself, U Cephei,
and Y Cygni specifically exemplify these differences. They manifest
themselves as well in non-eclipsing spectroscopic binaries. A minority,
headed by β Aurigæ, consist of globes fully luminous; a few, such as α
Virginis and ο Leonis, show traces of a second spectrum; but most are
real crypto-doubles, only one member of which emits any perceptible
light. With these the heavens may be said to swarm.
A number of interesting questions are suggested by the vast numbers and
singular relationships of dark stars. What is their history? What their
destiny? What their function in the universal scheme of things? The view
commonly taken of them is that they are antiquated suns—suns in a state
of senile decay—suns that, having used up their radiative vitality, are
relegated to a lower cosmic plane. Such there must be, unless
regenerative machinery be at work to supply, in some unimagined way, the
perennial waste patent to our senses. Suns wear out, yet do not incur
annihilation. They must survive their faculty of shining. Immense, but
strictly limited reservoirs of energy, they are constructed, not for its
economical storage, but for its rapid expenditure. Its exhaustion,
which, to our apprehension, is simply a question of time, will leave
them gigantic earths, solid, inert globes, with only their gravitative
force unimpaired. That quondam-suns of this description pursue their
unnoticed journeys through space, cannot reasonably be denied; they
undoubtedly swell the hosts of dark stars; yet there are indications
that these are not mainly of the effete quality—that they do not merely
represent the _caput mortuum_ resulting from one inexorable evolutionary
process.
The frequent association of obscure with brilliantly luminous stars is
very remarkable. Algol and its companion make a typical instance. They
are assuredly of contemporaneous, and, judging by the usual criteria, of
comparatively recent origin. The closeness of the system formed by them
denotes, on the theory of tidal evolution, that it has, during no very
long time, been subject to such modifying influence; and Algol itself
bears all the marks of stellar juvenility. Another specially instructive
example among non-eclipsing binaries is afforded by Castor. Here a great
Sirian sun has an obscure and a brilliant attendant circulating
respectively in less than three days and in several hundred years. Now
the remote component must have separated from the parent sphere long
before the close satellite came into being; yet it is at the acme of
vitality, while that of the junior star is, so far as radiative symptoms
can tell, already spent. Nor is there any warrant for assuming that they
differ considerably in mass. Indeed, whatever disparity there may be is
just as likely as not to be _in the wrong direction_. A doubt thus
arises whether the shining of suns depends solely upon temperature; for
gradations of cooling cannot possibly account for the abrupt contrasts
of luminosity met with in mutually revolving globes. Irresistibly the
hypothesis presents itself that, among the varieties of the cosmos,
there may be a multitude of bodies conditioned in most respects like
suns, but which have never attained to the possession of a photosphere,
and have consequently remained imperfect radiators. The formation of the
dazzling cloud-shell by which suns are visibly bounded, is a baffling
enigma. It must involve an intricate combination of circumstances, not
always perhaps realised. Should any of them fail, a “dark star” would
result—a body intensely hot, yet destitute of the apparatus requisite
for the effective diffusion of light and heat.
We can now see that the obscurity of temporary stars previous to their
sudden illumination does not necessarily imply that they were effete.
Their galactic preference also argues the contrary. The Milky Way is
apparently a region where development has not gone far. Things are there
more nearly in their primitive state than elsewhere in the sidereal
world. Growing and flourishing suns seem to congregate in that vast
gathering-ground, rather than those that have seen better days. Hence,
if stellar apparitions signified the rekindling of stars sunk to
extinction through age, the Milky Way is precisely the part of the
heavens in which we should least look for their occurrence. That they
frequent it almost exclusively, helps to convince us that their
antecedent dimness was not due to decrepitude. This inference is
confirmed by the character of their spectra. The light suddenly acquired
has the quality considered to mark an early stage of sidereal
development. The blaze of a helium star can with difficulty be supposed
to proceed from a highly-condensed or semi-solidified body.
Dark stars must then, on this showing, be divided into two classes, the
first consisting of suns on the retired list, the second, of those
incapacitated by nature for active service. Their discrimination in
practice can scarcely be effected, with our present resources, otherwise
than tentatively or by conjecture. Still it may not be in all cases
hopeless. To take one instance. The system of 61 Cygni is held by
Wilsing of Potsdam to include one or more unseen members. If so, their
place—since the associated group bears many marks of antiquity—should be
found among lapsed suns. And this suggests the further thought that, in
a few more ages, the entire family may have ceased from radiation, and
will continue, in the rayless void, to fulfil their mutual rounds and
pursue their way towards an unknown goal. That many such blind systems
exist, inaccessibly to observation, is a conjecture which can neither be
verified nor refuted.
CHAPTER XXVI.
THE GENERAL QUESTION OF STELLAR VARIABILITY
The variability of stars is a “wood of error.” There seems no exit from
it; the tracks that invite our ideas to enter upon them turn out
circuitous and misleading. Yet its exploration need not therefore be
abandoned as hopeless; and indeed lines of approach are converging upon
it from many directions in a manner that promises well for the future
elucidation of much that is still mysterious. In the meantime, we can at
least attempt to arrange the known facts in some kind of definite order.
Stellar light-change is of two fundamentally separate descriptions. The
first may be termed “extrinsic,” because it obviously depends upon
circumstances not inherent in the bodies it affects; the second,
“intrinsic,” as resulting from constitutional peculiarities. They are
readily distinguishable by the nature of their time-relations.
Punctuality marks the former kind, a large measure of irregularity the
latter. The difference becomes intelligible if we admit, as we
reasonably may, that short-period or extrinsic variability belongs
exclusively to compound objects, long-period variability to solitary
masses. In no case—except where darkening by eclipse unmistakably
occurs—can we define the kind of action to which the observed
vicissitudes are due; but the power lately acquired of discerning
between the “forced” periods prescribed by orbital relations and the
“free” periods determined by fluctuating interior conditions, marks an
important step in advance. The circumstance, although unaccountable,
should not be lost sight of that eclipsing and non-eclipsing variables
diverge somewhat emphatically in the character of their spectra. Those
of Algol-stars belong to the first type, often to the helium division of
it; those of Cepheid stars are mostly solar; and although enlarged
experience may tend to invalidate these generalisations, it may, on the
other hand, serve to reaffirm them. The spectrographic study of globular
clusters—a task for the immediate future—should help greatly to widen
the bases of knowledge regarding this curious relation; and the diligent
collection of illustrative instances, by limiting the extent of its
prevalence, may throw some light upon its cause.
The secret of short-period variability offers few points open to attack;
but an attempt might be made to penetrate some of its outworks by
comparing the systemic conditions of steadily lustrous close binaries
with those of pairs fluctuating in magnitude. Stars of the same spectral
class should, for the avoidance of complications, be chosen for
confrontation. Polaris, for instance, θ Ursæ Majoris, λ Andromedæ, are
solar orbs revolving swiftly round invisible companions. In what
respects, it will be of interest to ascertain, do their orbits differ
from those of the variables δ Cephei, η Aquilæ, and ζ Geminorum? Are
they markedly less eccentric? Can they be inferred to be more spacious?
No real discrimination may be possible, and if so, this line of search
fails, and some other must be struck out. But it is clear that only
tentative inquiries, pursued untiringly and successively, can avail to
bring us nearer to the central truth we are in quest of.
The light-changes of stars are determinate in amount just in proportion
as they are accurate in period. Irregularities of one kind are
concomitant with irregularities of the other. They reach, in long-period
variables, the limit of total disregard of traceable law. For one
settled cause of variability a number of consilient causes would seem,
in such objects, to be substituted. Should their co-operation break
down—should one among them become temporarily ineffective—every vestige
of cyclical recurrence may disappear. And it is important to note that
stars with disturbed periods merge, by insensible degrees, into stars
with no periods at all. Other stars show intermittent periodicity. They
start cyclical variations only to drop them after a few recurrences, the
alternations being often repeated at indefinitely prolonged intervals. A
law of order is present; it has not been abrogated; but its workings are
almost neutralised by adverse influences. Then again, stars are found
subject to uncertain accesses of light-change. Fits of instability of
the most marked kind may be followed by long epochs of constancy, as in
the case of P Cygni, and, perhaps we may add, of η Carinæ; and many
objects rejected from revised catalogues of variables as having failed
to make good their title to inclusion are similarly stars reposing after
more or less protracted crises of activity. The relations of stellar
fluctuations to time are indeed unlimitedly various. They are, however,
characterised by one feature which persists significantly, though far
from immutably, notwithstanding the disguising effects, in
Mira-variables, of accelerated, retarded, and duplicated phases. This is
the more rapid increase than decrease of light. It is strongly
pronounced in Cepheid and cluster variables; it asserts itself with
modifications in variables of long period; in the sun it is never
obliterated by the supervening irregularities of the spot-cycle. This
wide range of agreement hints at some deep-seated principle of
community, undefinable at present, and intangible to our mental grasp,
yet lying at the root of the diverse phenomena of stellar light-change.
Their specific association with particular spectral types is a fact
replete with meaning, no less than the exemption from variability,
complete or partial, of whole classes of stars. Taking first the
negative cases, we find the Wolf-Rayet family characterised by
exceptional stability. Not one of its five-score members, so far
discovered, shows the smallest symptom of luminous variation. Nor are
their congeners, bright-line helium stars, ordinarily affected by it.
Those that are so affected betray qualities in some way unusual. Among
them are to be reckoned nearly all temporary stars, the singular
short-period variable in Lyra, η Carinæ, and P Cygni. Nebulous stars are
not as a rule variable. The Pleiades shine with approximate constancy;
the star-groups at the hearts of the great Orion and Trifid nebulæ give
no conspicuous sign of fluctuation;[798] and isolated stars with nebular
appurtenances are, for the most part, steady light-givers. T Tauri,
however, makes an exception; and disappearances, transient or lasting,
of certain minute stellar denizens of planetary nebulæ have been, from
time to time, asserted or surmised. White stars of either spectral
variety are scarcely ever intrinsically variable. As spectroscopic
doubles they are indeed subject to obscuration by eclipse; as telescopic
doubles, to slight and capricious fluctuations in the magnitudes of one
or both components, disconnected, apparently, from any of their orbital
circumstances. Only three hydrogen or helium stars are known to be
variable in any true sense; these are u Herculis, U Geminorum, and R^2
Cygni; and the rarity of this association of qualities makes it
especially desirable that their spectral distinctions should be
scrutinised with the utmost care. Very few second-type stars vary
otherwise than extrinsically. Nor do they, that we yet know of, undergo
eclipses. Yet many are binaries of the Cepheid kind, and thus become
_compulsorily_ variable. The solar quality of light must be regarded as
highly favourable to steady radiation.
Just the reverse is true where banded absorption comes in. Stars of the
third and fourth spectral types are not only pre-eminently variable, but
they vary almost without exception irregularly, or in long periods. No
spectroscopic binaries,[799] no Cepheid or Algol-stars have been found
among them. In the character of their light-change, the two classes of
stars with banded spectra are indistinguishable. But their spectroscopic
study has brought into strong relief the different conditions of gaseous
incandescence prevalent in each. In fourth-type stars it keeps aloof
from fluctuations of brightness; the same vivid lines gleam in variables
and non-variables; nor do they seem to kindle or fade in correspondence
with the general light. Those that are visible in third-type stars, on
the contrary, are an exclusive and unfailing mark of extensive
variability, and attest, by a sympathetic course of development, their
intimate connection with its processes. All this teaches us that the
processes in question have their seat in limited regions, and are in a
measure detached from the action going forward in other parts of the
same stellar structures. Now variability of the kind prevalent in stars
with banded spectra (as we know from certain of their spectral traits)
originates in, or immediately above, the photosphere. Remote from the
photosphere, accordingly, must be the location of the emitting gaseous
strata in fourth-type stars, since the rays emanating from them shine
undisturbed by the progress of the most conspicuous luminous
vicissitudes. Hence the obvious deduction that these are conditioned by
internal peculiarities. The brilliancy of stars depends, _cœteris
paribus_, upon the rate of transport from within outward. Variability is
an index to circulatory changes. In addition to this immediate cause,
collateral influences doubtless come into play—forces which co-operate
and combine, or mutually nullify one another, but so obscurely to us
that there can be no profit, in the present state of knowledge, in
prosecuting what could be no more than a conjectural inquiry.
CHAPTER XXVII.
IRREGULAR STAR CLUSTERS.
Stars associated together into communities are probably subject to
mutual influences of special kinds. With the relative movements produced
in them by the stress or pull of gravity we are not here concerned; they
form an extraordinarily interesting subject for future inquiry, but as
yet no hint of their method is derivable from the scanty materials at
hand. The physics of clusters, however, falls within our scope, and is a
topic more immediately accessible. It is open to discussion chiefly in
two ways—by studying the spectral peculiarities, and the luminous
variability of the component objects.
It is not always easy to distinguish between a casual “sprinkle” and a
true cluster. The Pleiades are the only family of stars which shows a
wandering tendency; the rest are _adscripti glebæ_; they have no common
drift by which they could be set apart from casual inmates of the same
sky-region. There remains the argument from probabilities of
distribution, with its indefinite variations of conclusiveness according
to the circumstances of each particular case. Yet it usually suffices
for conviction. None, at least, of the five hundred registered clusters
present any real ambiguity of character, although many other groups
doubtless subsist unrecognised because poor in numbers and loosely
scattered.
Two varieties of stellar collections can be readily discriminated. One
is characterised by a spherical form; the constituent bright points
press inward towards a centre; they aggregate into “globular clusters.”
Those termed “irregular” appear to be constructed on a different plan.
Very slight traces of interior condensation are perceptible in them;
they are made up of star streams, branches, and spirals, more or less
closely intertwined and commingled. A glittering assemblage in Gemini
(Messier 35) has an obviously radiated structure. Lord Rosse was struck
with wonder at the arrangement into loops and arches of the stars in M
37, a similar object in Auriga. Still more definite and amazing are the
“patterns, consisting of lines, wreaths, and curves of stars,” in Dr.
Roberts’s photograph of a cluster in Cassiopeia (N.G.C. 7789), taken
with an exposure of ninety minutes, 26th November 1892. The effect of a
marshalled array is irresistible.
[Illustration:
PLATE XIX
Photograph of Messier 11. Taken by Dr. Roberts with an exposure of
90^m.
]
Interior vacancies seem correlative to a streaming conformation. They
perhaps represent spots denuded of their bright inhabitants by the
action of some unknown expulsive force. Irregular clusters, at any rate,
are often remarkably perforated or furrowed. Some are rendered, by the
development of “dark lanes,” essentially bifid or trifid. The well-known
star-throng in Antinous (M 11) is broken up by partial clearings in a
manner suggestive of eventual disruption. The breaches in the masonry
(so to speak) are finely shown in a photograph taken by Dr. Roberts,
10th August 1896, reproduced, by his kind permission, in Plate XIX. M.
Fenet, from an earlier Crowborough plate, mapped 395 components of this
cluster, which includes altogether about 1200, and remarked that most
were entitled, by the close attendance of satellite-stars, to be
regarded as forming multiple systems.[800] Their collection into seven
or eight separate allotments was evident to him, and could not, he
thought, fail to become further accentuated with time. All,
nevertheless, yield apparent allegiance to a ninth-magnitude star, which
fully sextuples the brightness of any of its followers. A spectroscopic
examination of this object would be desirable. The general quality of
its light might be readily ascertained, and the detection of
motion-displacements need not be despaired of. A catalogue of two
hundred members of this brilliant assemblage, referred to their leader,
was drawn up in 1870 by F. R. Helmert, and compared with measures
executed by Lamont in 1836–39.[801] The agreement was complete within
the limits of probable error; no discrepancies betrayed shiftings of
relative position, although sure provision was made against their
unnoticed occurrence in the future.
Helmert considered several of the stars observed by him to be slightly
variable; but their changes made no show on the Arequipa plates of the
cluster. Since, however, they were taken at an interval of a few days,
only variables of short periods or rapid vicissitudes could have been
disclosed, and none such, it is safe to say, are present. Yet there is
some probability that a temporary member was added to the group upwards
of sixty years ago. On the 12th of August 1839, Lamont entered No. 9 on
his list as a “new star”; he had not perceived it before, and on the
ensuing 9th of September it was gone. Helmert did not expressly look for
it, but could only have missed seeing it through its extreme
faintness.[802] Indeed, it was most likely, by that time, as hopelessly
extinct as Nova Andromedæ now is. The stars of M 11, although devoid of
nebulous attachments, are shown in one of Professor Barnard’s
small-scale photographs to form a knot at the margin of one of the great
cloudy formations in the Milky Way, their actual nuclear relation to
which he regards as “hardly questionable.”[803] This may be; the
supposition is plausible; yet it is a long way from being demonstrable.
Meanwhile the practical inquiries to be made in connection with the
cluster are these two: What is the spectrum of its leader star? and, Do
any of its components vary in light?
The lovely double cluster in Perseus (N.G.C. 869, 884) resembles it in
being non-nebulous, perhaps also in possessing galactic affinities. The
twin groups are known respectively as _h_ and χ Persei. Their connection
is remote; dynamically they can scarcely be mutually dependent, but they
are of closely analogous construction, and their components are
similarly linked into festoons and spirals.[804] None of them can be
perceived to drift, absolutely or relatively. This was put to the test
in 1884, when 172 stars in “χ Persei” were photographically determined
by O. Lohse for confrontation with the results of Vogel’s micrometrical
measures of them fourteen years earlier.[805] Their seeming immobility
will probably be maintained for many decades yet to come. Vogel’s
special catalogue of the thirty brightest among them (all above the
tenth magnitude) might, nevertheless, usefully be revised, from the
photometric point of view, for the purpose of detecting possible
alterations of brilliancy. The task would be the more hopeful that some
of the objects in question have a note of colour. A “ruby” star was
allotted a central position in χ Persei by Sir John Herschel;[806] the
Parsonstown reflectors displayed rosy, yellow, and bluish tints in many
of its sparkling associates; and Mr. Espin, much more recently, located
in the outskirts of the cluster eight reddish stars with fluted
spectra.[807] These may be expected to prove more or less variable,
though not in the prompt and definite fashion prevalent in globular
assemblages.
Professor Barnard recognises two varieties of irregular clusters—the
purely stellar and the nebulously stellar.[808] They can be
distinguished with certainty only by chemical means. Long photographic
exposures are needed to test satisfactorily the condition of grouped
stars. The characteristic nebulosity of many clusters may in fact be
counted one of the most important discoveries made with the assistance
of the camera. But at present we are dealing with the non-nebulous kind,
such as the Hyades and Præsepe in Cancer.
Aldebaran, the great red “eye of the Bull,” is in, but not of the
Hyades. The disconnection will be rendered obvious, and the relative
drift more precisely definable, when the radial movements of the several
stars can be spectroscopically fixed. This, indeed, is already feasible,
were it not that the great telescopes of the world are otherwise
occupied.
Forty-five stars in Præsepe have been located with rigid accuracy.
Winnecke’s observations of them in 1858, Asaph Hall’s in 1870, above
all, Schur’s Catalogue for 1875,[809] laboriously constructed from a
triangulation with the Göttingen heliometer, ensure the detection of
their future movements. They are extremely minute. In the course of
thirty-two years they produced effects so small as to be barely
determinable. The spectra of ninety members of this stellar family were
photographed in or about the year 1896 at Harvard College.[810] Owing to
their faintness—they ranged from 6·5 to 9·5 magnitude—the details of
their classification remained uncertain; but the one main fact was
brought out that the collection is of mixed quality. It does not seem to
be expressly assorted in any way; no spectral pattern can be called
predominant. Twenty-eight of the ninety associates were recorded as
Sirian, sixty-one as solar or intermediate stars, and the third type was
represented by a solitary specimen; that is to say, the percentage of
first-type spectra in Præsepe is thirty-one, while it rises to
sixty-five in the Pleiades, and sinks to fifteen in Coma Berenices. In
this last asterism, however, the stars, although crowded, are not
_clustered_.[811] Yet their crowding obtains significance through
Professor Pickering’s notice of their almost exclusively solar
character.
A spectrographic survey, carried out at Harvard College, of four
southern clusters of the coarse-grained or irregular description, has
lifted another corner of the veil from this department of astrophysics.
One of these objects, still uncatalogued, is situated in the
neighbourhood of η Carinæ; a second (N.G.C. 3523) is not far off; the
third and fourth (N.G.C. 6405 and 6475 = M 6, M 7) are found in Scorpio.
The plates exposed showed in the aggregate 705 spectra capable of
characterisation, of which 576 were unmistakably of the first type.[812]
The average proportion, then, of hydrogen stars in these four clusters
is 82 per cent. Half a dozen helium stars were identified in the
anonymous group, none in the rest. On the whole, spectral uniformity may
be considered the ideal state towards which most clusters tend; it would
perhaps, if accidental components could be eliminated, prove to be more
nearly realised than it seems. In this connection it is of interest to
note that M 37 in Auriga consists wholly of yellow stars, presumably
belonging to the solar class. The sky, in Admiral Smyth’s phrase,
appears in that spot as if strewn with gold dust. The companion cluster
in Gemini is, on the other hand, resplendently white; and Dunlop
registered at Paramatta a bluish globular cluster (N.G.C. 6723) likely
to be packed with Sirian stars.
The jewel-cluster about κ Crucis is differently organised. Harmonies of
contrast rather than of consonance may here be observed; but the
collected brilliants display their various tints effectively only in the
fields of large telescopes. From measures of 130 of them, Mr. Russell of
Sydney derived in 1872 ostensible evidence of comparatively rapid
interstitial movements during the thirty-five years elapsed since the
date of Sir John Herschel’s corresponding work; but until a fresh set of
determinations gives assurance that they are pursued systematically,
little weight can be laid upon discrepancies otherwise explicable. His
suspicions of variability in twenty-five components have not so far been
verified. Several are bright enough to show distinctive spectra, the
nature of which it would be particularly interesting to ascertain. This
beautiful object lies near the northern border of the “Coal Sack.”
Irregular clusters obviously form systems of extreme intricacy. They
cannot be pieces of mechanism set in action by some uniformly operating
motive power. Their aspect is in most cases irreconcilable with the
hypothesis of a dynamical equilibrium. Few, if any, betray by movement
or conformation the influence of a preponderating centre of attraction.
They rather suggest inconceivably complex aggregations of partial
systems bound together loosely nor perhaps indissolubly. The
investigation of their mutual relations will tax the resources of the
old as well as of the new astronomy. Some of the problems to be
confronted have just begun to take shape, others loom on a remote
horizon. As a prelude to dealing with them, the separation might
gradually be effected of the really physical from the merely optical
components of clusters. The process will be greatly facilitated by the
ready help of the camera; and the slow evolution of telescopic or
tangential displacements can already in part be forestalled by the
spectroscopic disclosure of radial velocities.
CHAPTER XXVIII.
NEBULOUS CLUSTERS—THE PLEIADES.
“Tangled in a silver braid” of shining world-stuff, the Pleiades stand
out as the typical nebulous cluster. They give signs of not being
indefinitely remote. The assembled stars have a common drift, which is
most likely a perspective effect of the sun’s advance in the opposite
direction. If this be so, their light spends just 200 years in reaching
the earth, the rate of our progress towards the constellation Lyra being
taken at twelve miles a second. Alcyone then radiates at the very least
190 times more powerfully than our sun; in its place, Sirius would
appear fainter than the fifth magnitude; it would be outshone, not only
by the _lucida_ of the group, but also by five of its companions—by
Atlas, Merope, Electra, Maia, and Taygeta. Thus the glory of the
Atlantids would be but slightly enhanced by the addition of the great
Dog star to their number, and the scale of the system must be
commensurate with the magnificent luminosity of its members.
A beginning has been made in the discrimination of the genuine Pleiades
from their optical companions. The definite character of their proper
motions has made actually feasible what in other similar collections is
only remotely possible. The outcome of Elkin’s measures with the Yale
heliometer in 1884–85[813] was to distinguish forty-five stars,
including Alcyone, as inseparable travellers, while eight proved their
independence by dropping out of the ranks. This group of forty-five
members may be regarded as a nucleus round which additional stars will
aggregate as their movements develop. How far it will extend, how many
of the small stars swarming on long-exposed negatives it will eventually
take in, remains conjectural. Its delimitation, however, should be
practicable in the course of a decade or two; for the comparison of
photographs taken about 1915 with those of 1885 and 1888 may be expected
to bring numerously into view relative displacements consequent upon the
abandonment, as it were in mid-ocean, of a multitude of pseudo-Pleiades
exempt from the drift belonging to the true cluster. There is reason to
think that this process of expulsion will have to be carried far. The
self-selected assemblage will probably not be overcrowded. Exact
numerical inquiry has led to many unexpected results, but to none more
surprising than that of the thinning-out of faint stars within the area
of the Pleiades. Professor Bailey counted nearly 4000 on a photograph
including it taken with the Bruce twenty-four-inch lens in 1897; but
their density, as a detailed examination made evident, fell off notably
and systematically inside the precincts of the system. “It therefore
appears,” Professor Pickering wrote,[814] “that the total number of
stars in the region of the Pleiades is actually less than in adjacent
portions of the sky of equal area, and it is much less than the
corresponding number in many parts of the Milky Way.” Regarded, then, as
a physical entity, the cluster includes only the brightest of the
spangled points thrown together into the field. The spangles of the
background would indeed presumably be still more numerous but for the
absorbent effect of the nebulous masses attached to the brilliant stars
in front of them. Their paucity, at least, must be somehow accounted
for, and this explanation of it, suggested by Professor Pickering, seems
admissible. M. Stratonoff,[815] too, was led, by a study of stellar
distribution in their neighbourhood, to the conclusion that the physical
associates of Alcyone are comparatively few; and the conclusion is the
more interesting from the sure prospect of bringing its truth to the
test.
The Pleiades might be described as not merely a nebulous cluster, but as
a cluster of nebulæ, so numerous and so sharply characterised are the
cloudy forms collected within its borders. All save one are photographic
revelations. The exception is the “Merope nebula,” discovered by Tempel
19th October 1859. A mere “breath stain” on the sky, it is, to
telescopic vision, a highly elusive object; yet it is always there,
striated and definite, when looked for by chemical means, and the
hypothesis of its variability has long ago been abandoned. The Maia
nebula has something of the same striped aspect, but clings in a
strongly curved whorl to the star which forms its nucleus. Mr. H. C.
Wilson of the Goodsell Observatory described as follows a photograph of
this object taken by him 30th January 1894.[816] “The region about Maia
is especially interesting. A very bright horn-shaped patch of nebula
runs out from the west edge of the star-image immediately northward, and
extends to a distance of 3′ north of the star. The nebula here is full
of irregularly parallel streaks similar to those about Merope, but
making only a very small angle with the meridian. Some of them run to
and beyond the bright stars north of Maia. A series of rather broad and
diffuse patches extend from the middle of the group on a diagonal toward
the north-west, reaching to a comparatively bright pair of stars in that
direction.”
A second Merope nebula, totally unlike the first, was discovered by
Professor Barnard with the Lick thirty-six-inch refractor 14th November
1890.[817] It is round, clearly terminated, and centrally condensed, 30″
in diameter, and presents the general effect of a distant comet. With
the adjacent star it forms so close a combination as to indicate, almost
of necessity, the slow progress of mutual revolution. Mr. Burnham, who
measured the new nebula at Lick in the autumn of 1891, regarded it as
“one of the most singular objects in the heavens,” and “unique with
respect to its nearness to a bright naked-eye star.”[818] It is the
brightest nebula in the Pleiades, and came out well on plates taken by
Professor Keeler in 1898 with the Crossley reflector.[819]
Another cosmic species singularly exemplified in this cluster might be
called “ribbon nebulæ.” They run in narrow, straight bands from star to
star, in one case stringing together six or seven, “like beads on a
rosary,” and they pursue with fair accuracy, along parallel lines, an
east and west direction. What manner of communication they establish
between the suns they connect it is impossible to divine. They may
conceivably be mere survivals of a prior order of things, belonging
rather to the past than to the present; but no structures more curious
have been brought to our notice by the camera than these long, luminous
highways built as if for the purpose of facilitating intercourse between
the cities of space. An _unfinished road_ starts from Electra towards
Alcyone; it has been completed over only about one-third of the way. Or
is it the wreck of a celestial causeway which formerly reached its
destination, but has been gradually, for some ages past, falling out of
use and repair? The question is a daring one; ultimately, however, the
comparative study of analogous objects may supply hints for answering
it, at least by a plausible surmise. The cluster is besides crowded,
especially in the neighbourhood of Alcyone, with irregular or
nondescript nebulæ, which choke the background as if with rolling fog.
But in general the tendency is unmistakable to assume filamentous
shapes, such as were shown with peculiar distinctness in two photographs
taken by M. Stratonoff at Tashkent early in 1896 with multiple exposures
of respectively ten and seventeen and a half hours.[820]
[Illustration:
FIG. 46.—Drawn from Photographs by E. Calvert.
]
The history of the Pleiades nebulosities does not end here. Professor
Barnard had long been aware of a dulling of the sky-ground over a vast
adjacent area; and at last, in December 1893, he put these vague
perceptions to the test by means of a ten hours’ exposure with the
Willard lens.[821] “The resulting picture,” he wrote, “showed a number
of singular curved and streaky nebulosities, apparently connected with
the Pleiades and extending all about the group.” Some of them he was
able to trace for several degrees on either side, especially towards the
east. Yet doubts were expressed as to whether spurious photographic
effects were not in question. The phenomenon disclosed was, indeed, so
amazing that some degree of scepticism was excusable. Its reality,
nevertheless, had to be admitted. Confirmatory photographs were produced
by Dr. Max Wolf,[822] by Mr. H. C. Wilson,[823] and by Professor Bailey.
A skilful drawing by Mr. E. Calvert, embodying the combined results, is
reproduced in Fig. 46. But its limits are too narrow to include the
whole of these far-spreading formations. There seems no end to them. The
interior nebulosities are left undepicted for the sake of clearness. We
are thus brought face to face with the “startling fact” (as Professor
Barnard calls it) “that the Pleiades and their involved nebulosities are
but the central condensation of an enormous nebula, intricate in
details, and covering at least a hundred square degrees of the
sky.”[824] The magnitude of this mixed system staggers belief and
confounds the imagination. We contemplate it with imperfect apprehension
of its scope and significance. For the present, the relations between
its various parts appear scarcely open to investigation. The possibility
even of speculating upon them will offer itself only when acquaintance
begins to be made with the spectral characteristics of the Pleiades
nebulæ. That they are all alike gaseous may safely be assumed;
nevertheless, their dispersed light, when it becomes practicable to
examine it, will perhaps offer diversities full of interest.
Among the stars of the cluster a single spectral type markedly
predominates. It may be distinguished as “late Orion.” Hydrogen
absorption is prominent; helium absorption also asserts itself, but with
less emphasis than in the earlier stars of the same class.[825] This
quality of light is common to all the principal stars; those that
deviate from it are of inferior grades of brightness, and may
eventually, through the effects of proper motion, be sifted out from an
assemblage to which they do not properly belong. Hence, when the group
comes to be organised on a definitive basis, the _gross_ percentage of
sixty-five helium stars may have to be raised very much higher. The
first example of a mixed hydrogen series was met with in Alcyone.
Professor Campbell was astonished to perceive in 1893 that its spectrum,
otherwise marked only by absorptive action, included a glowing crimson
C. Very remarkably, too, this solitary bright ray is coupled with a dark
streak, situated, as usual in cases of duplication, on its more
refrangible side. Which is the displaced line has still to be
ascertained. Pleione, the only other member of the family showing signs
of emission, is spectroscopically akin to γ Cassiopeiæ. Its bright rays
bisect obscure bands.[826] The recession at the rate of eight or ten
miles a second of the solar from the Atlantid system must occasion a
perceptible shift towards the red of all the spectral lines of its
members, to which constant element are superadded the varied, and
perhaps varying effects of their individual motions. Spectrographic
researches hold out, then, the best prospect of gaining, within a
reasonable lapse of time, some insight into the working of this amazing
piece of celestial mechanics. From a triangulation, executed with the
Göttingen heliometer in 1889–91, Dr. Ambronn derived, as he thought,
indications of a division of the cluster into several distinct parcels
of mutually dependent masses;[827] but his measures, which included only
sixteen stars, had too restricted a scope to be decisive of much.
No stars of assured variability, whether periodical or irregular, are
found among the Pleiades. Yet light-changes, eluding definite
recognition, are suspected to progress. Maia and Merope have both been
held to fluctuate slowly; and Atlas, divided into a pair by Struve in
1827, is now single, with all powers, in the serenest skies. Only a
twofold occultation, noted by Hartwig in 1876, seemed to intimate the
obscure survival of the vanished companion. Possibly it may make itself
_felt_ spectroscopically, even should it never again be seen. The
motion-displacements, accordingly, yielded by Atlas deserve attentive
scrutiny, since from them may be obtained the key to one of the long
outstanding enigmas of double-star astronomy. Another member of the
group appears to have an authentically variable attendant. In Wolf’s map
of 1874 an anonymous 7·2 magnitude star due south of Alcyone is marked
as a wide double; it was single in the Paris photograph of 1886, but
again double in that of 1888, when the satellite had risen to
eighth-magnitude brightness. Yet M. Chevremont, reviewing the collection
with a small refractor in November 1895, could find no trace of it.[828]
This was the most precisely defined among several cases of presumable
light-fluctuation met with in the course of his survey. Again, M.M.
Müller and Kempf constructed at Potsdam in 1899 a photometric catalogue
of ninety-six Pleiades,[829] forty-two of which, given in the Bonn
Durchmusterung as of 9·5 magnitude, seemed to have diminished so
considerably in brightness that their mean magnitude could not be placed
higher than 10·7.[830] Very little real change, however, may here have
been concerned, since the ordering of stars in the lower ranks of the
Durchmusterung is known to have been a highly casual process. It can
scarcely be doubted, indeed, that some components of the cluster are, in
a measure, variable, but their variability is of a kind not easily
certified; it follows no method; it obeys no time-prescription; its
effects, perceived when least looked for, cannot be counted upon to
recur. Hence the problems of light-change set by the Pleiades are of a
peculiarly baffling nature.
CHAPTER XXIX.
NEBULOUS CLUSTERS—_Continued._
Many nebulous clusters besides the Pleiades are known, but none in which
the relations of stars and nebulæ are so highly specialised. In general,
the stellar collection seems as if independently organised, and plunged
as a whole into an ocean of cosmic fog, without any strong tendency on
the part of its members to form individual nebulous attachments or
connections. When, however, the structural details of such formations
come more fully to our acquaintance, they may be found to contain
evidence of a closer association between particular objects of the two
species than can at present be vouched for. Such investigations can be
conducted effectively only by photographic means and with carefully
adapted instruments. For the purpose of bringing out the full extent of
the nebulosity, small portrait-lenses have prerogatives, illustrated
practically by Professor Barnard,[831] theoretically by Professor
Wadsworth.[832] But pictures on a larger scale than those obtained with
them are needed for the disclosure of many topographical minutiæ huddled
together by the strong concentration due to their short focal length.
For securing these, reflectors of powerful light-grasp are
unsurpassable. Thus two lines of photographic inquiry should be made to
converge upon nebulous clusters; one directed towards determining the
limits of the involving nebulosity, the other towards ascertaining its
constructive peculiarities.
To a superficial view the object we are now about to describe seems more
like a cluster and nebula than a nebulous cluster. The nebula is
“Messier 8”; it is just visible to the naked eye, and from the oval
vacancies which interrupt its light, it has received the descriptive
designation of the “Lagoon Nebula.”[833] The cluster—separately
catalogued as N.G.C. 6530—immediately follows it on the same parallel,
but not in complete detachment. The two formations unmistakably overlap,
and, to discerning vision, inextricably intermingle. Professor Barnard’s
negatives, taken with the Willard lens in 1892, showed the compound
object to be “a singular mixture of stars and nebulosity. East and west
its diameter is about 45′, and north and south some 42′. The southern
side is sharply defined and serrated, with three distinct pointed
projections. From its north-following corner a wisp of nebulosity
extends nearly to a group of nebulous stars, and possibly with a longer
exposure would be found to connect with them.”[834] A picture on so
small a scale could teach little regarding internal structure; the
“lagoons,” in fact, appear in it nearly _silted up_ with diffused
nebulosity. Many of the brighter stars are involved in the pervasive
haze, and one in particular occupies too critical a position at the edge
of “a very black hole” for the supposition of a mere chance arrangement
to be permissible. It may be doubted, however, whether the 5·7 magnitude
star, 9 Sagittarii, has more than an optical connection with the
cluster, and the multitude of stellar points glittering in the
background will almost certainly be separated from it by the slow
discrimination of drifting movement. A preliminary step towards applying
the test was taken by M. Comas Solà in 1898.[835] His photographic
triangulation of the group fixes a starting-point for comparative
inquiries which can yield tangible results only in the distant future.
His plates showed the stars to be doubly implicated with nebulosity. One
of the seventh magnitude acts as the focus to a cloudy mass, distinct,
apparently, from a diffuse, elongated structure projected upon the
central parts of the cluster, which it not impossibly encloses in
annular folds.[836] The Lagoon Nebula gives a spectrum of bright lines.
It will be interesting to learn whether they are displaced by motion,
and whether, if so, corresponding spectral shifts in the clustered stars
ratify the presumption of organic relationship between the two orders of
formation.
The optical history of nebulous objects is often curious and
instructive. As an open cluster, N.G.C. 2239, in Monoceros, was first
observed by Sir John Herschel. An adjacent patch of nebulosity, seen by
Swift in 1865,[837] again attracted Barnard’s attention in 1883. It
proved to be a kind of knot on a great nebulous ring discerned by the
latter in its entirety with the Lick twelve-inch refractor in 1889.[838]
This encloses the star group like a ring-fence, and may be said to
osculate with a second great filmy ellipse, of which only a section is
perceptible. Then came the turn of the camera. Professor Barnard
obtained a photograph of the complex arrangement 9th January 1894, which
confirmed visual impressions while demonstrating their inadequacy. They
were not misleading, but extremely partial. The photographed nebula is,
in Professor Barnard’s words, “about one degree in diameter, and very
irregular in brightness and outline.”[839] It involves the cluster with
unequal condensations, which are especially heavy north of the bright
stars. The nebulous knots, and the section of a large ellipse, fully
depicted in the sketch of 1889, reappeared in the chemical picture. In
this, the only effect of annularity left visible is that a vacant
interior space seems reserved for the conspicuously grouped stars. Their
chief being of the eighth magnitude, the determination of its spectral
character should present no difficulty, and would be of particular value
as a test of nebulous affinity.
The number of alternative titles by which the star catalogued by
Flamsteed as “15 Monocerotis” is known expresses the curious variety of
its claims to distinction. It is variable, multiple, and nebulous. In
the first capacity it is designated “S Monocerotis.” Its fluctuations
from 4·9 to 5·4 magnitude, in a period of 3^d 10^h 38^m, were noted by
Winnecke in 1867. These elements are indeed still somewhat uncertain.
They need verification and revision. The variable is, moreover, the
leading member of a triple combination, enrolled by Struve as “Σ 950.”
With a green companion at 2·8″, and a bluish one at 16·6″, it makes an
exquisite telescopic object, but gives no sign of orbital motion. It
occupies a dominating position in a collection of fifty or sixty stars,
ranging from the eighth to the thirteenth magnitude, one of which is
subject to fits and starts of extensive variation.[840] By virtue then
of its chiefship of a cluster, 15 Monocerotis is registered among nebulæ
as N.G.C. 2264. Nor solely on this account. Both Sir John Herschel and
Lord Rosse suspected it to be nebulous, and Bruno Peter, who measured
forty-five components of the group at Leipzig in 1879–82,[841] found
their leader visibly wrapt in a hazy envelope. But the wide extent of
its nebulous connections was entirely a photographic revelation. A
picture of the region about 15 Monocerotis, taken by Professor Barnard
on 1st February 1894,[842] showed it to be involved in a great nebula
some three degrees in diameter. “It clusters densely,” he says, “about
the groups of stars, and then spreads out in a weak, diffuse light, with
rifts in it, and irregularly terminated along the edges of a vast
vacancy in the Milky Way. The condensation, which is very strong, is not
at 15 Monocerotis, but twelve minutes south-preceding that star, where
it becomes a compact mass with numerous wisps and holes in it.” The
absence of nebulous concentration about the individual stars struck him
forcibly, especially by contrast with the different state of things in
the Pleiades.[843] The nebulosity is free and general; no single member
of the stellar assemblage appropriates a special share, or carves from
it an appendage of its own. Yet it is difficult to doubt that the
apparent association is real and physical.
The Wolf-Rayet spectrum is, broadly speaking, reversed in that of 15
Monocerotis. It includes both the hydrogen series and the upper “blue”
band; metallic lines scarcely appear, but those of oxygen, nitrogen, and
silicon are unlikely to be absent. About the peculiarities of the less
refrangible section nothing is yet known. Here, perhaps, symptoms of
emission will be found; indeed, a bright C might be looked for with
success not only in 15 Monocerotis, but in some one or two of its
principal associates.
An eighth-magnitude star in Auriga was noticed by Auwers about 1860 to
be projected on a hazy disc (N.G.C. 2175). The object is placed
centrally in a group of smaller stars, enveloped in nebulosity strongly
manifested in Professor Barnard’s photographs of 1894.[844] The
combination is quite similar to that just described in Monoceros. The
spectrum of N.G.C. 2175 was examined by Professor Keeler with an
indecisive upshot.[845]
Sir John Herschel observed at the Cape “a very remarkable object” in the
Milky Way, where it crosses the tail of Scorpio. It showed to him under
the guise of “a decided, tolerably defined, semi-nebulous mass, with
abundance of very small stars forming altogether a telescopic Magellanic
cloud. It fills about a field, and has branches and sinuses.”[846] This
miniature Nubecula (N.G.C. 6437) invites photographic delineation. Long
exposures may disclose in it constructive particulars of extreme
interest.
Nebulous clusters are connected, by insensible gradations, with certain
tracts of nebulosity in the Milky Way, which, since they are situated
in, or in the line of sight with a stellar stratum, necessarily appear
more or less densely star-strewn. The stars strewing them do not,
however, collect into groups capable of individualisation; hence they
cannot be termed “nebulous clusters.” These suggest some kind of
organisation; they are more or less isolated and coherent entities, and
are distinguishable as such from layers and beds of stars, however
closely packed. Not that a clear line separates the two kinds of
formation; the multiplicity of the heavens is too great for this to be
possible; but it is well to maintain differences ideally, even though
they be blurred, here and there in the concrete, beyond our perplexed
powers of recognition. Nebulous clusters, on the other hand, shade off,
as their members become fewer, into nebulous groups such as the Orion
Trapezium, and reduce, “in the limit,” to simple nebulous stars. These
will be the subject of a future chapter.
CHAPTER XXX.
GLOBULAR CLUSTERS.
There is no possibility of failing to recognise in a globular cluster a
true agglomeration—a structure _teres atque rotundus_. The systemic
unity of such objects is as evident as that of a “globe of dew,” though
it is by no means certain that they are not, like that “frail and fading
sphere,” in course of more or less speedy evaporation. A gradual process
of ejection is at least suggested by their streaming edges and
filamentous appendages, formed of branching rows of stars, apparently on
the move outward. One hundred and ten globular clusters were registered
by Sir John Herschel in 1864 in his “General Catalogue” of nebulæ, and
not many have since been identified. They are astonishing constructions.
Their silvery radiance is a delight to the eye; the imagination is
allured by their visionary beauty; reason is startled by the recondite
nature of the problems they intimate. What, we cannot but ask ourselves,
is the true nature of these mysterious “balls of stars”?[847] Are the
luminous particles composing them suns in the proper sense? What are
their mutual relations? How did they originate? In what are they to
eventuate? Can mechanical stability be claimed for them, or must they be
supposed to form temporary societies undermined by forces tending
towards dissolution? On all these points definite information is still
lacking; but there is no reason to despair of its future provision,
since the inclusion of globular clusters within the scope of organised
research is of quite recent date, and knowledge respecting them is
accordingly in a nascent stage.
It may, however, safely be affirmed that their components are sun-like
bodies—that they are spherical masses at an enormously high temperature,
radiating into space by means of suitably adapted photospheric
apparatus. Some are intensely actinic. They are much brighter chemically
than visually. They hence presumably emit light mainly of the shorter
wave-lengths, and are abnormally hot bodies. But the secret of their
nature cannot be divined in our present ignorance of their spectroscopic
peculiarities, and those of the individual star-points in clusters will
long remain inaccessible. Their combined light, nevertheless, where it
is concentrated into a “blaze” at the core, is capable of being
effectually analysed with powerful instruments, and the determination of
its quality is a _sine quâ non_ for progress in this branch. Until this
has been effected, there is no possibility of assigning to globular
clusters their proper place in the celestial hierarchy.
The great southern agglomerations, ω Centauri and 47 Toucani, seem
almost untouched by the wear and tear of time. They show few signs of
dilapidation. No dusky rifts, no glades or clearings are perceptible in
them; the subtraction of material (if it be going on) has made little
progress; they are as yet well compacted to the centre. Nor is a flow of
stars outward hinted at unless obscurely. They are _cleaner_ at the
edges than most objects of their class; tentacular appendages are
wanting; their components are not visibly in marching order. This may
mean that they have but newly arrived at their present state of being,
and if so, their spectra should be of an early type. To the component
stars, accordingly, a very low mean density may probably be attributed;
their attractive power will prove small relatively to their light; the
corresponding interstitial movements must be slow, and will be difficult
of detection.
[Illustration:
FIG. 47.—Photograph of ω Centauri (Bailey).
]
Just within the northern border of the Milky Way ω Centauri is visible
to the naked eye as a hazy star of the fourth magnitude.[848]
Telescopically it presents a grand aspect. Nothing more strikingly
effective can be imagined than the transformation, by optical means, of
a blurred light-spot into a glittering and multitudinous assemblage of
separate suns. Nearly 6400 can be distinguished on sensitive plates,
besides a crowd of others so small as to merge together into a grey
mottling. No true nebulosity seems to be present. The photograph from
which the counts were made by Professor and Mrs. Bailey is reproduced in
Fig. 47. It was taken at Arequipa 19th May 1893 with an exposure of two
hours in the thirteen-inch Boyden refractor. The area marked out for
investigation covered 900 square minutes. It was “fairly well filled”
with stars, and their statistical study defined the fact of their true
central crowding.[849] They condense not merely in appearance, through
the augmenting depth of spherical space in which they are distributed,
but also as a consequence of actual compression inward. The number of
stars per square minute was found, in fact, to increase in arithmetical
progression with approach to the centre. Away from it the diminution
ceased, a constant figure being reached along a line taken to be the
boundary of the cluster; and this constant, since it must represent the
areal population of the general sky, supplied a means of correcting the
apparent results for that of the cluster. Deduction was accordingly made
of 1616 unconnected stars, mere visual intruders from a limitless
background; and there remained 5050 true components, collected within a
circular space somewhat larger than that occupied by the full moon. They
average about 12·5 magnitude.
The south polar cluster, 47 Toucani, is of equal loveliness with ω
Centauri, although on a smaller scale. Its “computed diameter” is
22′[850]—that is to say, the _extra_ stars which it projects upon the
sphere die out completely at a distance of 11′ from its centre. They
number about 2300, and are distributed in accordance with the same law
noted as prevalent in the companion cluster. They are, however, even
more densely aggregated; the realm is less spacious than ω Centauri
proportionately to the throng of its inhabitants.
The great cluster in Hercules (M 13) presents a less uniform texture
than its prototypes in the southern sky. Yet its constituent orbs follow
virtually the same gradient of central compression.[851] Their
distribution is affected besides by influences of an unimaginable kind.
Three “dark lanes,” making an equiangular junction at a point south-east
of the centre, were detected with the Rosse reflector in 1850.[852] An
identical form of marking tends to recur in other parts of the cluster.
On the Lick photographs of 1890–91, Professor Holden was able to trace
no less than thirteen repetitions of it.[853] This insistence, he
remarked in a local paper, made it evident “that a definite law was
acting to produce this form, and that this law might be truly taken as
representative for this cluster. In some way there are dark lanes
produced and maintained among the hundreds of bright stars in this
globular mass, and there are _many_ such channels. How can we conceive
of such a system? It is tolerably clear that either the dark lanes are
absolutely empty of matter, or at least that they are empty of luminous
matter.” Yet neither of these alternatives seems to be in accord with
fact.
A plate exposed during ten minutes with the Crossley reflector by
Professor Keeler showed all the brighter stars in M 13.[854] Two hours
were, however, needed to bring the swarms of their faint associates into
view. In all, more than 5400 stars, fairly within the precincts of the
cluster, were counted on a negative taken 13th July 1899. A study of
their distribution, made by Mr. Palmer, Professor Keeler’s assistant,
elicited some noteworthy peculiarities.[855] The components separated,
speaking broadly, into two distinct orders of brightness, those of
intermediate magnitudes being comparatively scarce. Out of the total
number of 5482 counted on the plate, 1016 were classed as bright, 4466
as faint, or below 13·5 magnitude. Now the mode of scattering of these
two radial—they extend outward in curved rows; that of their minute
companions is more nearly globular. Moreover, the characteristic dusky
tracks are vacant only as regards the former class of objects. They are
lightly strewn with the diffusive star powder found everywhere in the
cluster. To its presence Mr. Palmer attributes the effects of nebulosity
noticed in earlier photographs. It requires a very high resolving power
to distinguish the stellar haze created by flocks and throngs of
sixteenth-magnitude stars from genuine “fluid haze”; but in this case
there seems little doubt that the feat was performed. Confirmatory
evidence of the best kind was afforded by Professor Barnard’s direct
observations with the forty-inch Yerkes refractor. They convinced him
that globular clusters are non-nebulous formations.[856] Spectrographic
impressions will probably before long add their testimony, at least in a
negative sense.
With the thirteen-inch photographic refractor of the Potsdam
Observatory, Dr. Scheiner obtained, 9th September 1891, the first plate
of M 13 on which the stars were sufficiently defined for exact
measurement.[857] He accordingly prepared a catalogue by which the
places of 833 were fixed with the utmost accuracy; and these fundamental
stars, henceforward kept under watch and ward, will perhaps one day
disclose the plan of their movements, and thus enable future astronomers
to attack, with some possibility of success, one of the most arduous
problems in celestial dynamics. It confronts them, under a still more
bewildering form, in a superb cluster in Scorpio (M 62 = N.G.C. 6266).
Here a second focus of condensation is obvious; two star-globes are
fused into one.[858] The contorted growth of a twin cherry may help us
to realise, however imperfectly, the attendant indefinite complexity of
the conflicting forces.
A compressed cluster in Serpens (M 5 = N.G.C. 5904), discovered by Kirch
in 1702, presents telescopically the appearance of a softly radiant
globe with divergent outliers. A photograph taken by Dr. Roberts 25th
April 1892, indicated a nebulous interior; but no such effect came out
on negatives exposed with Dr. Common’s five-foot reflector, nor was it
perceptible visually to Professor Barnard. It was due, presumably, to an
amalgam of faint stars forming a kind of matrix for those bright enough
to be individualised. The discovery of their variability was begun by
Mr. Packer in 1890.[859] He remarked the fluctuations of two components,
and Dr. Common suspected many more to share the same character.[860]
This premonition was followed up, though not until after five years, by
Professor Bailey’s announcement[861] that many globular clusters—say one
in five—are veritable nests of variables. Their abundance is such that
as many as a hundred—in Professor Barnard’s words—“have been found in a
space in the sky that would be covered by a pin’s-head held at the
distance of distinct vision.”[862] Of 3000 components of ω Centauri
examined within a radius of 22′, 128 were found to fluctuate to the
extent of half a magnitude or more.[863] In one case a range of five
magnitudes was observed; but that of most of the objects investigated
was limited to one and a half, or two magnitudes. Very short periods are
the rule; three are of less than seven hours; yet one is protracted to
475 days, and there will be a special interest in determining the nature
of the light-change comprised in so long a cycle. A true “Mira variable”
would seem an anomaly in a cluster made up of silvery white stars; since
we are taught by experience to associate periods of many months with
strong absorption and consequent redness of colour. But
cluster-variables belong, for the most part, to a type apart, the
character of which has been described in an earlier chapter. They have
long minima, and brief maxima attained with extraordinary rapidity. The
activity of their changes, when they set in, contrasts singularly with
the completeness of their suspension during the intervals of rest. There
is an entire absence of concert among the affected stars. Each is an
independent, self-regulated phenomenon. No more curious spectacle is
afforded by the heavens than that of a throng of seeming signal-lights
waxing and waning every few hours under the sway, obviously, of some
common law, yet with no trace of unanimity; some fading while their
neighbours are on the rise, others stationary and semi-extinct, though
only biding their time to enter upon a phase of renewed brilliancy; and
none deviating by a hair’s-breadth from the course of change
individually prescribed for it.
Eighty-five variable stars have up to the present been recognised in
Messier 5, and they agree, for the most part, quite closely in a mode of
fluctuation elucidated by Professor Bailey’s persevering inquiries.[864]
The form of their typical light-curve is not different from that
assignable to the components of ω Centauri; but they tend unmistakably
to obey a common period of approximately twelve hours, and the
oscillations of nearly all are between the fourteenth and the fifteenth
magnitudes.[865] Yet they are not executed simultaneously; a congruity
of epochs is not even distantly indicated. Professor Barnard took visual
charge in 1898 of some half a dozen of these strange objects,[866] and
his list included Packer’s original variables, Nos. 42 and 84 of the
Harvard enumeration, stars exceptional in the great assemblage of which
they form part, both as to the length of their periods and the manner of
their change. This copies the pattern set by δ Cephei; it proceeds
continuously, although not symmetrically, along a curve steep in its
upward branch, sloping gradually downward, and interrupted by a “hump,”
significant of an abortive second maximum.[867] Its time-measure is
about twenty-six days. Professor Barnard was struck with a number of
ink-black holes rending the brilliant surface of the star-globe in
Serpens. They are closely adjacent to the dense central portion, and
suggest tunnelling operations on the scale of those progressing in the
great Hercules cluster.
A cluster in Canes Venatici (M 3 = N.G.C. 5272) is similarly perforated.
Lord Rosse observed “several small dark holes” at its core, from which
“rays run out on every side.”[868] A “bifurcated dark lane” was,
moreover, perceived in the northern segment of the nuclear “blaze.” From
which we can gather that the distribution of stars in M 3 is controlled
by forces of the same nature as those ruling in M 13. Yet the two
clusters are markedly differentiated as regards light-stability. That in
Canes is already known to contain 132 variables; while in the Hercules
group diligent inquiry has failed to certify the presence of more than
two, its components actually shining much more steadily than the average
of the stellar multitude outside its limits. Eighteen hundred stars were
counted by M. Orbinsky from photographs of M 3 taken at Pulkowa in
1894,[869] and his measurement of their places will in the future supply
a test of their relative mobility.
A starry sphere in Pegasus (M 15 = N.G.C. 7078) seemed nebulous in Dr.
Roberts’s photographs;[870] but they were perhaps clouded by stellar
dust, not by true cosmical fog. Of 900 members of this collection
examined at Harvard College, 51 proved variable. The proportion in
Ihle’s great cluster in Sagittarius (M 22 = N.G.C. 4424) is much
smaller—16 to 1550; and only 10 among 600 stars tested for stability in
M 2 gave responsive signs of fluctuation. This cluster, which is
situated in Aquarius, might be the twin of that in Hercules plunged in a
deeper depth of space.[871] The clusters ω Centauri and 47 Toucani, so
much alike in other respects, deviate widely in the matter of
variability. Periodical stars by the score occur, as we have seen, in
the former stately assemblage; in the latter, only six have been
registered, notwithstanding the most careful scrutiny. Whence the
diversity? Professor Pickering[872] surmises that it depends upon the
relation of a common plane of revolution to the line of sight. Each
globular cluster would be, on this view, a system, the movements of
which, whether axial or orbital, are conducted on the same level. And
should this level happen to coincide with the visual ray, variability
would result, either through the rotation of such components as
possessed unequally luminous surfaces, or as a consequence of the
eclipses of those provided with closely revolving satellites, Yet
neither rationale of light-change can, without grave misgivings, be
admitted. Suns with dusky hemispheres, or permanently spotted, may be
treated as mathematical fictions. Nor is any evidence as yet forthcoming
that genuine eclipse-stars ever find a habitat in clusters. Certainly
none of the cluster-variables so far investigated can be accounted
such.[873] That they are rapid binaries may be plausibly surmised, but
they must be of the non-occulting sort. In eclipsing stars the maximum
is essentially permanent; the minimum is accidental. In
cluster-variables opposite conditions prevail. Habitually obscure, they
brighten incidentally.
Professor Bailey’s discovery throws open a spacious field of research.
Each variable cluster might well claim a sentinel appointed for the
exclusive following of the complex, elusive, and rapid changes which
ceaselessly develop within its compass. In December 1901, 509 components
of star-globes were reckoned as periodical. Every one of these is
perhaps a system apart; every one has its peculiarities, the inner
meaning of which can only be drawn out by sustained attention. Powerful
instruments are, moreover, required. The objects in question lie near
the limit of practicable observation; work upon them taxes modern
resources to the utmost. Nor can the photographic method alone be relied
on. Long exposures are needed to show the stars at all, and they can
naturally give no more than the “mean magnitude” during the intervals
they cover. But when these intervals bear a large proportion to the
entire period of change, such coarse-grained data cannot satisfactorily
represent the manner of its progress. The resulting light-curve, as
Professor Bailey says,[874] is always smoothed down; and it is smoothed
to the limit of a straight line, in the ultimate case of the exposure
equalling the period of a star’s variation. Hence the absolute necessity
for supplementary visual determinations to fill out the peaks and
corners rounded off by the camera. At the critical epoch, when the flash
is being turned on, every minute counts. Estimates of brightness at the
rate of ten or twelve an hour are not too numerous for the purpose of
keeping guard over the swift alterations going forward. The hourly or
two-hourly averages given on sensitive plates are wholly inadequate.
Two questions of fundamental interest present themselves in connection
with the variability of clusters. Why, we must ask, are the stars in one
globular assemblage luminously unstable, while in others, its strict
analogues, they shine quite steadily? The contrast is not explained by
any visible difference of constitution. Variability does not appear to
come in at any particular stage of growth; it is not associated with a
definite situation in the heavens; it does not characterise pierced and
outworn globes preferentially to compact ones, or _vice versâ_; it can,
in short, be correlated with no feature obvious to direct notice. It
remains to be seen whether any spectroscopic peculiarity corresponds to
it.
Again, the absence from, or extreme scarcity of Algol-stars in clusters
occasions perplexity. We are led to believe that rapid variables are, in
truth, binaries revolving in the light-period. But if so, the orbital
planes of a certain proportion of them ought to pass through the earth,
with the outcome of affording us the spectacle of so many occulting
pairs. If these do not exist, we shall be forced to conclude that
cluster-variables owe their punctuality to some other cause than the
strict time-keeping of satellites.
One of the most signal services rendered by photography to astronomy has
been in the facilities supplied by it for the measurement of
star-clusters. The relative positions, especially of the components of
compressed groups, can only with extreme difficulty be established by
direct triangulations; while every negative taken of them fixes their
configuration at a given epoch, and gives the means of determining its
changes, should they occur. Thus the places of sixty-two stars in M 5,
catalogued at Harvard College in 1897, were estimated by Professor
Pickering to be of so high a degree of accuracy that annual
displacements amounting to one-hundredth part of a second of arc can be
detected by their comparison with results similarly obtained from plates
taken a few years hence.[875] The foundations have then been laid for an
extensive superstructure of knowledge, as regards both the physical and
the dynamical condition of globular clusters; yet centuries may elapse
before it becomes possible—in Kepler’s phrase—to “think over again,”
with apprehensive minds, those wonderful “thoughts of God.”
CHAPTER XXXI.
WHITE NEBULÆ.
“White nebulæ”—so called by Professor Young[876]—are those giving
continuous spectra. They are in an immense majority. They are reckoned
by thousands, or tens of thousands, gaseous nebulæ by the score. True,
very little progress has been made with their actual spectroscopic
examination, the faintness of their rays forming, in general, an
insuperable obstacle to their analysis; but their shape and aspect
supply indications, rarely misleading, as to the quality of their light.
That of elliptical and spiral nebulæ is, to the best of our knowledge,
always continuous; and with these may be classed the round,
centrally-condensed objects which abound in every nebular region of the
sky. Several other varieties of this great sidereal family might be
indicated, but they are by comparison scantily represented, and have
been but little investigated. The paragon of white nebulæ is the grand
ellipse in Andromeda. No other is visible to the naked eye; it should
be, judging by appearances, much the nearest to the earth of the whole
tribe; its structure is splendidly definite, and profoundly significant;
its spectrum shows peculiarities challenging inquiries which must be
long-continued and arduous, but promise results of far-reaching
importance. In January 1899 Dr. Scheiner,[877] employing a small
spectrograph in combination with a mirror of nearly thirteen inches
aperture, and only forty inches focus—an apparatus specially adapted for
dealing to advantage with objects of extended surface—obtained in seven
hours a legible spectrograph of the nebula. The indications gathered
from it were of a most surprising kind. Dark rays were perceived to
interrupt the continuous light, and they seemed to agree with the
Fraunhofer lines in the solar spectrum. The Andromeda nebula was
accordingly inferred to be a genuine cluster of solar stars; but this
conclusion is very far from being securely established. No bright lines
could be made out in the Potsdam photograph, but many have been _seen_
at Tulse Hill.[878] On 13th November and 11th December 1897, when they
were particularly distinct, approximate wave-lengths were assigned to
six or seven, all of which fall near lines in the Wolf-Rayet stars. The
reality of the coincidences cannot at present be pronounced upon; they
are hinted at rather than asserted; but their verification would enforce
an entire recasting of ideas as to the nature of white nebulæ.
A photograph of the Andromeda ellipse, taken by Dr. Roberts 10th October
1887, set the example, since extensively followed, of resolving into
spirals, with the help of the camera, all sorts and conditions of
nebulæ. It was, indeed, a memorable picture. The vast structure is shown
in it and its successors[879] to be furrowed through and through by dark
channels, or rather by a single continuous channel, winding in
symmetrical convolutions in a left-handed direction from the compact
nucleus outward to the dim, indefinite margin. Thus the nebula is not
simply a concatenation of flat rings separated by vacant intervals; if
it were, the problem of its construction would be less difficult; since
the annular gaps might represent spaces cleared of their contents by
exceptionally acute gravitational disturbance, while the ejection of
matter along a spiral track belongs to a totally different order of
phenomena, and implies the operation of laws scarcely yet brought within
our ken.
The Andromeda nebula is presumably a round disc viewed obliquely. If so,
the angle of its inclination is about 25°.[880] Remarkably enough, the
nucleus does not share the elongation of the surrounding spires, as it
should if it were no more than a flat condensation in their plane. Its
outline is, on the contrary, circular,[881] and its true shape must be
that of a globe. There is no probability that the innumerable stars
strewing the formation have any physical connection with it. Two small
nebulæ in its immediate neighbourhood, on the other hand, certainly
belong to its system. The closer and brighter (M 32) was discovered by
Le Gentil in 1749; the other, which is situated in a nearly opposite
direction, by Caroline Herschel in 1783. Both can be seen with powerful
telescopes to be included within the limits of the primary
agglomeration.[882] Le Gentil’s nebula, indeed, appeared on a Meudon
negative to lie as a condensed knot upon one of its external
spires,[883] and the companion object doubtless owns a similar origin.
The latter is an oval, apparently amorphous mass; its longer axis is
inclined 60° to that of the great nebula. The two satellites may
eventually yield signs of orbital revolution; or the whole disc perhaps
rotates as one piece, and they along with it; we cannot attempt to
decide which condition is the more likely to prevail. Perhaps neither to
the exclusion of the other. It is conceivable that the more remote
member of the system circulates independently, while the inner companion
is borne onward with the general swirl.
Far inferior to this “Ajax” among the nebulæ, although eminent among the
“other Argives,” is a large lenticular object in Cetus (N.G.C. 252),
noticed by Caroline Herschel in 1783. Sir John Herschel[884] considered
its “streaky and knotty” texture to denote resolvability into stars; but
it came out instead as a fine spiral in a photograph taken by Dr.
Roberts 25th December 1899, a reproduction of which is given, by his
kind permission, in Plate XX. The whorls are evidently much
foreshortened. They are studded, as Dr. Roberts remarks,[885] “with
numerous condensations of a stellar character,” while six ordinary stars
are probably seen in projection upon them. Measures of their positions
relative to each other and to exterior stars might serve, he adds, for
the detection of any movements, rotational or translational, by which
the nebula may be affected. Its considerable south latitude brings it
within the spectrographic domain of the Cape Observatory, and the
McClean apparatus might be competent to obtain an impression of a
spectrum sure to prove interesting, if only it can be made distinctly
visible.
A nebulous “ray” in Ursa Major (M 82) was described by Lord Rosse as “a
most extraordinary object, at least ten minutes of arc in length, and
crossed by several dark bands.”[886] These run obliquely to the axis,
and give the nebula—as Mr. Ingall said—“a twisted appearance, like a
distaff of flax.[887] It appears to possess two centres of condensation,
which must lend no slight complexity to its internal economy. Each is
perhaps the starting-point of a separate arrangement of luminous coils,
but no fair view can be got of them; they are foreshortened into mere
broken lines.” The nebula, Dr. Roberts explains,[888] is presented to us
“in section, and the upper and lower surfaces are very rugged.” The
divisions between the rings hence took shape in a negative, to which he
gave three and a half hours’ exposure, 31st March 1889, as “rifts and
attenuated places” not obviously fitting together into a harmonious
plan. The profile of a corrugated disc is not an easily intelligible
object, and that is all that can be seen of M 82. Perspective has done
its utmost to disguise its true aspect. Turned edgewise towards the
earth, it betrays only by its indentations and rugosities the effects of
the ploughing action to which for ages it has been subjected. Swift’s
“hairline nebulæ” belong to the same category. They show as bare streaks
of nebulosity, bulging a little where the nuclei protrude. Presumably
they are flat, circular surfaces, the planes of which coincide with the
line of sight.
The “ray” in Ursa Major is not solitary. It is placed at a distance of
only 42′ from a larger structure (M 81), evidently of the same general
character. The two were photographed together by Dr. Roberts in 1889,
and cannot be wholly disconnected. The primary—if we may call it
so—resembles the great Andromeda nebula, and, like it, was resolved into
a fine spiral. The spectra of both objects were found by Sir William
Huggins to be continuous; but the significant details disguised by
apparent continuity have still to be revealed.
[Illustration:
PLATE XX.
Photograph of a Spiral Nebula in Cetus. Taken by Dr. Roberts, 25th
December 1899.
]
The essential formative law of white nebulæ is unmistakably that of
spirality. This conviction, strongly upheld by the long series of the
Crowborough pictures, was irrefutably established by Professor Keeler’s
photographic survey with the Crossley reflector.[889] Owing to the
strong light-collecting power of the instrument, the harvest of nebulæ
garnered was so plentiful that the number within its reach over the
whole heavens was estimated at no less than 120,000, and nearly all of
these can be inferred, from the preliminary results obtained, to have a
spiral shape. On the Lick plates, in fact, a small compact nebula, _not_
disposed in luminous coils, stood out as a rarity. All spindle-nebulæ
were resolved into spirals viewed aslant, but into spirals of various
degrees of complexity. Some consist merely of two curved branches,
shaped like the letter S, and diverging oppositely from a nuclear
condensation. An object of the kind situated in Pegasus (N.G.C. 7479) is
reproduced from Professor Keeler’s photograph in Plate XXI. Subjoined
are the drawings by J. Herschel, d’Arrest, Lord Rosse, and Tempel, with
which Keeler compared the autograph picture. They make an instructive
study. Herschel saw the object as a narrow spindle “extended between two
stars,” d’Arrest as a lozenge; Lord Rosse perceived, in addition, a mass
of spiral convolutions surrounding a faint star, while Tempel caught the
double effect of a round attached to an elongated patch of luminosity,
but failed to discern their true connection. At last on the Lick plates
the object disclosed itself under an intelligible aspect. “A glance at
the photograph,” Professor Keeler wrote,[890] “shows that the nebula is
a two-branched, left-handed spiral, with a nucleus or condensation near
the point of inflection. The preceding branch is strong and single, but
the following branch is split into two, which cross where their
curvature is greatest, at some distance from the centre of the spiral,
and unite again at their extremities. This appearance in the components
of the following branch, and the fact that the ends of both branches
curve around so as to approach the centre more closely than do the
intermediate parts, are doubtless effects of projection, the plane of
the spiral lying obliquely to the line of sight.”
Lord Rosse’s star occupies the centre of the space fenced round by the
preceding branch (that to the left). “It would be of great interest,”
the Lick astronomer continued, “to know whether this singular position
of the star is accidental, or whether the star and the nebula are
physically connected, and if so, in what way the star was left in its
present position during the process of contraction. On the first of
these questions an investigation of the spectrum, which will be made in
due time” (the time, alas! never came), “may throw light. Assuming for
the present that the star is physically connected with the nebula, it
seems to me possible that the proximity of this star may account for the
unsymmetrical appearance of the spiral, which may be due to an actual
difference in the dimensions of the two branches, or to their lying in
differently inclined planes.”
The first nebula in which a spiral conformation was recognised is still
unsurpassed as a specimen of its class. We are enabled, by Mr. W. E.
Wilson’s kindness, to reproduce in Plate XXII. his fine picture of this
stupendous object. The coils are left-handed; they follow, as they issue
from the nucleus, the line of movement taken by the hands of a watch.
Our view of them is straight and square; they can be little, if at all,
foreshortened. Yet they do not wind symmetrically round their origin.
Their flow is broken and distorted, like the current of a river by
jutting rocks. The spiral is fundamentally double. Two main streams
leave the nucleus at diametrically opposite points, and preserve their
separate individuality until they melt away into the outer darkness.
Their course seems to be prescribed essentially by the combination of an
ejective with a rotatory velocity; disturbances, however, manifestly
supervene. The branches divide and reunite; they are cloven and bossy;
they swerve widely from the circular track. This is especially
remarkable in the case of the longest and brightest arm, which stretches
irregularly outward to join a secondary exterior nucleus. This
circumstance alone suffices to prove that the diffusion of matter in
this formation has been outward. The perturbing mass was undeniably
there before the luminous stream which it diverted began to flow; and
its flow was quite plainly towards it from within. Other indications of
centrifugal action are visible. Mr. Wilson’s photograph shows “cometary
tails curved like a plume away from the central nucleus,” attached to
some of the denser knots on the convolutions of the spiral;[891] and
these effects of apparent repulsion are likewise clearly legible on Lick
and Crowborough plates of the same object.
[Illustration:
PLATE XXI.
1. Photograph of a Spiral Nebula in Pegasus. Taken by the late
Professor Keeler.
2. Drawings of the same Nebula by Herschel, d’Arrest, Rosse, and
Tempel.
]
Indraughts or infalls from space are not here concerned, whereas, in Mr.
T. C. Chamberlin’s words,[892] “the effects of explosive projection,
combined with concurrent rotation, must obviously give rise to a spiral
form.” Each such nebula (and there are tens of thousands of them)
results, in his view, from the “approach without collision” of a roving
star to a compact gaseous mass. Strained to the point of disruption by
tidal influences, this embryo vortex would, at a given moment, project
from both extremities of the ellipsoid into which it had become
elongated, a stream of material curved into whorls through the continual
slackening of its angular rate of rotation; and the double catastrophic
outrush served to constitute a great system of shining spires,
subsequently diversified by the supervening phenomena of minor
outbreaks. This rationale has much to recommend it, and probably rests
upon a substratum of truth; yet the events contemplated in it are on a
small scale by comparison with the grandiose dimensions which we must
ascribe to spiral nebulæ.
Lord Rosse described a nebula situated near the star 83 Ursæ Majoris (M
101 = N.G.C. 5457, 5458), as a large faintish spiral, with several arms
and knots, at least 14′ across.[893] A four hours’ exposure at Lick
brought into view a surprising wealth of intricate details. The
groundwork of the structure agrees closely with that of the great spiral
in the Hunting Dogs. It is composed of two main effusions, sweeping
round from left to right. But they spread, and split, and ramify, drawn
hither and thither by multiple attractions, while preserving in their
complex interlacings, the whirling impress of their origin.
“Three-branched spirals” still survive here and there in catalogues.
Such were supposed to be the delicate objects, M 99 in Virgo (N.G.C.
4254), and M 83 in the head of the Centaur. But the triplicate form
ascribed to them was most likely of optical creation. There is no
satisfactory evidence that it exists in nature. So far as we can judge,
the spiral type originated, by fundamental necessity, through a double
outflow, in contrary directions, from the parent mass. A mode of genesis
is intimated which recalls, though distantly, the diametrically opposed
eruptions not uncommonly witnessed on the sun. It may be added that no
genuine spiral appears to be a simple watch-spring coil. This, to be
sure, is, to some extent, a matter of definition. It depends upon what
we agree to call a spiral nebula. Yet the difference will most likely
prove to be radical between stars with curving trains, like Maia in the
Pleiades, and those cosmic “whirlpools,” every trait of which testifies
to the counterplay of multiple activities.
“Cometary nebulæ” are not very rare, and they present aspects of
considerable variety. The nuclei are not always stellar, nor are the
appendages attached to them in all cases inflected. A few have been
photographed. Thus an object (N.G.C. 1999) 50′ south of ι Orionis was
noticed by Lord Rosse as resembling “a comet coiled into a ring
nebula,”[894] and appeared under the same form on a plate exposed by Dr.
Common with his three-foot (now the Crossley) reflector in February
1883.[895] Its spectrum has not, that we are aware of, been examined.
“Reaping-hook” shapes also occur. West of the Argo nebula, a falcated
and forked tail, 10′ long, was observed by Sir John Herschel to issue
from a granulated, perhaps a double nucleus[896] (N.G.C. 3199). The
inner edge is sharp, but it fades gradually outward. Cometary, too, is
N.G.C. 520. It has an indistinct nucleus and a bifid train.[897] A
nebulous hyperbola with a star near the vertex (N.G.C. 2366) is met with
in Camelopardalis;[898] and fan-shaped appendages to stellar
condensations are a recognised variety of the species. A pair of these
singular objects were photographed by Professor Barnard, 2nd February
1894,[899] in the immediate neighbourhood of the bright-line star γ
Cassiopeiæ. His sketch, showing their positions with regard to it, is
copied in Plate XXIII. Fig. 1. These do not seem to be casual. The
opening out of the two fans straight away from the star suggests an
express plan of orientation. Each nebula is about 15′ in diameter. They
are “excessively faint and dilute,” and almost elude visual observation.
In Professor Barnard’s opinion, they would never have been detected
otherwise than by chemical means.[900] Yet they photograph easily
enough; and if this actinic quality denotes, as seems probable, a
gaseous constitution, mention of them should, properly speaking, be
postponed to a later chapter. Their spectral classification, however, is
likely, for some time to come, to remain matter of conjecture. Dr.
Roberts obtained, with ninety minutes’ exposure on 25th October 1895, an
excellent photograph of the twin fan nebulæ, in which traces of a
luminous connection are apparent.[901]
[Illustration:
PLATE XXII.
Photograph of Whirlpool Nebula (M 51). Taken by Mr. W. E. Wilson, 6th
March 1897.
]
“Rifted” nebulæ must be classed as a variety of the elliptical sort.
They appear either as cloven discs—N.G.C. 5128 in Centaur is an
example—or as parallel rays, such as a bifid streak in Leo (N.G.C.
3628). A probably analogous structure in Andromeda was photographed by
Dr. Roberts in 1891.[902] It came out immensely elongated, and with just
such a “chink in the middle” as had been seen by Sir John Herschel,
whose opinion that the nebula was the foreshortened representative of a
thin, flat ring of enormous dimensions thus received strong
confirmation. Yet the added light-power of the Parsonstown mirror had
extended Herschel’s “chink” into a channel, running from end to end of
the formation. Or rather the retinal impression afforded by it had been
misinterpreted in this sense; for that it was a misinterpretation the
camera incontrovertibly asserts. The error may serve as an illustration
of Professor Keeler’s remark that “the most obvious tendency of the
draughtsman is to prolong a line or curve beyond the point at which it
actually stops.”[903] A suspicion even arises that other telescopic
presentments of rays split throughout their length are similarly
misleading, and that they are, in fact, like the glimmering oval in
Andromeda, rings thrown into perspective. Otherwise why should the dark
rifts always coincide in direction with the major axes of such
formations? If these are really circular discs, they might as well run
across as along them; but they never do. We must then choose between two
inferences. Either the nebulæ are in fact, and not merely by optical
projection, elliptical, in which case a longitudinal line of cleavage
would be intelligible, or they are luminous rings viewed very obliquely.
It must indeed be admitted that the strong development of nuclear
condensations in some rifted nebulæ appears almost to exclude the latter
alternative. Dr. Roberts’s photographs, for instance, of N.G.C. 4565 in
Coma Berenices, and of N.G.C. 4594 in Virgo,[904] exhibit an arrangement
of parts insistently demanding a different explanation. What seems
certain is that no single principle is valid all round. Modifications
must be introduced to meet the exigencies of nature’s endless variety.
The great majority of white nebulæ might be called globular clusters in
disguise. They present a round surface, condensed centrally by
gradations testifying to their true spherical form. The only obvious
distinction between them and “balls of stars” is that they are
irresolvable by any telescopic powers that can be brought to bear upon
them. And the suggestion lies close at hand that this quality depends
wholly upon distance—that round nebulæ are neither more nor less than
remote globular clusters. Yet it cannot be adopted without hesitation.
The space-relations of the two classes of object are very different.
Clusters frequent the Milky Way; white nebulæ avoid it. The discrepancy,
it is true, may be capable of reconcilement, but by a somewhat elaborate
artifice of speculation. Nor is there any immediate prospect of solving
the difficulty by the aid of the spectroscope. We are unacquainted at
present with any criterion for distinguishing continuous nebular light
from that of compressed clusters. One may eventually be found, but its
application must always be a matter of extreme delicacy.
[Illustration:
PLATE XXIII.
1. Fan Nebulæ near γ Cassiopeiæ (Barnard).
2. Drawing of Struve’s Planetary with Spectrum (Keeler).
3. Drawing of Webb’s Planetary with Spectrum (Keeler).
4. Drawing of Annulated Planetary in Andromeda with Spectrum (Keeler).
]
Professor Max Wolf’s explorations of the heavens show them to be strewn
with an incredible number of small faint nebulæ.[905] Directly visible
only by elusive glimpses, they come out individually distinct and
measurable on sensitive plates; and the Heidelberg observer has already
laid his plans for the construction of a photographic catalogue of
nebulæ, likely to be at least twenty times more voluminous than the most
exhaustive visual enumeration. In certain regions he indeed found that
only two per cent of the dim objects delineated on his plates had been
previously recorded. The newly discovered crowd vary greatly in shape.
Some are round and compact; many more are round and diffuse; there are
spirals among them, and spindles, and draped or arched formations. A
surprising number are marked “planetary,” and are hence, presumably,
gaseous. The physical nature of the rest is inferable only when the
definiteness of their shapes prescribe their arrangement in some
established category.
Thus we are able to assert confidently that those disposed along coiling
lines or projected into rays and ellipses, give continuous light; but if
we attempt to go further, and obtain a clear conception as to how the
light originated, embarrassments beset our path. Let us confront them
fairly. The only white nebula of which the spectrum has been observed to
any purpose is the great elliptical spiral in Andromeda. It almost
certainly includes lines or bands of absorption; it is probably marked
by traits of emission as well. Plausibility is thus lent to the opinion
that the nebula is a genuine cluster of stars amalgamated by distance
into a soft haze. The haze, however, shines very dimly; its lustre is
almost evanescent comparatively to that of the sun. If, then, its
component particles are true suns, they must be inordinately far apart.
For the sake of giving some precision to our ideas on the subject, we
will attempt to illustrate this numerically. If we assume the central
parts of the nebula to possess ¹⁄₂₀₀th the intrinsic lustre of the full
moon, or (what comes to the same) ¹⁄₁₂₄ millionth that of the sun, while
consisting of scattered globes of solar brilliancy, it follows that this
also is the proportion between the total bright area covered by their
discs and the dark area of vacancy, the dimness of the nebula measuring
the spread of the interspace. Hence the component stars, taking each to
be half a million of miles in diameter, should be separated from its
next neighbour by an interval of more than 5000 millions of miles, as
seen projected upon a plane perpendicular to our line of vision. Their
real distances, since they are presented to us slantwise, would of
course be very much greater; but with them we are not just at present
concerned. We must next try to form an estimate of how close together
these sun-like bodies should appear to be in order to produce the
observed effect of a smooth luminous surface. If the gaps amounted to
one-tenth of a second, the nebula would certainly, with the powerful and
perfect telescopes now in use, show symptoms of resolvability. Yet none
appear. The fog does not even tend to condense into droplets, and the
temporary star of 1885 stood out, to the last hour of its visibility, by
contrasted light-quality from the soft surrounding glow. Allowing, then,
that the linear intervals of 5000 millions of miles between the
constituent bodies of the Andromeda nebula are represented by optical
intervals of ¹⁄₂₀th of a second, we arrive at a parallax for that vast
structure of less than ¹⁄₁₀₀₀″. In other words, its rays spend about
3300 years in travelling to the earth. At this distance, the stars we
have supposed aggregated in it would appear of fifteenth magnitude. Now
they should in the fainter outlying parts of the nebula be more sparsely
distributed than near the centre, and as they thinned off they would
inevitably appear in their proper guise as fifteenth-magnitude stars.
But the texture of the glimmering haze remains the same in every stage
of attenuation.
It may then be taken as certain that, if this object be of stellar
constitution, it is made up of stars smaller and closer together than we
have supposed; unless we are prepared to lengthen still further, and
very materially, a light-journey already protracted to the verge of the
incredible. It is not, however, easy to conceive that bodies much less
than half a million miles in diameter can be truly sun-like. An
outpouring of light and heat in the profuse measure exemplified by the
sun, implies storage-accommodation on a colossal scale; and the spectrum
of the Andromeda nebula, so far as it can be deciphered, seems to
correspond to a high standard of temperature.
Undoubtedly the path “of least resistance” is to accept the stellar
origin of nebular radiance. It is not entirely practicable, but every
other is impassable. The solar corona presents no real analogy to white
nebulæ, since it is kept incandescent by the potent agency of the sun,
while their glow is self-sustaining. This it can only be—setting aside
the vague possibility of electrical discharges—by the sacrifice of
motion in some form. According to Sir Norman Lockyer’s well-known
hypothesis, the collisions of swarming meteorites supply the evolved
energy; but there is little or no evidence that the cause acts, or would
be adequate if it did act. We, at least, have no experience of its
operation. The only meteoric collisions we know of are with the earth,
which spreads a wide net for the capture of flying cosmic particles.
As an alternative suggestion it may be worth considering whether the
shining of nebulæ might proceed from a very slow loss of circulatory
speed through the resistance of a gaseous medium. A pulverulent
constitution, resembling that of Saturn’s rings, should then be
attributed to them; they would consist of relatively small masses
interfused with some highly subtle aerial remnant, the distinctive
bright lines of which add complexity to the nebular spectrum. But the
velocity of circulation in such structures should increase outward.
Other things being equal, they should accordingly, if arrested motion
were the source of their luminosity, gain brightness with increasing
distance from the centre. The reverse is very markedly the case; but the
attendant conditions are so intricate that the contradiction need not be
fatal to the speculation. It cannot, however, be usefully discussed
apart from a profound study of the dynamical condition of such a
peculiar system as that just indicated; and this we must leave to more
competent authorities.
CHAPTER XXXII.
DOUBLE NEBULÆ.
“Double nebulæ,” Dr. See wrote in 1893,[906] “have been greatly
neglected since the time of Sir John Herschel, but it is to be hoped
that astronomers will again give adequate attention to these remarkable
objects, which should be at once systematically studied and
photographed. If accurate drawings or photographs of these objects were
now made, it is not to be doubted that important changes could be
observed fifty years hence.”
His special interest in them originated from a research into the
“evolution of stellar systems.”[907] Sir John Herschel’s drawings of
coupled nebulæ illustrated most aptly his theory of the origin by
“fission” of double stars, some appearing actually modelled upon
Poincaré’s “apioid”—the figure assumed by an ellipsoid when becoming
unstable under the stress of increased axial rotation, and about to
break up into unequal masses. Without entering into details regarding
the process, it may be explained that disparity in the products of
disruption indicates want of homogeneity in the parent body, so that the
sooner the components separate, the greater the chance of their
approximate equality. But the result of photographically investigating
the pattern-objects was completely to alter the point of view from which
they had to be regarded. “The actual nebulæ,” Professor Keeler
stated,[908] “have almost no resemblance to the figures. They are, in
fact, spirals sometimes of very beautiful and complex structure, and in
any one of the nebulæ the secondary nucleus of Herschel’s figure is
either a part of the spiral approaching the main nucleus in brightness,
or it cannot be identified with any real part of the object.” There had
been premonitions to this piece of “destructive criticism.” Many spirals
are readily seen to be essentially duplex. Such is their exemplar in
Canes Venatici, the second nucleus of which, separately catalogued by
Sir John Herschel,[909] was brought into connection with the first only
when the intervening whorl of nebulous matter disclosed itself at
Parsonstown in April 1845. In many other cases, the brighter knots which
tend to form on curving branches are seen isolated, for lack of light to
bring the linking filaments into view, with the outcome of visually
decomposing one formation into several. Thus telescopic improvements,
which avail to analyse stars, have frequently a synthetic effect upon
nebulæ. Even such adjacent objects as are presumably in mutual systemic
relation, often show signs of being bound together by organic ties as
well. Hence it is difficult to draw a line between single and double
nebulæ. A pronounced “dumb-bell” form graduates insensibly into a pair
of clearly individualised globes, barely united by a faint ligament. And
their condition seems less alien to our ideas when we remember that the
sun is nebulously connected with the earth by means of the zodiacal
light.
Unification with increase of optical power was exemplified by Burnham’s
observation, at the Lick Observatory in 1891,[910] of a nebula as
single, though bi-nuclear, which Herschel had registered under two
distinct headings (N.G.C. 7174, 7176). The condensations, which possibly
offer to our view a double star in the making, are just 26″ apart, and
belong to a nebular group in the Southern Fish. A more dubious object is
situated in Aquarius (N.G.C. 7287). Detected by Müller at the M‘Cormick
Observatory, it was described as an “excessively faint, slightly
nebulous double star.” Burnham found the object to be indeed double at
an interval of about 20″, yet not stellar, one component, at any rate,
and perhaps both, appearing as small dim nebulæ. An authentic example of
a double nebula was noted by Barnard in 1888, with the twelve-inch
refractor of the Lick Observatory, in the neighbourhood of the wide
double star 23 Orionis (Σ 696). The components are 36″ apart, faint and
uncondensed. A tenth-magnitude star forms with them an equilateral
triangle. Their measurement by Burnham in 1891[911] supplies a datum of
first-class importance for the future determination of change in the
system which they beyond question constitute.
Littrow described in 1835[912] a curious combination of three small
nebulæ marking the angles of a triangle, the sides of which are formed
by three nebulous bands, while a fine double star occupies the middle of
the enclosure. The arrangement, met with near γ Pegasi, would make a
promising subject for a photographic experiment. Close telescopic
scrutiny, on the other hand, might advantageously be brought to bear
upon a nebulous pair in Gemini (N.G.C. 2371, 2372). The distance from
centre to centre of the components is only 32″, and they were seen at
Parsonstown to be connected by “tails and filaments,” if not encircled
by a filmy annulus.[913] An intermediate star, noted as “bright” 19th
December 1848, was observed by Lassell in 1852,[914] and by d’Arrest in
1862, but has of late ceased to attract attention. Can it have lost
light? D’Arrest seems to have had no difficulty in seeing it with an
eleven-inch refractor, so the question might be readily answered. The
preceding member of the pair was remarked by Dr. Dreyer in 1887 to be
brighter and more condensed than its companion.[915] A similar nebular
and stellar group was discovered by Dr. Common in 1880[916] in the
constellation Crater. He regarded the nebulæ as planetaries; in the
absence, however, of information concerning their spectra this cannot be
held certain. The existence of the linking star—a feature of peculiar
interest—has not, we believe, been verified, but need not be doubted.
Fine telescopic seeing avails to resolve, no less than to unify nebulæ.
Some split up, like close double stars, under high powers. With a
magnification of 250 Professor Swift perceived a nebula (N.G.C. 6679),
earlier discovered by himself, to be a well-separated pair, and he
obtained a similar result for one of Sir William Herschel’s. “It would,”
he adds, “be a great satisfaction to be fully assured that they are
binaries.”[917] We fear that the satisfaction is reserved for a future
generation of astronomers. The Herschelian nebula in question was
doubtless N.G.C. 3690 in Ursa Major, which had already in 1852 been
divided with the Rosse reflector into two irregular masses at a distance
of about 60″.[918] This is a coarse object compared with two delicate
pairs discovered by Swift at Echo Mountain, California, in 1897. Each
resembles a “double nebulous Uranus,” the conjoined discs being 5″ or 6″
apart.[919] They are numbered 6 and 27 on his eleventh catalogue; yet,
although one seems the replica of the other, they are not near
neighbours in the sky. Mixed pairs, stellar and nebulous, are less
scarce than one might expect. Swift’s southern explorations yielded
nearly a dozen specimens. Two are situated in Argo, near the small round
nebula, N.G.C. 3267. Each proved resolvable, in the exquisite
Californian air, into a star and nebula at a distance of 4″;[920] and
the veteran observer’s concluding list of discoveries included five
analogous couples, the widest having a span of 8″. They should at once
be micrometrically measured; for until this is done they cannot be said
to have started on their career in scientific history.
Double elliptical nebulæ are picked up now and again. They are not
easily distinguishable from rifted nebulæ. Probably the true criterion
is the duplicity of the nucleus. Rays stretched parallel to the main
formation, but exhibiting no trace of independent condensation, can only
be regarded as outlying portions of it; where there are two nuclei,
there are, _in esse_ or _in posse_, two distinct bodies. As such two
lens-shaped objects in Pegasus (N.G.C. 7814), photographed by Dr.
Roberts,[921] should probably rank. The “dark lane” shown by the Rosse
reflector was perceived in the negative to bisect the globe-like
nucleus, and to widen out on either side of it; so that each oval is
complete in itself; neither looks like a fragment of the other. A true
pair seems also to be constituted by the lenticular nebulæ (N.G.C. 3786,
3788) delineated by Spitaler at Vienna in 1893;[922] while the status of
many more cannot be fixed until they have been photographed with a
variety of instruments and exposures. Among those of uncertain nature
should be reckoned a cloven ray in Leo (N.G.C. 3628), 15′ in
extent,[923] and probably annular;[924] a similar object in Centaur
(N.G.C. 5128), viewed with amazement by Sir John Herschel; a fissured
ellipse in Leo Minor (N.G.C. 2964), thought to be “almost double” at
Parsonstown; and a spindle in Draco (N.G.C. 5866), described by
Professor Keeler as “divided lengthwise by a narrow, perfectly dark
straight rift, on each side of which, near the north-preceding end, and
involved in the nebulosity, is a minute star of about the sixteenth
magnitude.”[925] A bifid beam in Coma Berenices (N.G.C. 4565), 14′ long,
and with a protruding nucleus, appears to be essentially single. Sir
John Herschel noticed that the segments of rifted nebulæ are sharp on
their confronted sides, diffuse outwardly.[926] They recall the gaping
shell of a bivalve; and this peculiarity, if photographically
persistent, might serve as a secondary mark of unity. The unity, as
already pointed out, may be that of a ring viewed edgewise; or, in some
cases, a formation primitively one may have been sundered by
disintegration, as a rock-ledge is cut by a mountain torrent. This is,
of course, said merely by way of illustration. We are unable to conceive
how disintegrating forces in a nebula really act. Nor should the
possibility be forgotten that the occurrence of black chasms may
indicate, not the removal of matter, but an abolition of light. These
apparently breached objects perhaps subsist, after all, in substantial
entireness.
Double nebular ellipses do not invariably lie parallel to one another.
The Andromeda nebula, for instance, is, in a manner, coupled with N.G.C.
205, the longer axis of which, as has been said, makes an angle of 60°
with that of its primary. A pair in Virgo (N.G.C. 4567, 4568) stand in
yet more singular relations to each other. They coalesce at their
following extremities, and diverge at an angle of about 45°.[927] They
might be conceived of—were this mechanically possible—as revolving on a
pivot. The combination is essentially reproduced by two spindle nebulæ
near the hind foot of the Great Bear (N.G.C. 3786, 3788), which meet
almost rectangularly. Dr. Spitaler’s drawing of them is copied in Fig.
48. A third pair, similarly composed, was noted by Swift, 23rd September
1897, at the Lowe Observatory.[928] A bright nebula in Sculptor (N.G.C.
55) was then too seen to have a dim companion. Both are elongated; they
meet obliquely and overlap. Possibly indeed they form together a single
curved nebula; yet the indications are more in favour of a genuine
coupled arrangement.
[Illustration:
FIG. 48.—Drawing of Spindle Nebulæ (Spitaler).
]
Elliptical nebulæ are sometimes found less congruously associated with
round, perhaps globular attendants. Thus an enormously long ray in Canes
Venatici (N.G.C. 4631) is preceded by a tenth-magnitude star, and that
again by a nebulous orb.[929] Of the ray, it was recorded at
Parsonstown, 26th March 1848, that “masses of light appear through it in
knots”;[930] and the drawing made there exhibits helical lines
corresponding presumably to an extensive system of foreshortened flat
spires. The star does not appear to have been seen; nevertheless it
ought, unless greatly diminished during the score of years elapsed since
Herschel’s observation of it, to have been conspicuous with the six-foot
speculum. A spindle-nebula in Eridanus (N.G.C. 1532) has also a round
companion;[931] and two are attached to an ellipse depicted by Spitaler
in 1893 (N.G.C. 2781, 2785). Triple combinations of round nebulæ are
fairly common. The varieties of multiple stars are recapitulated in
them. Single primaries have closely double satellites, or single
satellites wait upon compound primaries. Three nebulæ, which, from their
central brightening, may roughly be described as spherical, were
detected by Barnard in 1886 lying close together in the field of his
six-inch refractor.[932] Yet they are faint objects even with the Lick
thirty-six-inch. The intervals between them were determined by Burnham
in 1891 to be respectively 94″ and 78″. A century hence there will be
hope of eliciting evidence of incipient revolution by the renewal of
these measures. Another of Barnard’s new nebulæ (N.G.C. 6302) was
resolved by Swift into a triplet. It is plunged deep in the Milky Way in
Scorpio, and has a gaseous spectrum.[933]
Compound nebulæ lead the way to groups and clusters of such objects.
Swift counted twelve in a single field near Algol, and perceived at
least twenty collected into a slightly larger space between κ and γ
Herculis.[934] Barnard explored in 1890 a nest of eighteen small
separate nebulæ in Ursa Major; and Max Wolf observed in March 1901 a
real “Nebelhaufen” surrounding, though probably disconnected from the
star 31 Comæ Berenices.[935] No less than 108 components, some
elongated, some roundish and of various degrees of faintness, were found
included in a circle 30′ in diameter; and a similar group, photographed
in 96 minutes, has η Virginis for its centre.[936] White nebulæ, in
fact, tend very markedly to gather into flocks; whether as a consequence
of their mode of origin, or through the compulsion of their mutual
attraction, remains an open question. And since the masses of these
bodies are likely to be very small, and their real distances very great,
circulatory movements only of the most leisurely kind can be ascribed to
them. We are unable as yet to forecast, even remotely, the establishment
on a settled footing of the dynamics of nebular systems.
CHAPTER XXXIII.
NEBULOUS STARS.
“There is a vast difference,” Professor Swift wrote in 1897, “between a
nebulous star and a star in a nebula.”[937] Vast indeed, since a star
and nebula in reality billions of miles apart may be thrown by
perspective into the same visual line, while a true nebulous star claims
the ownership of its luminous appendage on the evidence of obviously
adapted form. The nimbus is not more unmistakably fitted to the head of
a saint in a picture than nebulous halos are, very often, to the stars
they encircle. The relationship is patent. That the glow emanates from
the star, and is no casual adjunct to it, the instinctive logic of the
eye suffices at once to decide.
Sir William Herschel was the first to give express attention to “stars
with burrs.” They struck him as remarkable, not only in themselves, but
for what they implied. They served as the basis of a memorable train of
reasoning.[938] Their atmospheres, he argued, being plainly “not of a
starry nature,” must be composed of a “shining fluid,” the same which is
seen to be diffused through space in milky tracts, or curdled into
fantastic shapes of chaotic irregularity. He was, in a word, led by them
to the capital discovery of _nebulæ_ as a distinct sidereal order. On
13th November 1790, he came across the “singular phenomenon” which
determined his abandonment of the view that the universe is constituted
exclusively of stars variously aggregated. This was “a star of about the
eighth magnitude, with a faint luminous atmosphere of a circular form,
and of about 3′ in diameter. The star is perfectly in the centre,” he
continued, “and the atmosphere is so diluted, faint, and equal
throughout that there can be no surmise of its consisting of stars, nor
can there be a doubt of the connection between the atmosphere and the
star.” This exemplar object, situated in Taurus (N.G.C. 1514), he
regarded as “decisive in every particular”; yet its nature is still to
some extent dubious. It has even been classed of late as a planetary
nebula, and certainly shares not a few characteristics of that family.
The uncertainty of its status renders its study especially instructive.
A planetary nebula is definitely terminated; a nebulous star fades off
into space. One shows a disc; the other is surrounded by an
“atmosphere.” Moreover, a star, or stellar nucleus, is subordinate in
the one formation, while it dominates the other. These distinctions,
however, cannot always be unhesitatingly drawn, since the relative
strength of the stellar and nebular elements varies widely in different
objects. Hence the doubt as to the category in which Herschel’s typical
specimen should be ranked. For the glow round it is uncommonly bright;
d’Arrest found it to strike the eye with a four-and-a-half-inch
refractor.[939] Nor is it equably diffused. The Parsonstown telescope
showed it as spotted and patchy, and very curiously “ragged” at the
edges;[940] and its aspect to Mr. Burnham in 1891 was essentially the
same. A “broken and mottled” surface, about 126″ across, emerged in the
field of the Lick thirty-six-inch.[941] He was inclined, though under
reserve, to agree with Barnard in considering the object planetary. Its
affinities might be settled by spectroscopic means; the attempt to do so
could, at any rate, hardly fail to have an interesting result. So far,
little has been ascertained about the spectra of true nebulous stars.
Their halos, certainly in some cases, presumably in all, emit bright
lines, and it might be expected that the same lines would show by
absorption in the spectrum of the central star. The presumption has not,
indeed, been fully verified; while the expectation founded on that
presumption is disallowed by the facts scantily at our disposal. Stars
with nebulous appendages are usually, if not invariably, distinguished
by “early Orion” spectra—a combination already noted as significant in
connection with the course of sidereal growth; but they show no special
lines that could be attributed to light-stoppage by the immense bulk of
rarefied incandescent matter interposed between our eyes and their
shining photospheres. This is one of the many perplexities involving the
luminous relations of nebular stuff, which, setting “Kirchhoff’s law” at
defiance, exercises no absorption correlative to its emission.
One of the few nebulous stars bright enough for easy spectroscopic
investigation is situated in Scutum Sobieski. Of 5·5 magnitude, it is
enrolled in the Bonn Durchmusterung under the heading S.D. −10°4713. On
a plate exposed by Professor Barnard with the Willard lens, 29th June
1892,[942] a large diffused nebulosity was seen to encircle it. The
appendage must be visually very faint to have escaped notice so long.
Its proper spectrum may then be nearly evanescent. The star it belongs
to is No. 8198 of the Draper Catalogue, where it is set down, although
doubtfully, as of the second type (Spectrum E). Now this is a point of
crucial importance to theories of stellar development. The pronounced
nebulous condition of a star near the solar stage would have a bearing
on such inquiries that could not be ignored. Should it be established,
current ideas will need revision. The spectral character of Barnard’s
_nebulosa_ in the Shield promises, indeed, to afford a test by which to
try the validity of reasonings on sidereal evolution. The test ought,
with the least possible delay, to be applied.[943]
A seventh-magnitude star in Eridanus was perceived by Swift in 1859 to
be almost centrally placed in a shining corona.[944] It is perhaps
identical with N.G.C. 5315. Its spectral classification should present
no difficulty. A similar object, equally adapted for spectroscopic
inquiry, was detected by the elder Herschel in Cepheus (N.G.C. 7023).
The nebulosity is particularly strong north and south of the star.
Irregularities of a more marked kind are apparent in other instances. A
tenth-magnitude star in Monoceros was found by Barnard visually nebulous
in 1888, photographically in 1894.[945] A “small dark space,” however,
interrupts the encircling halo. A subsequent exposure with the same
instrument disclosed as “closely nebulous” the 9·5 magnitude star, D.M.
+ 23° 1313.[946] And here again the illumination is unevenly
distributed, the “fuzzy” border to the star-disc being denser south and
east than elsewhere. This object lies almost midway between η Geminorum
and χ^2 Orionis. One in most respects analogous (N.G.C. 2247), detected
by Swift, 24th November 1883,[947] came out noticeably “blurred” on the
same plate with Barnard’s nebulous star in Monoceros. Two further
specimens of the class were photographed by Barnard in Sagittarius.
One—D.M. −19° 4948—is fainter than the ninth magnitude. It has a narrow
fringe of light.[948] The second—D.M. −19° 4953—is of 7·6 magnitude, and
is encompassed by a far-spreading halo, 15′ in diameter,[949]
conspicuous with the camera, although nearly invisible to the eye.
Nebulous stars are frequently compound—perhaps more frequently than
stars clear of cosmic fog. Sir John Herschel recorded at the Cape a
close pair (N.G.C. 5367) as involved in a bright glow two minutes of arc
in extent; and a faint star with an aureola, discovered by Tempel in
Cetus (N.G.C. 707), proved, when scrutinised by Burnham in 1891, to have
a minute attendant at an interval of 10″.[950] A still more interesting
detection concerned a nebulous triplet in Auriga (N.G.C. 1931).
Discovered by Sir William, and described by Sir John Herschel as “one of
the most curious objects in the heavens,” it consists of three stars,
the brightest of 9·5 magnitude, forming an equilateral triangle with a
side of about 8″, placed precisely at the centre of a small circular
nebula. Mr. Burnham had repeatedly inspected it with minor
instruments,[951] but it needed all the power of the Lick refractor to
bring into view a fifteenth-magnitude satellite at a distance of little
more than 2″ from one of the stars of the triangle. As a test for
“seeing” facilities, the pair is of unsurpassed delicacy. Again, a wide
double star occupies the middle point of a pretty large faint nebula in
Monoceros (N.G.C. 2182). The attendant may be only optically such; the
circumstances are on this point indecisive. The chief star, however, was
found by Mr. Burnham to be double in a perfectly unequivocal sense.[952]
A companion of nearly its own magnitude (8·6) is separated from it by a
spatial gap of less than half a second, and the two must assuredly
revolve round their common centre of gravity. Here, indeed, we are
confronted by a profoundly embarrassing question. The couple are
evidently plunged in nebulous matter; their movements must then,
according to received ideas, be impeded, with the result of an eventual
collapse of the system. We can see no escape from the dilemma except by
adopting the startling hypothesis that the nebulous fluid does not
constitute a resisting medium. The difficulty greatly enhances the
interest of spectroscopically determining the velocities of bodies
nebulously connected.
A 6·5 magnitude star in Cepheus (D.M. +57° 2309) appeared in a
photograph taken by Barnard in 1893 “surrounded by a rather
unsymmetrical dense nebulosity.”[953] A “hazy glow” could be seen with
the Lick thirty-six-inch, which, in Burnham’s employment, had already
revealed the star to be very unequally double at 4″.[954] A first-type
spectrum is dubiously ascribed to it in the Draper Catalogue.
The nebulous triplet, ι Orionis, has been more completely observed than
perhaps any of its congeners. It consists of a third and an
eighth-magnitude star 11″ apart, with an eleventh-magnitude satellite at
49″, described by Admiral Smyth as “grape-red” in colour.[955] Sir John
Herschel perceived the group to be “involved in a feeble nebula 3′ in
diameter,”[956] and in the nebula (N.G.C. 1980) there was apparent with
the Rosse reflector a central cavity containing the bright
star-couple.[957] Possibly the effect was an illusion due to their
effacing radiance; but this cannot be taken for granted, since “holes”
in nebulæ are an attested phenomenon. And the early observations at
Parsonstown approve themselves as singularly accurate through the
confirmatory evidence of the best recent photographs. The spectrum of ι
Orionis is of the helium type, and Dr. McClean identified in it three
members of the Pickering series of hydrogen, besides many oxygen
lines.[958] The surrounding glow emits the ordinary nebular rays, but
they make no show, either directly or by reversal, in the dispersed
stellar light. Yet before reaching outer space, that light has to
traverse enormous volumes of incandescent or luminescent nebulium. The
anomaly presented by the absence from the Fraunhofer spectrum of the
solar coronal green line is here repeated in an emphasised form. A long
nebulous streak, visible only on sensitive plates, links the hazy
appendage of ι Orionis with the great formation in the Swordhandle.[959]
Far away in the northern part of the constellation, there is found in λ
Orionis a combination very similar to that presented by the nebulous
trio just considered. A yellowish and purple pair (Σ 738), of 3·7 and
5·6 magnitudes, at 4·2″, are immersed, with a comparatively remote
twelfth-magnitude attendant at 29″, in a nebulous haze, photographed by
Barnard in three hours, 17th September 1893.[960] The discovery was at
once telescopically verified.
Five nebulous stars occur together in a narrow region of Sagittarius,
and three of them are double. The two apparently single are Barnard’s
stars, already mentioned. The pairs are N.G.C. 6589 and N.G.C. 6590,
both first noticed by Swift,[961] and N.G.C. 6595, delineated seventy
years ago by Sir John Herschel. The character of an object photographed
by Dr. Roberts[962] near the spiral nebula M 81 in Ursa Major, needs to
be more satisfactorily determined. Known to Herschel and d’Arrest as a
condensed nebula (N.G.C. 3077), it appeared on the sensitive plate with
a sharp, stellar nucleus in lieu of the woolly disc visually
perceptible.[963] The spectroscope may perhaps help towards its rightful
classification. Nebulous stars merge insensibly into stars with nebular
appendages, such as ω Orionis and σ Scorpii. The former has a dimly
luminous, curved spur running out from it, besides a larger mass hanging
like a cloud above it to the north; the latter is winged with
nebulosity, two pointed projections issuing from it in divergent
directions.[964] The whole of these appurtenances were detected
photographically by Professor Barnard. Both stars afford spectra marked
by helium absorption. The relations of stars and nebulæ are manifold.
Misty trains and tails of all sorts and sizes have stellar foci; they
emanate from stars, or condense into stars; but nebulous stars are,
properly speaking, what Herschel called “stars with burrs”; they give
the usual sharply defined diffraction-discs, although a dim halo spreads
more or less symmetrically round each. The nebular element in such a
combination is entirely subordinate to the stellar; while in stars with
appendages the disparity gradually becomes reversed.
“Rejected” nebulous stars are still worth attention in view of the
possibility that they may be subject to genuine change. The case of 55
Andromedæ is particularly instructive, if only as illustrating the
propagation of error. This is a 5·6 magnitude star, qualified as
_nebulosa_ by Flamsteed and Piazzi, and regarded by Sir John Herschel as
a typical specimen of a hazy star. It figures as No. 428 in his _General
Catalogue of Nebulæ_ (1863), but was omitted by Dreyer from the revised
edition of that work. Sir William Huggins, nevertheless, observed it in
1864 to be “a fine nebulous star with a strong atmosphere;”[965] and
since he used a very perfect refractor, his confirmation of what
Herschel had seen with a reflector had an independent value which might
seem to exclude the hypothesis of association in optical illusion. Yet
neither Lord Rosse in 1848 nor d’Arrest in 1856 had perceived any trace
of nebulosity, and Schjellerup, during some years previous to 1866,
always found the star sharp.[966] So again it appeared to Mr. Burnham in
1879–80,[967] and so it seems likely to remain. We can, however,
scarcely persuade ourselves that several eminent observers conspired to
blunder; and Schjellerup’s theory that Piazzi merely repeated
Flamsteed’s note, which crept, he supposes, into the British Catalogue
by a transference from the great Andromeda nebula, is rendered
unacceptable by the circumstance that 55 Andromedæ follows the nebulous
structure held to have been confused with it at an interval of
considerably more than one hour of right ascension. Nor even if so
extraordinary a mistake had been made, was Piazzi capable, one would
think, of copying it unawares. His high astronomical reputation suffices
in itself to clear him from the charge of such astounding carelessness.
The spectrum of the star resembles that of the sun, a type never yet
unequivocally associated with nebulous attachments. On the other hand,
the normal quality of the light renders their optical creation more
difficult of explanation. Thus the nebulous aspect of 55 Andromedæ must
stand over as one of the unsolved problems of astronomical history.
A similar, but less convincing case is that of 8 Canum Venaticorum. On
four separate occasions Sir John Herschel noticed this fourth-magnitude
star to be surrounded by a “considerable atmosphere.” Yet since no one
before or after him has vouched for its presence, he was presumably
deceived. The spectrum of 8 Canum is of the solar class.
Finally, a 7·5 magnitude star in Cetus, discovered as nebulous by
Stephan at Marseilles in 1880[968] (N.G.C. 988), appeared to Burnham and
Barnard in 1891 devoid of any such peculiarity.[969] A photograph taken
with suitable exposure would serve decisively to test its present
condition. The criterion might indeed fail with stars so bright as 55
Andromedæ and 8 Canum Venaticorum; for their imprinted discs would
become, through chemical irradiation in the time needed to bring out
faint glows, sufficiently distended for their obliteration.
In connection with nebulous stars two lines of inquiry open out. First,
the spectroscopic. The scanty evidence at our disposal is to the effect
that the stellar rays of such objects are of the “Orion” kind; that they
show the quality believed to characterise suns in a primitive stage,
while their aureolas shine like gaseous nebulæ. But these
generalisations rest on a very narrow basis of fact, and probably admit
of interesting and significant exceptions. Indeed, each nebulous star
should be treated as a separate spectroscopic problem, destined to
afford in the course of its solution insight into many obscure secrets.
A second branch of research relates to the structural peculiarities of
stellar halos. Their luminosity is seldom, perhaps never, equably
distributed. Its irregularity sometimes goes so far as to produce the
effect of dark vacuities, photographically attested to be no mere visual
deceptions. What their true nature and origin may be, is a subject for
inquiries likely to be long and arduous. It is scarcely credible that
they are what they appear to be, obscure tunnels, striking, in the
direction of the earth, right through the heart of immense spheres of
shining tenuous matter. The alternative view is preferable that the
so-called “atmospheres” of stars are really effluences—that they consist
essentially of spiral coils wound closely enough to merge ordinarily
into an approximately uniform surface, while leaving in certain
circumstances conspicuous gaps between their luminous folds. If this be
so, nebulous stars fall into line with cometary nebulæ, the trains of
which take a more or less completely annular shape; but their nearest
allies are unquestionably to be found in the planetary family; and this
brings us to the subject of our next chapter.
CHAPTER XXXIV.
PLANETARY NEBULÆ.
Planetary nebulæ seem to be intermediate between nebulous stars and
annular nebulæ. Indefinite aureolas are replaced in them, as if through
the spreading outward of nebulous matter towards a limiting spherical
surface, by compact discs, and with the further advance of this process
of exterior condensation, the discs become rings. A stellar nucleus
persists throughout these phases. They may not be strictly phases of
development. To establish an actual sequence of growth, facts of various
orders should be considered. As a simple matter of fact, however,
objects are found which combine so closely the visual features of
planetary nebulæ and nebulous stars on the one side, and of planetary
and annular nebulæ on the other, that their classification in the above
order is prescribed inevitably, if only for mental convenience.
As a preliminary to the physical study of these objects, we must try to
attain a clear conception of their real forms in solid space. This is
not easy when their structural complexities are taken into account; but,
setting these for the moment aside, we can gather some indications
regarding the general plan of their fabrication. As a rule, planetary
nebulæ are markedly elliptical. They may then be concluded to be
spheroidal in shape; and even those sensibly circular are probably
spheroids viewed along their shorter axes. That their compression is due
to axial rotation is a fair inference, verifiable, possibly, by
spectroscopic measurements. These were indeed ineffectually tried by
Professor Keeler, 3rd April 1891, on a bright planetary in Hydra (N.G.C.
3242);[970] but he doubted whether the conditions of the experiment
permitted the detection of a difference in the velocities of the
advancing and retreating limbs of the nebula of less than seven or eight
miles a second, and the movement of such bodies is likely to be
excessively slow. Otherwise they could scarcely be supposed capable of
holding together; for they are obviously of tenuous composition, and
gravity at their equators can act very feebly in equilibrating
centrifugal impulse.[971] And here an untrodden field opens to
enterprising inquirers. A theoretical investigation might, to begin
with, be attempted of the figures which should belong to rotating globes
of the kind on certain probable assumptions as to their nature and modes
of movement; and the results might be tested by the application to a
number of promising objects of Keeler’s spectrographic method, in which
the linear images of an equatorial slit serve, through the tilt imparted
to them by the contrary motions of the opposite limbs, to measure, in
miles per second, the speed of gyration. Small, lucid, strongly
elliptical planetaries would be the most hopeful subjects for
experiment; nor would a negative upshot be without value. The Hydra
nebula, for instance, which is nearly as much flattened as the globe of
Saturn, is equal in light to a seventh-magnitude star; it is about 20″
across, and somewhat diffuse at the edges.[972] With it may be compared
a condensed planetary in Ophiuchus (N.G.C. 6572), sometimes called
Struve No. 6 (Σ 6). Its light, which is comparatively intense, emanates
from a small disc 8″ in diameter (exclusive of a hazy margin), with an
ellipticity of about ¹⁄₂₀₀. Professor Keeler’s drawing of the object
with the visible part of its spectrum is copied in Plate XXIII. Fig. 2.
The nucleus is not stellar, although it gives strongly continuous light.
The lines in its spectrum were found by Keeler to be displaced upward by
an amount corresponding to a motion towards the earth of 6·3 miles a
second;[973] but this motion should be considered to belong to the sun,
not to the nebula. Judging by its appearance, axial acceleration may
have advanced farther in it than in most members of the class, and might
record itself in an equatorial spectrograph. The trial, at any rate,
should be made. Anticipation of failure need not be allowed to paralyse
effort. In science the rush of a forlorn hope often carries a fortress
that has obstinately held out against a siege in regular form.
The uniformity of aspect at first supposed to characterise planetary
nebulæ disappears before the searching scrutiny of the powerful
telescopes now in use. Their surfaces prove to be full of suggestive
detail. They are broken up by irregular condensations, or furrowed by
the operation of antagonistic forces; they betray, here possibly the
action of repulsive, there of attractive influences, and bear
inscriptions of no less profound historical import than the contortions
and faultings of terrestrial strata. They are quite commonly multiplex
formations. One glimmering disc is superposed upon another, intimating
the analogy of the successive filmy envelopes flung round the heads of
active comets. With the twenty-seven-inch Vienna refractor Vogel
succeeded, in 1883, in resolving the above-mentioned planetary in
Ophiuchus (N.G.C. 6572) into three distinct layers,[974] doubtless
representing concentric ellipsoids of unequal antiquity. A triple
outpouring of matter at age-long intervals seems recorded. And the case
is typical, although the nebulous shells are not often so clearly
defined. This, however, is, on the one hand, a matter of telescopic
seeing; on the other, perhaps of evolutionary progress. By way of
illustration, let us take as the first term of a sequence an average
nebulous star, such as that detected by Auwers in Auriga (N.G.C. 2175);
next in order we might place an object near 16 Cygni (N.G.C. 6826), in
which the “glow” has become compacted into a large, round, seemingly
uniform disc.[975] Sir William Herschel described it as “a beautiful
phenomenon, of a middle species between the planetary nebulæ and the
nebulous stars.”[976] The following term of the series may be found in a
planetary not far from γ Eridani (N.G.C. 1535), consisting of an
eleventh-magnitude star, embossed upon two concentric nebulous shields;
or rather, presumably, enclosed within a pair of nebulous globes. If we
suppose these to have been produced by successive outflows, checked at a
limiting surface, we could easily see that the occurrence of a third
access of ejective energy would complete the model of the triple
specimen in Ophiuchus. It may be added that the difficulty appears
almost insuperable of explaining the growth of multifold planetaries on
the hypothesis of simple contraction. A repulsive agency in a manner
asserts its past activity.
Let us now imagine one of these spherical envelopes to be hollow. The
effect to the eye would be that of a luminous ring. And, in fact, an
appreciable proportion of planetary nebulæ show an interior circlet of
dim radiance, which can hardly be otherwise interpreted than as the
projection on a plane of a vast nebulous bubble. This mode of
construction is fully carried out in “annular nebulæ”; it is partially
realised in not a few specimens in which a ring within a disc is
perceptible or conspicuous. All will be considered together in the next
chapter; in this we are concerned only with examples devoid of—so to
speak—symptoms of inflation.
[Illustration:
PLATE XXIV.
1. Photograph of the Owl Nebula (Roberts).
2. Photograph of the Orion Trapezium (W. H. Pickering).
]
The largest planetary in the heavens lies south-east of the second
Pointer, β Ursæ Majoris. Discovered by Méchain in 1781, it was numbered
97 on Messier’s list (N.G.C. 3587), and has been extensively and
carefully observed. Its dimensions were given by Lord Rosse in 1874 as
163″ by 147″;[977] but the major axis measures 203″ on a photograph
taken by Dr. Roberts, with an exposure of four hours, 20th April
1895,[978] and reproduced, by his kind permission, in Plate XXIV. Fig.
1. The observational history of the object is extremely curious. Sir
John Herschel saw “a large, uniform, nebulous disc, quite round, very
bright, not sharply defined, but yet very suddenly fading away to
darkness.”[979] On 2nd March 1848, however, Lord Rosse perceived a
doubly perforated surface, with a star in each cavity,[980] and his
observations were confirmed by Dr. Robinson. The drawing representative
of them is a record of permanent interest. The resemblance to an owl’s
face, given to it by the symmetrically placed _oculi_, is unmistakable;
and the great planetary in Ursa Major became known, from the middle of
the last century, as the “Owl Nebula.” The name has survived the
similarity. On 9th March 1850 the two stars were noted at Parsonstown
shining as usual in their respective excavations; five weeks later the
fainter one had vanished;[981] nor could it ever again be found, though
looked for about forty times during the ensuing quarter of a century. To
Professor Keeler in 1891[982] the nebula wore indeed an entirely
different aspect from that previously attributed to it. “There is but
one nucleus,” he stated, “which is by estimation almost exactly central,
at a place which in Lord Rosse’s drawing is occupied by a bridge of
light between two dark openings. There is also but one central dark
space.” Nevertheless, two appear in the Crowborough picture, like
lagoons separated by an isthmus, and on the isthmus there is planted, as
it were, a lighthouse, diffusing a brilliant illumination. Thus the
interior vacancies remain in _statu quo_, while the stars that formerly
occupied them have both faded out of sight, leaving the prodominance to
a third, not identical with either. The change, in short—admitting that
there has been change—relates, not to the structure of the nebula
itself, but to the relative brightness of three connected stars. The
question whether those at present extinct will ever become revivified,
can be answered only by prolonged experience. Dr. Roberts thought that
his photograph indicated for the nebula a combined ring and disc
formation;[983] but this is not manifest. The picture affords no
verification of the marginal inequalities recorded at Parsonstown; yet
there is reason to believe that they were not illusory. The torn and
jagged contour, which Professor Alexander expounded as the effect of
“disruption and dispersion outward,”[984] may come into view in
representations on a larger scale, taken with special precautions for
the definition of minute details.
The suspected alteration of the Owl planetary accentuates the need for
keeping watch over nuclear stars, more especially since they present
unexplained peculiarities. Their light is of purely stellar quality, but
of remarkable actinic power. They are very much brighter chemically than
visually. Further, they often give nebulous images on the sensitive
plate, while appearing sharp with the telescope. In some rare cases (as
in N.G.C. 6781), they are eccentrically situated. Burnham is probably
justified in regarding the possession of a central star as an essential
feature of planetary nebulæ,[985] although it is occasionally
undiscernible with the telescope. N.G.C. 6563 in Sagittarius, and N.G.C.
7354 in Cepheus, are examples of nebulous discs unrelieved to the eye by
the sparkle of any stellar points; and a beautiful little planetary in
Perseus, discovered by Barnard 11th December 1890,[986] is of similar
aspect. The missing nuclei, however, of all such objects would, it is
tolerably certain, become manifest in long-exposed photographs, which
should accordingly be taken for the purpose of deciding a point of
fundamental importance in the economy of planetary nebulæ.
Perhaps the most noted member of the class, from the numerous
experiments of which it has been made the subject, is located in Draco
(N.G.C. 6543), quite close to the pole of the ecliptic. To ordinary
observation it presents a greenish-blue surface, 22″ by 18″, centred on
a vividly white star of the tenth magnitude; but Professors Holden and
Schaeberle recognised, with the Lick refractor, its “helical”
conformation.[987] Two brighter intersecting hoops, perceived at a
glance to diversify the disc, fell into position on closer scrutiny as
the thread of a screw, uniting their curves into one continuous
tri-dimensional spiral. The discernment of this novel form—believed to
be typical—was a suggestive contribution towards what might be termed
the solid geometry of nebulæ. Speculations as to its mode of origin
would, however, be premature until, by the unerring testimony of the
camera, it has been definitively proved to subsist.
The Draco planetary was observed on the meridian by Lalande 26th July
1790, and there are no good grounds for holding it to have since shifted
appreciably from the place then assigned to it. Burnham, it is true, has
found a progressive diminution of the distance between the central star
of the nebula and one external to it, amounting to 0·033″ annually;[988]
but the movement, if real, may confidently be ascribed to the
disconnected star. The nebula is indeed very far from being stationary
in space. Professor Keeler determined for it spectroscopically in 1891 a
velocity of approach towards the solar system of forty miles a
second;[989] and its apparent fixity on the sphere is doubtless only an
effect of extreme remoteness. Some attempts to determine its annual
parallax[990] have resulted only in showing it to be small “below
compute.” From the prismatic examination of this object, 1st August
1864, Sir William Huggins learned the existence of gaseous nebulæ.
A nebula in Cygnus (N.G.C. 6826) was described by Mr. Burnham as “almost
an exact duplicate” of the planetary in Draco. It is nearly circular,
the longer diameter measuring nearly 27″, the shorter 24″. The nucleus
is very bright (8·8 magnitude), and gives a strong continuous spectrum.
The light from the disc, too, includes an unusually large _white_
ingredient.[991] The radial motion of this nebula is very small; it
approaches the sun by 3·3 miles a second. But the sun is travelling on
its own account towards its place at a much higher rate of speed, so
that the apparent sluggishness of the nebula indicates that it is really
moving away from us, its measured pace representing only the velocity
with which our system gains upon it.
The spectra of planetary nebulæ are fundamentally alike; they differ
only in details. About forty bright lines have been determined in them,
visually and photographically, and they are invariably fine and sharp,
as if emitted by materials of great tenuity. They seem hazy only just
where they cross certain diffuse nuclei; and this feature seems to imply
a gradual condensation of the nebulous stuff towards the central mass,
which, in such cases, cannot properly be called a star. The essential
characteristic, however, of nebular light is the presence in it of the
ray, or rays, of “nebulium.” This is the one sure criterion by which
gaseous nebulæ can be distinguished from stars.[992] The chief nebulium
line has a wave-length of λ 5007, and is of a clear green colour. With
it is constantly associated a ray about one-third as bright at λ 4959,
and the invariability of their relation lends strong probability to the
opinion that both emanate from the same substance.[993] A strong
ultra-violet line at λ 3727, photographed for the first time by Sir
William and Lady Huggins in 1882, perhaps claims an identical origin.
This, however, remains doubtful in the absence of decisive evidence that
it is an unfailing constituent of the nebular spectrum. The green gas
designated “nebulium” is unknown, so far, terrestrially; nor has it been
observed to shine in any of the heavenly bodies except nebulæ. There is
reason to believe it denser, or at any rate less diffusive than
hydrogen. An object catalogued in the Southern Durchmusterung as a
ninth-magnitude star under the title S.D.M. −12° 1172, was found by Mrs.
Fleming in 1891 to give the spectrum of a planetary nebula. But hydrogen
glows in it with unusual intensity. The relative brightness of the three
green lines is estimated by Campbell[994] to be ordinarily 10 : 3 : 1, F
(Ηβ) being the faintest and most refrangible. But the proportion in the
planetary near Rigel is 10 : 3 : 7. In other words, hydrogen is of seven
times its normal lustre comparatively to nebulium. Further, the three
lines, when viewed through an open slit, form discs of severally 11″,
9″, and 14″ diameter. Here then, apparently, a hydrogen-envelope
constitutes an outer shell to the nebulium-sphere; and a gas that rises
higher than another is presumably specifically lighter, although, in
view of the enormous altitudes attained by calcium vapour near the sun,
the inference must be regarded as subject to qualification. Certainty on
the point, and on many others connected with the physics and chemistry
of nebulæ, may be said to be unattainable until nebulium is captured in
the laboratory. And the prospect of this achievement, although not
hopeless, is remote.
The following table gives the wave-lengths and origins, when they are
known or can be conjectured, of forty bright lines in the spectra of
planetary nebulæ. A few are common to all, notably the trio in the
green, with about half a dozen of the blue and ultra-blue hydrogen
lines, while others are more individual in their occurrence; but, on the
whole, bodies of this class seem to be of remarkably uniform
constitution.
LINES OBSERVED IN THE SPECTRA OF
PLANETARY NEBULÆ.
Wave-Length. Origin.
6563 Hydrogen (Hα).
5876 Helium (D_{3}).
5751 Oxygen?
5680 Unknown.
5540 Unknown.
5412 Hydrogen; Pickering series?
5313 Unknown.
5183 Unknown.
5007 Nebulium.
4959 Nebulium?
4861 Hydrogen (Hβ).
4790 Unknown.
4743 Unknown.
4715 Unknown.
4688 Hydrogen; Rydberg series?
4662 Unknown.
4643 Nitrogen?
4610 Nitrogen?
4597 Nitrogen?
4574 Silicon?
4472 Helium.
4390 Helium.
4363[995] Unknown.
4341 Hydrogen (Hγ).
4265 Unknown.
4145 Unknown.
4122 Helium?
4102 Hydrogen (Hδ).
4067 Unknown.
4026 Helium.
3970 Hydrogen (Hε).
3968[995] Unknown.
3889 Hydrogen (Hζ).
3869[995] Unknown.
3836 Hydrogen (Hη).
3795 Hydrogen (Hθ).
3768 Hydrogen (Hι).
3727 Unknown.
3460 Unknown.
3390 Unknown.
Several of the Wolf-Rayet lines, it will be observed, are comprised in
this list, notably those at λ 541, λ 469, and λ 464. And the absence of
recognisable metallic rays strengthens the analogy with stars of that
peculiar description. As a rule, the hydrogen spectrum in planetaries
begins with the green line. The red line has been distinguished in only
a few specimens, which appear more condensed than the rest. Yet it would
be rash to assume that this is really the case.[996] The relative
intensity of the hydrogen lines in stars and nebulæ is an intricate
subject, the ramifications of which have yet to be tracked out. The
coincident appearance of D_{3} with C is worth notice as a hint that the
conditions favourable to the development of the slower light-vibrations
are the same for helium as for hydrogen. They are markedly present in
Struve’s planetary in Ophiuchus (N.G.C. 6572 = G.C. 4390), which gives a
complex spectrum of at least thirty lines,[997] accompanied by faint
continuous radiance. Three classes of fact regarding it are recorded in
Professor Keeler’s sketch of the portion of it accessible to
eye-observations (see Plate XXIII. Fig. 2): first, the positions of the
component rays in the scale of wave-lengths; next, their relative
lustre; thirdly, the extent of the nebula from which they are derived.
And it is of interest to perceive that only those associated with
nebulium, together with the green and blue lines of hydrogen (Hβ and
Ηγ), seem to reach the limits of the disc, while the others are radiated
only by its central parts. Still we have to remember that the length of
the lines must depend to some extent upon their intensity; and that they
may be short only because the sections of them given out by dim regions
of the nebula are of evanescent faintness. Professor Keeler suspected
the presence of dark bands interrupting its continuous light between
D_{3} and λ 5007,[998] and they will, if verified, supply the only
extant proof of absorption in gaseous nebulæ. The nucleus of Σ 6
apparently reinforces the emissions from the disc; but it is plainly not
a genuine star. Further spectrographic investigation of the object is
most desirable.
In the Draco planetary (N.G.C. 6543), on the contrary, the stellar and
nebular elements of the spectrum are perfectly distinct. Some of the
bright lines can indeed be seen only when the central star is outside
the slit, and are therefore due, in Professor Campbell’s words, “to the
nebula proper, as indeed are all the lines observed, and there is no
evidence to show that they exist at all in the central star.”[999] A few
of those registered in Struve’s planetary are missing here, particularly
the hydrogen line (Pickering series) at λ 541; but C, D_{3}, and both
the Wolf-Rayet blue radiations are perceptible, while the leading
ultra-violet line at λ 3727 appeared conspicuously in Von Gothard’s
photographs.[1000] The relative strength of the green lines in this
nebula, as determined by Campbell, is 10 : 3 : 2. It is of a
verd-antique hue, and, indeed, the nearly total suppression of red rays
in their light gives to all planetaries a blue or greenish tinge.
One of the Durchmusterung stars in Cygnus (D.M. + 41° 4004) was noticed
by the late Prebendary Webb, 14th November 1879, to have a hazy disc,
some 10″ in diameter.[1001] Stephan, at Marseilles, independently
detected its nebular character, and Winnecke compared it to a small
comet with a tenth-magnitude nucleus at its preceding end. Moreover, the
nucleus is double. Professor Keeler described the object (N.G.C. 7027)
as follows:[1002]—“This is the brightest nebula that I have examined,
and its spectrum is exceedingly interesting. The nebula is irregular in
outline, and contains two central condensations, one of which has an
oval and fairly well-defined outline. The other is much fainter and more
diffuse.” His drawing of it, made at the great telescope, is copied in
Plate XXIII. Fig. 3, together with a representation of its spectrum. The
nuclei are obviously non-stellar. The continuous spectrum of even the
more conspicuous member of the pair is not incomparably brighter than
that derived from the disc, and it claims all the emission rays as
properly, though not exclusively belonging to it. Professor Keeler
accordingly regarded it as “in a much less condensed state than the
nuclei of Σ 6 and many other nebulæ of its kind,” its exceptional
brilliancy notwithstanding. The spectrum of Webb’s planetary is
remarkable for the intensity of some usually quite subordinate lines,
especially of the Rydberg hydrogen ray (λ 4688), and of the unknown
lines at λ 4743 and λ 4363. Campbell could just identify the red glint
of C,[1003] which by its faintness evaded Keeler’s survey; D_{3} was
made out by both observers, and an unidentified line at λ 3869 was
photographed by Von Gothard in 1892, although his plates were blank at
the place where the significant λ 3727 was expected to appear.[1004]
A companion to Webb’s nebula, both in physical aspect and by vicinity in
the sky (N.G.C. 7026), was discovered spectroscopically by Dr. Copeland
in 1880.[1005] It had, however, been observed telescopically by Mr.
Burnham seven years earlier.[1006] It is small, bright, duplex, a pair
of diffuse nuclei 6″ apart sustaining a filmy structure which, viewed
with the Lick refractor, suggested a comparison to two sheaves of corn
laid side by side.[1007] Burnham refuses to admit the planetary nature
of either of the objects in Cygnus,[1008] yet their spectra scarcely
allow them to be relegated to a different class. The “Rydberg line” is
equally prominent, relatively to their brightness, in both objects. From
the cosmogonic point of view they are of high illustrative importance.
We seem to have before our eyes double stars in slow course of
formation, and preparing to break loose by the development of systemic
revolutions from the trammels of a joint rotation.
Bi-nuclear planetaries are not uncommon. One such (N.G.C. 3195) was
observed by Sir John Herschel in the south polar constellation of the
Chameleon, and is depicted in his volume of _Cape Results_. The nuclei
are fairly well matched in lustre, and will perhaps grow into a double
star like γ Virginis. Their spectrum is still unrecorded; but it will be
of considerable interest to determine whether it exhibits the
peculiarities visible in those of the double planetaries in Cygnus. Two
bright patches near opposite margins of the circumference give the
nebula in Chameleon somewhat the air of a reduced copy of the celebrated
“Dumb-Bell” in Vulpecula.
Of a planetary in the Poop of Argo, originally discovered by the elder
Herschel, Lassell wrote at Malta about 1851:[1009]—“No description can
do justice to this singular object,” which is “not beautiful, for it has
no symmetry, but wonderful.” His drawing shows the disc, which had
appeared to d’Arrest perfectly round, as pear-shaped, with multiple
condensations. So that a quadruple or quintuple star in embryo may here
be offered for our contemplation.
“Stellar” nebulæ have been mostly discovered by Pickering’s method of
sweeping with a direct-vision spectroscope. There is probably no radical
difference between them and planetaries, for their comparative
minuteness may be a simple effect of distance. Or they may be
constructed on a reduced scale. We should naturally expect to meet in
nebulæ a variety of dimensions not inferior to that existing among
stars. Moreover, they are all alike gaseous, and give—so far as is yet
known—perfectly similar spectra. Nevertheless, Mr. Burnham writes of the
stellar kind as “very small, bright, round nebulæ, which in a small
instrument would resemble stars slightly out of focus, but do not appear
to come within the planetary class.”[1010] An admirable specimen was
detected spectroscopically by Pickering 16th July 1882. Previously
registered as a 9·4 magnitude star, it took rank, on the strength of its
bright lines, as a stellar nebula (N.G.C. 6790). The Lick
thirty-six-inch showed it to be round and lucent, with a minute nuclear
point.[1011] A miniature of Struve’s planetary in Ophiuchus seemed to
float in the field of the telescope. Without a slit, the spectrum,
examined by Keeler, resembled three tiny greenish _stars_, that formed
on Hβ being much the faintest. This stellar nebula accordingly is
analogous to Webb’s planetary in the feeble glow of its hydrogen
constituent. Another pseudo-star in Aquila (N.G.C. 6891) disclosed
itself prismatically to Dr. Copeland in 1884. It has a disc just 4″
across, and its spectrum, photographed by Von Gothard 27th October
1892,[1012] includes the usual range of nebular lines up to λ 3727,
besides a fair admixture of continuous light. A nebula of the fourteenth
magnitude, visually a finished star, was noticed for the quality of its
light by Pickering, 25th November 1881, near the star _b_^2 Cygni.[1013]
More conspicuous members of the class have been identified in
considerable numbers on the Draper Memorial plates.
The crowd of small nebulæ photographed by Dr. Max Wolf in 1901 comprised
a remarkable proportion of seeming planetaries. They were collected on
his plates into pairs and groups in a manner recalling the distribution
of Wolf-Rayet stars, but not previously observed to characterise that of
planetary nebulæ. The question indeed arises whether they are really
such? Or do they rather belong to “that much less interesting class of
objects” designated by Mr. Burnham as “small circular patches of
nebulosity”? The spectroscope alone can decide, and its verdict should
be elicited without delay. Upon it will largely depend the conclusions
to be drawn respecting the affinities of the planetary family, their
mutual relations, and the mode of their scattering in space.
There is reason to believe them enormously remote. Four have been
directly measured for parallax, namely, the helical nebula in Draco, the
bi-nuclear planetary in Argo (N.G.C. 2440),[1014] Webb’s in Cygnus, and
a structure with a ring and disc in Andromeda (N.G.C. 7663).[1015] None
showed the least sign of perspective shifting when viewed from opposite
sides of the earth’s orbit; and the demonstration thus afforded of their
immense distance is confirmed by the insensibility of nebular proper
motion. This, since it is continually progressive, must eventually prove
determinable, and comparisons of its amount with the mean _radial_
velocity of these bodies will supply a criterion for their absolute
localisation. Professor Keeler derived from the line-displacements in
the spectra of eleven nebulæ a value for this quantity of sixteen miles
a second; and by sixteen miles a second they should, accordingly, on an
average, progress along each of the other co-ordinates fixing their
position in space. When corresponding angular advances have been
established, their average distance can then at once be estimated.[1016]
Let us assume, for instance, that the eleven nebulæ in question, taken
one with the others, have a secular proper motion of three seconds of
arc in declination, and as much in right ascension. This, although the
outside of what is probable, implies that their mean distance
corresponds to a light-journey of 580 years. A planetary not more than
10″ in diameter would, if thus remote, fill a globe about 600 times
wider in girth than one circled by the orbit of Neptune, and 216 million
times more capacious. The data supplied by Professor Keeler are indeed
an obviously insufficient groundwork for extensive generalisations; but
more of the same kind, and it may be hoped of not inferior quality,
cannot fail to be forthcoming shortly; while the precise visual
measurements executed by Burnham, Spitaler, Javelle, and others will
surely serve, after some decades, for the detection of genuine nebular
journeyings across the sky. A beginning of definite knowledge will then
have been made regarding the true magnitudes and place in the sidereal
scheme of these singular objects. It must not, however, be forgotten
that many of the faint stars taken as fiducial points for their
micrometrical determination may prove to be satellites of the
neighbouring gaseous globes drifting and shifting in their company. This
disquieting possibility, foreseen by Sir John Herschel and d’Arrest,
would, of course, if realised, vitiate conclusions as to parallax or
proper motion. Yet the work done for these purposes need not be looked
upon as wasted. By its aid, should the slow revolutionary movements of
stars round planetary nebulæ ultimately be brought to light, a new
department of sidereal mechanics may be founded. Exact determinations in
astronomy, made on a judicious plan, are rarely useless. If unprofitable
for their designed aim, they are pretty sure to turn to account for some
other, and perhaps a higher one.
CHAPTER XXXV.
ANNULAR NEBULÆ.
The model annular nebula is the beautiful filmy ellipse situated between
ß and γ Lyræ (N.G.C. 6720). Discovered by Darquier at Toulouse in 1779,
it appeared to Sir William Herschel a simple hoop of light, quite dark
within. The “gauze drawn over the hoop,” spoken of by Sir John Herschel,
had been perceived by Schröter in 1797, and with long exposures the
photographed ring fills up into a disc. Plate XXV. is taken from a
singularly perfect representation obtained by Mr. W. E. Wilson in twenty
minutes, while an hour’s exposure sufficed to blur beyond recognition
the characteristic annular aspect of the structure. Yet the havoc thus
wrought, in a pictorial sense, is compensated by the experimental
significance of a result proving the interior of the ring to be far from
vacuous of luminous or luminescent material.
[Illustration:
PLATE XXV.
The Ring Nebula in Lyra. Photographed by W. E. Wilson, F.R.S.
Exposure, 20^m.
]
The dimensions of the ring are about 87″ by 64″. It was shown in the
Lick negatives as an exceedingly complex structure. “It seems,”
Professor Keeler wrote,[1017] “to be made up of several narrower bright
rings, interlacing somewhat irregularly, the spaces between them being
filled with fainter nebulosity.” Many bright patches and condensations
diversify the main annulus, which is, nevertheless, fundamentally
continuous; it does not break up into detached knots of nebulosity. The
light is strongest near the extremities of the transverse axis, and its
failure at each vertex of the ellipse, conspicuously shown in our Plate,
was, already in 1785, noticed by the elder Herschel.[1018] It is
accompanied by symptoms of effusion, marking possibly equatorial
outflows due to rotative acceleration. Lord Rosse in 1863, and Schultz
in 1865, were alike struck with “nebulous radiations in the direction of
the longer axis, which seemed momentarily almost to destroy the annular
form.” They issued from the north-eastern side, while a similar
appearance on the south-westerly side of the nebula was observed by
Professor Holden at Washington in 1875.[1019] He perceived, too, with
surprise that the south end of the minor axis, which Lord Rosse had
represented as the best terminated, was unmistakably, after thirteen
years, less clearly finished than the north end. Similarly, the Lick
thirty-six-inch disclosed in 1888[1020] the whole southern margin as
filamentous, by a kind of alternation with the corresponding state of
the northern edge noticed at Parsonstown. This tufted or fringed effect
was caught, for the first time photographically, by Professor Keeler.
The oval imprinted on his plates was fringed on both sides, and his
measurement of eleven nebulous projections from it will give the means
of detecting any future variations in their luminosity or distribution.
The gauzy stuff in the interior is not a mere formless light-mist. A
drawing published by Lord Rosse in 1844[1021] represented it as divided
into longitudinal striæ, and their reality was confirmed by the Crossley
photographs after fifty-five years. “I have tried,” Professor Keeler
wrote, “to verify this band-structure by visual observation with the
thirty-six-inch refractor, and have fancied at times that I could catch
glimpses of it; but the observation is a most difficult one, the
contrast of the bright and dark bands, exaggerated by the photograph,
being almost too slight to affect the eye.” The surprising accuracy of
the delineations made with the Rosse speculum, while its burnish
continued unimpaired, was thus once more exemplified. It conveys a
warning against lightly setting aside any of the earlier records
concerning nebulæ obtained at Parsonstown, even when they imply
seemingly improbable changes. It is certainly by no accident that the
striæ within this nebula coincide in direction with its major axis; nor
can the termination of the transverse axis by maxima, of the
longitudinal axis by minima of brightness[1022] be regarded as casual
features. All the details of the edifice, in fact, are arranged with
obvious reference to its apparent shape, and this amounts to a
demonstration that the elongation is real. The nebula, then, is not
simply projected into an oval; it is not a circular formation viewed
obliquely. There seems no escape from the conclusion that it is an
ellipsoid of revolution—that the bands follow the line of the equator,
and originate under conditions prescribed by the rotation of the body;
while the partial interruption of luminosity at the ends of the oval
mark outflows of matter where centrifugal velocity overrides the holding
power of gravity. Everything, indeed, leads us to suppose that this
nebula, like the rest of its kind, is actually a hollow spheroid of
shining fluid, the marginal brightness resulting from the increased
thickness of the luminous shell penetrated by the visual ray. The “hoop”
and the “gauze” drawn round it are then two aspects of the same thing.
Nebulous rings, as such, probably do not exist. They would be subject to
perspective effects, no traces of which are to be found in the heavens.
Annularity in nebulæ may accordingly be considered as a purely optical
modification of a different structural plan.
A glittering point of light occupies the centre of the annulus in Lyra.
Many anomalies are connected with its visibility. The first heard of it
is in 1800, when Von Hahn of Remplin, in Mecklenburg, was surprised by
its disappearance.[1023] He attributed the change, not to loss of light
in the star, but to the nebulous clouding-over of the black background
upon which he was accustomed to see it relieved. Unperceived by the
Herschels, it was next seen by Lord Rosse in 1848,[1024] and attracted
Father Secchi’s attention at Rome in 1855.[1025] A ten-inch Steinheil
sufficed, in 1865 and 1867, even in unfavourable weather, to show Hahn’s
star to Hermann Schultz;[1026] yet it unaccountably evaded the
deliberate scrutiny, ten years later, of Professor Asaph Hall,[1027]
armed though he was with the twenty-six-inch Washington equatorial. The
same instrument, however, displayed its evasive sparkle to Mr. A. C.
Ranyard, 23rd August 1878;[1028] while to Dr. Vogel, with the Newall
telescope in 1875, and the Vienna twenty-seven-inch in 1883, it remained
consistently imperceptible.[1029] Very remarkable, too, is its
non-appearance to Dr. Spitaler at Vienna in 1885, when he carefully
delineated the nebula, as well as in 1886, during frequently repeated
verifying observations.[1030] The interior seemed then to contain only
dimly luminous floccules; nevertheless on 25th July 1887 the star caught
his eye at the first glance. It had, in the meantime, 1st September
1886, been photographed by Von Gothard, and has since abstained from
capricious disappearances. It is indeed of such exceptional actinic
power that the camera cannot easily lose sight of it. Fainter visually
than the fifteenth magnitude, it needed only an exposure of one minute
to come out distinctly on a Crossley negative, and it left a dim
impression in half that time.[1031] With all exposures the image was
clearly defined, although in photographs taken with other instruments it
had usually presented a hazy disc. Its light appears to be of normal
stellar quality. Its maximum of intensity, that is to say, falls in the
yellow part of the spectrum. Both Keeler and Barnard agreed that, with
the Lick and Yerkes refractors, the focus for the star was about
one-fifth of an inch shorter than for the encircling nebula. And a
similar disparity exists between the nuclei and discs of most
planetaries. This, however, leaves their special chemical effectiveness
unexplained; for Keeler’s suggestion of its being due to ultra-violet
emissions lacks the support of known facts.
In the field with the Lyra nebula, Barnard perceived, 2nd October 1893,
a second of about the fourteenth magnitude, 30″ in diameter, and
somewhat irregular in shape.[1032] Professor Keeler’s longest-exposed
negative showed the new object to be “a left-handed, two-branched
spiral.”
In 1891 Mr. Burnham measured the nucleus of the ring in Lyra with
reference to an external star of the twelfth magnitude which closely
follows it. Eight years later, Professor Barnard employed the forty-inch
Yerkes telescope to repeat his determinations,[1033] and was inclined to
attribute a slight discrepancy to real motion in the nebula. But the
Potsdam plates lent no confirmation[1034] to a suspicion which will
probably remain long unverified. His experience was entirely negative as
regards light-variability in any part of the object. Alleged changes in
the stellar kernel he translated into genuine changes in visual
facilities. “The fact,” he wrote, “that the nucleus is seen on a
nebulous background makes steadiness of the atmosphere a most important
factor in its distinctness—far more so than in the case of an ordinary
star in the open sky. When the seeing is exceptionally good, the nucleus
appears with a distinctness strikingly in contrast with its ordinary
condition, so much so that one has to guard against deception in
supposing a real change of light.”
Nine bright lines have been seen or photographed in the spectrum of the
Lyra annulus, that of shortest wave-length at λ 3727 being, according to
Von Gothard, the most intense.[1035] Hydrogen shines feebly; no C has
been recorded, but the Rydberg ray at λ 4688 was detected by
Campbell.[1036] Helium is unrepresented; since a violet line at λ 3869,
which appears prominently and persistently in nebular spectra, cannot be
ascribed to that substance. It chances, indeed, to coincide
approximately with one belonging to a known series, but is itself of
“rare and strange” origin.
Several analogues of the Lyra nebula have been discovered; none so large
or so bright. The best imitation is situated in Cygnus (N.G.C. 6894). It
measures 47″ by 41″, the inner vacuity 20″, and was marked “resolvable”
at Parsonstown.[1037] Needless to say that the sparkling effect which
conveyed the impression of a stellar constitution was altogether
illusory. A fifteenth-magnitude star near the interior border of the
ring to the north-west was measured by Burnham in 1891.[1038] The true
nucleus is considerably fainter. First brought to view in a photograph
taken by Dr. Roberts 31st August 1897,[1039] it duly reappeared in one
of the Crossley pictures of 1899,[1040] and was visually discerned with
the great Paris siderostat by M. Antoniadi, 17th July 1900.[1041] The
elongation of the annulus from north-east to south-west was obvious to
him no less than the faint haze with which it is filled. Nothing is
known about the spectrum of the ring nebula in Cygnus. It doubtless
resembles that of its prototype in Lyra.
Even more scanty is the information at hand regarding “a beautiful
delicate ring” in Scorpio, about 40″ in diameter (N.G.C. 6337). It is in
a field crowded with stars, two of which are projected upon, but may not
belong to it. Neither occupies the proper position of a nucleus, and the
one seen by Lassell at Malta had to him somewhat the aspect of a
nebulous knot.[1042]
An annular nebula in Ophiuchus (N.G.C. 6369) was found by Mr. Burnham
“very like the well-known example in Lyra, except in brightness.”[1043]
A central star of 14·5 magnitude was probably then first noticed. The
longest diameter of the ring measures 31″; the edges seemed to Sir John
Herschel “a very little cottony,”[1044] and they are doubtless fringed
with dim appendages, like those attached to its model in Lyra. This is
again copied in an object discovered by Mr. Gale of New South Wales in
1897, or perhaps a little earlier. It is bright enough to make its late
detection somewhat surprising.[1045]
Immersed in a fine cluster in Argo (Messier 46), a nebula of planetary
aspect, about 60″ in diameter (N.G.C. 2438), drew the attention of Sir
William Herschel. The Rosse reflector showed a central star dominating a
vacuous interior, besides two stars sparkling on the condensed
border.[1046] Lassell perceived in the object a resemblance to a large,
dim, compound planetary in Eridanus (N.G.C. 1535), but with the
qualifying circumstance that in N.G.C. 2438 only one “stratum of
nebulosity” was discernible.[1047] This singleness of construction
appears characteristic of perfected nebular rings, and such the inmate
of the cluster in Argo has declared itself to be. In a photograph taken
by Dr. Roberts, 24th February 1894,[1048] it is definitely and
unmistakably annular. Three stars are projected upon, or contained
within it, we cannot tell which; although one by its nuclear position
gives some assurance of being there through organic relationship. Nor
can we venture to assert that the nebula is really in the cluster. It
may only be thrown accidentally into line with it. Still the fact that
Sir John Herschel recorded in two cases similar collocations of
planetary nebulæ with clustered stars[1049] inclines the balance of
probability towards the side of genuine association. One of these groups
(N.G.C. 5979) is situated in the constellation Circinus, the other
(N.G.C. 2818) near the mast of Argo.
Certain complex formations, intermediate between planetary and annular
nebulæ, have now to be considered. A striking specimen of the kind is
met with in Andromeda (N.G.C. 7662). The disc, which includes perhaps
more than one ring, measures 32″ by 28″. Alexander, about the middle of
last century, and Lassell subsequently, thought the structure
bi-annular. Vogel[1050] and Holden remarked its warped and twisted
appearance, denoting possibly a multiple combination of rings thrown off
in various planes as the outcome of long-past crises in a slow process
of development. A central star surrounded by close spirals of nebulosity
was seen at Parsonstown,[1051] but evaded the scrutiny of O. Struve in
1847, of Searle in 1866, and of Vogel in 1883. Lassell perceived it
under the guise of a minute, bluish disc, Burnham as an ordinary
fifteenth-magnitude star.[1052] Fig. 4 in Plate XXIII. reproduces
Professor Keeler’s drawing of the Andromeda planetary with its visual
spectrum.[1053] “This nebula,” he wrote, “is annular, with a bright
inner ring and a very small nucleus. It is somewhat elongated north and
south.” The fourth line in the spectrum is the “fundamental” of
Rydberg’s hydrogen series; its unusual strength makes it the equal of
the ordinary hydrogen lines on either side of it. The spectrum of the
nucleus appears to be perfectly continuous, save for a possible bright
knot about the place of D_{3}. Campbell, however, caught no glimpse of
this radiation, although he determined, visually and photographically,
eighteen bright lines in the spectrum of the nebula.[1054] We subjoin
his list, with notes and comments between brackets.
BRIGHT LINES RECORDED IN N.G.C. 7662.
┌────────────┬────────────────────────────────────────────────────────┐
│Wave-length.│ Remarks. │
├────────────┼────────────────────────────────────────────────────────┤
│ 540│Very faint, difficult (Wolf-Rayet line; Pickering │
│ │ series). │
│ 532│Very faint, difficult (possibly the chromospheric K │
│ │ 1474). │
│ 5007│First nebular line, very bright. │
│ 4959│Second nebular line, very bright. │
│ 4861│Hβ, very bright. │
│ 4744│Faint (origin unknown). │
│ 4715│Faint (unknown). │
│ 4688│Very bright (Rydberg hydrogen line). │
│ 4663│Very faint (unknown). │
│ 4643│Faint (nitrogen ?) │
│ 4472│Very faint (helium; prominent in Orion stars). │
│ 4364│Bright (unknown). │
│ 4341│Hγ, very bright. │
│ 4102│Ηδ, very bright. │
│ 4067│Very faint (unknown). │
│ 4026│Very faint (helium). │
│ 3969│Hε, very bright. │
│ 3869│Very bright (unknown). │
└────────────┴────────────────────────────────────────────────────────┘
The brilliancy of the last line, in view of the faintness and fewness of
other helium emanations, confirms the inference that its association
with that substance is inadmissible. Experiments in the laboratory would
nevertheless be valuable on the behaviour, for instance, of the adjacent
violet line of helium in a mixture of that gas with hydrogen.
Campbell perceived this nebula to consist “of two nearly concentric
rings more or less broken up, with a fourteenth-magnitude stellar
nucleus near its centre.” Ingall described it, 18th December 1885,[1055]
as a “superb planetary,” of a bright greenish-blue tint, the centre not
quite dark, and thus fitly to be called annular. On applying a power of
500, “an extraordinary structure appeared to unfold itself. The bright
ring seemed very jagged and fringed at the edges, and the centre was
mottled with unequal shades, often as if of _two_ dark centres.” The
note made on it at Harvard College was: “Somewhat annular; edges
hazy.”[1056] The blue colour of the object faithfully corresponds to the
actinic energy of its rays. In two seconds of exposure to them the
Crossley reflector gave a weak image, including a barely visible central
star,[1057] and Professor Keeler obtained finished pictures in 20, 30,
and 60 seconds. At Potsdam, in 1892–93,[1058] thirty-three impressions
were taken for the purpose of parallax-determinations, but they yielded
no positive result. They afforded only the information that the nebula
could not have an annual parallax so large as one-fifth of a second;
and, indeed, the true value of the quantity, judging from other
indications, may very well fall short of one-tenth the assigned maximum.
A “sky-blue likeness of Saturn” in Aquarius (N.G.C. 7009) is built very
much on the lines of the Andromeda planetary, but with the addition of
“ansæ.” These, in August 1888, were resolved by Holden and Schaeberle,
with the aid of the Lick refractor, into a pair of attendant
nebulosities, situated in line with the major axis of a strongly
elliptical body, and subsensibly united to it by evasive gleams of
illumination.[1059] Yet the likelihood is small of their being really
satellite-globes revolving in the same track at an invariable interval
of two right angles. The probabilities of the case oblige us to believe
rather that the original interpretation of them as the extremities of an
annular appendage came nearer the truth. An analogy indeed suggests
itself between them and the nebulous effusions from the vertices of the
ring formation in Lyra.[1060] And here again we are assured that they
mark an equator—that the disc they seem attached to must be the
projection of a rotating spheroid. Intricacies of interior arrangement
are, however, visible, showing the progress of manifold activities. Two
dark cavities, extended parallel to the major axis, and helical wisps of
nebulosity, were observed by Vogel at Vienna in 1883.[1061] Keeler saw
and photographed a somewhat distorted condensed ring measuring 26″ by
16″,[1062] and Scheiner’s plates recorded curious spoke-like projections
from an intensely actinic central star.[1063] The spectrum differs from
that of the Andromeda planetary only by the inclusion of the enigmatic
“last line” at λ 3727. From the displacement of the green ray of
nebulium Keeler determined for the Saturn nebula a movement of approach
towards the sun at the rate of thirty-one miles a second, only a small
proportion of which can be due to our own journey through space.
Sir William Herschel observed in the constellation Gemini in 1787 “a
star of the ninth magnitude, with a pretty bright nebulosity equally
dispersed all around” (N.G.C. 2392). Lord Rosse found in it a dark hole
close to a slightly eccentric nucleus.[1064] D’Arrest thought the object
might be called annular;[1065] Lassell perceived a ring surrounding a
bluish disc;[1066] Secchi described it as a star with an annular
aureola.[1067] H. C. Key, using an eighteen-inch silver-on-glass
reflector, noticed about 1868[1068] a concatenation of bright and dark
rings besides the patch of interior obscurity detected at Parsonstown.
To Burnham the nebula in Gemini appeared “one of the most beautiful
objects of the kind in the heavens.”[1069] He assigned to it a diameter
of 45.″ Barnard, finally, was impressed by its “magnificent and
beautiful” effect in the Yerkes telescope.[1070] It disclosed to him a
ninth-magnitude star encircled not quite symmetrically by a brightish
oval ring, partially incomplete towards the south. “This ring,” he
continued, “which is well defined inside and out, is surrounded by a
vacuity, and this in turn by an almost circular broad ring of light less
intense than the inner ring, and with a distinct break in it
north-preceding. It breaks up into a clouded or unequal surface, and is
very irregular on its inner edge, but fairly uniformly circular on its
outside edge. The inner ring is filled with a nebulous light which has a
black spot in it, south-preceding the nucleus.”
That the perforated effect is no illusion may be taken as proved. Nor is
it subject to change. It has obtruded itself now for half a century upon
one observer after another, under divers conditions, both instrumental
and climatic. This nebula then, and a few others like it, betray the
action of some force strange to our experience, by which the matter
contained in an extensive region is either expelled thence, or its
light-giving faculty suppressed.
The annular nebula in Lyra is the only member of the class that has been
satisfactorily investigated. Until spectrographic possibilities are
further developed there seems little chance of dissipating the
perplexities that still envelop its nature. That is, by direct means;
for indirectly much may be done. Comparisons, for instance, of detailed
results for sister-objects ought to prove highly instructive as to the
laws governing the construction of all alike. Most of them have been
singularly neglected, considering the interest attaching to their
peculiarities. The hooped nebula in Cygnus, the “ghost” in Scorpio,
Gale’s annulus, the ring in Ophiuchus, should be photographed with long
exposures, on a scale sufficiently large to bring into view specialties
of texture and build. Their agreement in certain fundamental relations
would thus be tested, and its importance as a guide to theories of their
mode of origin cannot be overrated. Their self-delineation would,
however, doubtless accentuate besides that variety in similarity which,
throughout the whole created world, illustrates the wealth of the
resources disposed of by Nature, and the inexhaustible inventiveness of
the Mind revealed in Nature.
[Illustration:
PLATE XXVI.
Photograph of the Orion Nebula (W. H. Pickering)
]
CHAPTER XXXVI.
THE ORION NEBULA.
The first place among irregular nebulæ is, by universal consent,
accorded to the gleaming formation in the Sword handle of Orion.
Although incidentally referred to by Cysatus of Lucerne in 1618, it
received little attention until Huygens, in 1656, affixed his note of
admiration, and executed a drawing of the bright central part, still
known as the “Huygenian region.” Here is situated the trapezium, the
hub, as it might be called, in which all the spokes of the great wheel
are inserted. A photograph of the group is reproduced in Plate XXIV.
Fig. 2. It was taken by Professor W. H. Pickering from Mount Wilson,
California, 29th September 1889, with the thirteen-inch Boyden
telescope. The exposure allowed was only ten minutes, and already the
enveloping haze was beginning to cloud the images of the stellar
sextett. The companions (which seem to vary in light[1071]) of the two
brighter stars are distinct in the original negative, and can be made
out in the figure, one as a tiny blotch, the other as a mere deformation
of the lowest and largest disc. The physical association of these six
stars may be assumed without much risk of error; but there is nothing to
show that any real tie exists between them and four adjacent
star-points, at the limit of vision with the Lick thirty-six-inch,
detected by Barnard and Alvan Clark in 1889.
The nebula, as it developed on a plate exposed, under the same
circumstances, during 2^h 36^m, is shown in Plate XXVI. The stars of the
trapezium are here completely submerged; only their influence can be
traced, or suspected, in the symmetrical arrangement of the expansive
wings of light stretching away from their place. These are by no means
vague or indefinite outflows. Some of the long streamers have sharp
inner edges, peculiarly curved and notched. And the texture is generally
filamentous, like that of solar prominences, the characteristic forms of
which—as Mr. Ranyard effectively pointed out[1072] are faithfully
imitated in some of the minor features of the nebula. An outlying mass
to the north (N.G.C. 1977) not only claims affinity by some degree of
structural resemblance, but is seen, on Professor Pickering’s plate, to
be linked on to it by a faint intermediate extension. The gap between
them is absolutely black to telescopic vision.
[Illustration:
PLATE XXVII.
Nebulous Formation in Orion. Photographed, 14th January 1890, by W. H.
Pickering.
]
Improvements and modifications in photographic methods have led, by
successive steps, to a remarkable increase in the compass assignable to
this extraordinary formation. Portrait-lenses have played a leading part
in the process. Professor Wadsworth’s reasonings[1073] have made it
clear that small apertures, owing to the greater contrast afforded by
them with the sub-sensible illumination of the sky, are positively
advantageous for obtaining impressions of faint, extended objects. They
have the further prerogative of a wide field of view, giving room for
the grasp and delineation of large contours; so that, in certain
branches of celestial portraiture, they render indispensable and
invaluable services. By their aid the nebulous stars ι and c Orionis
were proved to be dimly connected with the trapezium nebula; a sharply
indented streamer became apparent, issuing towards it from the belt
star, ζ Orionis, long recognised as a separate focus of nebulosity;
while Max Wolf derived evidence of a luminous union between the nebulæ
surrounding ζ and ε Orionis.[1074] Finally, the combined picture, shown
in Plate XXVII., of all the nebulous patches in the constellation was
obtained by W. H. Pickering, 14th January 1890.[1075] The instrument
employed was a Voigtländer lens, 2·6 inches in aperture, and of 8·6
inches focal length; the time of exposure, 6^h 22^m. As the upshot, the
fragmentary condensations previously known became united into a vast
spiral formation, 15° across. Starting from near Bellatrix (which lies
outside the field), it sweeps round by κ Orionis and Rigel to the south,
then bends upward to η Orionis, and most probably effects a junction
with the nuclear mass in the Sword handle, although the further course
of the stream is rendered indistinct by the fogging of the plate. After
an interval of nearly five years, Professor Barnard independently
repeated and confirmed the Mount Wilson experiment.[1076] His small
“lantern lens” showed, with exposures of 2^h and 1^h 15^m, “an enormous
curved nebulosity encircling the belt and the great nebula, and covering
a large portion of the body of the giant.” In comparison with it, he
remarked, the old trapezium nebula “is but a pygmy.” The astonishing
extension given to the luminous structure in Orion was, however, of less
importance than the disclosure of its architectonic plan. Nor can it be
supposed that we are, even now, fully acquainted with it. Further
developments may be looked for. The nebulous connections of the various
parts will doubtless be more clearly expounded in future photographs.
Possibly an effective delineation might be obtained in sections with
instruments, like the Crossley or the Meudon reflector, too restricted
in angular grasp to embrace the whole at one view. The study of details
would thus be made feasible, their subordination to the general design
being at the same time kept well in sight. The relations of the members
to the main body of this nebula offer a problem of extreme complexity.
The stars of the trapezium, for instance, have an appreciable proper
motion of about 5″ a century. The nebulous plumage attached to them must
evidently partake in their displacement; but this cannot be
unhesitatingly asserted of the appendages to the belt stars, still less
of the ocean-river of nebulosity flowing outside them. Professor W. H.
Pickering discussed the movements of nineteen stars involved in the
spiral, though not of course necessarily connected with it.[1077] And so
far as any common character could be ascribed to them, it was that of
recession from the trapezium. It would be ridiculous to found any
conclusion upon so flimsy a basis; yet the indication harmonises with
hints, or nascent symptoms of a centrifugal tendency among clustered
stars such as the Pleiades. The determination and comparison of their
radial movements will be the best means of promoting knowledge on the
subject; and since Pickering’s nineteen stars are all, except one,
brighter than 6·2 magnitude, their spectroscopic measurement should
present no difficulty.
Meanwhile the original “Fish-mouth” nebula raises, even when treated
apart, questions of extreme intricacy. Seventy of the stars scattered
through it were photographically determined by Scheiner, and compared in
1898[1078] with Gould’s similarly deduced places for them. The close
agreement of the results showed them to have remained sensibly
stationary during about a score of years, not sharing the drift of the
central group. They must accordingly become, in course of time, detached
from it, and from the encircling nebula. They constitute then no
physical part of the structure, but are scattered casually over its
surface. The trapezium-stars, on the contrary, are really embedded in
the nebula. And the grounds for this assertion are twofold. Visual logic
by itself certifies its truth. The wings of a bird do not start more
definitely from the breastbone than the nebulous plumes from the stellar
group at their base. The relation is emphasised by the circumstance that
two of its members serve as the abutments of a luminous arch,
photographed at Potsdam,[1079] which spans the interval between them. A
similar nebulous bridge was remarked by Dr. Scheiner to connect two
other stars in the outskirts of the formation. Then, as we have already
seen, the spectrum of θ Orionis presents peculiarities indicative of a
close affinity with the nebulous masses around, and indeed suggests,
albeit doubtfully, an actual commingling of luminous substance. It
follows that the nebula shares the proper motion of the stars, the small
secular value of which (5″), at the probably vast distance of the
involved objects, corresponds to a very considerable real velocity.
Admitting for a moment the correctness of Professor Pickering’s estimate
of one thousand years for the duration of the light-journey from the
Orion nebula hither,[1080] we should have to ascribe to the trapezium
and its belongings a lateral speed of forty-nine miles a second, the
greater part of which would be inherent, since its direction precludes
the supposition that it is a perspective effect of the solar journey.
The receding radial motion of eleven miles per second, determined for
the nebula by Keeler, seems, on the other hand, to belong almost
entirely to the sun. Yet the line-displacements by which it is indicated
vary, in some degree, from tract to tract of the formation. Recent
spectrographic measures executed at Potsdam[1081] imply the progress of
interstitial movements, the nature and laws of which it will be of the
highest interest to ascertain.
Thirty bright lines, from λ 5007 up to the seventeenth member of the
hydrogen series, have been photographed in the spectrum of the Orion
nebula, besides the baffling groups faintly apparent on the Tulse Hill
plate of 1888. The yellow helium ray (D_{3}), visually detected by Dr.
Copeland in 1886, has at least three more refrangible associates; but
several lines met with in planetary nebulæ are missing, notably those
distinctive of the Wolf-Rayet class of stars. Thus the Pickering and the
Rydberg series are alike unrepresented in this grand object. Nor does it
glow with the light of any metallic vapour. Its hydrogen emissions are
in some respects peculiarly conditioned. The red line is invisible; the
green and blue lines are especially bright; beyond them there is a
progressive decrease of intensity, though the complete series, up to Hρ,
was photographed by Mr. S. A. Mitchell, with a concave grating of 11½
square inches, mounted on the twelve-inch refractor of the Yerkes
Observatory.[1082] Thus the state of the nebula is such as to favour,
apparently, the development of the quicker luminous vibrations. The
inference has, indeed, been controverted. Dr. Scheiner maintained that
the suppression of C in this spectrum is “due to purely physiological
causes, and warrants no conclusions as to the physical conditions of
luminosity,” in nebulæ.[1083] He regarded it, in fact, as an
illustration of the “Purkinje effect,” by which the eye, owing to its
differential colour-sensitiveness, loses sight of a red sooner than a
green ray, when the light-source from which both proceed is gradually
enfeebled. In the laboratory, accordingly, F survives alone in the
spectrum of a hydrogen-tube at minimum illumination. The plausible
deduction that there is no objective reality in the anomalous variations
of comparative strength affecting the hydrogen lines derived from
celestial objects is, nevertheless, untenable. It collapsed hopelessly,
on being confronted with the simple fact, noted by Keeler,[1084] that
the third hydrogen line (Hγ) always vanishes experimentally before the
first, while in nebulæ it shines unfailingly, although F be
imperceptible. The order of brightness, then, of the members of the
series is not, in nebulæ, prescribed by physiological causes alone; the
faintness of Hα is intrinsic. This is further proved by Professor
Keeler’s observation of “Hβ, and several of the hydrogen series above
it, glowing brilliantly in the spectrum of R Andromedæ, while not a
trace of Hα could be found.” So that “in some of the variable stars we
seem to have hydrogen in the same condition as in the nebulæ.”
Finally, Professor Campbell made the decisive experiment of directly
comparing the nebulæ with an artificial spectrum.[1085] The two kinds of
light being admitted through the upper and lower halves respectively of
the same slit, their spectra were seen side by side in the eye-piece,
and the corresponding lines in them could be at once equalised by merely
altering the distance of the hydrogen-tube from the slit. Combined
experiments with this apparatus by Keeler, Wright, and Campbell proved
that (1) when the F lines from the nebula and tube were matched in
brightness, the blue line above it (Hγ) was markedly stronger in the
nebular than in the tube-spectrum; (2) the equalisation of the blue
lines left the green line from the nebula conspicuously fainter than the
same line from the tube. “The relative intensities,” it was concluded,
“of the hydrogen lines from the nebula and from the tube are, therefore,
not the same; the nebular lines are relatively the stronger toward the
violet, the lines from the tube are relatively the stronger toward the
red end of the spectrum.” Absolute measures showed the three principal
lines in the spectrum of the Orion nebula to be of very low intensity.
Their faintness completely neutralised, to Professor Campbell’s eye,
their differences of tint. Hence the “Purkinje phenomenon,” which
depends upon the perception of colour, cannot in any degree affect their
comparative visibility. There is, however, another aspect to the
question. The spectrum of the nebula varies, not alone from the standard
of comparison supplied by the vacuum-tube, but also locally, within the
formation itself. The relative strength of the constituent rays is
different for its different sections. Dr. Runge of Hanover,[1086] who
devoted special attention to the point during a visit to the Lick
Observatory in September 1897, satisfied himself that the F line of
hydrogen, which, near the trapezium, had only one-third (or possibly
two-fifths) the intensity of the nebulium line, was ten times brighter
in the faint outlying sections of the nebula. The relative gain was then
twenty-five to thirtyfold; while the alleged physiological cause,
admitting that it had full scope and play, could at the utmost have
produced a gain of 1·8 times. The reality of the change was further
certified by observing the second nebular line (λ 4959), which in the
“Huygenian region” just equals F, to disappear as the slit was moved
outward, while F continued to shine with a very sensible lustre. And
since, in this case, the Purkinje effect was null (the lines being
almost indistinguishable in colour), a demonstration was afforded of a
genuine modification in the curve of emissive energy in passing from one
part of the great nebula to another.
Professor Keeler devised a completely novel method of demonstrating its
non-homogeneous character.[1087] Pictures of the nebula taken on
orthochromatic plates protected by colour-screens from the blue
radiations, were compared with impressions on ordinary unscreened
plates, all being exposed with the Crossley reflector, though during
very unequal times. The result was to show that, for equal intensity of
the Huygenian region, that of the remote parts and outlying streamers
fell greatly below its normal value in the screened photographs.
“Conversely,” to quote the words of the ingenious operator,[1088] “where
photographs made by the two methods, on the same night, show an equal
extent of nebulosity, the Huygenian region is very much more intense on
the orthochromatic plate. We infer, therefore, that in the remote parts
of the nebula the two lowest nebular lines are weak, or the hydrogen
lines strong, as compared with the Huygenian region. Thus the results of
spectroscopic researches are confirmed, and are extended to parts of the
nebula too faint for visual observation.” That is to say, the hydrogen
image of the Orion nebula is larger than its image in nebulium. And this
corresponds precisely with Campbell’s spectroscopic discovery about the
planetary nebula S.D. −12° 1172,[1089] already adverted to. When
nebulium comes to be examined in the laboratory—if that shadowy
possibility be ever realised—the cause of the discrepancy may be laid
bare. The most obvious is a difference of density between the two
substances; yet we cannot unreservedly assume its validity, considering
the noted effects of what we may call electrical preferences in
modifying the spectra of attenuated gaseous structures.
Two facts, then, have been definitely ascertained regarding the hydrogen
spectrum in the “Fish-mouth” nebula. The first is that its more
refrangible constituents are preferentially developed as compared with
the standard set by the vacuum-tube. The second is that of its
persistence in regions of the nebula too dim to glow with any other
species of light. Both peculiarities urgently demand explanations, which
can probably be afforded only by arduous experimental work. Dr. Scheiner
has not neglected this side of the inquiry; and although the outcome of
his efforts is negative, it serves none the less to answer a fundamental
question. He postulates in nebulæ extreme rarefaction and excessively
low temperature; and to test the effect of the latter condition he
plunged hydrogen-tubes into liquid air and examined the spectrum. He
found it entirely unchanged by cooling to −200° centigrade.[1090] This,
he pointed out, harmonises with the view that the luminosity of gases
originates solely through internal movements of the individual
molecules, and is hence independent of external temperature. Moreover,
one of the few means available for terrestrially altering the relative
strength of the red and green hydrogen lines is that of electrical
differentiation. Professor J. J. Thomson’s observation[1091] of C
bright, F invisible near the positive, F bright, and C invisible near
the negative electrode, offers a clue which has not yet been followed
up, for the threading of the labyrinth.
In another of its elements besides hydrogen, the spectrum of the Orion
nebula is suspected to vary regionally. The ultra-violet ray at λ 3727
is second to none in importance when photographed with suitable
apparatus. Nevertheless, it was missing from a plate exposed by Sir
William and Lady Huggins 28th February 1889,[1092] although the
impressions upon it of two much weaker lines near its place were clearly
to be seen. The anomaly of its total absence has not recurred. Professor
Keeler vainly went over the ground in 1892–93,[1093] groping with his
slit for the blank district. Professor W. H. Pickering, it is true,
derived indications of local diversities in the intensity of the line
from a photograph taken without a slit, 10th July 1888; but they were
rather suggestive than conclusive.
Suspected light-variations in the Orion nebula have not been confirmed
by modern research. There is absolutely no photographic evidence of
change, and visual discrepancies have not been attested with sufficient
precision for the support of any positive inferences. Professor Holden
was indeed disposed, after an exhaustive comparison of his own with
numerous recorded observations, to believe in luminous instability of a
partial kind;[1094] but the effects considered may have been only
apparent. Their production is at once rendered intelligible by Professor
Ormond Stone’s pertinent remarks:—“The general appearance of the
Huygenian region,” he wrote in 1896,[1095] “is very much like that of a
so-called ‘mackerel sky.’ Many of the condensations have pretty
well-defined nuclei, whose light diffuses, blending with the surrounding
nebulosity when the seeing grows poor. I have frequently been surprised
to find how greatly the definition changes the relative brightness of
the different condensations. This explains the many apparently
contradictory estimates.” So far as it is possible to judge, then, the
brightness of this nebula is not subject to change, either general or
local. Moreover, its component parts seem absolutely fixed in outline
and position. Internal movements, if in progress, will need the lapse of
ages to become sensible to the eye. No relative shiftings of knots or
nuclei have been detected; no alteration in shape of outgrowths and
effusions. Yet they are of an eminently unstable aspect, and might be
supposed no less essentially transitory than the appurtenances of
comets. And so most probably they are, although the unit of time by
which their duration is measured be long, and the scale of their
construction unimaginably vast.
Some of the stars, however, scattered near the trapezium preserve
anything but a constant lustre. One, catalogued as T Orionis, fluctuates
irregularly from 9·7 to 13·0 magnitude; and others vary as unmistakably,
though to a less extent. None are periodical, so that they belong to a
different category from the flash-lights of globular clusters. An
attentive study, photometric, photographic, and spectroscopic, of the
Orion variables could not fail to be fruitful and instructive. Their
instability is the more noteworthy from the whiteness of their light. T
Orionis (also known as “Bond 822”) is perhaps a unique example of a star
untinged with red losing and regaining nineteen-twentieths of its
visible radiance. Among its obviously variable neighbours are the
objects numbered by Bond in his survey of the nebula, 641, 647, 654, and
679. Scarcely any sustained attention has yet been paid to the group,
notwithstanding the many questions of interest connected with it. Enough
only is known to make it certain that its members exhibit no community
of character in their vicissitudes save that of exemption from any
traceable law of order. Thus Bond 654 was noticed by Holden, as it had
been noticed by Otto Struve twenty years earlier, to rise occasionally,
from habitual quasi-extinction, to brief maxima of about twelfth
magnitude. But Ormond Stone recorded none of these sudden brightenings,
although he observed the star, 30th September 1886, to rank higher than
the twelfth magnitude. Possibly the manner of its variability is itself
variable. The star Bond 647, on the other hand, has gained largely in
average lustre since 1837, when Sir John Herschel’s measures, reduced to
Struve’s scale, made it of 13·1 magnitude. Otto Struve chronicled its
disappearance in 1863, after a prolonged maximum at 12·5
magnitude;[1096] Bond found it to be of 11·6 magnitude in 1867; Ormond
Stone, of 10·9 magnitude, 1886 to 1894;[1097] and there is nothing to
show that this steady rise has reached its culminating point. A
compensatory decline may be anticipated, though not with entire
confidence. The situation of these stars lends a special meaning to
their variations. That they are physically connected with the nebula
they are seen projected upon, cannot indeed be proved, but it may
legitimately be assumed. Hence every advance in knowledge of their
vicissitudes cannot but help to elucidate the still obscure relations of
nebulous environment to stellar light-change.
CHAPTER XXXVII.
OTHER IRREGULAR NEBULÆ.
Irregular nebulæ are objects of large size, indeterminate outlines, and
capricious shapes. They become fully apparent, as a rule, only by
photographic means, their exterior sections and subordinate parts
shining too dimly for distinct visual perception. The chemical retina,
however, sees them with comparative ease; for their light consists
mainly of isolated short-period vibrations. They are hence known to be
of gaseous composition, and all are situated in or near the Milky Way.
Many observers, especially those armed with large reflectors, can pick
them out at sight by their greenish tinge; but they never appear blue
like planetaries. There seems, indeed, to be a definite difference of
hue between the two classes, approximate spectral identity
notwithstanding. This probably indicates superior strength of the
nebulium lines in irregular nebulæ; or the maximum of intensity in the
dim continuous spectrum may be situated lower in them than in the
planetary kind. Discrimination is not easy; for impressions of colour
are often too subtle to be analysed.
Irregular nebulæ are of surprising variety. Each specimen has individual
peculiarities, for the most part inimitable by any other. No copy of the
Orion nebula is to be found in the heavens, and the Argo nebula, which
comes next to it in importance, is equally _sui generis_. This
magnificent edifice has no corner-stone corresponding to the trapezium;
instead, a black opening of a lemniscate form, and as sharp to the eye
as if cut with a punching instrument, yawns in its brightest part. The
operation by which it came to be produced was, moreover, repeated in a
fainter nebulous tract farther south, and was hence evidently controlled
by some definite and special combination of circumstances. With reason,
then, in view of the unique character of this feature, the formation has
been surnamed the “Key-hole Nebula.”
At the eastern edge of the northern key-hole lies the extraordinary
variable, η Carinæ, the vicissitudes of which cannot but be related to
the tumultuous processes of change doubtless going on in the seething
chaos around. The general surface of the nebula is emblazoned besides
with a multitude of ordinary small stars, historically and
telescopically undistinguished. Their scattering, however, is not at
random; it has marks of _intention_. Sir John Herschel pointed out their
disposal along the margins of dark rifts in the nebula; and many such
allineations were traced by Mr. Ranyard[1098] in Dr. Russell’s
photographs, taken at Sydney in 1894. The object as a whole seemed to
him “a very fine specimen of a nebulous cluster with a central
condensation, associated with dark structures and radiating streams of
stars.” These “are in most cases accompanied by narrow black channels in
the general nebulosity, which run parallel to, and alongside of the
star-streams.” One is reminded of the dark lanes bordered with stars in
the Hercules cluster; and the analogy, if genuine, is of no slight
significance. Setting aside for the moment its implications of affinity
between stellar globes and nebulæ, it would afford a certainty that the
stars distributed over the surface of the Argo formation are really in
and of it; and since they are obviously galactic, this would amount to a
demonstration that the nebula too is galactic—that it belongs to the
Milky Way, not geometrically by projection, but physically by
collocation.
The first photograph of this fine object was obtained by Dr. Russell
with a six-inch portrait-lens in June 1891; but it had an experimental
rather than a delineative value. This could not be said of one taken
nine months later by Sir David Gill. An exposure of twelve hours, spread
over four nights, with the thirteen-inch photo-refractor of the
International Survey, yielded the remarkable picture which forms our
frontispiece. The advantages of the autographic process could not be
more forcibly exemplified than by comparing it with Sir John Herschel’s
drawing of the same object.[1099] Months of labour at the telescope were
of less avail than half a day’s “following” with the camera. The artist
fully recognised the inability of even his skilful hand to delineate the
endless gradations of light and shade which his eye perceived. The
elaborate pains taken by him tended, indeed, as in most similar cases,
to exaggerate contrasts, and so vitiate the general effect. In the
photograph, the disclosed nebulous fields are not only wider, but they
are more harmoniously related and more intelligibly arranged. Nor has
the distinctive trait of the nebula evaded chemical portraiture. The
“key-hole” is conspicuous on the plate, although deformed by luminous
inflows; Herschel’s “kidney-bean” opening to the south (sixth square
from the bottom, fourth from the left side of the Plate) is scarcely
encroached upon by chemical diffusion; and a third vacuity of similar
design, though less perfect execution, occurs to the north-west of the
key-hole (fourth square from the top, seventh from the left). There is,
nevertheless, one striking discrepancy between the drawing and the
photograph—a discrepancy which, on the face of it, implies the
occurrence of genuine and extensive change. An isolated, trident-shaped
structure prominent in the former is imperceptible in the latter, or
survives, at the most, fragmentarily. Its disappearance was due to no
accidental defect in the Cape negative; the Arequipa plates, exposed
with the Bruce twenty-four-inch lens, show a corresponding effacement.
It had, in fact, taken place even to the eye already in 1871, when Dr.
Russell failed to perceive the “swan” form (as Sir David Gill called it)
with the Sydney eleven-inch refractor. The evidence of light-extinction
is almost conclusive; yet it should not be admitted without further
question. Visual study of the nebula would perhaps be the most promising
road towards the end in view. Such objects are now rarely _looked at_;
observers adapt their apparatus and devote their energies to the
exposure of plates. Yet in some cases—and this is surely one of them—the
direct and indirect methods should, for completeness, be employed
concurrently. Photographic and photometric brightness are commonly
disparate in stars; they differ in nebulæ still more widely; it remains
to be proved whether their differences may not be irregularly
distributed or even variable with time. The spectrum of the Argo nebula
is of the usual gaseous type. Further particulars about it are wanting.
The Trifid nebula in Sagittarius (M 20 = N.G.C. 6514) affords another
instance of ostensible change. Discovered by Messier in 1764, it
appeared to Sir William Herschel in 1784 in the guise of “three nebulæ
faintly joined into a triangle. In the middle,” he added, “is a double
star.” And again, after two years, “About the double star is a black
opening,” the combined effect of which recalled the Orion trapezium. He
reiterated in 1811 that the position of the star was “in the middle” of
the obscure space between the nebulæ.[1100] Sir John Herschel similarly
assigned its place in 1827[1101] as “exactly in the central vacuity of a
large irregular nebula, which appears to have been broken up into three
portions by three rifts or cracks extending from its centre to its
circumference, and whose directions meet at the double star.” Nothing
could be more explicit; and his verbal description is authenticated by a
rough sketch of high evidential value, though laying no claim to
precision. Six years later, at Slough in 1833, he observed the double
(really a sextuple) star to occupy “the centre of the trifid nebula.”
Yet at the Cape in 1835, he drew it as adhering to the south-eastern
lobe, and—stranger still—without comment on the alteration. And
virtually under the same aspect the object was seen by the American
observers, Mason and Smith, in 1839, as well as by Lassell at Malta in
1862. The complete immersion of the star-group in nebulosity, and its
eccentric situation at the apex of a shining conical mass, are now
patent to the merest tiro in telescopic scrutiny. Autographic
impressions tell the same tale. One obtained by Dr. Roberts in ninety
minutes, 13th July 1899, is shown in Plate XXVIII. Fig. 1. The open fan
of nebulosity in the south-eastern quadrant has the multiple star at its
apex, but indistinguishably, owing to the burnt-up condition of the
plate in this bright region. The abruptness with which the luminous
masses abut upon the dark rifts that divide them is most remarkable. On
a Crossley plate of 6th July 1899,[1102] the small central block of
nebulosity came out semi-detached, while in the Crowborough picture it
appears as a simple prolongation of the great northern lobe. Here, too,
by a further effect of light-concentration, the dependent nebula to the
north is completely annexed by the adjacent triple structure. Its
nuclear star would make an interesting subject for spectroscopic study.
Now Herschel’s Cape drawing of 1835 is in substantial agreement with the
photographs of 1899. He saw about the same extent of nebulosity
disclosed in them, distributed very much in the same way, and similarly
related to the principal stars scattered through it. During sixty-four
years, at any rate, fixity has prevailed. Mutual displacements are not
sensibly in progress. The alleged variation must, we are driven to
infer, have taken place suddenly between 1833 and 1835. This is
certainly hard of credence; but it is still more difficult to admit that
both Sir William and Sir John Herschel erred so egregiously as to locate
the multiple star in the middle of a black space, if it really sparkled,
as it does now, upon a background of lucent silver.
Professor Swift mentions having observed about 1888 “a luminous filament
of the most delicate spider-like fineness stretched across the
north-west cleft” of the Trifid nebula.[1103] It reminded him of a cable
of the New York and Brooklyn suspension bridge, with the difference that
it did not sag in the middle, but went straight from shore to shore. The
installation of electric street lights at Rochester precluded him from
keeping watch over this delicate and perhaps novel feature. It would be
interesting to learn whether it continues visibly to span that strange
abyss.
[Illustration:
PLATE XXVIII.
1. Photograph of the Trifid Nebula. Taken by Dr. Roberts, 13th July
1899.
2. Photograph of Messier 77. Taken by Dr. Roberts, 26th November 1892.
]
The spectrum of this nebula was observed by Professor Keeler in 1890 as
“continuous but short, being apparently confined to the blue and
green.”[1104] Only a “brightening near the middle” could be detected.
Nevertheless, on 3rd August 1894, Professor Campbell perceived at a
glance the three usual nebula lines, the third (F) being relatively
strong.[1105] An auroral glow almost effaced whatever continuous light
was present; and the same accident, singularly enough, recurred at the
date of a second observation 24th May 1895. Much might be learned about
the nature of the Trifid nebula by a searching spectrographic inquiry.
The condition, for instance, of the unknown line at λ 3727 would be
important to ascertain. Its variations from one object to another, or
possibly from one region to the next of the same object, are doubtless
of a significance only to be fathomed by the patient collection and
comparison of facts. The spectrum of the multiple star, which seems to
have a nuclear relation to the south-eastern division of the nebula, was
stated by Keeler to be devoid of marked features. Yet such may present
themselves when its thorough examination becomes practicable. The
components of the object were successively discovered in the course of
nearly a century. Sir William Herschel saw it double in 1784; Herschel
and South about 1826 found it to be triple; and no more than three
members of the group were distinguished by Lassell with his four-foot
reflector. The fourth and fifth stars—neither much brighter than the
thirteenth magnitude—were added by Professor Langley, using the
fifteen-inch Harvard College refractor, in 1866; finally, the Washington
twenty-six-inch disclosed the sixth to Professor Holden 5th August 1875.
A fantastic structure, known as the “Omega” or “Horse-shoe” nebula (M 17
= N.G.C. 6618), is situated on the border of Scutum Sobieski, and, like
the Trifid, invites inspection from southern latitudes. Sir John
Herschel perceived it at Slough in the figure of a Greek Omega (Ω), with
the left-hand baseline turned upward.[1106] He was surprised to see at
the Cape a second arch springing from the same level as the first,[1107]
besides other suspected convolutions. The subordinate appendage was
again noticed by Swift in 1883.[1108] In a photograph taken by Dr.
Roberts, 5th August 1893, the “horse-shoe” resemblance is almost
obliterated.[1109] Much greater prominence is given to the
spindle-shaped axis originally noticed by Messier in 1764. On the plate
it is found to be encompassed by a dim envelope, uniting the various
patches of nebulosity into a large oval, 18′ by 12′, to the
north-western end of which an abortive “horse-shoe” is appended like an
excrescence. A picture obtained under more favourable circumstances
might bring these somewhat incongruous parts into an intelligible mutual
dependence. Professor Holden collected evidence suggestive of variation
in the Omega nebula,[1110] but none that could be regarded as
conclusive. “There has certainly not been any bodily shifting,” Dr.
Dreyer pronounced in 1887.[1111] He was not, however, equally clear that
partial fluctuations in brightness might not have taken place. The
question remains an open one.
The spectrum of the Omega nebula was recorded as gaseous by Sir William
Huggins in 1864. Nothing further is known about it.
Perhaps the most important of the nebulæ for purposes of comparative
study is “30 Doradûs.” Situated in the Greater Magellanic Cloud, the
“looped” nebula may exercise, in Professor Pickering’s opinion, a
dominating influence over that extraordinary mixed assemblage. Yet it
has the filmy and unsubstantial appearance of silver filagree torn in
shreds and hung in the black sky. Its spectrum offers a remarkable
combination of linear elements with strongly continuous radiance. Here,
if anywhere, a frontier-instance between “white” and “green” nebulæ is
to be found. Burton in 1874 affirmed the predominance in it of the
fundamental nebular line; but the Harvard observers are less explicit.
Professor Pickering briefly announced in 1892[1112] the spectrum of this
object to be “unlike that of other gaseous nebulæ”; adding in 1897[1113]
that its “constitution appears to be partly stellar and partly gaseous.”
Further, a sixth-magnitude star in Libra (A.G.C. 20,937) is said to
reproduce the peculiarities of its mixed light, a discovery ranking
among the most profoundly instructive of those made by Mrs. Fleming. Its
full import may, however, develop only through prolonged investigation.
Near the star ξ Persei on 3rd November 1885, Barnard discovered, at
Nashville with a six-inch telescope, a “very faint, very large,
diffused” nebula (N.G.C. 1499).[1114] Six years later it came
prominently into notice through a photograph taken at Halensee, near
Berlin, by Dr. Archenhold.[1115] The Willard lens was then repeatedly
brought to bear upon it at Lick, and one of the resulting pictures, to
which Barnard gave six hours’ exposure 21st September 1895, is
reproduced in Plate XXIX. The nebulosity extends over at least two
degrees, and includes many “angular condensations.” A round dark spot
near the northern border strikes the eye at once. That it is “doubtless
a hole in the nebula,” Professor Barnard avers. But in a gaseous mass a
“hole” could neither be produced nor maintained. Light in the perforated
region must be suppressed unless it be intercepted, and the latter
alternative involves consequences that may fairly be called
inadmissible.
A drawing of the ξ Persei nebula, published by Dr. Scheiner in
1893,[1116] embodies five photographic delineations obtained with a
Voigtländer “euryscope” of four inches aperture, in times of exposure
varying from one to six hours. He found it to be little inferior in size
to the great Orion nebula, but totally different in plan of
construction. It has strongly luminous borders, and these are connected
by bright causeways crossing a comparatively obscure interior. There is
no sign of a nucleus, nor any tendency towards the formation of one.
Portrait-lenses, or some modification of them, seem to be the only kind
of instrument with which impressions of this object can be secured; a
plate exposed during six hours at the focus of the Potsdam thirteen-inch
astrographic refractor showed no trace even of veiling from the
prolonged impingement of its rays. They are equally ineffective, Dr.
Archenhold states, upon orthochromatic plates—a fact reasonably held to
imply that the nebula emits chiefly light-waves of short periods. In
other words, it is a _green_ nebula, and all but certainly gaseous.
A “vast and magnificent nebula” near Antares, seen imperfectly and
fragmentarily, was disclosed in its entirety by Professor Barnard’s
photographic researches in 1895.[1117] Its primary gathering-ground is
about ρ Ophiuchi, a quadruple star of fourth magnitude giving a helium
spectrum; but σ, γ^2, and 22 Scorpii, besides other smaller stars, form
subordinate foci. Antares itself is involved in the trailing skirts of
this cosmic cloud, but may in reality lie far away from them. Furrowed
by an intricate system of rifts, and pierced by obscure cavities, the
Antares nebula is evidently in an agitated and unstable condition; and
its marked tendency to cling to individual stars suggests that its
development will take the direction of accentuating such local
condensations. The example of the Pleiades was recalled to Professor
Barnard; and it may be that the formation in Scorpio presents us with an
analogous aggregation in an earlier stage of growth.
One in many respects similar was photographed in Cepheus by Professor
Barnard 13th October 1893. He traced in it “numerous irregular vacancies
and zigzag lanes,” and noticed it to “mingle indefinitely with masses of
small stars and become part of them.”[1118] This nebula is two degrees
in diameter, and rudely circular in shape. Still more far-spreading and
complex is a wonderful nebulous maze, vaguely centred at a point near ξ
Cygni, but extending outward to a distance of at least eight degrees.
Dr. Max Wolf, who virtually discovered this vast formation by his
photographs of 1891, considers it to embrace all the stars, bright and
faint, that come within its scope;[1119] and we cannot doubt that a
heterogeneous system, partly stellar, partly nebular, is here presented
to view. But the particularities of its composition evade for the
present profitable inquiry.
Barnard’s circular nebula in Monoceros (N.G.C. 2237), which to the eye
seems to draw a line of circumvallation round the cluster within, but
loses in photographs all trace of annularity, may provisionally be
classed as “irregular.” A fine picture taken by Dr. Roberts 5th March
1899[1120] shows the nebulosity to extend over a space about 77′ by 67′,
in the form of a cloudy aggregation “broken up into wisps, streamers,
and curdling masses, densely dotted with stars,” and including “many
dark areas with and without either stars or nebulosity. Some remarkable
black tortuous rifts meander through the nebulosity on the
north-preceding half of the nebula; their margins are sharp and well
defined in the midst of dense nebulosity. They are as clearly cut as we
see the cañons of great rivers, but their width may in reality be
millions of miles.”
[Illustration:
PLATE XXIX.
Photograph of a Nebula in Perseus (N.G.C. 1499). By E. E. Barnard.
Exposure, 6^h.
]
Irregular, too, is a beautiful winged formation distantly resembling the
Orion nebula, photographed by Schaeberle in the vicinity of Nova
Aurigæ.[1121] The physical investigation of all these objects will prove
an arduous but interesting task. The measurement of their radial
movements, especially, should help to define ideas regarding their true
status in the heavens.
CHAPTER XXXVIII.
NONDESCRIPT NEBULÆ.
The “dumb-bell” nebula in Vulpecula (M 27 = N.G.C. 6853) might almost be
called a hybrid between the planetary and irregular species. Its
affinities are indeed so numerous as to embarrass its classification.
Imperfectly seen, it appears double, and such it was considered to be by
the elder Herschel. What survives of it when the fainter parts are
obliterated by dim air or bad definition, suggests the bell-shaped
receptacles detached from the neck of an hour-glass. And, in fact, the
succession of instances is unbroken, from bi-nuclear planetaries through
the “dumb-bell” stage to unmistakable pairs of nebulæ thinly connected
by hazy strips. This point has, however, been already referred to. The
dumb-bell nebula, moreover, possesses a nuclear star, situated at the
narrowest point of the neck; and it is difficult to conceive of the neck
as other than a temporary arrangement. Many indications favour the idea
that it will, sooner or later, break up and disappear, leaving the star
regent over the masses finally disjoined. And it is curious to remember
that just such an interjacent star occurs in several compound nebular
systems.
A tendency to form a marginal ring is, on the other hand, visible in
this nebula. A photograph taken by Mr. W. E. Wilson of Daramona,
Ireland, which we are permitted by him to reproduce in Plate XXX., shows
an incomplete bright border to be a leading feature in its construction.
Whether in the future it is destined to become predominant, we can
scarcely venture to surmise. The dumb-bell nebula is about 8′ in
diameter. The bright framework can be seen in the photograph to be
filled out into an elliptical shape by faint supplementary luminosity,
which, however, is much stronger in the original negative. Dim
protrusions at the ends of the major axis, traceable in Dr. Roberts’s
picture,[1122] intimate opposite outflows, similar in cause and
character to those suspected to proceed from the ring nebula in Lyra.
The texture of the “dumb-bell” is lumpy and irregular. Presumably it is
of an ellipsoidal form, and rotates on its transverse axis.
[Illustration:
PLATE XXX.
Photograph of the Dumb-Bell Nebula. By W. E. Wilson, F.R.S.
Exposure, 60^m.
]
The spectrum is purely gaseous, and is dominated visually, with more
than the usual emphasis, by the green nebulium line, while in the
photographic region the ray at λ 3727 was found by Von Gothard to be of
quite peculiar strength.[1123] Symptoms of helium-emission should be
looked for.
Several objects have been described as miniatures of the Vulpecula
dumb-bell, but the likeness may prove, on a more searching examination,
to be only skin-deep. Among its supposed analogues are: (1) N.G.C. 1365,
situated in Fornax, and described in Sir John Herschel’s _Cape
Results_.[1124] It belongs, he says, to the class of “annular nebulæ
with centres,” but the ring breaks off, leaving the oval incomplete at
its extremities, while the shorter axis terminates with two bright
masses, between which lies a “resolvable” nucleus. (2) N.G.C. 5189. In
Musca. A “general similitude” to the dumb-bell was noticed by J.
Herschel.[1125] (3) N.G.C. 3195. In Chameleon. A drawing is given in the
_Cape Results_. (4) N.G.C. 1978. One of the nondescript inmates of the
Magellanic Cloud. (5) N.G.C. 6905. A gaseous nebula in Cygnus, with a
star in the centre, and four attendant upon it, like satellites. (6)
N.G.C. 3226, 3227. A double nebula, near γ Leonis, consisting of two
hazy masses subsensibly united by a “neck,” and enclosed in a dim,
elliptical sheath. All these various structures may not, indeed, be
traversing the same line of development. In some, the annular tendency
will perhaps eventually prevail, in some the disruptive. The balance
between opposing forces is unlikely to incline everywhere the same way,
and upon its poise depends by supreme appointment the special form to be
assumed in the course of ages by each individual system.
The “Crab” nebula in Taurus, discovered by Dr. Bevis in 1731, was again
noted by Messier, 12th September 1758,[1126] as a “whitish light,
elongated like the flame of a taper.” He took it at first for the comet
he was in search of; and, to avert future confusion of the kind, adopted
the memorable expedient of constructing a catalogue of nebulæ.[1127] The
specimen at present in question measures 5′ along its greatest diameter,
and about 3′ across. Lord Rosse gave it its distinctive name from the
protruding filaments resembling the claws of a crustacean, brought to
view by his great reflector.[1128] These have not yet been photographed.
On a plate exposed during three hours by Dr. Roberts, 2nd February 1892,
there came out an ovate body composed of “dense masses of clouds, with
fainter areas between them,” a deep bay on one side being
counterbalanced by a projecting limb on the other.[1129] No tentacular
appendages were disclosed.
The spectrum of the Crab nebula appears to be rather more strongly
continuous than is usual in the gaseous class, to which it
unquestionably belongs. For at Harvard College in 1868 the green
nebulium ray was recorded, and was suspected to have _less_ refrangible
companions.[1130]
Another object of dubious relationships is Messier 77 (N.G.C. 1068),
Lord Rosse’s “blue spiral” in Cetus.[1131] The description intimates an
anomaly, since true spirals are “white,” and give a continuous spectrum.
Now the colour of this object corresponds, as might have been expected,
to a gaseous constitution, whether of the normal kind or in certain ways
peculiar, remains to be proved. Its form, too, is ambiguous. A drawing
published by Lord Rosse portrays a round, faint disc more than 2′ in
diameter, upon which are relieved the brighter coils of a definitely
separated central mass. In Dr. Roberts’s picture (see Plate XXVIII. Fig.
2), on the other hand, there is no trace of a convoluted structure. It
shows “a stellar nucleus with projecting ansæ of dense nebulosity”
surrounded by a faint zone, and that again “by a broad nebulous ring
studded with strong condensations” like inchoate stars. A different
aspect was, however, assumed by the object in a photograph taken by MM.
Baillaud and Bourget in 1899 with the thirty-three-inch reflector of the
Toulouse Observatory.[1132] The nucleus here again resolved itself into
the winding folds seen at Parsonstown, and a paler spiral formation
served for its background. Its character continues in many respects
enigmatical.
M 77 is copied, in Professor Holden’s opinion, quite accurately by a
nebula in the same constellation (N.G.C. 1084). Under the gaze of the
camera it may resume the spiral shape obscurely visible to the
Parsonstown observers, notwithstanding their final description of it as
“a fine oval, with ragged edge and a mottled look,” indented by “a dark
bay north of the nucleus.”[1133] Judging by analogy, it should yield a
gaseous spectrum; though the circumstance, considering its remoteness
from the Milky Way, would be exceptional.
Messier 78 (N.G.C. 2068), in Orion, is a “singular wispy nebula,” 6″ or
7″ across, enclosing a triple star, surmised to vary in light.[1134] An
arrangement of the more lucent nebulous tufts along a spiral curve,
affirmed visually,[1135] is denied photographically. Dr. Roberts’s plate
exhibits a central cumulus, sharply terminated on one side, vaguely
diffused on the other, the dense nuclear part being surrounded by dim
floccules with wide dark spaces between.[1136] A smaller adjacent nebula
(N.G.C. 2071) has a stellar focus, to which appurtenances like “mare’s
tails” are attached. The distance from centre to centre of the two
nebulæ is about 15′ of arc, and they can be made out to be in faint
nebulous connection; but neither, Dr. Roberts expressly states, gives
indications of possessing a spiral form. Their spectra, which are
probably discontinuous, have scarcely been examined.
That of “a fine, pale-white object”[1137] in Canes Venatici is known to
be continuous.[1138] Discovered by Méchain in 1781, M 94 (N.G.C. 4736)
is large and bright;[1139] its nucleus, which is granular in texture,
and evidently globular in shape, being surrounded by a zone of extremely
dim, and that again by a zone of relatively intense luminosity. The
outer annulus appears, in a photograph taken by Dr. Roberts,[1140] to be
broken up into nearly a dozen irregular star-like condensations. Two
abrupt opposite projections from it explain, possibly, the quasi-spiral
aspect of the nebula in the Rosse reflector.
A circular object, one minute of arc in diameter, and of a “lucid
pale-blue colour,” was met with by the elder Herschel in the
constellation Hercules, and ranged in the planetary class[1141] (N.G.C.
6299). D’Arrest described it as a “nebulous disc,” invested with a
“nebulous sheath,” so that its passing for a comet in 1819 is easily
understood. Nevertheless, Sir William Huggins found it to shine with
perfectly continuous light,[1142] and it was thought at Parsonstown, on
9th May 1872, to be resolved into a globular cluster.[1143] It should be
added, however, that an undoubted planetary, N.G.C. 2022, was seen with
the same instrument under a similar illusory aspect. There is indeed
much difficulty in admitting the nebula in Hercules to be a genuine
cluster. Its cometary envelope and the azure cast of its rays are almost
contradictory of a stellar composition. But until a photograph of its
spectrum has been obtained nothing can be definitely asserted on the
subject. The quantity of light available for analysis is about equal to
that given by a ninth-magnitude star.
A nebula in Virgo (N.G.C. 4900) looked, when ill-seen with the Rosse
reflector, something like the Owl planetary in Ursa Major.[1144] A
bright patch in the centre seemed to have dark spots on either side, the
whole being surrounded by a lucid annulus or coil. Yet on a plate
exposed at Crowborough, during three hours, no spiral structure emerged
to view, although the nebulous condensations visible in the interior
were judged to be of the kind usually found to accompany
spirality.[1145] The formation, indeed, looks completely amorphous. It
is probably of a non-gaseous nature.
The mutual relationships of many of the nebulæ just described are
doubtless very close. Their elucidation offers a tempting and profitable
field of research. Specimens in some respects anomalous are often the
most instructive to study. Abortive features may be found in them, or
half-developed characteristics, isolated from their accustomed
surroundings, and thereby laid bare to scrutiny. Advantageous
standpoints for comparison and correlation would thus be gained; and
where these are effectively practicable, science cannot miss the path of
progress.
CHAPTER XXXIX.
VARIABLE NEBULÆ.
The occurrence of local changes of brightness is reasonably certain, as
we have seen, in some of the great irregular nebulæ, and may be
suspected in others. Variability affecting smaller objects in their
entirety must then be admitted as possible. No doubt the phenomenon
would introduce ideas difficult to adjust and unexpected; but the
heavens are full of surprises. The immediate question to be put
regarding it is, Does it really subsist? The answer must be given with
extreme circumspection. The visibility of nebulæ depends upon contrast;
the blackness of the sky has as much to do with it as the brightness of
the filmy masses projected against it. They are besides apt to disappear
with high magnification, and that for two reasons. First, because of the
diffusion over a larger area of the same quantity of light; secondly,
because of the restriction of the background in narrower fields of view.
Hence there are drawbacks to the employment of large telescopes in
nebular observation. The history of Tempel’s Merope nebula, marked by
vicissitudes ascribed again and again to intrinsic causes, now fully
recognised as non-existent, is a warning against hasty conclusions on so
delicate a point. The lesson has indeed been so thoroughly learned that
changes of the sort have of late been announced only with a certain
timidity, and under reserve. Caution in the matter can, indeed, hardly
be blamed for exaggeration, in view of Swift’s remark that, after the
Krakatão eruption, many faint nebulæ absolutely disappeared.[1146] Nor
need we go beyond Chacornac’s “temporary nebula” for an exemplification
of optical caprices. On 19th October 1855 the French observer noticed a
striated haze (N.G.C. 1988) attached to the star ζ Tauri, which seemed
to have gained brightness in the ensuing January. No one else, however,
saw it, and it had vanished by 20th November 1862. According to Tempel
it never shone in the sky, but was a telescopic creation—a false image
of an eleventh-magnitude star near ζ Tauri;[1147] and Burnham
unhesitatingly adopts this opinion.[1148] Thirty years later its reality
could have been tested by photographic means; but astronomers in those
days had to rely upon the fallible human retina.
Chacornac’s phantom formation emerged near the site of two genuine
nebular Novæ. On 11th October 1852, Hind detected, close to a star, then
of the tenth magnitude, but since registered as an irregular variable
under the designation T Tauri, a dim, round nebula (N.G.C. 1555) which
brightened steadily until 1856, when it was obvious to general
observation. A comparatively rapid decline ensued. Auwers[1149] could
barely discern the object with the Königsberg heliometer in January
1858; to d’Arrest,[1150] using the eleven-inch Copenhagen refractor, it
was wholly invisible 3rd October 1861; in 1862 it was vainly sought at
Paris and at Malta with Foucault’s and Lassell’s great mirrors; Hind
himself was unable to find it; Secchi, under the pure Roman sky, was
equally unsuccessful; only at Pulkowa it continued to glimmer just
perceptibly for a few months longer. From 1863 the sky in its place
seemed a dead blank. At last, 15th October 1890, Mr. Burnham requested
his colleague, Professor Barnard, to examine the region with the Lick
thirty-six-inch, whereupon a nebulosity about 50″ in diameter, and so
faint as to be at the limit of vision, was detected[1151] (see Fig. 49).
Burnham too saw it, but believed that he could not have done so
independently, his splendid powers of sight being better adapted to the
discernment of concentrated than of diffused light-rays.
The nebula was again observed by Barnard—and with somewhat increased
facility—in February 1895.[1152] Seven months later he was amazed to
find it utterly gone! His search was repeated on three nights, under
supremely good conditions, with the same negative result. And the
object, so far from evading the grasp of large apertures, is peculiarly
fitted for observation with them, owing to its small size and compact
shape. Nevertheless, the forty-inch Yerkes refractor failed to show it
at all in 1897, and barely enabled Barnard to catch a glimpse of it,
28th September 1898.[1153] Finally, Professor Keeler obtained faint
images of it on two Crossley plates, exposed during four hours each, in
December 1899.[1154] A copy of his drawing from them is given in Fig.
50. It exhibits the nebula as composed of three vaguely defined patches,
united by a dim haze, the camera having, as usual, descried structural
complexities inappreciable by the eye. On 20th January 1900, the great
refractor just availed to bring it into view, and it has not since been
heard of. To Professor Keeler it appeared inconceivable that in its
present obscure state it could ever have been seen with small
telescopes; and indeed the evidence of variability is conclusive. Will
it ever recover any of its lost brightness? It may be doubted. The
changes so far undergone by it have been, though comparatively slow,
strictly analogous in character to those of “new” stars; a presumption
hence arises that it will share their fate of permanent extinction.
There is much reason to suppose that Hind noted in 1852 an early stage
of its kindling; that its maximum in 1855–56 was solitary, its
declension irretrievable.
[Illustration:
FIG. 49.—Sketch of T Tauri and Hind’s Nebula, 15th October 1890
(Barnard).
]
Strange to say, the phenomenon was duplicated. While looking fruitlessly
for Hind’s nebula, Otto Struve came upon another unfamiliar object
(N.G.C. 1554) 4′ east of its predecessor’s empty place. This was early
in 1868; and the Nova—for d’Arrest was sure of its previous
non-existence[1155]—was kept in view until 1877, when absolute
obliteration covered it. Even Barnard’s quest for it in 1890, 1895, and
1899 was ineffectual. Its former position is marked in Keeler’s drawing
(Fig. 50) by the thirteenth-magnitude star _b_, but no nebulous
impression was there made upon the plates. Dr. Roberts had indeed
already, in 1890,[1156] vouched for its photographic disappearance. When
last seen by Tempel,[1157] 8th November 1877, the nebula was 90″ across,
and d’Arrest had expressly recorded the presence in it of an
eccentrically situated, though definite nucleus.
[Illustration:
FIG. 50.—Region of T Tauri and Hind’s Variable Nebula. Drawn from
Photographs by Professor Keeler.
]
Hind’s and Struve’s nebulæ were bright enough, as Professor Barnard
recalled wonderingly,[1158] to be ranked, soon after the middle of the
last century, in Herschel’s first and second classes respectively. Both
were watched “and measured by the best observers then living,” and “were
easily visible in ordinary telescopes.” Yet one survives only as a
fitful shimmer; the other is utterly, and probably for ever, quenched.
They are perhaps obscurely connected, and the system—if such it can be
termed—may include the nebulous variable T Tauri; although nebulæ and
star alike seem to fluctuate in complete independence one of the others.
These are the only authentic instances of temporary nebular
developments; but allegations of nebular light-change are common. Some
have proved groundless; not a few, however, rest on a solid substratum
of fact. The following may serve as specimens.
On 17th October 1785 William Herschel discovered, not far from Algol, “a
pretty bright star with two faint branches” (N.G.C. 1186). Sir John
verified the observation, while estimating the star at only fourteenth
magnitude (about twelfth on the modern scale). Yet neither it nor its
appendages could be seen with the Parsonstown reflector, and d’Arrest,
after diligent and repeated search, affirmed decisively, _Nostra ætate
in hac regione tale quid non exstat in cœlo._[1159] The lost object,
nevertheless, came again into view in 1891. On 31st January and 26th
February of that year, Bigourdan, using the thirteen-inch Paris
equatorial, perceived it as a twelfth-magnitude star, with an
unmistakable “fan” of nebulosity spreading from it over 1′ of arc. Only
a fortnight later, Spitaler made a drawing of the object with the Vienna
twenty-seven-inch, and described it as an elongated nebula, not regular
enough to be called elliptical, 2′ in extent, and including a focal star
of the eleventh magnitude.[1160] And Burnham, in the following August,
“readily found in the proper place a _tenth_-magnitude star involved in
a faint elongated nebula” measuring at least 2′ or 3′.[1161] During the
course of 1891, accordingly, the star seems to have been progressively
gaining light and the nebula compass. But if the star only were
variable, the attached nebula would have appeared to shrink and become
effaced as the bright point within it acquired intensity. Its
simultaneous increase could not have been counterfeited. Proof was
afforded by it that the growth, like the previous failures of
luminosity, were due to influences diffused throughout every part of the
formation.
The next variable nebula was a “find” of Barnard’s. It was conspicuous
to him 30th November 1888. He judged it equal to a ninth or
tenth-magnitude star, and remained convinced that its lucidity was of
recent origin. Three years later it had parted with quite four-fifths of
its lustre, and had faded down nearly to evanescence.[1162] It is
situated in Cetus. No information is at hand as to whether its decline
has continued since 1891. Unless arrested, it must, before the century
closed, have carried it out of sight even of the chemical retina, and
the object should then probably be relegated to the class of
“temporaries.” The question is of great interest, and might be answered
by taking one long-exposed photograph with a portrait-lens or a large
reflector.
This is not the only case in which an accession of brightness has been
thought to be demonstrated by the lateness of discovery. A nebula was
encountered by Tuttle in Draco, 1st September 1859 (N.G.C. 6643), which,
in d’Arrest’s opinion, should certainly have been caught in the meshes
of the Herschelian nets unless in their time comparative obscurity had
enveloped it. Similarly, a nebula in Camelopardalis detected by Barnard
in 1889,[1163] and again independently by Denning in 1890,[1164] could
not long, Barnard considered, have been thus readily apparent. He
recommended its being kept under surveillance as a probable variable,
but as yet it has shown no sign of being so;[1165] unless, indeed,
Swift’s earlier observation of the same object, recorded without date by
a simple entry on a star-map,[1166] indicated a previous maximum.
Another instance of a possible rise in the scale of luminosity is
afforded by a small, fairly bright nebula in the Camelopard, first
observed by Denning 30th September 1891, and casually again four times
in the ensuing month. Yet his many previous reviews of the sky-contents
in that neighbourhood had failed to elicit any trace of its existence.
Like Barnard’s and Tuttle’s “new” nebulæ, however, it has apparently
come to stay; and since variability is in sidereal bodies usually an
ineradicable property, the hypothesis of an ascent from invisibility
cannot safely be accepted until a corresponding descent has been entered
upon.
The irregular variability, on the other hand, of two nebulæ adverted to
by Winnecke in 1877–78[1167] is almost incontestable. The first (N.G.C.
3666) is in Leo. Elliptic in shape, in size 90″ by 40″, it was marked
“very bright” by the elder Herschel 15th March 1785, but “extremely
faint,” 23rd March 1830, by the younger, who added the comment, “This
nebula must have changed greatly if ever it belonged really to the first
class.” But its waning was not definitive. Boguslawski inscribed it as a
_bright object_ in 1840 on the Berlin Academy star-map of that region;
and Winnecke found it, 10th April 1878, of unquestionable primary rank.
Yet meantime, in 1863, d’Arrest had described it as _subobscura_, and
manifestly of third-class lustre; while again, on 24th May 1887, Dr.
Dreyer perceived its diminished radiance only with the utmost
difficulty. Its further history remains untold.
Winnecke’s second variable (N.G.C. 955) is an inmate of the crowded
nebular district in Cetus. It consists, Burnham says,[1168] of long,
narrow “nebulous wings on either side of a bright central condensation.”
“On the whole,” he continues, “it is rather a curious object, and should
be easily found and seen.” This was in 1891, and agrees quite well with
Dreyer’s notice of the object in November 1887 as “fully of the second
class.”[1169] The case for change rests upon its invisibility to
Schönfeld in December 1861, and to Vogel in November 1865; although in
1856, 1863, and 1868 it had been seen at a glance by Schönfeld himself,
no less than by d’Arrest and Winnecke.
Winnecke’s nebulæ were at first held by him to be periodical; but this
they certainly are not. No fixed relation to time has so far been shown
to govern nebular fluctuations. They either consist—according to the
best evidence at command—of a solitary maximum, analogous to the
outburst of a new star, or of irregular accessions and losses of light.
No case of cyclical recurrence is on record. Photography is clearly
destined to play an important part in the investigation of this
difficult subject; its aid will be peculiarly welcome where visual
faculties are often baffled, embarrassed, and deceived.
Nebular variability is indeed a phenomenon not only evasive to the
senses, but startling to thought. It cannot be even remotely assimilated
to the light-changes that progress in certain globular clusters; it is
independent of geometrical conditions, of orbital movements, of planes
and periods. Its cause defies conjecture; we can only be sure that it
acts upon a prodigious scale. Thus Hind’s nebula in Taurus measured at
least 2′ across. Its parallax was almost certainly less than one-tenth,
and may not have exceeded one-hundredth of a second. The larger value
would give, for the smallest admissible linear diameter of the object,
1200 astronomical units (radii of the earth’s orbit) or 111,000 million
miles. Centred on the sun, it would extend on every side to twenty times
the distance of Neptune, the equation of light within the vast formation
being six and a half hours. Yet it kindled as a whole, through the
pervading influence of some far-reaching event. Did another dark nebula
sweep through it? We dare not pronounce. Its mysterious brightening,
however, hints at the existence of an indefinite multitude of similar
bodies lurking in the obscurity from which, by some rare chance, it
emerged. It introduces us, in fact, to a realm of invisible nebulæ,
impenetrable by observation, and hence pre-eminently adapted for the
sports of scientific fancy.
Hind’s and Struve’s vanished nebulæ were presumably of gaseous
composition, like the adjacent glow round T Tauri;[1170] Winnecke’s
variable pair doubtless shine with the white light characteristic of the
elliptic family to which they structurally belong. Their remoteness from
the Milky Way points to the same conclusion. Luminous instability does
not then appear to be associated in nebulæ with any special radiative
quality. Those giving continuous, and those giving discontinuous spectra
may equally be affected by it.
CHAPTER XL.
THE NATURE OF NEBULÆ.
The relations of white to green nebulæ are obscure. Unitive links
between the two classes have yet to be established. In most respects
they stand at present widely apart. They present a superficial likeness,
but their dissimilarities seem to be radical. They are unconnected by
any marked spectral affinities; they differ organically in structure;
their distribution on the sphere is regulated by opposite principles.
Hence their genealogical precedence remains unsettled. It would be rash
to say that either family developed from the other, or even that they
are collateral offshoots from a common stock. That a line of continuity
will, sooner or later, become traceable is more than likely, but we must
wait for the guidance of facts with regard to it; premature divinatory
efforts are usually good for less than nothing.
As to the constitution of white nebulæ, we seem on the verge of knowing
something definite. Premonitions of their being a species of
fine-grained star-cluster have become audible. The subject, however, is
not ripe for discussion. A comparatively advanced stage has, on the
other hand, been reached by the problems connected with gaseous nebulæ,
since their spectra let us, to a certain extent, into the secret of
their composition. This, on the whole, seems to be remarkably uniform.
Individual differences, it is true, both physical and chemical,
distinguish the various members of the class; but they are of a
subordinate kind. We may then safely attempt to generalise as regards a
few of their more obvious properties. Three of these can be at once
enumerated:—
(1) Gaseous nebulæ are almost perfectly transparent.[1171]
(2) They shine with extreme feebleness.
(3) Their mass is vanishingly small in proportion to their bulk.
We will take each point separately. That nebulæ offer no appreciable
obstacle to the transmission of light is attested by the unaltered
radiance of stars shining through them. No absorption that can possibly
be due to the cosmic fog in which they are plunged to depths of many
millions of miles, is traceable in the spectra of such objects as Θ
Orionis, of ρ Ophiuchi, of Maia or Merope in the Pleiades. Similarly,
the central stars of planetary nebulæ shine through an interposed
medium, the extent of which is measured by the radius of each gaseous
globe; and this, by a rough minimum estimate, can rarely be less, and
must often be a great deal more than 50,000 to 60,000 million miles. Yet
from the heart of these extraordinary formations the light of their
nuclei comes to us, so far as it is possible to judge, absolutely
intact. The impotence of comets for light-stoppage is thus vastly
enhanced in nebulæ.
The feebleness of their luminosity is a matter of direct observation. A
shining superficies, unlike a shining point, loses none of its lustre
with increased distance. Its area of course diminishes according to the
law of inverse squares, but every minute element of that area continues
to radiate with the same intensity as before. The sun, for instance, is
no less _bright_ as viewed from Neptune than when it crosses the
meridian of Khartoum, but it is 900 times _smaller_. So with the nebulæ.
They are really as faint as they appear. Using the best available data,
Mr. Ranyard arrived at the conclusion that a planetary of the most vivid
kind emits per square mile less than (1)/(22,000) millionth of the light
sent abroad by the solar photosphere. This implies—adopting the result
of Langley’s experiment at Pittsburg—that white-hot iron glows at least
4,000,000 times more powerfully than the bluish disc of the “Saturn” or
the “Owl” nebula. Moreover, the differences in areal lustre between one
nebula and another represent actual varieties of emissive strength.
Remoteness has nothing to do with producing them. A _debilissima_—a
“breath-stain” on the sky—may be as near to us as the great hiatus in
the vault through which Huygens half-imagined the blazing of empyrean
fires.
Finally, nebulæ being prodigiously voluminous and of apparently
insignificant mass, must be of exceedingly low mean density. This
fundamental fact was realised with uncompromising distinctness by Mr.
Ranyard in 1892.[1172] Taking, for illustrative purposes, the Orion
nebula to be a sphere 20′ in diameter, composed uniformly of materials
1,000,000 times rarer than atmospheric air at sea-level, he found that
its mass would be such as to impart, to a body at a distance from its
centre equal to that of α Centauri from ourselves, a circular velocity
of 180, or a parabolic velocity of 255 miles per second. In the
neighbourhood of the nebula, accordingly, there should be a marked
prevalence of large proper motions. A star, travelling across the line
of sight under the influence of its attraction at the rate of 100 miles
a second, would, it was shown, have an annual displacement on the sphere
of no less than 25·5″, and this independently of remoteness. For with
the same angular dimensions, the solid contents of the nebula would
increase as the cube of the distance assumed for it; while the seeming
velocities of bodies in gravitational dependence upon it would undergo
no change. This is rendered obvious by a moment’s consideration. For
take any given star circulating round its centre of gravity at the rate,
let us suppose, of 100 miles a second. And let us further suppose, to
begin with, that the distance of the system is such that light would
spend ten years on the journey thence to our eyes. Let us now double
that distance and follow out the consequences. First, the nebula is
eight times more massive than would have comported with the previous
arrangement. Next, the revolving star is twice as far from it as before,
since the apparent interval has not changed. Whence we easily gather, by
the application of Kepler’s third law, that it now moves with double its
previous speed. But its distance has, by hypothesis, also been doubled;
consequently, its proper, or apparent motion remains just what it was.
If, then, the stars about the Orion nebula were really in swift
circulation, they should appear to be conspicuously progressive. This,
however, is so far from being the case that the region is one of
exceptional fixedness. The spectroscopic information at command is to a
corresponding effect. The six brightest stars of the constellation,
measured at Potsdam in 1892, proved to be all affected, in varying
degrees, by the retreat of our system from that locality of the heavens,
but gave no signs of travelling rapidly on their own account. The
conclusion is inevitable that the Orion nebula—and it may be accepted as
typical—contains inestimably less matter than should be comprised by it
if its average density were that of a Crookes vacuum. Mr. Ranyard,
indeed, assigned to it a consistence not exceeding (1)/(10,000)
millionth that of air at standard pressure, which, he continued, “would
about correspond to the mean density of the solar nebulous mass,
supposing it to have been spherical when its radius was a little more
than 107 astronomical units, or when the sun occupied a sphere with a
radius of a little more than three and a half times the distance of
Neptune.” The potential solar system in those days lay muffled in the
haze of a small planetary nebula.
Yet it is impossible to conceive of nebulæ as formed simply of matter in
an aerial condition. They are no mere vague effusions. They possess
definite and characteristic structure. Lord Rosse,[1173] indeed, thought
sharpness of contours distinctive of the gaseous kind. Mr. Maunder[1174]
speaks of their “strange and complicate shapes, showing here and there
strongly-marked outlines”; and he adverts to the difficulty of
explaining this peculiarity in vast, uncontrolled extensions of rarefied
gas. The abolition of this incongruity was one of the strong points of
Sir Norman Lockyer’s “meteoritic hypothesis” of nebular constitution.
The spectroscope, it is true, pronounced against it; nebular chemistry
has very little in common with the chemistry of “uranoliths”; yet amid
much that was precarious or unsound, the valuable idea was introduced
that a proportion of solid matter must enter into the composition of
nebulæ. Its condition and distribution, however, remain unknown. It
would seem to be devoid of light, for the faint continuous spectrum
accompanying the nebular bright lines would be displayed even by a
homogeneous gaseous mass, unless its radiations were of purely
superficial origin.[1175] But here we meet the unresolved enigma of
nebular luminosity. How do they shine? Is it through the direct agency
of heat? Experimental evidence does not countenance this view. In the
laboratory, hydrogen and helium can be induced to give out their
characteristic rays only under the stress of electrical excitement. The
concomitant high temperature might—as Sir William Huggins pointed out in
1891[1176]—prevail only along the path of the discharge, while the
surrounding gases remained cool, producing inequalities in
heat-distribution similar to those believed to exist in vacuum tubes.
If, on the other hand, nebular emissions were of simply thermal
production, the whole radiating mass should be at nearly the same pitch
of incandescence; for if the temperature were lower in some than in
other of its parts, absorption lines or reversals would betray the fact,
and the nebular spectrum bears no legible marks of selective
light-stoppage. Yet it is eminently improbable that formations so
circumstanced are, in their entirety, excessively hot. Their solid
inclusions should, if they were, glow powerfully, and strong continuous
radiance would replace the dim band, grey through faintness, actually
seen. On the supposition, however, that electrical discharges cause the
glow of nebulæ, their average temperature, judging by Dr. Scheiner’s
experimental results, might approximate to absolute zero.
The word “temperature,” indeed, when applied to matter in the last stage
of attenuation, has an extremely dubious meaning. Taking the kinetic
theory of gases to represent the literal truth, we find the effects of
heating upon them to be twofold, namely, increase (1) of translatory,
(2) of internal energy, the latter being perhaps a consequence of the
former. Since, then, their constituent particles travel faster when heat
is applied, they come into mutual collision more frequently and more
violently. That is to say, if their number per unit volume remains the
same. Let, however, the density of the gas be reduced, other things
being unchanged, what will ensue? There will be fewer encounters in
equal times, but they will _not_ be less violent, since the mean rate of
the molecules has undergone no alteration. Hence, apart from one
qualifying circumstance, the same species of vibration should be set
going as when the gas was denser and equally hot. The qualifying
circumstance is this. During the lengthened intervals between the
collisions, the molecules are radiating—that is, imparting energy to the
ether. If their free paths were sufficiently protracted, they would—if
that be possible—lose all they possess, and need to be fully restocked
at each encounter. The comparative isolation of the molecules would
accordingly result in their being mostly dark, and what we must call
cold; so that the extreme subtlety of matter seems incompatible with a
high temperature in any intelligible sense of the term. But there is
more. Reduction of pressure would have for its immediate result a
general diminution of luminosity. In all probability, it would also
produce a selective effect, for if, as seems likely, certain modes of
molecular vibration are more persistent than others, they would, with
the progress of cooling after each impact, tend to predominate, and the
balance of intensity among the spectral lines of a glowing gas would
thus be sensibly altered. Here, then, an explanation might be sought of
the spectral anomalies of hydrogen in nebulæ. Nevertheless, it does not
run quite smoothly, since up to the limit of possible rarefaction in
vacuum tubes the crimson line of hydrogen gains consistently in
strength.[1177] A corresponding difficulty presents itself in connection
with the helium spectrum.[1178] “Green tubes” are at a higher degree of
exhaustion than “yellow.” Yet in nebulæ the green ray is invisible;
nebular helium radiates only D_{3} with a few members of the same “set.”
By means of artificial exhaustion, accordingly, the nebular spectrum
cannot apparently be imitated. The only experimental clue to its origin,
in fact, is in Professor J. J. Thomson’s observation (already mentioned)
that F and C are of interchangeable intensity at the negative and
positive poles of a hydrogen tube.
To recapitulate. Two fundamental problems regarding gaseous nebulæ press
for solution—one connected with their structural forms, the other with
the nature of their light. Do they continuously fill the spaces they
appear to occupy, or should we figure them to ourselves as collections
of discrete bodies comparatively wide apart? In the former case the dark
gaps and chasms which form one of their leading features should be
ascribed, not to the absence of matter, but to defect of shining power,
illuminative, not architectural contours being disclosed by them. In the
latter, chiaroscuro effects would be reliable indices to the
distribution of material.
Secondly, we are confronted with the mystery of their shining. Why and
how are they lucent? Are they bright as a consequence of thermal or
electrical stimulation, or is their radiance some undefined species of
luminescence or phosphorescence? The answer must be given in view of the
three following peculiarities of their spectra: (1) The leading hydrogen
line in them is F, not C. (2) Helium shines in them as if in a “yellow”
tube. (3) Reversals are absent; lines of absorption make no assured
appearance, either as superimposed upon, or as subjacent to lines of
emission. None of these points are easy to decide. Their consideration
involves doubts and queries of an abstruse nature; but it will not
therefore be neglected. Indeed, such a region of inquiry as is here
presented, where we feel that at every step the Unknown may merge into
the Unknowable, has a particular and an illimitable fascination.
CHAPTER XLI.
THE PHYSICS OF THE MILKY WAY.
The Milky Way is an integral part of the great sidereal system. It marks
the equatorial girdle of a sphere containing stars and nebulæ variously
scattered and aggregated. The whole material creation is, to our
apprehension, enclosed within this sphere. We know nothing of what may
lie beyond. Thought may wander into the void, but observation cannot
follow. And where its faithful escort halts, positive science comes to a
standstill. Fully recognising the illimitable possibilities of
Omnipotence, we have no choice but to confine our researches within the
bounds of the visible world. That it _has_ bounds is evident from the
consideration that it possesses shape and parts. Indefinitely extended,
star-filled space could have neither. Hence it offers to the human mind
an intelligible problem—a problem perhaps too intricate for definitive
solution, yet coming well within range of attack. The siege operations
may be protracted through many a campaign; but in conducting them we
shall climb from peak to peak of the Alpine chain of truth, and gain
continually wider views of the majestic scene that encompasses and
enchants us.
The structural relations of the cosmos may evidently be looked at from
many sides; our immediate concern is with but one of its aspects. We
have only to consider the nature of the materials used for the building
of the edifice and the plan of their apportionment to its different
sections. These materials consist of gaseous and white nebulæ in all
their varieties; of star clusters, globular and irregular, and of the
sundry species of stars; and even a cursory inspection shows that they
are not piled together at random. Each class, on the contrary, obeys its
own law of distribution; and the distribution of sidereal, as of animal
species, is the outcome of their history, a test of their longevity, an
index to their nature. They are _where_ they are, because they are
_what_ they are.
There seemed, twenty years ago, very little reason to anticipate that
the photographic method could ever be used to advantage for
investigating the physics of the Milky Way; yet it has, especially in
Professor Barnard’s hands, proved most effective in that difficult
branch of inquiry. The requisites were peculiar. The galactic drifts are
made up of very small stars—usually of fourteenth to sixteenth
magnitude, or even fainter,—and the sensitive plate perceives them, not,
as the human eye does, collectively, merged into a nebulous surface, but
one by one as light-points. Hence their rays need to be powerfully
concentrated in order to make any chemical effect with a moderately long
exposure. Moreover, the field must be large enough to show the colossal
forms into which these stellar units are grouped. For each picture a
canvas of at least 100 square degrees must be available. Hence
telescopes of the ordinary type, however powerful, are, for this
purpose, entirely useless. An ordinary village photographer’s apparatus
would be better adapted to it. Professor Barnard obtained his remarkable
series of delineations with the “Willard lens,” a doublet of six inches
aperture and thirty-one focal length, constructed at New York in
1859.[1179] They extend over a large part of the Milky Way, but leave
lacunæ, the filling up of which should be the diploma-performance of the
new Bruce lens. It might indeed be supposed that a few specimen
sky-scapes in galactic regions would suffice to afford practical
acquaintance with the entire round; but this is very far from being the
case. No feature of the Milky Way is more surprising than its
inexhaustible variety. No “law of condensation,” such as prevails in
globular clusters, is there traceable. Each section follows its own
method of aggregation. In one, cloud-forms are met with of the cirrus
type; in another, they recall breaking waves or tossing spray;[1180]
again, groups of irregular bright spots alternate with extensive
ramifications and rifts; here the starry fabric is coarse-grained, there
of microscopic fineness; while for heterogeneous scattering in many
quarters, there is substituted in others an apparently designed
arrangement of the stars into rings, chains, and ellipses.
Barnard’s photograph in Ophiuchus, reproduced in Plate XXXI., has
characteristics demanding earnest attention. The bright star below the
middle of the plate encircled by a halation ring is θ Ophiuchi, an
object spectrally akin to Bellatrix, radiantly white and marked by
helium absorption. A “long, dull vacancy,” visible to the naked eye,
stretching east and west below (south of) Theta, comes out as “an
irregular rift in the sheeting of stars,” of highly complex relations.
It circuits the east as well as the north and south sides of the bright
mass in which the star is placed, breaks up to the north into scattered
dark openings, and straggles on to the western edge of the plate,
whence—as other photographs of the series show—it communicates with some
of “the great vacant lanes” in the “wonderful nebulous region about ρ
Ophiuchi.”[1181] The most singular feature, however, of the galactic
chasms visible in our figure is the presence in them of two gradations
of obscurity. Darker details can be made out on the dark background,
recalling the analogy (which did not escape Professor Barnard’s notice)
of the “black holes” in the umbræ of active sun-spots. The conviction
was thus, he added, almost enforced that the Milky Way rests here upon a
far-reaching stratum of nebulous, or quasi-nebulous matter.
[Illustration:
PLATE XXXI.
Photograph of the Milky Way in Ophiuchus (Barnard).
]
The reality, the frequency, and the determinateness of the black
openings in the Milky Way, dimly seen with the naked eye, constitute one
of the most important facts regarding the nature of that formation
attested by the camera. It is true that their great exemplar in the
southern heavens—the “Coalsack” in Crux—seemed, in a photograph taken by
Dr. Russell of Sydney, 13th August 1890, to be embroidered with small
stars over three-fourths of its area;[1182] but this is an unessential
trait of the phenomenon. The fundamental circumstance connected with it
is the extensive and complete perforation of the dense galactic stratum
lying behind the starry network; and of this stratum there is no trace
in the Sydney picture. Only the foreground of the scene is delineated in
it; the backward stretching ranges remained inaccessible with the means
employed. The true Coalsack, then, has not yet been photographed, and we
are ignorant of the precise form which it will take upon the sensitive
plate. The exposures made at Lick, however, effectively reached the
piles of luminous dust which form the ultimate reality of the Milky Way,
and brought into view with insistent clearness the pits and furrows of
almost absolute darkness by which they are frequently interrupted. One
is perpetually reminded, in looking at these autographic records, of
Herschel’s exclamation of amazement when his telescope plunged suddenly
into an unfathomable abyss in Scorpio: _Da ist wahrhaftig ein Loch im
Himmel!_
Now “holes” of the kind have a very wide cosmical significance. The
great rift, by which the entire galactic structure is divided throughout
one-third of its circumference, is only a magnification of innumerable
cracks and fissures yawning amid its component star-masses. And the dark
lanes in globular clusters are plainly of kindred origin. Moreover, the
riddled aspect of the Milky Way in certain of its sections is
continually reproduced both in nebulæ and clusters—in nebulæ, whether
gaseous or stellar, as well as in diverse species of clusters. The fact
is a general one, that in all the forests of the universe there are
glades and clearings. How they come to be thus diversified we cannot
pretend to say; but we can see that the peculiarity is structural—that
it is an outcome of the fundamental laws governing the distribution of
cosmic matter. Hence the futility of trying to explain it as of
incidental origin, as a consequence, for instance, of the stoppage of
light by the interposition of obscure bodies, or aggregations of bodies,
invisibly thronging space. That dark stars exist, singly and in systems,
and dark nebulæ no less, we have been almost inevitably led to conclude;
but the galactic clefts cannot be reckoned among their manifestations.
They are, on the contrary, what they appear to be, intervals of starless
space between neighbouring star-clouds, and suggest processes of
disintegration[1183] advancing with inconceivable slowness towards
unimagined issues. These wonderful collections are then in a state of
flux; they are passing from one condition to another; Supreme Power is
at work in dispersing or refashioning them, sending abroad their
aggregated suns like flying sparks from the anvil.
But are those sparks indeed suns on the scale of our own? The answer
to be given depends upon our estimate of their average distance. They
are undoubtedly very remote. It may be taken as certain that they lie
beyond the sphere occupied by the Durchmusterung stars—that is, by the
ordinarily distributed stars of our system down to 9·5 magnitude. How
much farther beyond, we are, however, unable to say. The thronging
orbs of the Galaxy are by no means of uniform brightness. A recent
inquirer[1184] concludes that they range from the sixth to the
sixteenth magnitude; and the statement, which probably falls short of
the truth, implies that stars enormously disparate in apparent lustre
are at sensibly the same distance from ourselves; hence, that the
crowd of small or medium-sized bodies collected in the Milky Way are
dominated by veritable giants, more sparingly distributed. Professor
Barnard endeavours to bring this state of things into conformity with
received standards by levelling down the dimensions of galactic
constituents.[1185] To the ruck of them he assigns scarcely more than
planetary rank, so as to escape the necessity of admitting fabulous
magnitudes for their leaders. But in truth we have no means of fixing
a scale for such valuations. We know that there are suns hundreds,
even thousands, of times larger than the sun. Why should there not be
others larger still in a similar proportion? _Non est naturæ mensura
hominis electio_, Kepler wrote to Herwart in 1599. Our estimates are,
in many cases, minimum values; we can place no upper limit to the
vastness of the orbs of space. It is only safe to affirm that the
greatest and the least of them are associated and conjoined in the
immense aggregations of “this gorgeous arch, with golden worlds
inlaid, built with Divine ambition.”
The sidereal tribes are very differently affected towards the central
plane of the system. Some, as it were, deliberately withdraw from it;
others are exclusive in their preference for it; many press towards it,
while maintaining a cosmopolitan status. That white stars largely
preponderate in the Milky Way, is a fact made evident by Kapteyn’s
discussion of the Cape Durchmusterung photographs.[1186] It might,
nevertheless, as Mr. Monck of Dublin first noted, be due to their
superior areal brilliancy, which would cause them, with increase of
distance, to come into view preferentially, and at last exclusively. The
percentage of white stars must, indeed, apart from very unlikely
specialties of distribution, augment among the lower magnitudes, since a
star emitting light of the Sirian quality would appear equally bright
when fully twice as far off as one of the same size clothed with a solar
atmosphere. Moreover, Kapteyn’s researches did not extend below the
tenth magnitude, and the genuine galactic particles are of much lower
grades of brightness. So that the nature of their spectra has not, up to
the present, been disclosed to us. Yet a surmise regarding it may be
hazarded. Mr. McClean perceived the bright helium stars included in his
survey to be disproportionately numerous in the Milky Way zone. This is
probably one among several symptoms of their great remoteness. They are
sufficiently removed, perhaps, from the centre of the sidereal sphere to
come within the sweep of the current of tendency setting in towards its
equatorial regions. How it seems probable that fainter specimens of the
type are similarly, but more strongly condensed; and a further step on
the tempting road of conjecture leads us to the inference that the dim
aggregations girdling the heavens are mainly composed of stars of the
Orion family—of stars reversing the duplicate series of hydrogen lines,
besides those of oxygen, helium, nitrogen, and silicon.
We have seen in an earlier chapter that the Milky Way is the chosen
resort of Wolf-Rayet stars. They doubtless belong to it intimately and
entirely; and the same may be said of Novæ. This involves the strange
consequence that, amid the radiant galactic hordes, there must circulate
a multitude of large obscure bodies, fitted on occasion to blaze into
sudden conflagration, on a scale startling to intelligent beholders in
every quarter of the universe. Must we then conclude that dark stars are
relatively more plentiful in the Milky Way than elsewhere? Not
necessarily, perhaps, for its scarcely infringed monopoly in the
production of temporary stars might be explained equally well by the
virtual limitation to it of the conditions needed for luminous
explosions, as by the abundance in it of their appropriate fuel.
Very few gaseous nebulæ have any considerable galactic latitude; they
are characteristically Milky Way objects. There are, indeed, exceptions.
Some noted planetaries—those in Draco and Ursa Major for example—are
situated far outside the zone of concentration, and they present the
appearance of being nearer to the earth than most members of the class.
The “stellar” variety, on the other hand, which are presumably small
through remoteness, are, to the best of our knowledge, limited to the
Milky Way; while irregular nebulæ occur either in the main stream or in
some of its affluents. Among these the Magellanic Clouds are, in a
sense, to be counted, though they should rather be described as pools,
left behind as the waters contracted into their present bed. They are
composed, like the Milky Way, of mixed ingredients, stellar and
nebulous; they seem to reproduce its condition; they bear the same
primitive stamp. Their globular shape, however, suggests an autonomous
constitution, each being probably a self-regulated body; while the
galactic aggregations may be supposed exempt from the efficient control
of a central authority. Dr Russell’s photographs disclosed spiral
tendencies in the Nubeculæ,[1187] destined perhaps to become more and
more pronounced with time.
Through the agency of the portrait-lens and the sensitive plate, the
nebulous affinities of the Milky Way have been more fully recognised
than was possible by visual means alone. Several of Professor Barnard’s
pictures exhibit an intermixture of vaguely diffused lucid matter with
layers of minute stars, especially in parts of Cygnus, Cepheus, Perseus,
Monoceros, and Scorpio. We see, then, that the great cosmic zone is not
only frequented by nebulous objects, but is nebulous in itself. Yet it
seems to repel from it the multitude of white nebulæ which tend to
collect about its poles. This, at least, is the law of their visual
distribution; but the camera threatens to abrogate it. Dr. Max Wolf
holds that the results of his preliminary photographic surveys prove
nebulæ to be in reality scattered pretty evenly over the heavens. Only
their average brightness, he thinks, varies, the so-called “nebular
regions” in Cetus and Virgo having acquired their reputation as such not
because nebulæ are more numerous, but because they are there more
conspicuous than in the intervening celestial tracts. The subject, in
fact, of nebular distribution, which had been supposed practically
disposed of, has been, by photographic explorations, reopened for fresh
discussion.
The crowding of globular clusters upon the Milky Way is unmistakable;
and the inwardness of their relation to its condensations is manifested
by their avoidance of the vacuous rifts, and their adherence to the
stream-lines of luminosity. Hence arises the logical necessity for their
radical separation from white nebulæ, to which they seem, in most
respects, so near akin that one might be led to believe mere difference
of distance to occasion the distinction between resolvable and
irresolvable objects. Only the opposite galactic proclivities of the two
classes decisively place them apart.
Within sight of that ultimate problem, the structure of the sidereal
universe, we pause. Our thoughts meet, but they cannot grapple with it;
nor does it come within the scope of our present purpose to make the
attempt. We must be content to register the marks of growth and change
legible in the Milky Way; to note the evidence of its comparatively
recent origin and inchoate state; to avow our impotence to comprehend
the Supreme design which it is directed to realise; and to bend in awe
and admiration before the unfathomable depths of difficulty and mystery
towards which the study of sidereal development, in its larger bearings,
inevitably leads. _Die Schöpfung_, as Kant discerned, _ist niemals
vollendet_. _Sie hat zwar einmal angefangen, aber sie wird niemals
aufhören._
APPENDIX
TABLE I
STARS WITH VARIABLE SPECTRA
┌───────────────┬────┬───────────────┬────────────────────────────────┐
│ Name of Star. │Mag.│Position, 1900.│ Remarks. │
├───────────────┼────┼──┬──┬─────────┼────────────────────────────────┤
│ „ │ „ │R.│A.│ Dec. │ „ │
├───────────────┼────┼──┼──┼─────┬───┼────────────────────────────────┤
│ │ │h.│m.│ │ │ │
│γ Cassiopeiæ │ 2·3│ 0│51│+ 60°│10′│Hα usually brilliant; at times │
│ │ │ │ │ │ │ invisible. D_{3} │
│ │ │ │ │ │ │ intermittently bright. Further│
│ │ │ │ │ │ │ variations apparent. │
│8 Schjellerup │ 7·2│ 1│12│+ 47°│ 9′│Spectrum continuous, 14th │
│ │ │ │ │ │ │ November 1887; fluted, 6th │
│ │ │ │ │ │ │ October 1891. │
│ο Ceti │var.│ 2│14│− 3°│26′│Hγ and Ηδ triple, 29th August; │
│ │ │ │ │ │ │ single, 6th November 1898. │
│ │ │ │ │ │ │ Iron (?) line at λ 4308, dark │
│ │ │ │ │ │ │ at maxima of 1896 and 1897, │
│ │ │ │ │ │ │ bright in 1898. │
│ψ Persei │ 4·2│ 3│29│+ 47°│52′│Bright Ηβ shifts capriciously │
│ │ │ │ │ │ │ from one to the other side of │
│ │ │ │ │ │ │ a broad absorption-band, on │
│ │ │ │ │ │ │ which it is normally central │
│ │ │ │ │ │ │ (A. C. Maury). │
│11 Monocerotis │ 4·7│ 6│24│ − 6°│58′│Bright Ηβ less refrangible in │
│ │ │ │ │ │ │ 1888–90, more refrangible in │
│ │ │ │ │ │ │ 1891–2 than dark Ηβ. │
│A.G.C. 9181 │ 5·4│ 7│10│− 26°│10′│Ηβ and Hγ vary from dark to │
│ │ │ │ │ │ │ bright, not always in concert │
│ │ │ │ │ │ │ (A. J. Cannon). │
│J Velorum │ 4·4│10│17│− 55°│33′│Ηβ and Hγ intermittently bright │
│ │ │ │ │ │ │ (A. J. Cannon). │
│A.G.C. 14,686 │ 7·0│10│40│− 59°│ 1′│H · lines alternately bright and│
│ │ │ │ │ │ │ dark (_Harvard Circular_, No. │
│ │ │ │ │ │ │ 32). │
│η Centauri │ 2·5│14│29│− 41°│43′│Composite spectrum, subject to │
│ │ │ │ │ │ │ intricate changes, partly │
│ │ │ │ │ │ │ explicable relative │
│ │ │ │ │ │ │ motion-shifts of bright and │
│ │ │ │ │ │ │ dark Ηβ (A. J. Cannon). │
│κ′ Apodis │ 5·6│15│21│− 73°│ 2′│Ηβ variably bright (A. J. │
│ │ │ │ │ │ │ Cannon). │
│R Coronæ │var.│15│44│+ 28°│28′│Spectrum at times continuous, at│
│ │ │ │ │ │ │ times banded. Bright lines │
│ │ │ │ │ │ │ occasionally visible. │
│214 Schjellerup│ 7·0│18│28│− 5°│13′│Spectrum third type, 1889; │
│ │ │ │ │ │ │ second type, 1892. │
│R Scuti │var.│18│42│− 5°│49′│Type III.; bands effaced at │
│ │ │ │ │ │ │ maxima. Unidentified bright │
│ │ │ │ │ │ │ lines recorded by Espin in │
│ │ │ │ │ │ │ 1890. │
│β Lyræ │var.│18│46│+ 33°│15′│Composite spectra with variable │
│ │ │ │ │ │ │ and shifting bright lines. │
│υ Sagitarii │ 4·7│19│16│− 16°│ 8′│Hβ variably bright (A. C. │
│ │ │ │ │ │ │ Maury). │
│ε Capricorni │ 4·5│21│36│− 19°│54′│Composite helium spectrum. │
│ │ │ │ │ │ │ Changes possibly explicable by│
│ │ │ │ │ │ │ motion-shifts of two │
│ │ │ │ │ │ │ dissimilar sets of dark lines │
│ │ │ │ │ │ │ (A. J. Cannon). │
└───────────────┴────┴──┴──┴─────┴───┴────────────────────────────────┘
TABLE II
LIST OF SPECTROSCOPIC BINARIES
┌─────────────┬───────┬─────────┬────────┬──────┬─────────────────────┐
│Name of Star.│ R. A. │ Dec. │ Mag. │Period│ Remarks. │
│ │ 1900. │ 1900. │ │ in │ │
│ │ │ │ │Days. │ │
├─────────────┼───────┼─────────┼────────┼──────┼─────────────────────┤
│ │h. m.│ │ │ │ │
│η Andromedæ │ 0 52│+ 22° 52′│ 4·6│ │Variable radial │
│ │ │ │ │ │ motion discovered │
│ │ │ │ │ │ by Campbell, 1900. │
│ │ │ │ │ │ Spectrum of │
│ │ │ │ │ │ companion faintly │
│ │ │ │ │ │ visible. Solar │
│ │ │ │ │ │ type. │
│Polaris │ 1 22│+ 88° 46′│ 2·1│ 3·97│Variable velocity │
│ │ │ │ │ │ detected by │
│ │ │ │ │ │ Campbell, 1899. │
│ │ │ │ │ │ Spectrum early │
│ │ │ │ │ │ solar. Companion │
│ │ │ │ │ │ dark. Further │
│ │ │ │ │ │ disturbance │
│ │ │ │ │ │ indicated. │
│φ Persei │ 1 37·4│+ 50° 11′│ 4·2│ │Variable velocity │
│ │ │ │ │ │ detected by │
│ │ │ │ │ │ Campbell in 1902. │
│ │ │ │ │ │ Bright-line helium │
│ │ │ │ │ │ spectrum. │
│ξ Piscium │ 1 48│+ 2° 42′│ 4·7│ │Variable velocity │
│ │ │ │ │ │ discovered by │
│ │ │ │ │ │ Campbell, 1900. │
│ │ │ │ │ │ Companion sensibly │
│ │ │ │ │ │ dark. │
│ξ_{1} Ceti │ 2 8│+ 8° 23′│ 4·4│ │Variable velocity │
│ │ │ │ │ │ discovered by │
│ │ │ │ │ │ Campbell, October │
│ │ │ │ │ │ 1900. Companion │
│ │ │ │ │ │ dark. │
│12 Persei │ 2 36│+ 39° 46′│ 4·9│ │Discovered by │
│ │ │ │ │ │ Campbell, January │
│ │ │ │ │ │ 1900. Two similar │
│ │ │ │ │ │ spectra visible at │
│ │ │ │ │ │ elongations. │
│τ Persei │ 2 47│+ 52° 22′│ 4·0│ │Radial velocity found│
│ │ │ │ │ │ variable by │
│ │ │ │ │ │ Campbell in October│
│ │ │ │ │ │ 1900. Spectrum │
│ │ │ │ │ │ previously observed│
│ │ │ │ │ │ as composite by A. │
│ │ │ │ │ │ C. Maury. Solar │
│ │ │ │ │ │ type. │
│β Persei │ 3 2│+ 40° 34′│ var.│ 2·87│Alternations of │
│ (Algol) │ │ │ │ │ approach and │
│ │ │ │ │ │ recession │
│ │ │ │ │ │ discovered by │
│ │ │ │ │ │ Vogel, 1889. │
│ │ │ │ │ │ Companion obscure. │
│ο Persei │ 3 38│ +31° 58′│ 4·0│ │Found by W. S. Adams │
│ │ │ │ │ │ in 1902 to vary in │
│ │ │ │ │ │ radial velocity to │
│ │ │ │ │ │ the extent of 251 │
│ │ │ │ │ │ kil. Single │
│ │ │ │ │ │ spectrum of helium │
│ │ │ │ │ │ type. │
│λ Tauri │ 3 55│ +12° 12′│ var.│ 3·95│Changes of velocity │
│ │ │ │ │ │ measured by │
│ │ │ │ │ │ Bélopolsky, 1897. │
│ │ │ │ │ │ Spectrum │
│ │ │ │ │ │ periodically │
│ │ │ │ │ │ double. Helium │
│ │ │ │ │ │ type. │
│α Aurigæ │ 5 9│ +45° 54′│ 0·2│ 104│Discovered by │
│ (Capella) │ │ │ │ │ Campbell and │
│ │ │ │ │ │ Newall, 1899. │
│ │ │ │ │ │ Spectrum composite.│
│η Orionis │ 5 19·4│ −2° 29′│ 3·5│ │Variable velocity │
│ │ │ │ │ │ detected with the │
│ │ │ │ │ │ Bruce spectrograph │
│ │ │ │ │ │ in 1901 by W. S. │
│ │ │ │ │ │ Adams and E. B. │
│ │ │ │ │ │ Frost. │
│δ Orionis │ 5 27│ −0° 22′│ 2·4│ 1·92│Discovered by │
│ │ │ │ │ │ Deslandres, 1900. │
│ │ │ │ │ │ Spectrum of helium │
│ │ │ │ │ │ type. Companion │
│ │ │ │ │ │ obscure. │
│β Aurigæ │ 5 52│ +44° 56′│ 2·1│ 3·98│Discovered by Miss │
│ │ │ │ │ │ Maury in 1889. │
│ │ │ │ │ │ Spectrum first │
│ │ │ │ │ │ type; doubled │
│ │ │ │ │ │ periodically. │
│η Geminorum │ 6 8·8│ +22° 33′│ 3·2–4·2│ │Variable velocity │
│ │ │ │(period,│ │ detected by │
│ │ │ │ 231^d)│ │ Campbell in 1902. │
│ │ │ │ │ │ Close visual │
│ │ │ │ │ │ companion │
│ │ │ │ │ │ discovered by │
│ │ │ │ │ │ Burnham, 1881. Slow│
│ │ │ │ │ │ revolution │
│ │ │ │ │ │ indicated. The │
│ │ │ │ │ │ variable shows a │
│ │ │ │ │ │ fluted spectrum. │
│ζ Geminorum │ 6 58│ +20° 43′│ var.│ 10·15│Discovered by │
│ │ │ │ │ │ Bélopolsky, 1898. │
│ │ │ │ │ │ Subordinate period │
│ │ │ │ │ │ of 3·38^d detected │
│ │ │ │ │ │ by Campbell. │
│ │ │ │ │ │ Spectrum solar │
│ │ │ │ │ │ type. Companion │
│ │ │ │ │ │ dark. │
│γ Canis │ 7 23│ +9° 8′│ 4·6│ │Discovered by │
│ Minoris │ │ │ │ │ Campbell in 1902 to│
│ │ │ │ │ │ vary in velocity to│
│ │ │ │ │ │ the extent of 8 │
│ │ │ │ │ │ miles per second. │
│α′ Geminorum │ 7 28│ +32° 7′│ 2·0│ 2·95│Discovered by │
│ (Castor) │ │ │ │ │ Bélopolsky, 1896. │
│ │ │ │ │ │ First type │
│ │ │ │ │ │ spectrum. Companion│
│ │ │ │ │ │ obscure. Line of │
│ │ │ │ │ │ apsides found to │
│ │ │ │ │ │ revolve in 2100^d. │
│V Puppis │ 7 55│ −48° 58′│ var.│ 1·45│Periodical doubling │
│ │ │ │ │ │ of lines detected │
│ │ │ │ │ │ by Pickering from │
│ │ │ │ │ │ Arequipa plates in │
│ │ │ │ │ │ 1896. A. W. Roberts│
│ │ │ │ │ │ finds components to│
│ │ │ │ │ │ revolve in contact.│
│ │ │ │ │ │ Helium spectrum. │
│ε Hydræ │ 8 42│ +6° 48′│ 3·6│ │Variable velocity │
│ │ │ │ │ │ detected by │
│ │ │ │ │ │ Campbell, December │
│ │ │ │ │ │ 1900. Solar │
│ │ │ │ │ │ spectrum. │
│ο Leonis │ 9 36│ +10° 21′│ 3·8│ 14·5│Discovered by │
│ │ │ │ │ │ Campbell, 1898. │
│ │ │ │ │ │ Spectrum previously│
│ │ │ │ │ │ noticed by Miss │
│ │ │ │ │ │ Maury to be │
│ │ │ │ │ │ compounded of two │
│ │ │ │ │ │ varieties of the │
│ │ │ │ │ │ Sirian type. │
│ξ Ursæ │11 13│ +32° 6′│ 3·8│ │Principal component │
│ Majoris │ │ │ │ │ of visual binary │
│ │ │ │ │ │ found by Wright in │
│ │ │ │ │ │ 1900 to be │
│ │ │ │ │ │ spectroscopically │
│ │ │ │ │ │ double. Spectrum │
│ │ │ │ │ │ solar. │
│93 Leonis │11 43│ +20° 46′│ 4·6│ │Variable velocity │
│ │ │ │ │ │ detected by │
│ │ │ │ │ │ Campbell, 1900. │
│ξ Ursæ │13 20│ +55° 27′│ 2·4│ 20·6│Discovered by │
│ Majoris │ │ │ │ │ Pickering, 1889. │
│ │ │ │ │ │ Period fixed by │
│ │ │ │ │ │ Vogel, 1901. │
│ │ │ │ │ │ Spectrum advanced │
│ │ │ │ │ │ helium type. │
│ │ │ │ │ │ Components equally │
│ │ │ │ │ │ bright. │
│α Virginis │13 20│ −10° 38′│ 1·2│ 4·0│Discovered by Vogel, │
│ │ │ │ │ │ 1890. Companion │
│ │ │ │ │ │ faintly luminous. │
│ │ │ │ │ │ Helium spectrum. │
│ζ Centuari │13 49│ −46° 47′│ 2·8│ 8·02│Discovered by Mrs. │
│ │ │ │ │ │ Fleming, 1899. │
│ │ │ │ │ │ Components │
│ │ │ │ │ │ unequally bright. │
│ │ │ │ │ │ Spectrum helium │
│ │ │ │ │ │ type. │
│d Boötis │14 6│ +25° 34′│ 4·8│ │Variable velocity │
│ │ │ │ │ │ discovered by │
│ │ │ │ │ │ Wright, April 1900.│
│β Lupi │14 52│ −42° 44′│ 2·7│ │Discovered by Mrs. │
│ │ │ │ │ │ Fleming, 1897. │
│ │ │ │ │ │ Components equally │
│ │ │ │ │ │ luminous. Helium │
│ │ │ │ │ │ spectrum. │
│δ Libræ │14 56│ −8° 8′│ var.│ 2·33│Variable velocity │
│ │ │ │ │ │ detected by Adams, │
│ │ │ │ │ │ 1902. Spectrum │
│ │ │ │ │ │ advanced helium │
│ │ │ │ │ │ type. │
│ε Libræ │15 19│ −9° 57′│ 5·2│ 90+│Discovered by │
│ │ │ │ │ │ Campbell, 1899. │
│ │ │ │ │ │ Solar spectrum. │
│ │ │ │ │ │ Companion obscure. │
│π Scorpii │15 53│ −25° 49′│ 3·1│ 1·57│Discovered by Miss │
│ │ │ │ │ │ Cannon, 1899. │
│ │ │ │ │ │ Helium spectrum. │
│ │ │ │ │ │ Components │
│ │ │ │ │ │ unequally bright. │
│Θ Draconis │16 0│ +58° 50′│ 4·2│ 9│Discovered by │
│ │ │ │ │ │ Campbell, 1899. │
│ │ │ │ │ │ Spectrum solar │
│ │ │ │ │ │ type. Companion │
│ │ │ │ │ │ obscure. │
│β Herculis │16 26│ +21° 42′│ 2·8│ │Variable velocity │
│ │ │ │ │ │ detected by │
│ │ │ │ │ │ Campbell, 1899. │
│ │ │ │ │ │ Companion dark. │
│ │ │ │ │ │ Arcturian spectrum.│
│μ_{1} Scorpii│16 45│ −37° 53′│ 3·3│ 1·45│Discovered by Bailey,│
│ │ │ │ │ │ 1896. Components │
│ │ │ │ │ │ unequally bright, │
│ │ │ │ │ │ perhaps variable. │
│ │ │ │ │ │ Spectra of helium │
│ │ │ │ │ │ type. │
│_h_ Draconis │16 55│ +65° 17′│ 4·7│ │Variable velocity │
│ │ │ │ │ │ detected by │
│ │ │ │ │ │ Campbell, 1899. │
│ │ │ │ │ │ Early solar │
│ │ │ │ │ │ spectrum. Companion│
│ │ │ │ │ │ dark. │
│ε Ursæ │16 56│ +82° 12′│ 4·5│ │Discovered by │
│ Minoris │ │ │ │ │ Campbell, 1899. │
│ │ │ │ │ │ Spectrum solar. │
│ │ │ │ │ │ Companion obscure. │
│ω Draconis │17 38│ +68° 48′│ 4·9│ │Discovered by │
│ │ │ │ │ │ Campbell, 1899. │
│ │ │ │ │ │ Spectrum solar. │
│ │ │ │ │ │ Companion obscure. │
│χ Draconis │18 23│ +72° 42′│ 3·7│ 282│Variable velocity │
│ │ │ │ │ │ detected by │
│ │ │ │ │ │ Campbell, 1898. │
│ │ │ │ │ │ Period computed by │
│ │ │ │ │ │ Wright. Early solar│
│ │ │ │ │ │ spectrum. Companion│
│ │ │ │ │ │ dark. │
│2 Scuti │18 37│ −9° 9′│ 4·8│ │Variable velocity │
│ │ │ │ │ │ detected by Wright,│
│ │ │ │ │ │ 1900. Companion │
│ │ │ │ │ │ dark. │
│β Scuti │18 42│ −4° 51′│ 4·4│ │Variable velocity │
│ │ │ │ │ │ detected by Wright,│
│ │ │ │ │ │ 1900. Companion │
│ │ │ │ │ │ obscure. │
│β Lyræ │18 46│ +33° 15′│ var.│ 12·91│Binary character │
│ │ │ │ │ │ discovered by │
│ │ │ │ │ │ Pickering, 1891. │
│ │ │ │ │ │ Bright-line helium │
│ │ │ │ │ │ spectrum. │
│ │ │ │ │ │ Components │
│ │ │ │ │ │ luminous. │
│113 Herculis │18 50│ +22° 32′│ 4·6│ │Variable velocity │
│ │ │ │ │ │ detected by Wright,│
│ │ │ │ │ │ July 1900. │
│ │ │ │ │ │ Companion obscure. │
│υ Sagittarii │19 16│ −16° 8′│ 4·7│ │Variable velocity │
│ │ │ │ │ │ discovered by │
│ │ │ │ │ │ Campbell, 1899. │
│ │ │ │ │ │ Spectrum noted as │
│ │ │ │ │ │ composite by Miss │
│ │ │ │ │ │ Maury. Of helium │
│ │ │ │ │ │ type with bright │
│ │ │ │ │ │ lines. │
│η Aquilæ │19 47│ +0° 45′│ var.│ 7·18│Variable velocity │
│ │ │ │ │ │ detected by │
│ │ │ │ │ │ Bélopolsky, 1895. │
│ │ │ │ │ │ Spectrum solar. │
│ │ │ │ │ │ Companion dark. │
│ο_{1} Cygni │20 10│ +46° 26′│ 3·8│ │Variable velocity │
│ │ │ │ │ │ detected by │
│ │ │ │ │ │ Campbell, July │
│ │ │ │ │ │ 1900. Spectrum │
│ │ │ │ │ │ previously │
│ │ │ │ │ │ described by Miss │
│ │ │ │ │ │ Maury as including │
│ │ │ │ │ │ Sirian and solar │
│ │ │ │ │ │ ingredients. │
│β Capricorni │20 15│ −15° 5′│ 3·4│ │Variable velocity │
│ │ │ │ │ │ detected by │
│ │ │ │ │ │ Campbell, 1899. │
│ │ │ │ │ │ Spectrum previously│
│ │ │ │ │ │ perceived by Miss │
│ │ │ │ │ │ Maury to be of │
│ │ │ │ │ │ Sirian and solar │
│ │ │ │ │ │ composition. │
│α Equulei │21 11│ +4° 50′│ 4·1│ │Variable velocity to │
│ │ │ │ │ │ the extent of 15 │
│ │ │ │ │ │ miles a second │
│ │ │ │ │ │ discovered by │
│ │ │ │ │ │ Campbell in 1902. │
│ │ │ │ │ │ Miss Maury had │
│ │ │ │ │ │ recorded a │
│ │ │ │ │ │ composite spectrum,│
│ │ │ │ │ │ solar and Sirian. │
│κ Pegasi │21 40│ +25° 11′│ 4·2│ 6±│Smaller member of the│
│ │ │ │ │ │ visual binary found│
│ │ │ │ │ │ by Campbell in 1900│
│ │ │ │ │ │ to be in rapid │
│ │ │ │ │ │ revolution round a │
│ │ │ │ │ │ dark companion. │
│ │ │ │ │ │ Sirian spectrum. │
│ι Pegasi │22 2│ +24° 51′│ 4·0│ │Variable velocity │
│ │ │ │ │ │ detected by │
│ │ │ │ │ │ Campbell, 1898. │
│ │ │ │ │ │ Spectrum of Procyon│
│ │ │ │ │ │ class. Companion │
│ │ │ │ │ │ obscure. │
│δ Cephei │22 25│ +57° 54′│ var.│ 5·37│Binary character │
│ │ │ │ │ │ discovered by │
│ │ │ │ │ │ Bélopolsky, 1894. │
│ │ │ │ │ │ Solar spectrum. │
│ │ │ │ │ │ Companion dark. │
│η Pegasi │22 38│ +29° 42′│ 3·1│ 818·0│Variable velocity │
│ │ │ │ │ │ detected by │
│ │ │ │ │ │ Campbell, 1898. │
│ │ │ │ │ │ Spectrum solar. │
│ │ │ │ │ │ Companion dark. │
│ο Andromedæ │22 57│ +41° 47′│ 3·8│ │Variable velocity │
│ │ │ │ │ │ detected by │
│ │ │ │ │ │ Campbell, 1902. │
│ │ │ │ │ │ Composite │
│ │ │ │ │ │ spectrum—Sirian and│
│ │ │ │ │ │ Orion—recorded by │
│ │ │ │ │ │ Miss Maury. │
│π Cephei │23 5│ +74° 1′│ 4·5│ │Variable motion │
│ │ │ 5 │ │ │ detected by │
│ │ │ │ │ │ Campbell, 1900. │
│ │ │ │ │ │ Companion dark. │
│λ Andromedæ │23 33│ +45° 56′│ 4·0│ 19·2│Variable motion │
│ │ │ │ │ │ detected by │
│ │ │ │ │ │ Campbell, 1899. │
│ │ │ │ │ │ Spectrum Arcturian.│
│ │ │ │ │ │ Companion dark. │
└─────────────┴───────┴─────────┴────────┴──────┴─────────────────────┘
INDEX
Abbe, structures in solar corona, 129
Abbot, measurements of coronal heat, 131, 138
Abney, Sir William, photography of infra-red solar spectrum, 25;
theoretical effects of rotation in stellar spectra, 280
Airy, the solar sierra, 102
Alcyone, a helium star, 194;
red hydrogen line bright in spectrum, 229, 230, 279, 284;
coupled with a dark line, 420;
radiative power, 415;
attendant nebulosities, 418
Aldebaran, spectral character, 183, 207;
parallax and luminosity, 208;
a perspective member of the Hyades, 412
Alexander, contour of Owl nebula, 473;
bi-annular nebula, 490
Algol, density, 192, 303;
spectrum, 194, 197, 283;
supposed colour-change, 256;
probable velocity of rotation, 283;
eclipses, 302;
system, 303–306;
dark satellite, 302, 401
Al-Sûfi, redness of Algol, 256;
magnitude of Θ Eridani, 373
Altair, peculiar spectrum, 280;
rotational hypothesis, 281;
inapplicability, 282, 283;
incipient bright lines, 284
Ambronn, triangulation of the Pleiades, 421
Ames, spectral researches, 34, 53
Anderson, variability of T Andromedæ, 358;
fading of Θ Eridani, 373;
discoveries of Nova Aurigæ, 375;
and of Nova Persei, 388
Andromeda nebula, temporary star in, 385;
spectrum, 439, 449;
spirality, 440;
attendants, 441, 457;
constitution, 450
Ångström, C. J., recognition of hydrogen-absorption in the sun, 52
Ångström, K., bands of telluric absorption traced to carbon dioxide,
24;
atmospheric heat-stoppage, 65
Annular nebulæ, relation to planetaries, 469, 472;
examples, 484–493;
nuclear stars, 486, 489, 490, 493;
spectra, 488, 491;
structures of intermediate character, 490, 493;
desirability of further research, 494
Antares, shows a typical fluted spectrum, 179, 184, 210, 211;
adjacent nebula, 190, 514;
actual brightness, 213, 214
Antarian stars, 179, 184, 209–214;
development, 272, 276, 277, 278;
probable massiveness, 275
Antoniadi, nucleus of ring-nebula in Cygnus, 489
Aquilæ, η, light-change, 321;
orbit, 322, 323
Archenhold, photograph of a nebula in Perseus, 513
Arcturus, spectrum, 182, 183, 205, 207;
luminous power, 205
Argelander, period of ζ Geminorum, 328;
light-curve of β Lyræ, 337;
inequalities of Mira, 348, 349;
of χ Cygni, 352;
period of R Lyræ, 368;
disappearances of R Coronæ, 371
Argon, non-apparent in the sun, 28, 30
Argûs, γ, bright-line spectrum, 187, 238, 239
Astrophysics, prevision of, 1;
development, 2–5;
widening of scope, 5–8;
character and future, 9, 10
Atlas, a suspected binary star, 421
Aurigæ, β, a spectroscopic binary, 291
Auroræ, analogy with solar corona, 139;
abnormal scarcity, 156
Auwers, detection of a nebulous star, 471;
observation of Hind’s variable nebula, 523
Bacon, prevision of astrophysics, 1, 2
Bacon, Miss, coronal photographs, 135
Bailey, double spectrum of μ Scorpii, 292;
of V Puppis, 334;
discovery of cluster-variables, 336, 434;
their mode of light-change, 435, 437;
spectrograph of Nova Normæ, 382;
counts of the Pleiades, 416;
encircling nebulosity, 419;
photograph of ω Centauri, 431
Baillaud and Bourget, photograph of a nondescript nebula, 519
Ball, Sir Robert, parallax of 1618 Groombridge, 206
Balmer, law of series, 52–54
Barnard, photographic investigation of the Milky Way, 7, 539, 540, 544;
observations of cluster-variables, 177, 434, 435;
photographs of a nebula in Monoceros, 190, 425, 426;
of galactic nebulosities, 205, 513, 514;
of Messier 11, 411;
of nebulosities encircling the Pleiades, 418, 420;
of Messier 8, 424;
of fan nebulæ, 447;
of a nebular group, 458;
of nebulous stars, 462, 463, 464, 465, 466;
of nebulous formation in Orion, 497;
detection of stars in trapezium, 230, 495;
observation of Τ Tauri, 372, 373;
nebulous and non-nebulous clusters, 412;
visual discoveries of nebulæ, 417, 474, 487;
portrait-lens photography, 423;
globular clusters non-nebulous, 433;
Stephan’s star non-nebulous, 467;
central star in Lyra nebula, 487;
structure of an annular nebula, 493;
notices of variable nebulæ, 523, 524, 525, 526, 527;
composition of the Milky Way, 542
Baxendell, eclipses of λ Tauri, 307;
minima of S Ursæ Majoris, 353;
periodicity of R Lyræ, 368
Bayer, star-maps of 1603, 371
Becker, telluric spectrum, 23
Bell, nebulous formation round Nova Persei, 395
Bellatrix, a typical helium star, 191
Bélopolsky, solar eclipse of 1896, 128;
correspondence of coronal changes with surface-phenomena, 129;
rotation of faculæ, 145;
of the corona, 147;
nitrogen lines in P Cygni, 235;
system of Castor, 293, 294;
radial motion of λ Tauri, 307;
of δ Cephei, 320;
of ζ Geminorum, 328;
system of β Lyræ, 339, 340
Berberich, effects on Encke’s comet of solar periodicity, 158
Bessel, dark stars, 400
Betelgeux, spectral affinities, 183, 210, 211;
real magnitude, 213, 214;
irregular variability, 369
Bevis, discovery of Crab nebula, 517
Bigelow, magnetic theory of corona, 138, 139;
solar rotation, 143;
modes of propagation of solar energy, 157
Bigourdan, observation of a variable nebula, 526
Birmingham, colours of stars, 257, 258
Boguslawski, record of a variable nebula, 528
Boisbaudrin, discovery of gallium, 28
Bolometer, applied to investigate infra-red solar spectrum, 22;
to measure coronal heat, 131
Boltzmann, law of radiation, 64
Bond, measurement of stars in Orion nebula, 504
Bouguer, darkening of the sun’s disc, 70
Boys, invention of radio-micrometer, 64
Bradley, measures of β Cygni, 264;
of Castor, 293
Brown, Miss, drawing of a spot-group, 79
Burckhalter, graduated coronal photographs, 133
Burnham, division of σ Orionis, 261;
of η Geminorum, 267;
observations of T Tauri, 372;
of Barnard’s Merope nebula, 417;
of double and triple nebulæ, 453, 458;
of nebulous stars, 461, 463, 464, 466, 467;
of planetary nebulæ, 474, 475, 480, 483;
of stellar nebulæ, 481;
of nuclear stars in annular nebulæ, 487, 489, 490;
of variable nebulæ, 523, 526, 528
Burns, magnitudes of galactic stars, 542
Buss, helium absorption in sun-spot spectra, 94
Calcium, isolation of its violet ray for solar photography, 18, 99,
101;
a constituent of the sun, 27;
emission lines in sun-spots, 95;
in faculæ, 100, 101;
in prominences, 110, 114, 115, 120;
in stars, 232;
absence from corona, 136;
absorption by in stellar spectra, 182, 185, 191, 192, 193, 200, 201,
203, 207, 209, 212, 225, 226, 390
Calvert, drawing of the Pleiades nebulosities, 419
Campbell, coronal spectrographs, 131, 132, 136;
bright and dark spectral series, 175, 229, 230, 238, 420;
spectra of bright-line helium stars, 186, 233, 420;
of Wolf-Rayet stars, 237, 239, 242;
spectrum of Rigel, 191;
of Mira, 223–225;
of γ Argûs, 238;
bright lines in fourth-type spectra, 218;
star with a hydrogen envelope, 240, 244;
detection of spectroscopic binaries, 266, 296–298;
orbit of ζ Geminorum, 328–330;
constitution of Nova Aurigæ, 378;
its spectrum, 380, 381;
spectrum of Nova Normæ, 383;
of Nova Centauri, 384;
of Nova Sagittarii, 385;
of Nova Aquilæ, 386;
of planetary nebulæ, 479, 480, 491;
of Orion nebula, 500, 501;
of Trifid nebula, 503;
relative brightness of nebular lines, 476;
description of a bi-annular nebula, 491
Cannon, Miss, bright-line stars, 186, 235, 239;
variable stellar spectra, 250;
spectrum of η Carinæ, 370, _note_
Canopus, nature of spectrum, 182;
remoteness, 204
Cantor, electrically illuminated gases non-absorptive, 140
Capella, spectral relationships, 183, 205;
duplicity, 294–296, 297
Carbon, a solar constituent, 29, 31, 219;
spectral effacement in presence of iron, 34;
condensed in photospheric clouds, 63;
present in the chromosphere, 118;
in stars, 180, 184, 216, 219, 278;
no trace of in Wolf-Rayet spectrum, 242;
bright in R Geminorum, 246
Carbon stars, spectrographed, 177;
nature of spectra, 179, 184, 185, 215–220;
bright lines in, 185, 216–219, 408;
white specimens, 220, 258;
distribution, 221;
affinities, 238;
development, 277, 278;
luminous instability, 358–360
Carinæ, η, nature of spectrum, 235, 370;
light-changes, 370, 371, 406;
position in Key-hole nebula, 507
Carlyle, colour of Sirius, 256
Carrington, movements of sun-spots, 84;
law of the sun’s rotation, 143, 146, 149;
cyclical shiftings of spot-zones, 151
Cassiopeiæ, γ, bright-line spectrum, 186, 233, 234, 279;
spectral fluctuations, 248, 249
Castor, spectrum, 203;
a triple system, 293, 294;
dark companion, 293, 401
Centauri, α_{2}, spectrograph of, 182;
a model sun, 192, 199, 205, 269
Centauri, ω, variable stars in, 333, 436;
telescopic aspect, 429;
central crowding of components, 430
Cephei, δ, a typical short-period variable, 319;
orbital movements, 320, 321, 323;
analogous objects enumerated, 324–326;
their distinctive qualities, 327
Ceraski, discovery of U Cephei, 309
Ceraski, Madame, discoveries of variable stars, 316
Cerium, a solar element, 27, 30;
base of a rare earth, 33
Chacornac, supposed observation of a temporary nebula, 522, 523
Chamberlin, theory of spiral nebulæ, 445
Chandler, movements of the earth’s axis, 159;
disturbances of Algol, 304–306;
phases of eclipsing stars, 311, 314;
discovery of a Geminid variable, 326;
distinctive character of the class, 328;
variations of U Pegasi, 332;
cycle of Mira, 348;
of R Leonis, 356;
redness and periods of variable stars, 358
Chase, parallax of Algol, 306
Chemistry, scope of celestial, 6;
solar, 18, 21, 25–34;
of sun-spots, 91, 163;
of helium stars, 189;
of hydrogen stars, 197;
of Antarian stars, 210;
of carbon stars, 220;
of Mira variables, 226;
of Wolf-Rayet stars, 241
Chevremont, variable stars in the Pleiades, 421
Chromosphere, composition and depth, 16, 102, 109;
deficiency of absorptive power, 47, 161;
spectrum, 112–114, 116, 118, 119, 163
Clark, Alvan G., black markings in prominences, 110;
detection of additional trapezium stars, 230, 495
Cleve, rare metals, 33;
analysis of clevite, 57
Clevite, helium occluded by, 56
Colour variability, elusive of observation, 253;
relation to light-change, 254, 255;
incidental occurrence, 255–258;
in double stars, 259–262
Comets, mimicked by coronal rays, 127;
variation of conspicuousness with the solar condition, 158
Common, photograph of a globular cluster, 433;
variability of components, 434;
photograph of a cometary nebula, 446;
a triple group of nebulæ, 454
Comte, illusory forecast, 1
Copeland, helium-line in spectrum of Orion nebula, 174, 499;
colours of stars, 256, 257;
spectrum of U Orionis, 350;
of U Geminorum, 366;
spectroscopic detection of nebulæ, 480, 482
Cornu, telluric absorption, 22;
spectral series, 53
Corona, problem presented by, 9, 140;
tenuity, 16;
periodical variations, 20, 127–129;
structure, 123–127;
chromospheric relations, 129;
spectrum, 130, 131;
polarisation, 132;
photography, 132–136, 141;
dark markings in, 134;
rotation, 136, 141;
theories regarding, 137–140
Coronium, importance of in solar physics, 52;
subtlety, 60;
probable qualities, 61;
spectrum, 130;
non-absorptive, 140;
an unknown element, 141, 163
Cortie, structure of sun-spots, 76, 77, 80;
their alternations in activity, 85;
average duration, 86;
their spectra, 90, 91;
rotation of faculæ, 146;
spectral anomalies of Nova Persei, 392
Crab nebula, discovery, 517;
spectrum, 518
Crookes, Sir William, meta-elements, 33, 34
Crucis, β, a typical oxygen star, 190, 193
Cygni, β, colours and spectra, 264, 265
Cygni, χ, bright-line spectrum, 226, 227;
colour-changes, 255
Cygni, P, nature of spectrum, 186, 233, 234, 235;
irregular variability, 406
Cysatus, first notice of Orion nebula, 495
Dark stars, problem they present, 176, 177, 189;
probable origin, 276, 277;
coupled with bright stars, 289, 292, 293, 298, 317, 400, 401, 403;
two classes, 403;
galactic relations, 543
Darquier, discovery of annular nebula, 484
D’Arrest, drawing of a nebula, 443;
observations of nebulous stars, 454, 461, 465;
sharpness of 55 Andromedæ, 466;
stellar satellites to nebulæ, 483;
annular nebula, 493;
nondescript nebula, 520;
variable nebulæ, 523, 525, 526
Davidson, black markings in prominences, 110
Dawes, black openings in sun-spots, 75;
colour of δ Cygni, 261
De la Rue, stereoscopic view of a sun-spot, 81
Dembowski, colours of double stars, 260, 261
Deneb (α Cygni), spectral relationships, 193, 197
Denning, observations of a variable nebula, 528
Deslandres, photography of prominences, 9, 18, 98, 103;
selective photography of the sun, 49, 99;
facular flames, 100;
electrical origin of chromospheric luminosity, 115;
ultra-violet prominence spectrum, 119;
the sun as a bright-line star, 121;
coronal spectrograms, 132;
experiments on coronal rotation, 136, 137;
coronal heat, 141;
spectrum of Altair, 281, 284
Dewar, researches at low temperatures, 60
Dissociation, in sun-spots, 90, 96, 163;
of calcium, 114
Doberck, colour of Hind’s star, 257;
of γ Delphini, 260
Doppler’s principle, 4, 19, 106, 297, 390
Double stars, evolution, 176;
variable tints, 259–262;
spectra, 263–270, 275, 276;
indistinguishable from spectroscopic binaries, 297
Draco planetary, helical conformation, 474;
spectrum, 475
Draper Catalogue, 173
Draper, Henry, illusory bright oxygen-lines in solar spectrum, 28
Draper, J. W., illusory dark oxygen-lines in solar spectrum, 28
Dreyer, observation of a double nebula, 454;
non-nebulosity of 55 Andromedæ, 466;
alleged displacement of Omega nebula, 512;
observations of variable nebulæ, 528
Dumb-Bell nebula, analogous objects, 481, 517;
structure, 516;
spectrum, 517
Dunér, spectroscopic measures of the sun’s rotation, 19, 144, 145, 146,
147, 283;
spectra of sun-spots, 89, 90;
spectra of fourth-type stars, 216, 218, 219, 220;
colours of 95 Herculis, 260;
revolutions of Y Cygni, 311, 312;
minima of Z Herculis, 314
Dunlop, a blue cluster, 414
Dyson and Lewis, elongation of Capella, 295
East, refraction in sun-spots, 80
Eberhard, spectrum of χ Cygni, 226
Ebert, temperature of the sun, 66;
theory of the corona, 139, 140
Eclipses, solar, observations during, 9, 19, 20;
disclosure by of reversing layer, 44–46;
of prominences, 109, 120;
of coronal halo, 123, 125–128, 130, 162
Eclipses, stellar, 299–301, 317
Eclipsing stars, mean density, 272, 303, 307, 310, 313, 314;
rotational speed, 282, 283;
enumerated and described, 299, 318;
light-curves, 309, 317;
spectra, 316;
absent from clusters, 438
Eddie, light-change of δ Cephei, 321
Eisig, spectrum of oxygen, 28
Electricity, spectral influences imperfectly understood, 6;
undulatory, 22;
supposed effectiveness in chromospheric illumination, 115;
action of in corona, 137–140
Elkin, parallax of Capella, 295;
measurements of the Pleiades, 415
Ellerman, spectrographs of carbon stars, 185, 216;
of a Wolf-Rayet star, 239
Ellis, solar and magnetic periodicity, 154
Engelmann, colour of δ Cygni, 261
Espin, spectrum of α Herculis drawn by, 183;
catalogue of fourth-type stars, 215;
spectrum of φ Persei, 235;
of R Coronæ, 247;
colours of stars, 255, 256, 257;
light-change of R Cygni, 357;
suggested period of 63 Cygni, 369;
red stars in Perseus cluster, 412
Evershed, photographs of flash spectrum, 46;
constitution of reversing layer, 47, 48;
light from spot-umbræ, 75;
their spectra, 93;
prominence-spectrum, 112, 116, 119;
titanium in the chromosphere, 118
Fabricius, variability of Mira, 348, 349
Faculæ, connection with spots, 74, 101, 151, 153, 161;
status and distribution, 98, 100;
photographed, 99, 100;
rotation, 145–147
Faye, the sun’s rotation, 149
Fenet, map of stars in M 11, 410
Fényi, disturbance of reversing stratum, 50;
depth of chromosphere, 102;
observations of prominences, 103–108, 116
Finlay, parallax of Antares, 213;
magnitude of η Carinæ, 371
Fitzgerald, electrical relations of the solar corona, 140
Flammarion, colours of double stars, 260, 261
Flamsteed, nebulosity of 55 Andromedæ, 466, 467
Flanery, variations of R Scuti, 363, 364
Fleming, Mrs., discoveries and classification of variable stars, 186,
227;
detection of an eclipsing star, 316;
duplicity of β Lyræ, 338;
variability of W Puppis, 352;
of V Delphini, 356;
discovery of Nova Normæ, 382;
of Nova Centauri, 384;
of Nova Sagittarii, 395;
of a planetary nebula, 476;
spectrum of a star in Libra, 512
Fomalhaut, character of spectrum, 201, 202
Fowler, photographs of Indian eclipse, 130
Franks, colours of stars, 257, 260
Fraunhofer, survey of the solar spectrum, 21, 22
Fraunhofer-lines, their interpretation, 13, 17, 21, 26, 29, 34;
motion-displacements, 19, 42, 144;
unidentified residuum, 36;
variability, 37;
effects of pressure on, 38, 51;
structural complexities, 40, 41, 43, 243;
tranquillity of originating strata, 50, 154;
in sun-spots, 93, 146;
in faculæ, 98, 146;
in reflected coronal light, 132;
in spectrum of Andromeda nebula, 440
Frost, level of sun-spots, 79;
thermal power of umbræ, 81, 82;
refraction in the sun, 167
Gadolin, discovery of yttria, 33
Gale, discovery of an annular nebula, 489
Gallium, recognition in the sun, 28, 118;
in α Cygni, 193
Galton, description of solar corona, 123
Gautier, sun-spot and magnetic periods, 3
Geminorum, R, carbon-bands bright in spectrum, 246
Geminorum, U, diffuse aspect, 361;
light-changes, 365, 366
Geminorum, ζ, a typical short-period variable, 319, 328;
orbit, 329, 330;
members of its class, 331–335
Gill, Sir David, Cape Durchmusterung, 173;
spectrograph of α_{2} Centauri, 182, 269;
oxygen in stars, 191;
α_{2} Centauri a replica of the sun, 192, 199;
parallax of Sirius, 198;
of Fomalhaut, 202;
of Canopus, 204;
spectrum of η Carinæ, 235;
photograph of Argo nebula, 507, 508
Gilliss, redness of η Carinæ, 371
Goodricke, variability of β Lyræ, 337
Gore, discovery of U Orionis, 350
Gothard, bright lines in γ Cassiopeiæ, 248, 249;
spectral changes in β Lyræ, 338;
ultra-violet spectrum of Nova Aurigæ, 381;
of planetary nebulæ, 479, 482;
of annulus in Lyra, 488;
of dumb-bell nebula, 517;
photograph of nuclear star in Lyra nebula, 487
Gould, red stars, 258, 261;
variability of RS Sagittarii, 308;
of R Trianguli, 324;
of L_{2} Puppis, 352
Gravity, undiscriminating action, 2;
universality, 3;
maintenance of sun’s heat by, 14;
neutralisation of effects upon solar appendages, 16, 51, 60;
effectiveness in stars, 194, 201, 206, 207;
gain of power with condensation, 272;
influence on stellar temperatures, 274
Groombridge, magnitude of R Cephei, 369
Grosch, corona of 1867, 127
Grover, observations of variable stars, 254, 361
Gruss and Laska, spectrum of R Leonis, 227
Guthnick, long inequality of Mira, 348
Hagen, minimum of U Geminorum, 366;
periodicity of S Persei, 370
Hahn, nuclear star in Lyra annular nebula, 486
Hale, photography of prominences, 9, 18, 99, 110;
duplicity of D_{3}, 57;
carbon in the chromosphere, 118;
spectra of prominences, 118, 119, 121;
prominence-forms in daylight and during eclipses, 120;
magnetic relations of chromosphere, 157;
spectrographs of carbon stars, 177, 185, 216, 220;
bright lines in, 185, 217–219, 238;
spectrum of Nova Persei, 392
Hall, measurement of stars in Præsepe, 412;
invisibility of star in Lyra nebula, 486
Halley, aurora of 1716, 156;
magnitude of θ Eridani, 373
Halm, function of the sun’s smoke-envelope, 72;
effects of solar periodicity upon terrestrial movements, 158;
theory of temporary stars, 398
Hansky, dark markings on prominences, 111;
drawings of coronal types, 128;
coronal structure, 130, 140
Hartley, spectral series, 53;
gallium a chromospheric constituent, 118
Hartley and Ramage, detection of gallium-lines in solar spectrum, 28
Hartwig, minima of Z Herculis, 314;
light-change of S^2 Cygni, 367
Harzer, solar rotation, 148
Hasselberg, investigations of metallic spectra, 26;
vanadium in the sun, 32
Hastings, constitution of solar atmosphere, 71
Helium, a constituent of prominences, 19, 113, 119;
non-absorptive in the sun, 29, 56, 60, 93;
terrestrial discovery, 29, 56, 174;
spectrum, 57, 58, 113, 114;
properties, 59;
absorption in sun-spots, 94, 195;
a constituent of gaseous nebulæ, 174, 477, 478, 480, 499;
absorption in stars, 174, 181, 190, 191, 195, 225, 228;
emission in stars, 175, 186, 187, 226–228, 350;
in Nova Aurigæ, 376;
in Nova Persei, 393;
mixed bright and dark series, 229, 230, 238, 239;
in β Lyræ, 338;
yellow series represented in nebulæ, 536, 537
Helium stars, distinguished by Vogel, 174, 181;
characteristics and varieties, 189–195;
nebular relationships, 189, 190, 272, 277, 279, 462;
distribution, 196, 543;
pass into Sirian stars, 197, 272, 277;
with bright lines, 229–236, 248, 251;
mean density, 272
Helmert, catalogue of stars in M 11, 410;
their variability, 411
Helmholtz, gravitational hypothesis of solar sustentation, 14
Herschel, Alexander, spectral series, 53
Herschel, Caroline, discoveries of nebulæ, 441
Herschel, Sir John, nebulosity round ι Orionis, 190;
colours of stars, 258, 259, 412;
observations of clusters, 414, 425;
nebulosity of 15 Monocerotis, 426;
globular clusters catalogued, 428;
observations of nebulæ, 441, 443, 446, 447, 456, 458;
double nebulæ, 452, 453;
nebulous stars, 463, 464, 465, 466;
Owl nebula, 472;
bi-nuclear planetary, 480;
star satellites to planetaries, 483;
description of Lyra nebula, 484;
planetaries in clusters, 490;
brightness of a star in Orion nebula, 504;
stars in Argo nebula, 507;
drawing of Argo nebula, 508;
multiple star in Trifid nebula, 509, 510;
observation of Omega nebula, 511, dumb-bell nebula, 516;
its miniatures, 517;
observations of variable nebulæ, 526, 528
Herschel, Sir William, discovery of binary stars, 3;
division of 11 Monocerotis, 233;
colours of double stars, 259, 260;
observations of Mira, 348, 349;
nebulous stars, 460, 462, 463;
planetary nebulæ, 471, 481;
annular nebulæ, 484, 489, 493;
aspect of Trifid nebula, 509;
multiple star in, 509, 511;
variable nebulæ observed by, 526, 528;
dark hole in Scorpio, 541
Hevelius, magnitude of R Cephei, 369
Hind, red star, 256;
variations of S Cancri, 306;
of U Geminorum, 365;
detection of T Tauri, 372, 523;
of a temporary nebula, 523
Höffler, parallax of stars in Ursa Major, 290
Holden, polar rays of the corona, 139;
dark lanes in Hercules cluster, 432;
helical nebulæ, 474;
structure of annular nebulæ, 485, 490;
ansæ of Saturn nebula, 492;
suspected changes in Orion nebula, 503;
variability of a star within its compass, 504;
evidence of alteration in Omega nebula, 512;
duplicate nondescript nebulæ, 519
Hornstein, the sun’s rotation, 146
Howlett, conformation of sun-spots, 80, 81
Huggins, Sir William, foundation of spectrography, 3;
application of Doppler’s principle, 4;
daylight coronal photography, 20, 135;
atmospheric absorption, 22;
hydrogen spectrum, 52;
duplicity of D_{3}, 57;
electrical theory of corona, 138;
spectra of double stars, 166, 265, 266;
stellar classification, 180;
constitution of Nova Aurigæ, 378;
spectra of nebulæ, 443, 475, 512, 520;
nebulosity of 55 Andromedæ, 466;
temperature in nebulæ, 535
Huggins, Sir William and Lady, spectrum of calcium, 115;
nitrogen-absorption in stars, 175, 191;
spectrographs of Rigel, 181; of Vega, 182;
of Arcturus, 182;
of α Aquilæ, 282;
effect of gravity on stellar spectra, 201, 274, 276;
spectrographic investigation of trapezium stars, 231, 232;
Wolf-Rayet spectrum, 242;
spectra of double stars, 264, 270;
stellar temperatures, 273;
oxygen in β Lyræ, 338;
unknown ultra-violet nebular ray, 476, 503
Humphreys and Mohler, effects of pressure on spectra, 39
Hussey, observations of Capella, 295;
glimpse of Nova Centauri, 395
Huygens, variability of P Cygni, 234;
notice of Orion nebula, 495
Hyades, a non-nebulous cluster, 412
Hydrogen, ultra-violet spectrum, 3, 52;
a chromospheric constituent, 16, 19, 56, 112, 116, 117;
harmonic relations of spectral lines, 52, 53;
a second series observed in stars, 54, 175, 181, 187, 189, 195, 237,
239, 240, 465;
in nebulæ, 479;
escape from earth’s atmosphere, 55;
spectrum truncated in the sun, 56;
bright in sun-spots, 95;
in faculæ, 101;
absorptive effects in stars, 179, 181, 182, 191, 197, 203, 210;
bright in variables, 185, 222, 352;
in helium stars, 229–335;
in Wolf-Rayet stars, 237–241;
in β Lyræ, 338, 339, 340;
in Nova Aurigæ, 376;
in Nova Persei, 390;
in nebulæ, 476, 478, 488, 500–502, 536, 537;
triple structure of bright lines in Mira, 224, 225, 238
Hydrogen stars, spectral characteristics, 179, 182, 197–202;
development, 272;
temperature, 273
Ingall, structure of a ray nebula, 442;
of an annular nebula, 492
Innes, investigation of eclipsing stars, 316;
magnitude of η Carinæ, 371
Iron, absorption in solar spectrum, 30, 40, 41;
in spot spectra, 90, 92, 96;
in stellar spectra, 182, 184, 191, 210, 212, 220;
spectral prepotency, 34;
bright lines of, in Mira, 225;
in χ Cygni, 226
Janson, discovery of P Cygni, 234
Janssen, solar photographs, 17, 73, 76;
corona of 1871, 125;
Fraunhofer-lines in coronal spectrum, 132
Javelle, measures of a planetary nebula, 483
Jewell, variable Fraunhofer-line, 37;
effects of pressure on wave-length, 38;
structure of Fraunhofer-lines, 40, 41, 163;
titanium-lines in chromospheric spectrum, 118;
rotation of reversing layer, 147
Julius, solar phenomena explained by anomalous refraction, 147
Kant, nature of the creative process, 545
Kapteyn, stellar distribution and spectral diversity, 173, 543;
variability of R^2 Puppis, 316;
progressive illumination of nebulosity round Nova Persei, 395
Kayser, spectral series, 53
Kayser and Runge, metallic spectra, 26
Keeler, import of spectroscopy, 3;
calcium in the sun, 115;
helium-absorption in Rigel, 174;
radial velocities of nebulæ, 177, 470, 475, 483, 493, 499;
spectroscopic indication of high temperature, 181, 201;
bright lines in fourth-type spectra, 218;
spectrum of a Wolf-Rayet star, 239;
hydrogen-envelope of another specimen, 241;
spectrum of γ Cassiopeiæ, 249;
D-lines in β Lyræ, 344;
photographs of T Tauri, 373;
of nebulæ, 417, 443, 444, 487, 492, 493;
of cluster in Hercules, 432;
errors of draughtsmen, 447;
double nebulæ, 452;
rifted nebula, 456;
spectroscopic experiment on rotation of a nebula, 469;
drawings of planetaries, 470, 478, 479, 490;
description of Owl nebula, 473;
Webb’s planetary, 480;
spectrum of a stellar nebula, 481;
structure of Lyra nebula, 485;
central star, 487;
hydrogen spectrum in nebulæ, 499, 500;
screened photograph of Orion nebula, 501;
ultra-violet ray in spectrum, 503;
spectrum of Trifid nebula, 510;
multiple star in, 511;
photographs of Hind’s variable nebula, 524, 525
Kepler, laws enounced by, 1;
prescience of, 2;
letter to Herwart, 542
Key, structure of annular nebula, 493
Key-hole nebula, relations to of scattered stars, 507;
extinction of one of its parts, 508;
spectrum, 509
Kirch, discovery of a globular cluster, 433
Kirchhoff, interpretation of Fraunhofer-lines, 13
Kirchhoff’s law, disregarded by solar helium, 93;
by nebular gases, 462, 467
Klinkerfues, parallax of Mizar, 290
Knopf, optical rationale of reversing layer, 166
Knott, nebulous aspect of U Geminorum, 361
Koch, variability of R Leonis, 355
Köhl, fading of a red star, 374
Konkoly, spectrum of γ Cassiopeiæ, 249;
of U Orionis, 350
Krüger, spectrum of R Leonis, 227;
of R Scuti, 248;
colours of stars, 256–258
Lagoon nebula, involves a cluster, 424
Lalande, observation of S Ursæ Majoris, 353;
of Draco planetary, 474
Lamont, measurements of M 11, 410;
possible temporary star in, 411
Lane, cometary forms in solar corona, 126;
heating of gaseous globes through condensation, 272, 273, 276
Langley, bolometric survey of infra-red spectrum, 22, 24;
selective absorption in solar atmosphere, 64, 70;
heat stoppage in terrestrial atmosphere, 65;
relation of temperature to spectral energy-curve, 66;
heat of sun-spots, 82;
observations during solar eclipses, 123, 127;
multiple star in Trifid nebula, 511;
glow of white-hot iron, 532
Laplace, self-absorption of sunlight, 70;
dark stars, 400
Lassell, star in a double nebula, 454;
description of a planetary nebula, 481;
observations of annular nebulæ, 489, 490, 493;
of Trifid nebula, 509;
multiple star in, 511
Le Chatelier, temperature of the sun, 66
Le Gentil, companion to the Andromeda nebula, 441
Lehmann-Filhés, orbit of β Aurigæ, 291
Lewis, spectra of mixed vapours, 34
Liais, black-edged prominence, 110;
drawing of solar corona, 127
Lindemann, observations of V Cygni, 359, 361
Littrow, triple nebula, 454
Liveing and Dewar, spectrum of liquid oxygen, 23;
heat-relations of magnesium-lines, 49;
spectral series, 53;
spectra of rare atmospheric gases, 114, 120
Lockyer, Sir Norman, application of Doppler’s principle to the sun, 19;
detection in the sun of carbon and vanadium, 31, 32;
disappearance of a Fraunhofer-line, 37;
photographs of flash spectrum, 46;
periodical changes in spot spectra, 89;
solar dissociation, 90, 96, 114;
enhanced lines in chromospheric spectrum, 119;
corona of 1871, 125;
wave-length of green coronal ray, 130;
spectrum of Bellatrix, 191;
of α Cygni, 193;
of α Aquilæ, 281;
of S Sagittæ, 325;
temperature of electric spark, 273;
system of β Lyræ, 339, 340;
meteoric hypothesis, 451, 534
Lockyer, Dr., disturbances of η Aquilæ, 323
Lohse, O., investigation of cerium spectrum, 30;
photographic measures of cluster in Perseus, 411
Looped nebula, peculiar spectrum, 251, 512
Lord, spectrum of a prominence, 116
Luizet, phases of U Vulpeculæ, 324
Lunt, spectrum of ε Canis Majoris, 182;
silicon in stars, 182, 191, 193
Lyræ, β, variability in light, 337;
spectrum, 338;
changes in, 339, 343–345;
system, 338–342, 345;
analogues, 345, 346
Lyra, ring nebula in, 484–488
McClean, photograph of a telluric band, 23;
spectroscopic character of the Milky Way, 173, 196, 543;
oxygen in stars, 175, 232, 239;
stellar classification, 180, 197;
importance of helium stars, 181;
spectral quality of early stars, 189;
spectrograms of γ Cygni, 204;
of Antarian stars, 211;
of η Carinæ, 235;
system of β Lyræ, 339;
spectrum of ι Orionis, 465
Mädler, corona of 18th July 1860, 123;
satellite of Mizar, 291
Magellanic clouds, analogy to Milky Way, 187, 242, 544;
Wolf-Rayet stars contained in, 242;
nebulæ, 512, 517
Magnetism, terrestrial, periodicity coincident with that of sun-spots,
3, 154;
sympathetic response to solar agitations, 156–159
Maia, light-power, 415;
attached nebulosity, 417, 446, 522;
suspected slow light-change, 421;
spectrum, 532
Mason and Smith, observations of Trifid nebula, 509
Maunder, pressure in reversing layer, 49;
helium spectrum, 58;
structure of sun-spots, 83;
duplicated disturbances, 85;
white prominences, 110;
coronal extensions, 134;
coronal photographs, 135;
eruptive nature of prominences, 137;
movements of spot-zones, 152;
solar periodicity, 156, 157;
constitution of nebulæ, 534
Maunder, Mrs., coronal photographs, 134;
nature of prominences, 137
Maury, Miss, stellar classification, 180, 181, 182, 183, 185, 186, 189,
192, 194, 197, 202, 205, 207, 210, 218;
spectrum of Fomalhaut, 201;
composite spectra, 265, 266, 298;
diffuse lines in stellar spectra, 282;
duplicity of β Aurigæ, 291;
system of β Lyræ, 339, 340
Maw, black holes in sun-spots, 76;
submerged bridges, 79
Méchain, discovery of a nondescript nebula, 519
Mendenhall, characterisation of astrophysics, 9
Merope, brightness, 415;
attendant nebulæ, 416, 417;
suspected variability, 421;
spectrum, 532
Messier, description of Crab nebula, 518
Metalloids, in the sun, 29;
facility of spectral effacement, 34
Metals, in the sun, 26, 30;
in stars, 181, 182, 183, 184, 189, 195, 197, 201, 203, 209;
emission by in stellar spectra, 225, 234, 284;
absence from nebulæ, 278
Michelson, temperature and energy spectrum, 66
Milky Way, photographic study of, 7, 539, 540;
a distinctive spectroscopic region, 173, 542;
frequented by Wolf-Rayet stars, 187, 242, 543;
by helium stars, 196, 236, 543;
by carbon stars, 221;
by temporary stars, 396, 402, 543;
by gaseous nebulæ, 398, 506, 544;
by globular clusters, 448, 545;
problem of construction, 538;
dark openings in, 540, 541;
magnitudes of component stars, 542
Mira, character of spectrum, 185, 222, 223;
multiple hydrogen rays in, 224, 225;
radial motion, 224;
redness, 254;
vicissitudes detected, 348;
inconstancy of their method and amount, 349
Mitchell, hydrogen spectrum in Orion nebula, 499
Mizar, a spectroscopic binary, 289;
nature of system, 290
Monck, theory of temporary stars, 379;
preferential visibility of white stars, 543
Moon, projectile character, 2;
photographically studied, 7
Morin, drawing of the corona, 133
Motion-displacements of spectral lines, 4, 5, 286;
in the sun, 19, 38, 42, 105, 108, 109, 283;
in Mira, 224;
by rotation in stars, 280–283;
in binary systems, 288, 289, 291, 292, 295;
in eclipsing pairs, 300, 302, 303, 307;
in short-period variables, 320, 329, 334;
in β Lyræ, 338–341, 343
Müller, F., double nebula, 453
Müller, G. and Kempf, variability of ST Cygni, 325;
of U Vulpeculæ, 334;
of X Persei, 369;
photometric catalogue of the Pleiades, 421
Myers, system of U Pegasi, 332;
of β Lyræ, 341, 342
Naegamvala, photographs of flash spectrum, 46;
helium-absorption in sun-spots, 94
Nebulæ, connected with temporary stars, 384, 394, 395;
photographic surveys, 443, 449;
irregular, 506;
nondescript, 516–521;
variable, 522–530;
distribution, 544, 545
Nebulæ, gaseous, peculiarities of light, 173, 251, 536, 537;
helium emissions from, 174, 175, 491, 499;
radial velocities, 177, 470, 475, 483, 493, 499;
the matrices of stars, 271, 469, 486, 490, 495;
temperature, 502, 535, 536;
feeble luminosity, 532;
tenuity, 533, 534, 536;
meteoric hypothesis of constitution, 534, 535
Nebulæ, white, difficulty of spectral investigation, 174, 439;
spirality, 440–446;
rifted, 447, 448;
avoidance of Milky Way, 448;
double, 452–458;
clustering tendency, 458;
probable composition, 531
Nebular physics, inseparable from stellar physics, 171;
conditions of progress, 173, 174
Nebulium, absorptive incapacity, 465, 532;
denser than hydrogen, 476;
terrestrially unknown, 476, 502
Nebulous stars, luminous stability, 406;
Herschel’s notice of, 460;
distinction from planetary nebulæ, 461;
spectra, 462, 465, 467;
frequently compound, 463–465;
doubtful instances, 463–467;
desirable inquiries, 467, 468;
non-absorptive surroundings, 532
Newall, experiments on coronal rotation, 136;
duplicity of Capella, 294
Newcomb, constitution of the solar corona, 138
Nitrogen, absorption-lines in stellar spectra, 175, 181, 191, 195, 232,
235
Nordenskiöld, notice of clevite, 56
Nova Andromedæ, 385, 411, 450
Nova Aquilæ, nebular transformation, 386
Nova Aurigæ, photographic investigation, 175, 376, 382;
sudden rise, 375;
theories regarding, 377–379;
nebular phase, 379, 381;
light-curve, 380
Nova Carinæ, changes in spectrum, 383
Nova Centauri, apparition, 251;
spectrum, 384
Nova Cygni, nebular transformation, 382
Nova Normæ, photographic discovery, 382;
metamorphosis into a nebula, 383
Nova Persei (1887), spectrographic record of apparition, 384
Nova Persei (1901), sudden rise, 388;
blazing spectrum, 390;
light-curve, 391;
spectral alterations, 392, 393;
nebulous environment, 394, 395
Nova Sagittarii, spectrographic records, 385
Olbers, electrical theory of comets, 3
Omega nebula, conformation, 511;
alleged, change, 512
Oppolzer, solar absorption, 71;
meteorological theory of the sun, 164
Orbinsky, measurements of a globular cluster, 436
Orion nebula, helium-ray in spectrum, 174, 499;
photographic extensions, 190, 495–497;
nuclear star-group, 230–232, 495–497;
scattered stars in, 498;
hydrogen-radiations from, 499–502;
unknown ultra-violet line in spectrum, 502, 503;
suspected variations, 503;
variable stars in, 504;
low mean density, 533
Owl nebula, observational history, 472, 473;
photographs, 473
Oxygen, telluric absorption by, 22, 23;
presence in the sun, 26, 27, 29;
dark lines of in stellar spectra, 175, 181, 191, 195, 232, 239, 241,
338, 465
Packer, variable stars in cluster, 433
Palmer, distribution of stars in Hercules cluster, 432
Parkhurst, phases of Y Boötis, 313, 314;
irregularity of U Geminorum, 366;
light-curve of S^2 Cygni, 367
Paschen, temperature and energy spectrum, 66
Peek, Sir Cuthbert, work at observatory, 254;
light-curve of S Ursæ Majoris, 353;
light-change of R Lyncis, 356
Pereira, description of a sun-spot, 75;
drawing of a sun-spot, 78
Perrine, expansion of nebulosity about Nova Persei, 394
Perrotin, colours of double stars, 259, 261
Perry, Father, dark helium-line in sun-spot spectra, 94
Perry, Professor, masses and temperatures of stars, 274
Peter, measurements of clusters, 189, 426
Photography, advantages, 3, 5;
lunar, 7;
double slit, 9, 18, 50;
solar, 17, 25, 49;
chromospheric, 18, 99, 100, 114;
of reversing layer, 45, 46;
of corona, 132–135;
of temporary stars, 175, 376, 382, 387, 388, 392;
of nebulous stars, 190, 462, 464, 465, 466;
of irregular nebulæ, 205, 507, 508, 509, 511, 513–515;
of Nova Persei nebula, 394, 395;
of Pleiades nebulosities, 416–420;
of white nebulæ, 442–444, 448, 449, 455, 458;
of planetary and annular nebulæ, 472, 484, 487, 490, 493;
of Orion nebula, 495–497;
of variable nebulæ, 525, 529;
of Milky Way, 539–541;
with portrait-lenses, 423, 513
Photometry, its dual aspect, 8;
of the Pleiades, 421
Photosphere, a limiting surface, 15, 16;
temperature, 49, 64–66, 69;
constitution, 62, 63, 69, 70;
problem of formation, 402
Piazzi, nebulosity of 55 Andromedæ, 466, 467
Pickering, E. C., discovery of a second hydrogen series, 54, 237;
general absorption in solar atmosphere, 70;
spectral character of galactic stars, 173, 196;
fifth stellar type, 188;
spectrum of Procyon, 203;
of 11 Monocerotis, 233;
of Wolf-Rayet stars, 239, 240;
composite stellar spectra, 265;
rotation of Altair, 281;
eclipses of Algol, 302, 303;
light-curves of eclipsing stars, 309;
phases of U Cephei, 310;
of W Delphini, 315;
of S Antliæ, 331;
binary character of V Puppis, 334;
of β Lyræ, 338;
light-curve of T Andromedæ, 357;
spectrum of U Geminorum, 366;
of Nova Persei, 392;
photograph of Nova Persei, 388;
spectra of stars in Præsepe, 413;
counts of Pleiades, 416;
surmised cause of cluster-variability, 436;
catalogue of stars in M 5, 438;
detection of stellar nebulæ, 481, 482;
spectrum of Looped nebula, 512
Pickering, W. H., spectrum of white prominences, 110;
photographs of nebulæ, 190;
of Orion trapezium;
of colossal involving nebulosity, 496;
movements of the entangled stars, 497;
estimated distance, 498;
spectrum of Orion nebula, 503
Pigott, discovery of R Scuti, 363;
of R Coronæ, 371
Planetary nebulæ, affinities, 188;
criteria of distinction from nebulous stars, 461, 469;
spheroidal in shape, 469;
radial velocities, 470, 475, 483;
complex structure, 471;
nuclear stars, 473, 474, 532;
spectra, 475–479;
bi-nuclear, 480;
distribution, 544
Plassmann, light-change of λ Tauri, 307
Pleiades, helium spectra, 181, 194, 413, 420;
nebulously involved, 189, 230, 416–420;
general stability in light, 406, 421;
drifting movement, 409, 415;
a typical nebulous cluster, 415;
fewness of real components, 416;
recession from the sun, 420;
centrifugal tendency, 497
Pluvinel, Fraunhofer-lines in coronal spectrum, 132
Pogson, variability of S Ursæ Majoris, 353;
of R Cephei, 369
Polaris, spectrum, 205;
triple system, 298;
constant brightness, 327
Pollux, estimate of mass, 206
Porro, light-curve of U Orionis, 350
Præsepe, measures of components, 412;
their spectra, 413
Pritchard, parallax of β Aurigæ, 291
Proctor, refraction in sun-spots, 80
Procyon, spectral relationships, 182, 203;
mass, 204;
semi-obscure companion, 400
Prominences, chromospheric outgrowths, 16, 19, 101;
photography, 18, 99, 109;
varieties, 102, 103;
movements in, 103, 106–109;
structure, 103, 105, 109;
diametrical situations, 106;
white and black, 110, 120;
spectrum, 115–121;
relations with coronal jets, 129, 130;
ejective character, 137;
movements in latitude, 153
Pupin, coronoidal discharges, 140
Pye, observation of reversing layer, 44
Rambaut, orbit of β Aurigæ, 291
Ramsay, discovery of helium, 29, 56;
its electric conductivity, 59
Ranyard, photograph of a sun-spot, 76;
coronal structures, 126;
coronal types, 128;
central star in Lyra nebula, 487;
conformation of Orion nebula, 496;
allineation of stars in Key-hole nebula, 507;
dimness of nebulæ, 532;
their small mean density, 533, 534
Red stars, Antarian, 179, 183, 209;
carbon type, 179, 184, 215;
variable in tint, 254–258;
in light, 352, 370, 374;
relation of colour-intensity to length of period, 358
Reed, halts in stellar light-change, 359
Refraction, effects of in sun, 165–167
Regulus, a helium star, 184
Reversing layer, disclosure during eclipses, 9, 44;
position in the sun, 16, 46, 161;
photographed, 45, 46;
analysis, 47, 48, 162;
temperature, 49;
tranquillity, 50;
slight pressure in, 51;
optical rationale, 166, 167
Riccò, level of sun-spots, 80
Rigel, absorption in by helium, 174, 181, 191;
by silicon, 193;
luminosity, 192
Ritchey, expanding nebula round Nova Persei, 394
Ritter, star-masses and temperatures, 274, 276
Roberts, A. W., system of RS Sagittarii, 308;
detection and investigation of eclipsing stars, 313, 314, 316;
phases of δ Cephei, 320;
of R Trianguli, 324;
of R^2 Centauri, 331;
of V Puppis, 334;
of S Aræ, 336;
magnitude of η Carinæ, 371
Roberts, Isaac, photographs of a circular nebula, 190, 514;
of clusters, 410, 433, 436;
of the Andromeda nebula, 440;
of other spirals, 441, 442;
of rifted nebulæ, 447, 448;
of a nebulous star, 465;
of Owl nebula, 472, 473;
of annular nebulæ, 489, 490;
of Trifid, 509;
of Omega nebula, 511;
of nondescript nebulæ, 517, 518, 519, 520;
photographic search for a vanished nebula, 525
Robinson, observation of the Owl nebula, 472
Rosetti, solar temperature, 65
Rosse, Lord, arrangement of stars in M 37, 410;
nebulosity of 15 Monocerotis, 426;
perforated cluster, 435;
ray nebula, 442;
spiral nebulæ, 443, 444, 445;
cometary nebula, 446;
observation of 55 Andromedæ, 466;
Owl nebula, 472, 473;
central stars in annular nebulæ, 486, 489, 493;
Crab nebula, 518;
contours of gaseous nebulæ, 534
Rotation, solar, spectroscopically determined, 7, 19, 144, 145;
from spot-movements, 83, 142, 143;
from faculæ, 145, 146;
attempted explanation of anomalies, 147–149;
theoretical effects in stellar spectra, 280, 281;
unapparent to observation, 282–284;
of planetary nebulæ, 469, 470
Rowland, invention of concave gratings, 18;
map of solar spectrum, 25;
solar chemistry, 25, 26, 30, 31, 32;
photographs of metallic spectra, 26, 91;
variable Fraunhofer-line, 37
Rubens, temperature and spectral energy-curve, 66
Runge, duplicity of clevite-ray, 57;
hydrogen-envelope of a star, 241;
spectrum of Orion nebula, 501
Runge and Paschen, oxygen triplet in solar spectrum, 29;
spectral series, 53, 54;
spectrum of helium, 57, 58
Russell, H. C., movements of stars in Jewel cluster, 414;
photographic and visual observations of Key-hole nebula, 507, 508;
photographs of Coalsack region, 540;
of nebulæ, 544
Russell, H. N., density of eclipsing stars, 307, 308, 310
Rydberg, spectral series, 53;
third hydrogen series, 187, 190, 232, 237, 238, 240
Sabine, sun-spot and magnetic periods, 3, 13
Safarik, periodicity of S Persei, 370
Sampson, origin of the solar rotation, 148
Savélieff, solar radiation, 68, _note_
Sawyer, variability of U Ophiuchi, 311;
of R Canis Majoris, 313;
of RX Herculis, 315;
of T Vulpeculæ, 325;
luminous outbursts of R Lyræ, 368;
minimum of R Coronæ, 372
Schaeberle, perspective effects in corona, 124;
coronal photographs, 126, 129;
ejective theory of corona, 137, 165;
conformation of nebulæ, 474, 492;
photograph of a nebula, 515
Scheiner, heat-relations of magnesium-lines, 49, 181, 187, 200, 201;
locus of general absorption in sun, 71;
temperature of the corona, 138;
search for electrical vibrations in sunlight, 159;
carbon bands in stars, 184;
constitution of solar stars, 207;
temperature of electric spark, 273;
incipient brightening of stellar hydrogen bands, 284;
photographic catalogue of stars in M 13, 433;
spectrograph of Andromeda nebula, 439;
photographs of nebulæ, 493, 513;
measurement of stars in Orion nebula, 498;
hydrogen spectrum in nebulæ, 499, 502
Schiaparelli, colour of Sirius, 255
Schjellerup, red stars, 256–258;
observation of 55 Andromedæ, 466
Schmidt, A., optical theory of the sun, 165, 166
Schmidt, J., colour of Algol, 256;
low minimum of S Cancri, 306;
detection of a group of variables, 325;
fluctuations of ζ Geminorum, 328;
periodicity of υ Herculis and R Lyræ, 368;
minima of R Coronæ and of ε Aurigæ, 372
Schönfeld, period of R Lyræ, 368;
variable nebula, 528
Schultz, central star in Lyra nebula, 486
Schumann, constitution of Nova Aurigæ, 378
Schur, light-curve of η Aquilæ, 321;
catalogue of stars in Præsepe, 412
Schuster, spectral series, 53;
variations in the electrical conductivity of space, 158
Schwab, phases of U Sagittæ, 316
Schwabe, discovery of sun-spot period, 13
Schwarzschild, orbit of β Aurigæ, 291
Searle, invisibility of star in bi-annular nebula, 490
Secchi, darkening of sun’s limb, 70;
rosy veils in spot umbræ, 77;
eruptive hypothesis of solar constitution, 165;
localisation of stellar varieties, 173, 179, 181, 185, 186;
bright lines in fourth-type spectra, 185, 216;
spectrum of α Herculis, 211;
of “La Superba,” 217;
of γ Cassiopeiæ, 233, 249;
of β Lyræ, 338;
colours of double stars, 260, 261;
central star in Lyra nebula, 486;
annular nebula in Gemini, 493;
disappearance of Hind’s nebula, 523
See, evolution of double stars, 166, 333, 452;
elements of Sirius, 198;
colour of Sirius, 255;
double nebulæ, 452
Seeliger, selective absorption in the sun, 71;
solar refraction, 167;
encounter-hypothesis of Nova Aurigæ, 377;
theory of stellar outbursts, 379
Shackleton, photograph of flash spectrum, 19, 45
Sidereal physics, defined, 171;
an extension of solar physics, 172;
scope, 177;
connection with sidereal mechanics, 287
Sidgreaves, variable Fraunhofer-line, 37;
convexity of spot-umbræ, 80, 81;
observation of a prominence, 119;
rotation of faculæ, 146;
eruptive hypothesis of solar constitution, 165;
spectrographs of Mira, 185, 223;
of γ Cassiopeiæ, 187;
of β Lyræ, 339;
spectrum of Nova Aurigæ, 379;
of Nova Persei, 390, 391, 392
Silicon, a constituent of the sun, 27, 29;
of stars, 181, 182, 191, 193, 195, 198, 232
Sirius, silicon-absorption in, 193;
spectral character, 197, 200, 201;
mass and brightness, 198, 199;
colour in antiquity, 255, 256;
dusky satellite, 400
Smyth, Admiral, colours of double stars, 259, 260;
gold-dust cluster, 414;
a red satellite star, 464
Smyth, Piazzi, experiments on solar heat-radiation, 68;
colours of 95 Herculis, 260;
redness of η Carinæ, 370
Solà, Comas, triangulation of cluster in Sagittarius, 424
Solar physics, wide range of, 8;
definition, 13;
altered standpoint, 14;
methods and character of progress, 17, 20, 150;
illustrated by sidereal research, 172
Solar stars, characteristics, 182, 205;
transition from hydrogen stars, 203;
giant specimens, 205, 206;
minor orbs, 206, 207;
transition to Antarian stars, 207
Spectra, nebular, difficulty of investigating, 173, 174;
continuous, 439, 440;
gaseous, 475–480, 488, 491;
relative strength of constituent lines, 476, 479, 499–502;
of variable formations, 529
Spectra, stellar, local diversities, 173, 187, 196;
classification, 179–188;
banded, 179, 183, 184, 207, 209–220;
with bright lines, 185–188, 217–220, 222–235, 237–245, 407, 408;
variable, 227, 247–252;
anomalous, 246, 247, 251;
dissimilarly coupled, 263–268, 276;
relations to colour, 263, 270;
evolutionary succession, 271–278;
of Algol variables, 316;
of Cepheid variables, 327;
of Mira variables, 360;
of Novæ, 397;
of clusters, 413, 414;
free from nebular absorption, 532;
galactic types, 543
Spectrography, importance of, 3, 4, 38, 122, 234;
detection of binary systems by, 289;
applied to temporary stars, 376, 384, 385;
to clusters, 413;
to nebulæ, 439, 476, 502
Spectroscopic binaries, discoveries, 176, 289–298;
of dissimilar light-quality, 266, 295;
effects of rotation on, 283;
nature of evidence regarding their movements, 288;
classification, 289
Spectrum, solar, visible section, 21;
infra-red, 24;
photographic, 25;
thronged with absorption-lines, 36;
peculiarities, 37–42, 162;
telluric, 23, 24;
of oxygen, 28;
of helium, 29, 57, 58, 113, 114, 174;
of carbon, 31;
of reversing layer, 44–48;
of hydrogen, 52–54;
of coronium, 60, 130;
of sun-spots, 88–96;
of faculæ, 98, 100;
of chromosphere and prominences, 112–121, 163;
of the corona, 130–132, 163
Spectrum analysis, import of, to astronomy, 3;
to solar physics, 14
Sperra, light-change of S Antliæ, 331
Spica, a spectroscopic binary, 292
Spiral nebulæ, examples, 440–446, 487;
prevalence, 443;
explosive theory, 445;
simulate duplicity, 453
Spitaler, drawings of nebulæ, 456, 457, 458, 526;
measurements of nebulæ, 483;
invisibility of central star in Lyra nebula, 487
Spörer, shiftings of sun-spot zones, 151, 152
Star-clusters, globular, structural plan, 409, 428;
examples, 429–436;
non-nebulous, 433;
nests of variables, 434, 435, 436–438;
frequent the Milky Way, 448
Star-clusters, irregular, conformation, 409;
examples, 410–414;
nebulous, 423–427
Stars, classification, 179;
evolution, 271–279;
rotation, 280–285
Stefan, law of radiation, 64
Stellar nebulæ, limited to the Milky Way, 544
Stephan, nebulous star, 467;
discovery of a nebula, 479
Stone, Ormond, aspect of Orion nebula, 503;
varying brightness of contained stars, 504
Stoney, spectral series, 53;
escape of gases from planetary atmospheres, 55;
composition of photosphere, 63
Stratonoff, rotation of the sun, 83;
of faculæ, 145, 147;
counts of the Pleiades, 416;
nebulosities of the Pleiades, 418
Struve, O., division of γ Andromedæ, 266;
mass of η Cassiopeiæ, 268;
observation of bi-annular nebula, 490;
disappearance of a star in Orion nebula, 504;
discovery of a temporary nebula, 524
Struve, W., colours of double stars, 259, 261;
duplicity of Atlas, 421;
triplicity of 15 Monocerotis, 426
Sun, new views regarding, 14, 15;
appendages, 15;
chemistry, 18, 21, 25–34, 163;
spectral peculiarities, 36–43, 162, 163;
conformation, 62, 161;
general atmospheric absorption, 64, 65, 69, 70, 71;
radiation, 64, 65, 68, 151, 159;
temperature, 65–67, 69;
photospheric veil, 71, 72;
a bright-line star, 121, 154;
mode of rotation, 142–149, 161;
periodicity, 150–160, 162;
a variable star, 151, 362;
theories of constitution, 164–167;
a solitary body, 172, 282;
motion in space, 192, 415;
stage of development, 274
Sun-spots, periodicity, 3, 13, 150–152;
doubts regarding, 9, 86, 87;
spectra, 72, 88–97, 163, 195;
structural features, 73, 75–77;
relations to faculæ, 74, 161;
groupings, 77–79, 85;
question of level, 79–83, 86;
heat-emissions, 81, 82, 164;
movements, 83–85, 96, 142, 144;
dimensions, 85;
chemistry, 91;
scarcity in seventeenth century, 156;
magnetic relations, 154–158;
nature and origin, 162, 164;
refractive effects in, 167
Swift, black lines in prominences, 110;
nebulosity in Monoceros, 425;
hairline nebulæ, 442;
double nebulæ, 455, 457;
nebular groups, 458;
nebulous stars, 462, 463;
new features in Trifid and Omega nebulæ, 522;
observation of a variable nebula, 527
Sykora, measures of the sun’s diameter, 74
Tacchini, daylight extent of chromosphere, 109;
white prominences, 110, 120;
coronal relations of prominences, 129;
magnetic influences of the chromosphere, 157
Telluric absorption, 22–25
Tempel, discovery of Merope nebula, 416, 522;
drawing of a spiral nebula, 443;
Chacornac’s nebula, 523;
observation of Struve’s temporary nebula, 525
Temperature, of reversing layer, 49;
relations of to hydrogen spectrum, 54, 201;
in solar photosphere, 63–67, 69;
in sun-spots, 92, 96, 164;
in solar corona, 138, 140;
in stars, 193, 195, 199–201, 273, 274;
in nebulæ, 502, 535
Temporary nebulæ, 524, 526, 528, 529
Temporary stars, spectral character, 175, 397;
recent apparitions, 375–396;
photographic discoveries, 382–387;
summary of facts regarding, 396–398;
galactic proclivities, 396, 402, 543
Theodorus, observation of η Carinæ, 371
Thome, colour of η Carinæ, 371
Thomson, electrical affinities of hydrogen-lines, 112
Tisserand, disturbances of Algol, 305, 306;
of U Cephei, 310
Titanium, absorption by in the sun, 32, 49;
in spots, 91, 92;
in stars, 192, 193, 198, 226, 232;
bright lines of in flash and chromospheric spectra, 47, 118
Toucani, 47, globular cluster, 431, 436
Trapezium stars, composition of group, 230;
spectrum, 231, 232, 498, 532;
inconspicuously variable, 406;
photographed, 495;
proper motion, 497, 498
Trifid nebula, nuclear multiple star, 406, 510;
conformation, 509, 510;
spectrum, 510
Trouvelot, veiled spots, 102;
observations of prominences, 106, 110
Trowbridge, spectroscopic experiments, 34
Trowbridge and Hutchins, carbon in the sun, 32
Turner, photographs of the corona in polarised light, 132
Tuttle, discovery of a variable nebula, 527
Vanadium, a constituent of the sun, 32, 49;
of prominences, 119;
of Sirius, 198;
prominence of absorption in spot-spectra, 91, 92;
emission in γ Cassiopeiæ, 234, 249
Variable nebulæ, evasive of recognition, 522;
authentic examples, 523–528;
problem of, 529
Variable stars, solar analogy, 151, 176, 362, 406;
their helium emissions, 175, 226, 228;
open to research, 175, 176;
in clusters, 177, 319, 335, 336, 433–438;
with fourth-type spectra, 215, 358–360;
with bright-line fluted spectra, 222–228, 407;
with anomalous spectra, 246–248;
redness, 253–255, 352, 358;
of short period, 319–336, 404;
of long period, 347–362;
irregular, 363–374, 405;
correlation of spectrum and period, 406–408
Variable stellar spectra, an inexplicable phenomenon, 227;
examples, 247–252, 338, 339, 344
Vega, spectrographed, 182;
character of spectrum, 197, 198, 201;
remoteness, 200
Very, photospheric radiation, 67, 68;
nebulous formation round Nova Persei, 395
Vogel, H. C., spectrographic investigation of radial motion, 4;
solar atmospheric absorption, 70, 71;
helium stars, 174, 181;
stellar classification, 180, 277;
carbon stars, 184, 277;
fluted spectra, 211, 277;
spectrum of Mira, 223;
of a Wolf-Rayet star, 239;
of R Geminorum, 246;
of γ Cassiopeiæ, 248;
of β Cygni, 265;
of 95 Herculis, 267;
of Capella, 294;
of β Lyræ, 339, 344;
colour of γ Delphini, 260;
rotation of Altair, 281;
period of Mizar, 290;
duplicity of Spica, 292;
composition of Nova Aurigæ, 378;
radial motion of Nova Persei, 390;
structure of a planetary nebula, 471;
observations of annular nebulæ, 487, 490, 493;
invisibility of a nebula, 528
Vogel, H. W., fifth hydrogen-line, 52
Wadsworth, small apertures for nebular photography, 423, 496
Webb, colours of double stars, 259, 261;
discovery of a planetary nebula, 479
Weiss, coronal structure, 126
Wells, Miss, discovery of W Delphini, 315;
of S^2 Cygni, 366
Wendell, photometric measures of U Pegasi, 332
Wesley, study of, and drawings from coronal photographs, 125, 126, 134,
135
Wiedemann, effects of luminescence, 243, 397
Wilczynski, mode of solar rotation, 148
Williams, Stanley, discoveries of eclipsing stars, 316;
of V Puppis, 334
Wilsing, rotation of faculæ, 145;
of the sun, 148, 149;
absence of Hertzian vibrations in sunlight, 159;
luminescence of temporary stars, 397;
system of 61 Cygni, 403
Wilson, Alexander, perspective effects in sun-spots, 79, 80
Wilson, H. C., structure of Maia nebula, 417;
exterior Pleiades nebulosity, 419
Wilson, W. E., heat of sun-spots, 82;
nebula round Nova Persei, 395;
photographs of nebulæ, 444, 445, 484, 516
Wilson and Gray, determination of the sun’s temperature, 64, 65, 67;
variable radiation, 68
Wilson and Rambaut, arrest of heat in solar and terrestrial
atmospheres, 65, 69
Winkler, detection of germanium, 33
Winnecke, coronal observations, 123, 126;
measures of stars in Præsepe, 412;
variability of 15 Monocerotis, 425;
description of Webb’s nebula, 479;
indications of nebular variability, 528, 529
Wolf, C., map of the Pleiades, 421
Wolf, Max, photographs of galactic nebulosities, 205, 514;
of the place of Nova Aurigæ, 375;
of the Nova Persei nebula, 394;
of the Pleiades nebulosities, 418;
of planetaries, 482;
electrical rationale of Nova Persei nebula, 395;
photographic nebular survey, 449, 544;
a cluster of nebulæ, 458;
nebular distribution, 544, 545
Wolf, R., sun-spot and magnetic periods, 3, 154
Wolf-Rayet stars, metals unapparent in, 175;
spectrographs, 177;
galactic distribution, 187, 242, 543;
nature of spectra, 188, 237–240, 284;
affinities, 195, 217, 218, 246, 278, 440;
specimen with a hydrogen-envelope, 240, 241, 244;
stability in light, 406
Wolfer, solar rotation, 83, 147
Woods, eclipses of S Velorum, 314
Wright, orbit of η Aquilæ, 322;
spectrum of Nova Sagittarii, 385;
of Nova Aquilæ, 386
Yendell, orbit plane of Algol, 303;
light-change of d Serpentis, 335
Young, prominence photography, 18;
spectroscopic measurement of the sun’s rotation, 19, 283;
observation of reversing layer, 19, 44;
description of flash photograph, 45;
thermal variations of the sun, 67;
structure of sun-spots, 77;
their spectra, 88, 91, 93, 94;
helium absorption in, 94;
calcium in the sun, 114;
chromospheric spectrum, 119;
mode of solar rotation, 149;
eruptions in the sun, 165
Zeeman effect, described, 6;
possibly present in spectrum of Mira, 224
THE END
_Printed by_ R. & R. CLARK, LIMITED, _Edinburgh_
-----
Footnote 1:
Fowler, Introduction to _Novum Organum_, p. 38; _Descriptio Globi
Intellectualis_, chaps. v., vii.; _De Augmentis_, iii. 4.
Footnote 2:
_Astrophysical Journal_, vol. vi. p. 273.
Footnote 3:
_The Observatory_, vol. xx. p. 144.
Footnote 4:
The indications of this beautiful instrument, which depend upon the
changes of electrical resistance in a strip of platinum, produced by
differences of temperature, are believed to be reliable to
one-hundred-millionth of a degree centigrade. Langley, _Phil. Mag._
July 1901, p. 123.
Footnote 5:
_Proc. Royal Society_, vol. xlvi. p. 134.
Footnote 6:
_Journal de Physique_, t. x. p. 16.
Footnote 7:
Cortie, _Astronomy and Astrophysics_, vol. xi. p. 399.
Footnote 8:
_Trans. Royal Society of Edinburgh_, vol. xxxvi. part i.
Footnote 9:
_Phil. Mag._ August 1892.
Footnote 10:
A micron = one-thousandth of a millimetre.
Footnote 11:
_Annals of the Smithsonian Astrophysical Observatory_, vol. i. p. 170.
Footnote 12:
_Trans. Swedish Acad. of Sciences_, 1889; _Journ. de Physique_, t. x.
p. 141.
Footnote 13:
_Phil. Trans._ vol. clxxvii. p. 457.
Footnote 14:
_Ibid._ p. 462.
Footnote 15:
Wiedemann’s _Annalen_, Bd. xlvii. p. 208 (1892). See also E. P.
Lewis’s important investigations of “The Wave-Lengths of Infra-Red
Lines,” _Astr. Ph. Journal_, vol. ii. p. 1.
Footnote 16:
Published in its enlarged form in 1889.
Footnote 17:
_Die Spectralanalyse der Gestirne_, p. 177.
Footnote 18:
_Astrophysical Journal_, vol. ix. p. 214.
Footnote 19:
_Astr. and Astrophysics_, vol. xiii. p. 506.
Footnote 20:
_Comptes Rendus_, t. cxi. p. 431.
Footnote 21:
_Astr. and Astrophysics_, vol. xiii. p. 216.
Footnote 22:
Runge and Paschen, _Wiedemann’s Annalen_, Bd. lxi. p. 641 (1897).
Footnote 23:
_Astroph. Journ._ vol. iv. p. 317, vol. vi. p. 426. A second triplet
is also possibly present, but it is less obvious than the first.
Footnote 24:
_Astroph. Journ._ vol. vi. p. 101.
Footnote 25:
_Proceedings Royal Society_, vol. xxvii. p. 308.
Footnote 26:
_American Journ. of Science_, vol. xxxiv. p. 348.
Footnote 27:
Kayser and Runge, _Wied. Annalen_, Bd. xxxviii. p. 80; Crew and
Basquin, _Astroph. Journ._ vol. ii. p. 103.
Footnote 28:
Hartley, _Proc. Royal Society_, vol. lv. p. 348.
Footnote 29:
_Astroph. Journ._ vol. iv. p. 232.
Footnote 30:
_Ibid._ vol. v. p. 194.
Footnote 31:
Moissan, _Le Four Électrique_, pp. 257, 261.
Footnote 32:
_Journal Chemical Society_, 1887, vol. lv. p. 284.
Footnote 33:
Cleve, _Trans. Chem. Society_, 1895, p. 470.
Footnote 34:
_What is Electricity?_ p. 299.
Footnote 35:
_Amer. Journ. of Science_, vol. i. p. 331, 4th series.
Footnote 36:
_Astroph. Journ._ vol. i. p. 89.
Footnote 37:
_Ibid._ vol. x. p. 161.
Footnote 38:
R. A. Porter, _Astroph. Journ._ vol. xv. p. 281.
Footnote 39:
Fowler, _Knowledge_, vol. xxiii. p. 11.
Footnote 40:
_Astr. and Astrophysics_, vol. xi. p. 79.
Footnote 41:
Cortie, _ibid._ p. 591.
Footnote 42:
_Astroph. Journ._ vol. iii. p. 106.
Footnote 43:
_Astroph. Journ._ vol. iii. p. 157.
Footnote 44:
_Ibid._ pp. 92, 93.
Footnote 45:
_Astroph. Journ._ vols. iii. p. 114, iv. p. 175, vi. p. 169.
Footnote 46:
_Ibid._ vol. vi. p. 225.
Footnote 47:
_Astroph. Journ._ vol. xi. p. 240.
Footnote 48:
_Ibid._ vol. iii. p. 100.
Footnote 49:
_Astroph. Journ._ vol. iii. pp. 102, 103 (Jewell), p. 158 (Hale).
Footnote 50:
Jewell, _Astroph. Journ._ vol. xi. p. 236.
Footnote 51:
_Memoirs Royal Astr. Society_, vol. xli. pp. 339, 434 (Ranyard).
Footnote 52:
_Ibid._ p. 115.
Footnote 53:
_The Sun_, ed. 1897, p. 358.
Footnote 54:
Only the more refrangible parts of the portrayed spectra have been
reproduced. The complete images reach down to λ 6000 in the orange.
Footnote 55:
_Phil. Trans._ vol. cxcvii. A. p. 402.
Footnote 56:
S. A. Mitchell, _Astroph. Journ._ vol. xv. p. 118.
Footnote 57:
_Monthly Notices_, vol. lviii. p. 300 (Evershed).
Footnote 58:
Hale, _Astroph. Journ._ vol. iii. p. 160.
Footnote 59:
_Proc. Royal Society_, 17th Jan. 1901.
Footnote 60:
_Knowledge_, vol. ix. p. 50.
Footnote 61:
W. E. Wilson, _Proc. Royal Soc._ 12th Dec. 1901.
Footnote 62:
_Ibid._ vol. xxx. p. 93.
Footnote 63:
_Sitzungsberichte Berlin Acad. der Wiss._ March 1894.
Footnote 64:
_Comptes Rendus_, 9th July 1894.
Footnote 65:
_Publicationen des Haynaldschen Observatoriums_, Heft vi. p. 15, 1892.
Footnote 66:
See _Knowledge_, vol. ix. p. 49 (E. W. Maunder).
Footnote 67:
The third hydrogen line (Hγ), often called “G,” is adjacent to the
band at λ 431 named G by Fraunhofer.
Footnote 68:
_Wiedemann’s Annalen_, Bd. xxv. p. 80.
Footnote 69:
_Phil. Mag._ vol. xli. p. 294 (1871).
Footnote 70:
_Trans. Royal Soc. of Edinburgh_, vol. xxxii.
Footnote 71:
_Journ. Chem. Society_, vol. xliii. p. 390.
Footnote 72:
_Journ. de Physique_, t. v. p. 341 (1886).
Footnote 73:
_Phil. Trans._ vol. clxxiv. p. 187.
Footnote 74:
_Nature_, vol. lv. p. 200.
Footnote 75:
_Phil. Mag._ vol. xxix. p. 331; _Wied. Ann._ Bd. lii. p. 119.
Footnote 76:
_Ibid._ Bd. lxi. p. 641.
Footnote 77:
_Berlin Abhandl._ 1890
Footnote 78:
_Phil. Mag._ vol. xxx. p. 33 (1890).
Footnote 79:
_Astroph. Journ._ vol. v. p. 209.
Footnote 80:
_Ibid._ vol. v. p. 92; Kayser, _ibid._ pp. 95, 243.
Footnote 81:
Rydberg, _ibid._ vol. vi. p. 233.
Footnote 82:
Dewar, _Trans. Chem. Society_, 1898, p. 535.
Footnote 83:
_Astroph. Journ._ vol. vii. p. 25. The idea seems to have been
anticipated by Waterston in 1845. E. Rogovsky, _ibid._ vol. xiv. p.
251, _note_.
Footnote 84:
S. R. Cook, _ibid._ vol. xi. p. 36; but cf. Stoney, _ibid._ pp. 251,
357.
Footnote 85:
Rowland observed in his photographs some “thin haze,” which he
regarded as possibly due to diffuse absorption by four ultra-violet
hydrogen lines, but the connection is very doubtful. Huggins, _Atlas
of Stellar Spectra_, p. 150.
Footnote 86:
_Chemical News_, 29th March 1895; _Nature_, 19th December 1901.
Footnote 87:
_Nature_, 6th June 1895.
Footnote 88:
Their wave-lengths are λλ 7066, 5016, 4472, and 4026.
Footnote 89:
Lockyer, _Nature_, 3rd October 1895.
Footnote 90:
Runge and Paschen, _Nature_, 26th Sept. 1895; _Sitzungsberichte Berlin
Akad._ 20th June 1895; _Astroph. Journ._ vol. iii. p. 4; Maunder,
_Knowledge_, vol. xix. p. 285.
Footnote 91:
Ramsay and Collie, _Proc. Royal Society_, vol. lx. p. 56.
Footnote 92:
_Ibid._ vol. lix. p. 222.
Footnote 93:
See _Nature_, 17th and 24th May 1900 (Cook and Johnstone Stoney).
Footnote 94:
_Fifth Robert Boyle Lecture_, 2nd June 1895.
Footnote 95:
Ramsay, Collie, and Travers, _Journ. Chem. Society_, 1895, p. 697.
Footnote 96:
Collie and Ramsay, _Proc. Royal Society_, vol. lix. p. 264.
Footnote 97:
_The Sun_, p. 110, edit. 1897.
Footnote 98:
_Proc. Royal Society_, vol. xvi. p. 29.
Footnote 99:
Scheiner, _Strahlung und Temperatur der Sonne_, p. 27.
Footnote 100:
_Proc. Royal Society_, vol. lviii.
Footnote 101:
Young, _The Sun_, p. 302.
Footnote 102:
_Phil. Trans._ vol. clxxxv. p. 396.
Footnote 103:
_Phil. Mag._ vol. viii. pp. 324, 550.
Footnote 104:
_Wiedemann’s Annalen_, Bd. xxxix. p. 309.
Footnote 105:
_Proc. Royal Society_, 12th December 1901.
Footnote 106:
_Comptes Rendus_, t. cxiv. p. 737, 1892.
Footnote 107:
_Journ. de Physique_, t. vi. p. 474, 1887.
Footnote 108:
_Wied. Ann._ Bd. liii. p. 284, 1894.
Footnote 109:
_Astroph. Journ._ vols. ii. p. 202, x. p. 40, xi. p. 288.
Footnote 110:
_Ibid._ vol. ii. p. 57.
Footnote 111:
F. W. Very, _Astroph. Journ._ vol. ii. p. 317.
Footnote 112:
_Ibid._ vol. iv. p. 44.
Footnote 113:
The “solar constant” is the number of units of heat per unit of area
which would be received in unit of time by the earth’s surface if its
atmosphere were removed. The most approved value is three (small)
calories per square centimetre per minute, a “small” calorie being the
quantity of heat requisite to raise one gramme of water one degree
centigrade.
Footnote 114:
Hence Savélieff’s experiments, according to which the solar energy
progressively augmented with the increase in the number of spots in
the years 1890, 1891, and 1892, are suggestive rather than conclusive
(_Comptes Rendus_, t. cxviii. p. 62, and _Astroph. Journ._ vol. xiii.
p. 346).
Footnote 115:
_Astroph. Journ._ vol. vii. pp. 255, 264.
Footnote 116:
_Proc. Royal Irish Acad._ vol. ii. p. 299, third series.
Footnote 117:
_Monatsberichte_, Berlin, 1877, p. 104.
Footnote 118:
_Astroph. Journ._ vol. i. p. 261.
Footnote 119:
_Proceedings Amer. Acad. of Sciences_, 1880; _Amer. Journ. of
Science_, vol. xxi. p. 41, 1881.
Footnote 120:
_Strahlung und Temperatur der Sonne_, p. 49.
Footnote 121:
Vol. i. p. 74, 1902.
Footnote 122:
_Astr. Nach._ Nos. 3330, 3410.
Footnote 123:
Rev. W. Sidgreaves, _Astr. and Astrophysics_, vol. xi. p. 212.
Footnote 124:
_Astroph. Journ._ vol. v. p. 250; _Observatory_, vol. xxi. p. 404.
Footnote 125:
_Memoirs Royal Astr. Society_, vol. xli. p. 8 (Ranyard).
Footnote 126:
_Memoirs Brit. Astr. Ass._ vol. v. p. 84.
Footnote 127:
A. J. S. Adams, _Journ. Brit. Astr. Ass._ vol. iv. p. 203; E. Brown,
_ibid._ p. 300.
Footnote 128:
_The Sun_, p. 125.
Footnote 129:
_Il Sole_, p. 62, ed. 1884.
Footnote 130:
Suffusions of dark red matter were observed in the umbræ of a multiple
spot by E. S. Martin, 17th September 1893, _Popular Astronomy_, vol.
i. p. 91.
Footnote 131:
_Memoirs Brit. Astr. Ass._ vol. iv. p. 92.
Footnote 132:
Maunder, _Observatory_, vol. xxi. p. 403.
Footnote 133:
_Ibid._ p. 402, vol. xxiii. p. 233.
Footnote 134:
_Monthly Notices_, vol. lv. p. 73.
Footnote 135:
_Ibid._ vol. lviii. p. 91.
Footnote 136:
_Old and New Astronomy_, p. 382.
Footnote 137:
_Knowledge_, vol. xxi. p. 89.
Footnote 138:
_Astroph. Journ._ vol. vi. p. 91.
Footnote 139:
_Monthly Notices_, vol. lv. p. 282.
Footnote 140:
_Astroph. Journ._ vol. vi. p. 366.
Footnote 141:
_Astr. and Astrophysics_, vol. xi. p. 734; _Astroph. Journ._ vol. iv.
p. 201. Cf. J. Halm, _Annals Edin. Observatory_, vol. i. p. 145.
Footnote 142:
_Monthly Notices_, vol. lv. p. 458.
Footnote 143:
_Ibid._ vol. xxxvii. p. 5.
Footnote 144:
_Astr. and Astrophysics_, vol. xii. p. 739.
Footnote 145:
Frost, _Astroph. Journ._ vol. iv. p. 200.
Footnote 146:
_Journ. Brit. Astr. Ass._ vol. vii. p. 121.
Footnote 147:
Maunder, _Knowledge_, vol. xvii. p. 198.
Footnote 148:
_Observatory_, vol. xxiii. p. 230.
Footnote 149:
_Knowledge_, vol. xvii. p. 199.
Footnote 150:
Cortie, _Observatory_, vol. xxiii. p. 230.
Footnote 151:
_Recherches sur la Rotation du Soleil_, p. 12.
Footnote 152:
Young, _The Sun_, p. 132.
Footnote 153:
_Astr. and Astrophysics_, vol. xi. p. 242.
Footnote 154:
_Proc. Royal Society_, vol. xv. p. 257.
Footnote 155:
_Chemistry of the Sun_, p. 324.
Footnote 156:
_Ibid._ p. 319.
Footnote 157:
_Memoirs Royal Astr. Society_, vol. l. p. 50.
Footnote 158:
_Monthly Notices_, vol. lviii. p. 373.
Footnote 159:
Frost-Scheiner, _Astronomical Spectroscopy_, p. 177; J. S. Ames,
_Astroph. Journ._ vol. i. p. 89; Hartley, _Proc. Royal Society_, vol.
lvi. p. 192.
Footnote 160:
_Monthly Notices_, vol. xlvii. p. 19.
Footnote 161:
_Nature_, 12th Dec. 1872.
Footnote 162:
_Astroph. Journ._ vol. v. p. 248.
Footnote 163:
Gases glowing electrically in vacuum tubes were found by M. Cantor
incapable of absorption. But the results of further experiments must
be awaited before conclusions in so delicate a matter can be availed
of to elucidate the state of helium in the sun.
Footnote 164:
_Nature_, 12th Sept. 1895; _The Sun_, p. 134.
Footnote 165:
_Monthly Notices_, vol. lii. p. 424.
Footnote 166:
_Journ. Brit. Astr. Ass._ vol. ix. p. 253.
Footnote 167:
_Ibid._ vol. vii. p. 191.
Footnote 168:
_Astr. and Astrophysics_, vol. xi. p. 313.
Footnote 169:
Mascari, _Astroph. Journ._ vol. vi. p. 372.
Footnote 170:
_Astr. and Astrophysics_, vol. xi. pp. 159, 414.
Footnote 171:
_Comptes Rendus_, 8th Feb. 1892; _Knowledge_, vol. xvi. p. 230.
Footnote 172:
Deslandres, _L’Astronomie_, Dec. 1894.
Footnote 173:
Deslandres, _Comptes Rendus_, t. cxiv. No. ii.; _Journ. Brit. Astr.
Ass._ vol. ii. p. 237.
Footnote 174:
Evershed, _Astr. and Astrophysics_, vol. xi. p. 240.
Footnote 175:
_Amer. Journ. of Science_, vol. xi. p. 169 (1876).
Footnote 176:
_Publicat. Haynald Observatory_, Bd. vi. p. 40.
Footnote 177:
_Comptes Rendus_, 25th Jan. 1897.
Footnote 178:
_Astr. and Astrophysics_, vol. xiii. p. 124.
Footnote 179:
_Astroph. Journ._ vol. i. p. 212.
Footnote 180:
_Comptes Rendus_, t. ci. pp. 50, 475.
Footnote 181:
Trouvelot, _Bull. Astr._ t. iii. p. 21.
Footnote 182:
Fényi, _Astr. and Astrophysics_, vol. xii. p. 38.
Footnote 183:
Deslandres, indeed, observed velocities to become, as a rule,
accelerated in the upper sections of prominences (_Éclipse du 16
Avril, 1893_, p. 60); but this need not imply that they spring up in a
perfect vacuum.
Footnote 184:
_Astr. and Astrophysics_, vol. xi. p. 63.
Footnote 185:
H. H. Turner, _ibid._ p. 67.
Footnote 186:
Brester, _Théorie du Soleil_, p. 54.
Footnote 187:
Hale, _Astroph. Journ._ vol. iii. p. 377.
Footnote 188:
_Phil. Trans._ vol. clxxx. p. 345.
Footnote 189:
_L’Astronomie_, t. xii. p. 11.
Footnote 190:
_Lick Reports on Total Eclipse of 1st January 1889_, p. 204.
Footnote 191:
_Memoirs Royal Astr. Society_, vol. xli. p. 519 (Ranyard).
Footnote 192:
_Phil. Trans._ vol. cxc. p. 204 (Wesley).
Footnote 193:
_Bull. de l’Acad. Imp. des Sciences_, Mars 1897, p. 253.
Footnote 194:
Frost-Scheiner, _Astronomical Spectroscopy_, p. 189.
Footnote 195:
_Proc. Royal Society_, vol. lviii. p. 255.
Footnote 196:
Some American photographs taken during the eclipse of 28th May 1900
(_Astroph. Journ._ vol. xii. p. 63) show a brightening upward from the
limb, as if through increased rarefaction, of a few lines belonging to
green helium. This behaviour runs in the direction indicated by
vacuum-tube experience.
Footnote 197:
See _ante_, p. 60.
Footnote 198:
_Proc. Royal Society_, vol. lxvii. p. 467.
Footnote 199:
_The Chemistry of the Sun_, p. 194.
Footnote 200:
_Proc. Royal Society_, vol. lxi. p. 433. Besides H and K, _echoes_ of
them survive in a pair situated so high in the ultra-violet that they
can only be photographed with a specially adapted apparatus. Their
wave-lengths are λ 3179 and λ 3159. They may or may not be
chromospheric lines. The records so far obtained do not extend to
their remote position.
Footnote 201:
_Bulletin of the Yerkes Observatory_, No. 4, 1897.
Footnote 202:
_Observations de l’Éclipse Totale du Soleil du 16 Avril_, 1893, p. 64.
Footnote 203:
Hale, _Astroph. Journ._ vol. v. p. 225.
Footnote 204:
Evershed, _Nature_, 9th Sept. 1897.
Footnote 205:
_Astr. and Astrophysics_, vol. xi. p. 738.
Footnote 206:
_Ibid._ p. 430.
Footnote 207:
_The Indian Eclipse_, p. 70.
Footnote 208:
_Astroph. Journ._ vol. xi. p. 243.
Footnote 209:
_Ibid._ p. 165.
Footnote 210:
_Ibid._ vol. vi. p. 412.
Footnote 211:
_Ibid._ vol. x. pp. 112, 287.
Footnote 212:
_Astr. and Astrophysics_, vol. xi. p. 66.
Footnote 213:
_Astr. and Astrophysics_, vol. xi. p. 821.
Footnote 214:
_Proc. Royal Society_, vol. lxviii. p. 396.
Footnote 215:
See a valuable discussion by Hale, _Astroph. Journ._ vol. iii. pp.
374–387.
Footnote 216:
_Phil. Trans._ vol. cxcvii. A, p. 399.
Footnote 217:
_Astr. and Astrophysics_, vol. xi. p. 431; _Comptes Rendus_, 17th Aug.
1891 (Fényi).
Footnote 218:
_Astr. and Astrophysics_, vol. xiii. p. 119.
Footnote 219:
_Comptes Rendus_, 25th July 1892; _Knowledge_, vol. xvi. p. 146 (A. M.
Clerke).
Footnote 220:
_The New Astronomy_, p. 40.
Footnote 221:
_Memoirs Royal Astr. Society_, vol. xli. p. 563 (Ranyard).
Footnote 222:
_Publications Pacific Society_, vol. ii. p. 68.
Footnote 223:
_Memoirs Royal Astr. Society_, vol. xli. p. 656.
Footnote 224:
Ranyard, _ibid._ p. 652.
Footnote 225:
_Ibid._ p. 589 (Pope Hennessy).
Footnote 226:
Perrine, _Lick Bulletin_, No. 9.
Footnote 227:
Ranyard, _Memoirs Royal Astr. Society_, vol. xli. p. 687.
Footnote 228:
_Lick Reports_, p. 193.
Footnote 229:
_Phil. Trans._ vol. cxc. p. 204.
Footnote 230:
_Knowledge_, vol. xii. p. 145, 1889.
Footnote 231:
_Report on the Eclipse_, p. 96.
Footnote 232:
_Ibid._ p. 100; _Observatory_, vol. xvii. p. 350.
Footnote 233:
_Mémoires Acad. de St. Pétersbourg_, t. iv. p. 38, 1862.
Footnote 234:
_Memoirs Royal Astr. Society_, vol. xli. p. 602.
Footnote 235:
_The New Astronomy_, p. 55.
Footnote 236:
_Memoirs Royal Astr. Society_, vol. xlvi. p. 238.
Footnote 237:
Young, _The Sun_, p. 264.
Footnote 238:
_Bull. de l’Acad. de St. Pétersbourg_, t. vi. p. 286.
Footnote 239:
_Atti dell’ Accad. dei Lincei_, 1889, p. 763.
Footnote 240:
Schaeberle, _Report on the Eclipse of 16th April 1893_, pp. 92–98.
Footnote 241:
_Bull. de l’Acad. de St. Pétersbourg_, t. vi. p. 253, 1897.
Footnote 242:
_Nature_, vol. lix. p. 529.
Footnote 243:
_Astroph. Journ._ vol. x. p. 190.
Footnote 244:
S. J. Brown, _Astroph. Journ._ vol. xii. p. 63.
Footnote 245:
_Ibid._ p. 73.
Footnote 246:
Frost-Scheiner, _Astr. Spectroscopy_, p. 192; _Comptes Rendus_, t.
cxvii. p. 25.
Footnote 247:
_Astroph. Journ._ vol. xi. p. 232.
Footnote 248:
Schuster, _Monthly Notices_, vol. xl. p. 35.
Footnote 249:
_Observatory_, vol. xxi. p. 157.
Footnote 250:
Burnham and Schaeberle, _Report on Eclipse of December 1889_, p. 38.
Footnote 251:
_Bull. de l’Acad. de St. Pétersbourg_, t. iv. p. 275.
Footnote 252:
_Popular Astronomy_, vol. viii. p. 369.
Footnote 253:
_The Indian Eclipse_, p. 114.
Footnote 254:
_Ibid._ p. 118.
Footnote 255:
Wesley, _Knowledge_, Oct. 1900, p. 227.
Footnote 256:
Maunder, _Knowledge_, vol. xxi. p. 109. Cf. the visual observations of
Seagrave in 1900, _Astroph. Journ._ vol. xii. p. 99.
Footnote 257:
Hale, _Astr. and Astrophysics_, vol. xiii. p. 662.
Footnote 258:
_Observations de l’Éclipse du 16th Avril 1893_, p. 43.
Footnote 259:
_The Observatory_, vol. xxi. p. 188.
Footnote 260:
_Astroph. Journ._ vol. x. p. 186.
Footnote 261:
_Loc. cit._ p. 50.
Footnote 262:
_Total Solar Eclipse_, Dec. 1889, p. 47.
Footnote 263:
_The Indian Eclipse_, p. 121.
Footnote 264:
_Popular Astronomy_, p. 260 (edit. of 1878).
Footnote 265:
_Astroph. Journ._ vol. xii. p 25.
Footnote 266:
_Proc. Royal Society_, vol. xxxix. p. 108.
Footnote 267:
Deslandres, _loc. cit._ p. 62.
Footnote 268:
_The Solar Corona discussed by Spherical Harmonics_, 1889.
Footnote 269:
_Public. Pacific Society_, vol. iii. p. 216.
Footnote 270:
_Astr. and Astrophysics_, vol. xii. p. 804.
Footnote 271:
_Magnetic Fields of Force_, p. 73 (1897).
Footnote 272:
_Reports on Eclipse of 1st January, 1889_, p. 10.
Footnote 273:
_Bull. de l’Acad. des Sciences de St. Pétersbourg_, t. vi. March 1897.
Footnote 274:
_Astr. and Astrophysics_, vol. xi. p. 483.
Footnote 275:
_Amer. Journ. of Science_, vol. xi. p. 253, 1901.
Footnote 276:
_Annalen der Physik_, 1900, p. 462; quoted by G. F. Fitzgerald,
_Nature_, vol. lxii. p. 7.
Footnote 277:
_Comptes Rendus_, t. cxxx. p. 1691; _Nature_, 5th July 1900.
Footnote 278:
Bigelow, _Astr. and Astrophysics_, vol. xii. p. 823.
Footnote 279:
_Astr. and Astrophysics_, vol. xii. p. 825.
Footnote 280:
_History of Astronomy_ (A. M. Clerke), 4th edit. p. 202.
Footnote 281:
_Recherches sur la Rotation du Soleil_, p. 55.
Footnote 282:
_Potsdam Publ._ Bd. iv. pt. ii.; _Astr. Nach._ Nos. 3000, 3153, 3287.
Footnote 283:
_Astr. and Astrophysics_, vol. xii. p. 632.
Footnote 284:
_Astr. Nach._ Nos. 3275, 3344.
Footnote 285:
_Monthly Notices_, vol. lv. No. 1; _Astroph. Journ._ vol. xiv. p. 317.
Footnote 286:
See _ante_, p. 98.
Footnote 287:
_Vierteljahrsschrift Naturforsch. Ges. in Zürich_, Bd. xli. 1896.
Footnote 288:
_Astroph. Journ._ vol. iv. p. 138.
Footnote 289:
_Bull. de l’Acad. de St. Pétersbourg_, t. vi. No. 3, p. 293.
Footnote 290:
_Astr. Journ._ No. 416; _Astroph. Journ._ vol. iv. p. 101.
Footnote 291:
_Ibid._ vol. v. p. 37.
Footnote 292:
_Astr. Nach._ No. 3039; _Astroph. Journ._ vol. iii. p. 247.
Footnote 293:
_Memoirs Royal Astr. Society_, vol. li. p. 123.
Footnote 294:
_Astroph. Journ._ vol. iii. p. 248.
Footnote 295:
Cf. Halm, _Annals Edin. Observatory_, vol. i. p. 99, 1902.
Footnote 296:
Clerke, _Hist. of Astr._ pp. 148, 149, 4th ed.; Spoerer, _Potsdam
Publ._ Bd. x. part i. 1894.
Footnote 297:
_Knowledge_, vol. xv. p. 131.
Footnote 298:
Fényi, _Publ. Haynald Observ._ Bd. vi. p. 41.
Footnote 299:
Mascari, _Astroph. Journ._ vol. ii. p. 119; Evershed, _Astr. and
Astroph._ vol. xi. p. 426.
Footnote 300:
Mascari, _Astroph. Journ._ vol. vi. p. 371.
Footnote 301:
_Proc. Royal Society_, vol. lxiii. p. 64; _Monthly Notices_, vol. lx.
p. 142.
Footnote 302:
_Knowledge_, vol. xvii. p. 175.
Footnote 303:
_Ibid._ p. 206 (A. M. Clerke).
Footnote 304:
_Trans. Astr. Society of Toronto_, 1897, p. 80.
Footnote 305:
_Observatory_, vol. xiv. p. 328.
Footnote 306:
_Ibid._ vol. xv. p. 143.
Footnote 307:
Maunder, _Knowledge_, vol. xv. p. 89.
Footnote 308:
_Ibid._ p. 93.
Footnote 309:
_Astr. and Astrophysics_, vol. xi. p. 437.
Footnote 310:
_Ibid._ p. 925.
Footnote 311:
_Astr. and Astrophysics_, vol. xii. p. 717.
Footnote 312:
_Report Brit. Ass._ 1892, p. 634.
Footnote 313:
_Astr. Nach._ Nos. 2836, 2837.
Footnote 314:
_Nature_, vol. lxi. p. 445; vol. lxii. p. 460; _Astr. Nach._ No. 3619.
Footnote 315:
_Wiedemann’s Annalen_, Bd. lix. p. 782, 1896.
Footnote 316:
Bigelow, _Report on Solar and Terrestrial Magnetism_, p. 15.
Footnote 317:
_Astroph. Journ._ vol. iii. p. 112.
Footnote 318:
_Astr. and Astrophysics_, vol. xii. p. 740.
Footnote 319:
_Astr. and Astrophysics_, vol. xiii. p. 278.
Footnote 320:
_The Sun_, p. 187.
Footnote 321:
_Astr. and Astrophysics_, vol. xii. p. 833.
Footnote 322:
_Die Strahlenbrechung auf der Sonne_, Stuttgart, 1891.
Footnote 323:
_Monatsberichte_, Berlin, 1860, p. 405.
Footnote 324:
Knopf, _Astr. Nach._ No. 3199.
Footnote 325:
_Sirius_, 1893, p. 176.
Footnote 326:
_Astr. Nach._ No. 3187.
Footnote 327:
Frost, _Astroph. Journ._ vol. iv. p. 196.
Footnote 328:
_Ibid._ vol. xii. p. 185.
Footnote 329:
_Monatsberichte_, Berlin, 24th October 1895.
Footnote 330:
_Harvard Annals_, vol. xxviii. part i. 1897.
Footnote 331:
S. A. Mitchell, _Astroph. Journ._ vol. x. p. 32.
Footnote 332:
_Astr. and Astrophysics_, vol. xiii. p. 661.
Footnote 333:
This “short title” was first applied to them by Sir Norman Lockyer.
See _Nature_, 18th May 1899.
Footnote 334:
_Potsdam Publicationen_, No. 14, p. 26, 1884.
Footnote 335:
_Die Spectralanalyse der Gestirne_, p. 320.
Footnote 336:
Dunér, _Astroph. Journ._ vol. ix. p. 131.
Footnote 337:
_Astroph. Journ._ vol. x. p. 108.
Footnote 338:
_Astroph. Journ._ vol. viii. p. 233.
Footnote 339:
Campbell, _Astroph. Journ._ vol. ii. p. 181.
Footnote 340:
_Harvard Annals_, vol. xxviii. pp. 49, 93, 100, 142.
Footnote 341:
_Monthly Notices_, vol. lix. p. 507.
Footnote 342:
_Comptes Rendus_, t. lxxiv. p. 516.
Footnote 343:
Pickering, _Astroph. Journ._ vol. vi. p. 459.
Footnote 344:
_Astr. Nach._ No. 3025.
Footnote 345:
Campbell, _Astr. and Astrophysics_, vol. xiii. p. 474.
Footnote 346:
_Phil. Trans._ vol. cxci. p. 129.
Footnote 347:
Maury, _Harvard Annals_, vol. xxviii. p. 15.
Footnote 348:
_Abhandl. der kön. Sächs. Gesellschaft_, Bd. xv. Th. 1.
Footnote 349:
One taken 1st February is shown in _Knowledge_, vol. xix. p. 109.
Footnote 350:
_Astr. and Astrophysics_, vol. xiii. p. 178.
Footnote 351:
_Knowledge_, vol. xix. p. 39.
Footnote 352:
_Harvard Annals_, vol. xxxii. p. 66.
Footnote 353:
ε Orionis = N.G.C. 1990.
Footnote 354:
Barnard, _Pop. Astr._ Sept. 1897, p. 232.
Footnote 355:
_Spectra of Southern Stars_, p. 11.
Footnote 356:
_Proc. Royal Society_, 27th April 1899.
Footnote 357:
_Astr. and Astrophysics_, vol. xiii. p. 491.
Footnote 358:
_Astr. Nach._ Nos. 3565, 3583. Bellatrix was one of McClean’s original
oxygen stars.
Footnote 359:
_Nature_, vol. lxi. p. 263.
Footnote 360:
_Astroph. Journ._ vol. xi. p. 262.
Footnote 361:
_Ibid._ vol. x. p. 31 (Mitchell).
Footnote 362:
_Atlas of Stellar Spectra_, p. 152.
Footnote 363:
Scheiner, _Sitzungsberichte_, Berlin, 13th February 1890.
Footnote 364:
_Nature_, vol. lix. p. 342.
Footnote 365:
Hartley, _Astroph. Journ._ vol. x. p. 164.
Footnote 366:
_Astroph. Journ._ vol. ix. p. 267.
Footnote 367:
_Nature_, 16th March 1899.
Footnote 368:
Cf. A. Cotton on “Kirchhoff’s law,” _Astroph. Journ._ vol. ix. p. 244,
note.
Footnote 369:
Huggins, _Atlas of Spectra_, p. 148.
Footnote 370:
_Astr. Nach._ No. 3025.
Footnote 371:
_Spectra of Southern Stars_, p. 3.
Footnote 372:
A. C. Maury, _Harvard Annals_, vol. xxviii. p. 24.
Footnote 373:
Frost-Scheiner, _Astronomical Spectroscopy_, p. 243.
Footnote 374:
_Proc. Royal Society_, vol. xlviii. p. 216.
Footnote 375:
_The Sun’s Place in Nature_, p. 305.
Footnote 376:
_Sitzungsberichte_, Berlin, March 1894.
Footnote 377:
_Astr. and Astrophysics_, vol. xiii. p. 660.
Footnote 378:
_Harvard Annals_, vol. xxviii. p. 27.
Footnote 379:
A. C. Maury, _Harvard Annals_, vol. xxviii. p. 30.
Footnote 380:
_Astr. and Astrophysics_, vol. xii. p. 720.
Footnote 381:
_Phil. Trans._ vol. cxci. plate 6.
Footnote 382:
Fowler, _Knowledge_, vol. xx. p. 78.
Footnote 383:
_Harvard Annals_, vol. xxviii. p. 39.
Footnote 384:
_Astronomical Spectroscopy_ (Frost), p. 266.
Footnote 385:
_Harvard Annals_, vol. xxviii. p. 40.
Footnote 386:
Dunér, _Sur les Étoiles à Spectres de la Troisième Classe_, p. 9.
Footnote 387:
Thiele, _Astroph. Journ._ vol. viii. p. 1.
Footnote 388:
_Harvard Annals_, vol. xxviii. p. 42.
Footnote 389:
Frost-Scheiner, _Astr. Spectroscopy_, p. 308.
Footnote 390:
_Astroph. Journ._ vol. vi. p. 423.
Footnote 391:
_Potsdam Publicationen_, No. 14, p. 22.
Footnote 392:
_Spectra of Southern Stars_, Plate iv.
Footnote 393:
_Astroph. Journ._ vol. vi. p. 424.
Footnote 394:
The star was named _Ant-Ares_ (“like Mars”) because of its resemblance
to the planet in colour.
Footnote 395:
_Monthly Notices_, vol. lii. p. 154.
Footnote 396:
_Monthly Notices_, vol. lviii. p. 443.
Footnote 397:
_Astroph. Journ._ vol. x. p. 93.
Footnote 398:
Espin, _loc. cit._
Footnote 399:
_Astroph. Journ._ vol. ix. p. 132.
Footnote 400:
_Astroph. Journ._ vols. viii. p. 237, ix. p. 271, x. p. 87.
Footnote 401:
Hale, _First Annual Report_, p. 7. Spectrographs of 152 Schjellerup,
taken by McClean with two hours’ exposure, were described by him in
1897 as showing a multitude of dark lines.—_Monthly Notices_, vol.
lvii. p. 8.
Footnote 402:
_Astroph. Journ._ vol. viii. p. 239.
Footnote 403:
_Sugli Spettri Prismatici delle Stelle Fisse_, Mem. ii. 1869.
Footnote 404:
_Sur les Étoiles à Spectres de la Troisième Classe_, pp. 10, 82.
Footnote 405:
_Astroph. Journ._ vol. ix. p. 119.
Footnote 406:
_Ibid._ vol. x. p. 110.
Footnote 407:
_Astroph. Journ._ vol. x. p. 112.
Footnote 408:
_First Annual Report_, p. 8.
Footnote 409:
_Sur les Étoiles_, p. 45.
Footnote 410:
_Astroph. Journ._ vol. ix. p. 274.
Footnote 411:
They are catalogued in the southern zones of the Bonn Durchmusterung
as −10°5057 and −10°513.
Footnote 412:
Hale, _Astroph. Journ._ vol. viii. p. 239; Espin, _ibid._ vol. x. p.
170.
Footnote 413:
Sidgreaves, _Monthly Notices_, vol. lviii. p. 345.
Footnote 414:
_Sitzungsberichte_, Berlin, 26th March 1896.
Footnote 415:
_Monthly Notices_. vol. lviii. p. 344.
Footnote 416:
_Astroph. Journ._ vol. ix. p. 31; _Observatory_, vol. xxii. p. 152.
Footnote 417:
Krüger, _Astroph. Journ._ vol. ii. pp. 151, 158.
Footnote 418:
Keeler, _Lick Publ._ vol. iii. p. 225.
Footnote 419:
Espin, _Astr. Nach._ No. 2859.
Footnote 420:
Maunder believed it to be of fourth type, 1st Oct. 1888. _Monthly
Notices_, vol. xlix. p. 303.
Footnote 421:
_Astr. Nach._ No. 3765.
Footnote 422:
_Astroph. Journ._ vol. viii. p. 233.
Footnote 423:
_Astroph. Journ._ vol. ii. p. 178.
Footnote 424:
Runge, _Astr. Nach._ No. 3471.
Footnote 425:
Professor Frost’s conjecture that such stars may possess atmospheres
preferentially absorbent of the shorter radiations did not seem to
himself plausible enough for publication. _Astroph. Journ._ vol. ii.
p. 182. A similar explanation, however, has lately been recommended by
Dr. Kayser. _Ibid._ vol. xiv. p. 313.
Footnote 426:
_Astroph. Journ._ vol. ii. p. 181.
Footnote 427:
Evidently analogous is Hale’s result that the lines first reversed in
metallic spectra are the most refrangible. _Astroph. Journ._ vol. xv.
p. 227.
Footnote 428:
A. C. Maury, _Harvard Annals_, vol. xxviii. p. 104.
Footnote 429:
Keeler, _Astr. and Astrophysics_, vol. xiii. p. 486. The helium line,
λ 4472, is specially described. It is bright in the Orion nebula, dark
in all Orion stars.
Footnote 430:
Proc. _Royal Society_, vol. xlvi. p. 41.
Footnote 431:
Campbell, _Astr. and Astroph._ vol. xiii. p. 397.
Footnote 432:
_Comptes Rendus_, 11th Oct. 1897; _Astroph. Journ._ vol. vi. p. 322;
_Atlas of Spectra_, p. 138.
Footnote 433:
A. C. Maury, _Harvard Annals_, vol. xxviii. p. 104.
Footnote 434:
_Astroph. Journ._ vol. ii. p. 180.
Footnote 435:
A. C. Maury, _loc. cit._ pp. 49, 100.
Footnote 436:
Sidgreaves, _Monthly Notices_, vol. lix. p. 506.
Footnote 437:
_O Gyalla Beob._ Bd. vii. p. 14. For Keeler’s failure to perceive
D-absorption see _Publ. Pacific Society_, vol. i. p. 80.
Footnote 438:
_System of the Stars_, p. 70.
Footnote 439:
Bélopolsky, _Astr. Nach._ No. 3603; _Astroph. Journ._ vol. x. p. 319.
Footnote 440:
_Harvard Annals_, vol. xxviii. p. 175.
Footnote 441:
_Astr. Nach._ No. 2963.
Footnote 442:
_Astr. and Astroph._ vol. xiii. p. 158.
Footnote 443:
Frost, _Astroph. Journ. vol._ x. p. 365.
Footnote 444:
A. C. Maury, _loc. cit._ p. 104.
Footnote 445:
McClean, _Proc. Royal Society_, vol. lxii. p. 419.
Footnote 446:
_Astroph. Journ._ vol. iv. p. 369. For a list of analogous objects see
_Observatory_, vol. xxii. p. 54.
Footnote 447:
_Observatory_, vol. xxii. p. 52.
Footnote 448:
_Astr. and Astroph._ vol. xiii. p. 467.
Footnote 449:
_Astroph. Journ._ vol. ii. p. 177.
Footnote 450:
_Harvard Annals_, vol. xxviii. p. 247.
Footnote 451:
_Spectra of Southern Stars_, p. 15, Plate xii.
Footnote 452:
Campbell, _Astr. and Astroph._ vol. xiii. p. 449; _Astroph. Journ._
vol. ii. p. 178.
Footnote 453:
_Publ. Pac. Society_, vol. i. p. 81.
Footnote 454:
Hale, _Yerkes Observ. Report_, ii. p. 6.
Footnote 455:
_Potsdam Publ._ No. 14, p. 15.
Footnote 456:
_Astr. and Astroph._ vol. xiii. pp. 460, 468.
Footnote 457:
Campbell, _Astr. and Astroph._ vol. xiii. pp. 456, 468; Pickering,
_Astr. Nach._ No. 3025; Vogel, _Potsdam Publ._ No. 14, p. 17; A. J.
Cannon, _Harvard Annals_, vol. xxviii. pp. 147, 248.
Footnote 458:
Pickering, _Astr. Nach._ No. 2986.
Footnote 459:
_Astr. and Astroph._ vol. xiii. p. 461.
Footnote 460:
_Ibid._ vols. xii. p. 913; xiii. p. 461.
Footnote 461:
_Astr. Nach._ No. 3471.
Footnote 462:
_Astroph. Journ._ vol. viii. p. 113.
Footnote 463:
_Proc. Royal Society_, vol. xlix. p. 33.
Footnote 464:
Jewell, _Astroph. Journ._ vol. iii. p. 99.
Footnote 465:
_Astr. Nach._ No. 2000.
Footnote 466:
Frost-Scheiner, _Astr. Spectroscopy_, p. 323.
Footnote 467:
Argelander, _Astr. Nach._ No. 624.
Footnote 468:
_Monthly Notices_, vol. li. p. 12; Krüger, _Cat. der färbig. Sterne_,
p. 81.
Footnote 469:
_Astr. Nach._ No. 3200.
Footnote 470:
_Ibid._ No. 3633.
Footnote 471:
_Monthly Notices_, vol. li. p. 12.
Footnote 472:
_Cat. der färbig. Sterne_, p. 99.
Footnote 473:
_Bothkamp Beob._ Heft ii. p. 146.
Footnote 474:
Copeland, _Monthly Notices_, vol. xlvii. p. 92.
Footnote 475:
_Astr. Nach._ No. 2539.
Footnote 476:
_Proc. Royal Society_, vol. lvii. p. 174.
Footnote 477:
_Monthly Notices_, vol. lix. p. 505.
Footnote 478:
_Sugli Spettri Prismatici delle Stelle_, Mem. ii. 1868.
Footnote 479:
Frost-Scheiner, _Astr. Spectroscopy_, p. 256.
Footnote 480:
Lockyer, _Proc. Royal Society_, vol. lvii. p. 173.
Footnote 481:
_Publ. Pacific Society_, vol. i. p. 80.
Footnote 482:
_Astr. Nach._ No. 3129.
Footnote 483:
A. C. Maury, _Harvard Annals_, vol. xxviii. pp. 124, 126.
Footnote 484:
_O Gyalla Beob._ Bd. vii. p. 14.
Footnote 485:
_Harvard College Circular_, No. 32; _Annals_, vol. xxviii. p. 183.
Footnote 486:
_Harvard Circular_, No. 32.
Footnote 487:
_Ibid._ No. 21; _Annals_, vol. xxviii. p. 184.
Footnote 488:
_Astroph. Journ._ vol. ii. p. 180.
Footnote 489:
_Fifty-fifth Harvard Report_, p. 10; _Harvard Circular_, No. 60;
_Annals_, vol. xxviii. pp. 180, 183.
Footnote 490:
Fleming, _Monthly Notices_, vol. liii. p. 275; _Harvard Circular_, No.
4.
Footnote 491:
_Astroph. Journ._ vol. i. p. 411.
Footnote 492:
_Harvard Circular_, No. 32.
Footnote 493:
Fleming, _Astr. Nach._ No. 3054; _Astr. and Astrophysics_, vol. xi. p.
27.
Footnote 494:
_Astroph. Journ._ vols. x. p. 365; xvi. p. 114.
Footnote 495:
A. C. Maury, _Harvard Annals_, vol. xxviii. p. 104.
Footnote 496:
_Astr. and Astrophysics_, vol. xi. pp. 269, 372, 550.
Footnote 497:
_Atti dell’ Accad. degli Agiati_, t. ii. 1896.
Footnote 498:
_Astr. Nach._ No. 1099.
Footnote 499:
Krüger, _Cat. der färbig. Sterne_, p. 9.
Footnote 500:
Known as “64 _b_ Schjellerup” = Krüger 513. See _Cat. der färbig.
Sterne_, p. 33.
Footnote 501:
No. 6803 of the _Copenhagen Catalogue_ = 214 Schjellerup = Krüger
1436.
Footnote 502:
Krüger 504.
Footnote 503:
Binary in slow motion (Innes). Composite spectrum (A. J. Cannon).
Footnote 504:
S.D.M. −10° 513 and S.D.M. −10° 5057 of eighth and seventh magnitudes
respectively.
Footnote 505:
_Mensuræ Micrometricæ_, p. lxxxi.
Footnote 506:
_Intellectual Observer_, vol. ii. p. 138, 1863.
Footnote 507:
_Annales de l’Observatoire de Nice_, t. ii.
Footnote 508:
Webb, _Celestial Objects_, 4th ed., p. 297; _Knowledge_, vol. xiii. p.
250.
Footnote 509:
_Astr. Nach._ No. 3023.
Footnote 510:
_Journ. Brit. Astr. Ass._ vol. v. p. 457.
Footnote 511:
_System of the Stars_, p. 159.
Footnote 512:
_Les Étoiles_, p. 690.
Footnote 513:
Webb, _Cel. Objects_, vol. ii. p. 182.
Footnote 514:
Burnham, _Astr. Nach._ Nos. 2875, 3114.
Footnote 515:
A. C. Maury, _Harvard Annals_, vol. xxviii. pp. 93, 99.
Footnote 516:
_The Observatory_, vol. xxii. p. 387.
Footnote 517:
_Phil. Trans._ vol. cliv. p. 431.
Footnote 518:
_Bothkamp Beob._ Heft ii. p. 28.
Footnote 519:
_Astr. Nach._ No. 3034.
Footnote 520:
_Harvard Annals_, vol. xxviii. p. 93.
Footnote 521:
A. C. Maury, _Harvard Annals_, vol. xxviii. pp. 92, 100.
Footnote 522:
Leavenworth, _Publ. Leander M‘Cormick Observatory_, vol. i. pt. iv. p.
96.
Footnote 523:
A. C. Maury, _Harvard Annals_, vol. xxviii. pp. 17, 119.
Footnote 524:
Pickering, _Astroph. Journ._ vol. vi. p. 350.
Footnote 525:
Huggins, _Atlas of Spectra_, p. 164, plate xii.; Bélopolsky, _Astr.
Nach._ No. 3510.
Footnote 526:
_Astroph. Journ._ vol. viii. p. 307.
Footnote 527:
_Nature_, vol. lx. p. 249.
Footnote 528:
Maunder, _Knowledge_, vol. xiv. p. 73.
Footnote 529:
_Astroph. Journ._ vol. vi. p. 326.
Footnote 530:
_Atlas of Spectra_, p. 160.
Footnote 531:
_Monthly Notices_, vol. xxxvii. p. 278. The difficulties in applying
the theory were pointed out by Vogel, _Astr. Nach._ No. 2141.
Footnote 532:
Scheiner, _Astr. Nach._ No. 2924; _Potsdam Publ._ Bd. vii. p. 232.
Footnote 533:
_Comptes Rendus_, t. cxxi. p. 629.
Footnote 534:
_Sitzungsberichte_, Berlin, 17th Nov. 1898.
Footnote 535:
_Astr. and Astrophysics_, vol. xii. p. 719.
Footnote 536:
_Phil. Trans._ vol. clxxxiv. p. 696.
Footnote 537:
_Atlas of Spectra_, Plate ix.
Footnote 538:
_Atlas of Spectra_, p. 69.
Footnote 539:
_Comptes Rendus_, t. cxxi. p. 629.
Footnote 540:
The method usually employed is that given by Lehmann-Filhés, _Astr.
Nach._ No. 3242.
Footnote 541:
A. C. Maury, _Astroph. Journ._ vol. viii. p. 174.
Footnote 542:
_Harvard Annals_, vol. xxvi. p. xviii.; _Harvard Circular_, No. 11;
_Astr. Nach._ No. 3017; _Monthly Notices_, vol. l. p. 297.
Footnote 543:
_Sitzungsberichte_, Berlin, 2nd May 1901; _Astroph. Journ._ vol. xiii.
p. 324.
Footnote 544:
_Astr. Nach._ No. 3456.
Footnote 545:
_Comptes Rendus_, t. xiii. p. 438; quoted by Gore, _Knowledge_, vol.
xxii. p. 201.
Footnote 546:
_Monthly Notices_, vol. li. p. 316.
Footnote 547:
_Astr. Nach._ No. 3242.
Footnote 548:
_Ibid._ No. 3629.
Footnote 549:
Vogel, _ibid._ No. 3017.
Footnote 550:
_Astroph. Journ._ vol. viii. p. 173.
Footnote 551:
_Harvard Circular_, Nos. 11, 21.
Footnote 552:
_Sitzungsberichte_, Berlin, 24th April 1890; _Astr. Nach._ No. 2995.
Footnote 553:
_Astroph. Journ._ vol. v p. 1.
Footnote 554:
_Memorie degli Spettroscopisti Italiani_, vols. xxvi., xxviii.;
_Bulletin de l’Acad. des Sciences_, St. Pétersbourg, t. vi. No. 1, t.
viii. No. 2.
Footnote 555:
See _Observatory_, vol. xxiii. p. 127.
Footnote 556:
_Astroph. Journ._ vol. x. p. 177; _Observatory_, vol. xxiii. p. 92.
Footnote 557:
_Ibid._ vols. xxii. p. 436, xxiii. p. 93; _Monthly Notices_, vol. lx.
p. 418.
Footnote 558:
_Astr. Journ._ No. 484.
Footnote 559:
_Monthly Notices_, vols. lx. p. 595, lxi. p. 70.
Footnote 560:
_Astroph. Journ._ vol. xiii. p. 89.
Footnote 561:
See Appendix, Table II., for a list of the spectroscopic binaries
discovered down to the middle of the year 1902.
Footnote 562:
_Harvard Annals_, vol. xxviii. p. 104.
Footnote 563:
_Observatory_, vol. xxiii. p. 128 (A. M. Clerke).
Footnote 564:
_Proc. Amer. Academy_, vol. xvi. p. 27.
Footnote 565:
_Popular Astronomy_, Oct. 1897, p. 306.
Footnote 566:
_Sitzungsberichte_, Berlin, 28th Nov. 1889; _Astr. Nach._ Nos. 2947,
2960.
Footnote 567:
_Astr. Journ._ vol. vii. p. 165.
Footnote 568:
_Knowledge_, vol. xv. p. 86 (A. M. Clerke).
Footnote 569:
_Astr. Journ._ Nos. 255, 256, 257.
Footnote 570:
Bauschinger, _V. J. S. Astr. Ges._, Jahrgang xxix.; Chase, _Astr.
Journ._ No. 218.
Footnote 571:
Boss, _Astr. Journ._ No. 343; Yendell, _Pop. Astr._ Dec. 1897, p. 401.
Footnote 572:
_Comptes Rendus_, t. cxx. p. 125; _Bull. Soc. Astr. de France_, 1895,
p. 73.
Footnote 573:
_Astr. Journ._ No. 318.
Footnote 574:
_Astr. Nach._ No. 804.
Footnote 575:
Schönfeld, _V. J. S. Astr. Ges._, Jahrgang ix. p. 230; _Sirius_, Bd.
x. p. 68; Argelander, _Bonner Beob._ Bd. vii. p. 397.
Footnote 576:
_Astroph. Journ._ vol. x. p. 317.
Footnote 577:
Schönfeld, _Jahresbericht_, Mannheim, Bd. xl. p. 76.
Footnote 578:
_Beobachtungen Veränderlicher Sterne_, Th. iii. p. 21.
Footnote 579:
_Astr. Nach._ No. 3474.
Footnote 580:
_Journ. Brit. Astr. Assoc._ vol. i. pp. 137, 255.
Footnote 581:
Chandler, _Astr. Journ._ No. 205.
Footnote 582:
_Astr. Journ._ No. 373; _Astroph. Journ._ vol. iv. p. 267.
Footnote 583:
Roberts, _Astroph. Journ._ vol. x. p. 313.
Footnote 584:
_Proc. Amer. Acad._ vol. xvi. p. 350; _Astr. Nach._ No. 3385.
Footnote 585:
_Astr. Journ._ No. 199; _Popular Astr._ Sept. 1897, p. 239.
Footnote 586:
_V. J. S. Astr. Ges._ Jahrgang xxvi. Th. ii.; _Potsdam Annual Report_.
Footnote 587:
Chandler, _Astr. Journ._ Nos. 199, 294, 396.
Footnote 588:
Tisserand, _Bull. de la Soc. Astr. de France_, Mars 1895.
Footnote 589:
_Astr. Journ._ Nos. 162, 294.
Footnote 590:
_System of the Stars_, p. 142.
Footnote 591:
_Astr. Nach._ Nos. 3091, 3467; _Astr. Journ._ No. 265; _Trans. Swedish
Acad. of Sciences_, 1892, No. 7.
Footnote 592:
Yendell, _Astr. Journ._ Nos. 163, 185; _Knowledge_, vol. xvi. p. 166
(A. M. Clerke).
Footnote 593:
_Astroph. Journ._ vol. xi. p. 190.
Footnote 594:
_I.e._ the interval between one perihelion passage and the next.
Footnote 595:
Russell, _Astroph. Journ._ vol. x. p. 318.
Footnote 596:
_Astr. Journ._ Nos. 327, 383.
Footnote 597:
_Ibid._ No. 383; _Astroph. Journ._ vol. iv. p. 272.
Footnote 598:
Chandler, “Third Catalogue of Variable Stars,” _Astr. Journ._ No. 379.
Footnote 599:
_Astr. Journ._ No. 326.
Footnote 600:
_Ibid._ Nos. 329, 384, 415.
Footnote 601:
_Monthly Notices_, vol. lv. p. 211.
Footnote 602:
_Astr. Journ._ No. 327; _Astroph. Journ._ vol. iv. p. 270.
Footnote 603:
_Ibid._ vol. x. p. 314.
Footnote 604:
Keeler, _ibid._ vol. i. p. 262.
Footnote 605:
_Astr. Journ._ No. 328.
Footnote 606:
_Astroph. Journ._ vols. i. p. 294; iii. p. 348; _Astr. Journ._ Nos.
374, 384, 422.
Footnote 607:
Yendell, _ibid._ No. 366.
Footnote 608:
_Harvard Circulars_, Nos. 3, 4.
Footnote 609:
_Astroph. Journ._ vol. iii. p. 213.
Footnote 610:
Pickering, _ibid._ vol. vii. p. 23.
Footnote 611:
_Astr. Journ._ Nos. 447, 450; Luizet, _Astr. Nach._ No. 3596.
Footnote 612:
_Astr. Journ._ No. 468.
Footnote 613:
_Astroph. Journ._ vol. xiii. p. 177.
Footnote 614:
_Astr. Nach._ Nos. 3572, 3567; Parkhurst, _Astr. Journ._ No. 475.
Footnote 615:
_Harvard Circulars_, Nos. 44, 47.
Footnote 616:
_Astr. Journ._ No. 508; _Proc. Royal Society of Edinburgh_, 4th Nov.
1901.
Footnote 617:
_Astr. Nach._ Nos. 3748, 3765; _Astr. Journ._ No. 517.
Footnote 618:
Such have recently been detected by Chandler in Algol, _Astr. Journ._
Nos. 509, 511.
Footnote 619:
_Astr. Nach._ Nos. 3257, 3338; _Bull. de l’ Acad. St. Pétersbourg_,
1894, p. 268; _Astroph. Journ._ vol. i. pp. 160, 263.
Footnote 620:
_Astroph. Journ._ vol. ii. p. 288.
Footnote 621:
_Astroph. Journ._ vol. iii. p. 227.
Footnote 622:
_Ibid._ vol. i. p. 160.
Footnote 623:
_Astroph. Journ._ vol. vi. p. 393; _Bull. de l’Acad. de St.
Pétersbourg_, t. vii. No. 4, 1900.
Footnote 624:
_Astroph. Journ._ vol. ix. p. 59.
Footnote 625:
Chandler, _Third Catalogue_, No. 8073 and _note_.
Footnote 626:
_Journ. Brit. Astr. Assoc._ vol. vii. p. 471.
Footnote 627:
_Astr. Nach._ No. 3483; Luizet, _ibid._ No. 3570; Flanery,
_Knowledge_, vol. xxiii. p. 134; _Harvard Circular_, No. 41.
Footnote 628:
Sometimes designated SU Cygni.
Footnote 629:
Lockyer, _Proc. Royal Society_, vol. lix. p. 101.
Footnote 630:
Its fourth member, Y Sagittarii, was added by Sawyer in 1886, _Astr.
Journ._ No. 328; Yendell, _Pop. Astr._ vol. ii. p. 364.
Footnote 631:
Chandler, _Astr. Journ. Nach._ No. 2749; Yendell, _Astr. Journ._ Nos.
157, 321; _Pop. Astr._ vol. ii. p. 207.
Footnote 632:
_Astr. Journ._ No. 374.
Footnote 633:
_Bonner Beob._ Bd. vii. p. 393.
Footnote 634:
_Astr. Nach._ Nos. 1745, 2420.
Footnote 635:
_Ibid._ No. 3565.
Footnote 636:
_Astroph. Journ._ vol. xiii. p. 94.
Footnote 637:
_Astr. Journ._ Nos. 378, 384; _Astroph. Journ._ vol. x. p. 312;
_Nature_, vol. lxiv. p. 469.
Footnote 638:
_Astr. Journ._ vols. ix. pp. 180, 183, 190; x. p. 11.
Footnote 639:
_Astroph. Journ._ vol. iv. p. 141.
Footnote 640:
_Astr. Journ._ No. 413.
Footnote 641:
Pickering, _Astr. Nach._ No. 3008.
Footnote 642:
Chandler, _Astr. Journ._ Nos. 358, 374, 426.
Footnote 643:
_Harvard Circular_, Nos. 23, 25.
Footnote 644:
_Astroph. Journ._ vol. viii. p. 163.
Footnote 645:
_Harvard Circular_, No. 33; _Harvard Annals_, vol. xxxviii. pp. 144,
160 (Bailey).
Footnote 646:
Cf. _Observatory_, vol. xx. p. 54.
Footnote 647:
_Astr. Nach._ No. 3410.
Footnote 648:
_Astr. Journ._ No. 477; _Astroph. Journ._ vol. xiii. p. 177; _Nature_,
12th September 1901.
Footnote 649:
_Harvard Circular_, No. 21.
Footnote 650:
Since the above was written, Miss A. J. Cannon has made the brief
statement (_Harvard Annals_, vol. xxviii. p. 177) that the 34^h period
satisfies all observations.
Footnote 651:
_Astr. Nach._ No. 3483.
Footnote 652:
_Astr. Nach._ No. 3570.
Footnote 653:
_Harvard Circular_, No. 41.
Footnote 654:
_Astr. Journ._ No. 331.
Footnote 655:
S. I. Bailey, _Astroph. Journ._ vol. x. p. 257.
Footnote 656:
S. I. Bailey, _Harvard Annals_, vol. xxxviii. p. 209.
Footnote 657:
_Monthly Notices_, vol. lxi. p. 163.
Footnote 658:
Bailey, _Harvard Annals_, vol. xxxviii. p. 234.
Footnote 659:
See _Knowledge_, vol. xvii. p. 128 (A. M. Clerke).
Footnote 660:
Frost, _Astroph. Journ._ vol. ii. p. 383.
Footnote 661:
_Astr. Nach._ No. 3565.
Footnote 662:
_History of Astronomy_, 4th ed. p. 379 (A. M. Clerke).
Footnote 663:
_Astr. Nach._ No. 3051.
Footnote 664:
_Astr. and Astrophysics_, vol. xiii. p. 575.
Footnote 665:
_Monthly Notices_, vol. lvii. p. 6; _Observatory_, vol. xx. p. 87.
Footnote 666:
_Harvard Annals_, vol. xxviii. p. 103.
Footnote 667:
_Sitzungsberichte_, Berlin, 8th Feb. 1894.
Footnote 668:
_Monthly Notices_, vols. liv. p. 94; lvii. p. 515.
Footnote 669:
_Memorie degli Spettroscopisti Italiani_, t. xxvi., June 1897.
Footnote 670:
_Astroph. Journ._ vol. vii. p. 1.
Footnote 671:
_Astr. and Astrophysics_, vol. xii. p. 356.
Footnote 672:
Baxendell, _Proc. Manchester Phil. Society_, vol. xxiv. p. 14.
Footnote 673:
Flanery, _Knowledge_, vol. xix. p. 230.
Footnote 674:
_Bonner Bcob._ Bd. vii. Th. ii. p. 332.
Footnote 675:
_Phil. Trans._ vol. lxx. p. 342.
Footnote 676:
Schmidt, quoted by Schönfeld, _Mannheimer Jahresbericht_, Bd. xl. p.
74.
Footnote 677:
Nijland, _Astr. Nach._ No. 3733.
Footnote 678:
Guthnick (_Astr. Nach._ No. 3745) holds that the star rises to
exceptionally high maxima once in 59½ years. As the Italian proverb
says, _Chi vivrà, vedrà_.
Footnote 679:
_Berliner Jahrbuch_, 1841, p. 93 (Olbers).
Footnote 680:
_Miscellanea Berolinensia_, t. i. p. 208.
Footnote 681:
_Bonner Beob._ Bd. vii. p. 340.
Footnote 682:
Humboldt’s _Cosmos_ (Otté’s trans.), vol. iii. p. 236.
Footnote 683:
Roberts, _Astr. Journ._ Nos. 295, 491–2.
Footnote 684:
A. J. Cannon, _Harvard Annals_, vol. xxviii. p. 189.
Footnote 685:
Roberts, _Astr. Journ._ No. 462.
Footnote 686:
_Astroph. Journ._ vol. ii. p. 198.
Footnote 687:
Baxendell, _Journ. Liverpool Astr. Soc._ vol. iii. p. 52.
Footnote 688:
Schönfeld, _Astr. Nach._ No. 2066.
Footnote 689:
Peek, _Journ. Brit. Astr. Assoc._ vol. ix. p. 261.
Footnote 690:
_Astr. Nach._ No. 3025; Hartwig, _ibid._ Nos. 3211, 3596.
Footnote 691:
Hale, _Yerkes Observatory Bulletin_, No. 13.
Footnote 692:
_Journ. Brit. Astr. Assoc._ vol. ix. p. 260.
Footnote 693:
_Astr. Nach._ No. 2859.
Footnote 694:
Peek, _Variable Star Notes_, No. 4.
Footnote 695:
Townley, _Publ. Washburn Observatory_, vol. vi. pt. iii. p. 60.
Footnote 696:
_Astroph. Journ._ vol. i. p. 305.
Footnote 697:
Parkhurst, _Popular Astronomy_, vol. i. p. 462.
Footnote 698:
Schmidt, _Astr. Nach._ No. 1897; Chandler, _Astr. Journ._ Nos. 186,
193.
Footnote 699:
Parkhurst, _Pop. Astr._ vol. i. p. 266.
Footnote 700:
Reed, _Astr. Journ._ No. 330.
Footnote 701:
Peek, _Variable Star Notes_, No. 2, p. 13.
Footnote 702:
Knott, _Observatory_, vol. xiii. p. 111.
Footnote 703:
_Astr. Nach._ No. 3184.
Footnote 704:
_Bulletin de l’Acad. Imp._ t. xxix. p. 302.
Footnote 705:
Peek, _Knowledge_, vol. xv. p. 52.
Footnote 706:
_Yerkes Observatory Bulletin_, No. 13.
Footnote 707:
_Knowledge_, vol. xix. p. 230.
Footnote 708:
_Ibid._ vol. xx. p. 237.
Footnote 709:
_Pop. Astr._ vol. v. p. 164.
Footnote 710:
Yendell, _ibid._ p. 17.
Footnote 711:
_Astr. Journ._ No. 400.
Footnote 712:
Pickering, _Astroph. Journ._ vol. iv. p. 370.
Footnote 713:
Parkhurst, _Pop. Astr._ July 1900, p. 343.
Footnote 714:
_Astroph. Journ._ vol. xii. p. 268.
Footnote 715:
_Vierteljahrsschrift Astr. Ges._ Jahrgang xxxiv. p. 315.
Footnote 716:
_Astr. Nach._ No. 3553.
Footnote 717:
_Astr. Nach._ Nos. 2075, 2420, 2491, 2492.
Footnote 718:
_Astr. Journ._ Nos. 491, 492 (1901).
Footnote 719:
_Astr. Nach._ No. 1905.
Footnote 720:
_Journ. Brit. Astr. Assoc._ vol. v. p. 262.
Footnote 721:
Pickering, _Proc. Amer. Acad._ vol. xii. p. 403.
Footnote 722:
Schönfeld, _Mannheimer Jahresberichte_, Bd. xl. p. 113.
Footnote 723:
_Monthly Notices_, vol. xvii. p. 23.
Footnote 724:
B.D. + 30° 591, now known as X Persei.
Footnote 725:
_Astr. Journ._ Nos. 434, 462; _Astr. Nach._ No. 3577.
Footnote 726:
Safarik, _Journ. Brit. Astr. Assoc._ vol. ii. p. 293.
Footnote 727:
_Astr. Nach._ No. 3011.
Footnote 728:
_Astr. Journ._ No. 231.
Footnote 729:
Peek, _Journ. Brit. Astr. Assoc._ vols. ix. p. 260; x. p. 156.
Footnote 730:
Miss Cannon found indications, in the spectrum of η Carinæ, of a
composite origin from a solar, and a bright-line helium star. _Harvard
Annals_, vol. xxviii. p. 175. Sir David Gill recorded numerous
spectral coincidences between the southern variable and Nova Aurigæ.
_Monthly Notices_, vol. lxi. p. 456.
Footnote 731:
_Monthly Notices_, vol. vi. p. 244.
Footnote 732:
_System of the Stars_, p. 116.
Footnote 733:
Roberts, _Astr. Journ._ No. 492, p. 89.
Footnote 734:
_Astr. Nach._ Nos. 624, 796, 806.
Footnote 735:
Gore, _Revised Cat. of Var. Stars_, 1888.
Footnote 736:
_Astr. Journ._ No. 151.
Footnote 737:
H. Corder, _Memoirs Brit. Astr. Assoc._ vol. v. pt. ii. p. 32.
Footnote 738:
_Astr. Nach._ No. 2704.
Footnote 739:
A. C. Maury, _Harvard Annals_, vol. xxviii. p. 31.
Footnote 740:
_Monthly Notices_, vols. l. p. 94; lv. p. 445.
Footnote 741:
_Monthly Notices_, vol. lix. p. 374.
Footnote 742:
_Ibid._ vol. lx. p. 425.
Footnote 743:
_Knowledge_, vol. xvi. p. 124.
Footnote 744:
Gore, _ibid._ vol. xxii. p. 176.
Footnote 745:
_Astr. Nach._ No. 3475.
Footnote 746:
_Astr. Nach._ No. 3118.
Footnote 747:
_Astr. and Astrophysics_, vol. xi. p. 393.
Footnote 748:
_Ibid._ p. 810.
Footnote 749:
_Ibid._ vol. xii. p. 159.
Footnote 750:
_Abhandlungen der Kön. Akad._ Berlin, 1893, p. 58.
Footnote 751:
_Discourse at the Royal Institution_, 13th May 1892.
Footnote 752:
_Astr. and Astrophysics_, vol. xi. p. 607.
Footnote 753:
_Ibid._ p. 907.
Footnote 754:
Campbell, _Astr. and Astrophysics_, vol. xi. p. 715.
Footnote 755:
Huggins, _Proc. Royal Society_, vol. liv. p. 30.
Footnote 756:
_Astr. and Astrophysics_, vol. xii. p. 728.
Footnote 757:
_Astr. and Astrophysics_, vol. xii. p. 54.
Footnote 758:
_Publ. Pacific Society_, vol. iv. p. 246.
Footnote 759:
Campbell, _Astroph. Journ._ vols. i. p. 49; v. p. 240.
Footnote 760:
Huggins, _Astr. and Astrophysics_, vol. xiii. p. 239.
Footnote 761:
Peek, _Journ. Brit. Astr. Assoc._ vol. ix. p. 260.
Footnote 762:
Rambaut, _Observatory_, vol. xxiv. p. 261.
Footnote 763:
_Ibid._ p. 360; Barnard, _Astroph. Journ._ vol. xiv. p. 152; _Monthly
Notices_, vol. lxii. p. 61.
Footnote 764:
Campbell, _Astroph. Journ._ vol. i. p. 51.
Footnote 765:
Pickering, _Astr. and Astrophysics_, vol. xiii. pp. 40, 398.
Footnote 766:
_Astr. and Astrophysics_, vol. xiii. p. 311; _Astr. Nach._ No. 3211.
Footnote 767:
_Harvard Circular_, No. 1.
Footnote 768:
_Harvard Circular_, No. 4.
Footnote 769:
_Ibid._ No. 4; _Astroph. Journ._ vol. iii. p. 214.
Footnote 770:
W. J. Hussey, _Astr. Journ._ No. 383; _Publ. Pacific Society_, vol.
viii. p. 220.
Footnote 771:
_Astroph. Journ._ vol. v. p. 233.
Footnote 772:
_Harvard Circular_, No. 42; _Astroph. Journ._ vol. ix. p. 269
(Pickering).
Footnote 773:
_Ibid._ p. 308.
Footnote 774:
E. C. Pickering, _Astroph. Journ._ vol. xii. p. 52; _Harvard
Circular_, No. 56; _Astr. Nach._ Nos. 3651, 3664.
Footnote 775:
_Astroph. Journ._ vol. xii. p. 258.
Footnote 776:
_Harvard Circular_, Nos. 56, 57.
Footnote 777:
_Sitzungsberichte_, Berlin, 21 März 1901.
Footnote 778:
Rev. W. Sidgreaves, _Observatory_, vol. xxiv. p. 192.
Footnote 779:
_Monthly Notices_, vols. lxi. pp. 388, 462; lxii. p. 521.
Footnote 780:
_Ibid._ vol. lxi. p. 464.
Footnote 781:
_Harvard Circular_, No. 59.
Footnote 782:
_Yerkes Observatory Bulletin_, Nos. 16, 17.
Footnote 783:
This was assured by Campbell’s determinations (_Lick Bulletin_, No.
8).
Footnote 784:
_Lick Bulletin_, No. 8; Sidgreaves, _Astr. Nach._ No. 3741; Von
Gothard, _ibid._ No. 3738.
Footnote 785:
_Astr. Nach._ Nos. 3752, 3753.
Footnote 786:
_Astroph. Journ._ vol. xiv. p. 167.
Footnote 787:
_Ibid._ p. 293.
Footnote 788:
_Lick Bulletin_, No. 10.
Footnote 789:
_Ibid._ No. 14.
Footnote 790:
_Astr. Nach._ No. 3756.
Footnote 791:
_Nature_, 30th Jan. 1902.
Footnote 792:
_Astr. Nach._ No. 3771.
Footnote 793:
_Astroph. Journ._ vol. xvi. p. 38.
Footnote 794:
_Astr. Nach._ No. 3603.
Footnote 795:
_Vierteljahrsschrift Astr. Ges._ Jahrg. xxxi. p. 258.
Footnote 796:
_Astr. Nach._ Nos. 3118, 3187, 3598.
Footnote 797:
_Nature_, vol. lxiv. p. 253.
Footnote 798:
The fifth and sixth stars of the trapezium have, however, certainly
gained brightness of late. Comas Solà, _Astr. Nach._ No. 3751.
Footnote 799:
The radial velocity of η Geminorum proved variable on Campbell’s
examination in 1902. _Lick Bulletin_, No. 20.
Footnote 800:
_Bull. Société Astr. de France_, Mars 1895, p. 85.
Footnote 801:
_Publicationen der Hamburger Sternwarte_, 1874, No. 1.
Footnote 802:
_Publicationem der Hamburger Sternwarte_, p. 79.
Footnote 803:
_Astroph. Journ._ vol. i. p. 11.
Footnote 804:
Roberts, _Celestial Photographs_, vol. i. p. 39.
Footnote 805:
_Der Sternhaufen_ χ _Persei_, 1878, p. 30.
Footnote 806:
_Phil. Trans._ vol. cxxiii. p. 373.
Footnote 807:
Webb, _Cel. Objects_, ed. Espin, vol. ii. p. 200.
Footnote 808:
_Astr. and Astrophysics_, vol. xiii. p. 180.
Footnote 809:
_Astron. Mittheilungen der kön. Sternwarte zu Göttingen_, Th. iv.; see
also Schlesinger’s “Measurement of the Rutherfurd Photographs of the
Præsepe Group,” _Contributions from the Columbia University_, No. 15,
1898.
Footnote 810:
_Harvard Annals_, vol. xxvi. pt. ii. p. 264.
Footnote 811:
W. C. Kretz, _Contributions from the Columbia University_, No. 16,
1900, p. 478.
Footnote 812:
_Harvard Annals_, vol. xxvi. p. 283.
Footnote 813:
_Transactions Yale Observatory_, vol. i. pt. i. 1887.
Footnote 814:
_Astroph. Journ._ vol. v. p. 352.
Footnote 815:
_Astr. Nach._ No. 3441.
Footnote 816:
_Astronomy and Astrophysics_, vol. xiii. p. 193.
Footnote 817:
_Astr. Nach._ Nos. 3018, 3032.
Footnote 818:
_Ibid._ No. 3074; _Publ. Lick Observatory_, vol. ii. p. 174.
Footnote 819:
_Journ. Brit. Astr. Assoc._ vol. ix. p. 133.
Footnote 820:
_Astr. Nach._ No. 3441.
Footnote 821:
_Ibid._ No. 3253; _Monthly Notices_, vol. lx. p. 258.
Footnote 822:
_Astr. Nach._ No. 3275; _Sirius_, 1891, p. 106. Wolf’s photographs
were taken previously, but interpreted subsequently to Barnard’s.
Footnote 823:
_Pop. Astr._ Feb. 1899.
Footnote 824:
_Monthly Notices_, vol. lx. p. 259.
Footnote 825:
A. C. Maury, _Harvard Annals_, vol. xxviii. p. 20.
Footnote 826:
See _ante_, p. 230.
Footnote 827:
_Astr. Mittheilungen_, Th. iii.; quoted in _Observatory_, vol. xvii.
p. 309.
Footnote 828:
_Bull. Société Astr. de France_, 1896, p. 293.
Footnote 829:
_Astr. Nach._ Nos. 3587, 3588.
Footnote 830:
Vogel, _Jahresbericht_, 1899.
Footnote 831:
_Monthly Notices_, vol. lvii. p. 16.
Footnote 832:
_Knowledge_, vol. xx. p. 194.
Footnote 833:
Clerke, _System of the Stars_, p. 284.
Footnote 834:
_Astr. Nach._ No. 3111; _Astr. and Astrophysics_, vol. xiii. p. 792.
Footnote 835:
_Astr. Nach._ No. 3535.
Footnote 836:
Cf. photographs of the cluster and nebula by Naegamvala, _Knowledge_,
vol. xix. p. 183, and by Roberts, _ibid._ vol. xxiii. p. 132.
Footnote 837:
_Sidereal Messenger_, vol. ix. p. 47.
Footnote 838:
_Astr. Nach._ No. 2918; _Astr. and Astrophysics_, vol. xiii. p. 178.
Footnote 839:
_Ibid._ p. 643.
Footnote 840:
Gore, _Knowledge_, vol. xxii. p. 201.
Footnote 841:
_Abhandl. der kön. Sächs. Ges._ Bd. xv. Th. i. 1889.
Footnote 842:
Reproduced in _Knowledge_, vol. xix. p. 109.
Footnote 843:
_Astr. and Astrophysics_, vol. xiii. p. 642.
Footnote 844:
_Astr. and Astrophysics_, vol. xiii. pp. 180, 182.
Footnote 845:
_Publ. Lick Observatory_, vol. iii. p. 202.
Footnote 846:
_Cape Results_, p. 115.
Footnote 847:
See _Knowledge_, vol. xxi. p. 279.
Footnote 848:
Bailey, _Astr. and Astrophysics_, vol. xii. p. 689.
Footnote 849:
Pickering, _Harvard Annals_, vol. xxvi. p. 218.
Footnote 850:
_Ibid._ p. 221.
Footnote 851:
_Ibid._ p. 220.
Footnote 852:
_Phil. Trans._ vol. cli. p. 732.
Footnote 853:
_Publ. Pacific Society_, vol. iii. p. 375.
Footnote 854:
_Ibid._ No. 70, p. 201.
Footnote 855:
_Astroph. Journ._ vol. x. p. 246.
Footnote 856:
_Astroph. Journ._ vol. xii. p. 181.
Footnote 857:
_Abhandl. der Kön. Preuss. Akad._ 1892, Anhang.
Footnote 858:
J. Herschel, _Cape Results_, pp. 23, 113, Plate vi. Fig. 14.
Footnote 859:
_Engl. Mechanic_, vol. li. p. 378; _Sidereal Messenger_, vols. ix. p.
380; x. p. 107.
Footnote 860:
_Monthly Notices_, vol. 1. p. 517.
Footnote 861:
_Astroph. Journ._ vol. ii. p. 321; _Harvard Annals_, vol. xxxviii.,
contains a complete discussion of these variables by Bailey.
Footnote 862:
_Address on Astronomical Photography_, p. 26; _Knowledge_, vol. xxi.
p. 280 (Clerke).
Footnote 863:
_Harvard Circular_, No. 33.
Footnote 864:
_Astroph. Journ._ vol. x. p. 255.
Footnote 865:
_Publ. Pacific Society_, No. 70, p. 210.
Footnote 866:
_Astr. Nach._ No. 3519.
Footnote 867:
Parkhurst, _Astr. Journ._ No. 482.
Footnote 868:
_Trans. Royal Dublin Society_, vol. ii. p. 132.
Footnote 869:
Bélopolsky, _Astr. Nach._ No. 3338.
Footnote 870:
_Celestial Photographs_, vol. i. p. 119.
Footnote 871:
Secchi, _Atti dell’ Accad. Pont._ t. vii. p. 90, 1853.
Footnote 872:
_Harvard Circular_, No. 33.
Footnote 873:
_Harvard Annals_, vol. xxxviii. p. 234.
Footnote 874:
_Astroph. Journ._ vol. x. p. 263.
Footnote 875:
_Harvard Annals_, vol. xxvi. p. 247.
Footnote 876:
_General Astronomy_, section 891.
Footnote 877:
_Astr. Nach._ 3549; _Astroph. Journ._ vol. ix. p. 149.
Footnote 878:
_Atlas of Stellar Spectra_, p. 125.
Footnote 879:
_Celestial Photographs_, vol. ii. p. 63.
Footnote 880:
Scheiner, _Photographie der Gestirne_, p. 332.
Footnote 881:
This is denied by M. Antoniadi, on the strength of an observation with
the giant siderostat of Paris, 1st Sept. 1900 (_Knowledge_, vol.
xxiii. p. 251).
Footnote 882:
Swift, _Pop. Astr._ vol. i. p. 112.
Footnote 883:
Rabourdin, _Cosmos_, 19 Fevrier 1898, p. 232.
Footnote 884:
_Cape Results_, p. 53.
Footnote 885:
_Knowledge_, vol. xxiii. p. 132.
Footnote 886:
_Trans. Royal Dublin Society_, vol. ii. p. 79.
Footnote 887:
_Engl. Mechanic_, vol. xlii. p. 311.
Footnote 888:
_Celestial Photographs_, vol. i. p. 78.
Footnote 889:
_Astr. Nach._ No. 3601.
Footnote 890:
_Astroph. Journ._ vol. xi. p. 3.
Footnote 891:
_Astronomical and Physical Researches at Daramona_, Appended
Illustrations.
Footnote 892:
_Astroph. Journ._ vol. xiv. p. 34.
Footnote 893:
_Trans. Royal Dublin Society_, vol. ii. p. 135.
Footnote 894:
_Trans. Royal Dublin Society_, vol. ii. p. 50.
Footnote 895:
_Observatory_, vol. xii. p. 84.
Footnote 896:
_Cape Results_, pp. 20, 94.
Footnote 897:
Rosse, _Trans. Royal Dublin Society_, vol. ii. p. 19.
Footnote 898:
_Ibid._ p. 57.
Footnote 899:
They seem to have been depicted two months earlier by Max Wolf. See
_Astr. Nach._ Nos. 3214, 3217.
Footnote 900:
_Astr. and Astrophysics_, vol. xiii. p. 183.
Footnote 901:
_Celestial Photographs_, vol. ii. p. 159.
Footnote 902:
_Ibid._ vol. i. p. 41.
Footnote 903:
_Astroph. Journ._ vol. xi. p. 4.
Footnote 904:
_Celestial Photographs_, vol. ii. p. 135.
Footnote 905:
_Sitzungsberichte der kgl. bayer. Akad. der Wissenschaften_, Bd. xxxi.
Heft ii. p. 111 (1901).
Footnote 906:
_Astr. and Astrophysics_, vol. xii. p. 300.
Footnote 907:
Published as an “Inaugural Dissertation” at Berlin in 1892.
Footnote 908:
_Astroph. Journ._ vol. xi. p. 347.
Footnote 909:
_Gen. Cat._ Nos. 3572, 3574; _Phil. Trans._ vol. cxxiii. p. 496.
Footnote 910:
_Monthly Notices_, vol. lii. p. 458.
Footnote 911:
_Monthly Notices_, vol. lii. p. 453.
Footnote 912:
_Sterngruppen und Nebelmassen_, p. 33.
Footnote 913:
_Phil. Trans._ vol. cxl. p. 512, plate xxxvii. fig. 6.
Footnote 914:
_Memoirs Royal Astr. Society_, vol. xxiii. p. 62.
Footnote 915:
_Monthly Notices_, vol. xlvii. p. 416.
Footnote 916:
_Copernicus_, vol. i. p. 50.
Footnote 917:
_Popular Astronomy_, vol. i. p. 370.
Footnote 918:
_Trans. Royal Dublin Society_, vol. ii. p. 97.
Footnote 919:
_Astr. Nach._ No. 3517.
Footnote 920:
_Monthly Notices_, vol. xlviii. p. 331.
Footnote 921:
_Celestial Photographs_, vol. i. p. 131.
Footnote 922:
_Astr. Nach._ No. 3168.
Footnote 923:
J. Herschel, _Phil. Trans._ vol. cxxiii. p. 403; Rosse, _Trans. Royal
Dublin Soc._ vol. ii. p. 95; Spitaler, _Astr. Nach._ No. 3168.
Footnote 924:
See a photograph by Roberts, _Celestial Photographs_, vol. ii. p. 135.
Footnote 925:
_Lick Publications_, vol. iii. p. 204.
Footnote 926:
_Cape Results_, p. 20.
Footnote 927:
_Trans. Royal Dublin Society_, vol. ii. p. 118.
Footnote 928:
_Astr. Nach._ No. 3517.
Footnote 929:
J. Herschel, _Phil. Trans._ vol. cxxiii. p. 431.
Footnote 930:
_Ibid._ vol. cxl. p. 512.
Footnote 931:
J. Herschel, _Cape Results_, p. 22.
Footnote 932:
_Astr. Nach._ No. 2755.
Footnote 933:
Pickering, _Harvard Circular_, No. 12.
Footnote 934:
_Popular Astronomy_, vol. i. p. 370.
Footnote 935:
_Astr. Nach._ No. 3704.
Footnote 936:
_Sitzungsberichte_, Munich, 23rd March 1901, p. 111.
Footnote 937:
_Astr. Nach._ No. 3517.
Footnote 938:
_Phil. Trans._ vol. lxxxi. p. 71.
Footnote 939:
_Leipzig Abhandl._ Bd. iii. p. 314.
Footnote 940:
_Trans. Royal Dublin Society_, vol. ii. p. 40; _Phil. Trans._ vol.
cli. p. 714.
Footnote 941:
_Lick Publications_, vol. ii. p. 161.
Footnote 942:
_Astr. Nach._ No. 3111.
Footnote 943:
Since the above words were written it has been applied. Professor
Frost examined, in September 1902, a spectrograph of S.D. −10° 4713
taken by Mr. Ellerman in 1899, and finds it to bear authentic marks of
the helium type.
Footnote 944:
_Sidereal Messenger_, vol. iv. p. 39.
Footnote 945:
_Astr. and Astrophysics_, vol. xiii. p. 177.
Footnote 946:
_Ibid._ p. 181.
Footnote 947:
_Astr. Nach._ No. 2683.
Footnote 948:
_Ibid._ No. 3111.
Footnote 949:
_Ibid._ No. 3101.
Footnote 950:
_Monthly Notices_, vol. lii. p. 442.
Footnote 951:
_Ibid._ p. 454.
Footnote 952:
_Publications of the Yerkes Observatory_, vol. i. p. 75.
Footnote 953:
_Knowledge_, vol. xvii. p. 17; _Astr. Journ._ No. 447.
Footnote 954:
_Publications of the Yerkes Observatory_, vol. i. p. 227.
Footnote 955:
_Celestial Cycle_, p. 152 (ed. 1881).
Footnote 956:
_Phil. Trans._ vol. cxxiii. p. 380.
Footnote 957:
_Ibid._ vol. cxl. p. 514, fig. 16; _Trans. Royal Dublin Society_, vol.
ii. p. 50.
Footnote 958:
_Phil. Trans._ vol. cxci. p. 129.
Footnote 959:
W. H. Pickering, _Harvard Annals_, vol. xxxii. p. 66.
Footnote 960:
_Knowledge_, vol. xvii. p. 17.
Footnote 961:
_Astr. Nach._ No. 2707.
Footnote 962:
_Monthly Notices_, vol. xlix. p. 363.
Footnote 963:
Ingall, _ibid._ p. 420.
Footnote 964:
_Popular Astronomy_, Sept. 1897, pp. 229, 232.
Footnote 965:
_Phil. Trans._ vol. cliv. p. 442.
Footnote 966:
_Astr. Nach._ No. 1613.
Footnote 967:
_Monthly Notices_, vol. xlii. p. 446.
Footnote 968:
_Comptes Rendus_, t. xc. p. 837.
Footnote 969:
_Monthly Notices_, vol. xlii. p. 446.
Footnote 970:
_Publ. Lick Observatory_, vol. iii. p. 203.
Footnote 971:
Cf. J. H. Jeans, “On the Stability of a Spherical Nebula,” _Phil.
Trans._ vol. cxcix. A, p. 1.
Footnote 972:
Ginzel, _Astr. Nach._ No. 2829.
Footnote 973:
_Lick Publications_, vol. iii. p. 209.
Footnote 974:
_System of the Stars_, p. 256, Fig. 35.
Footnote 975:
_Publ. Lick Observatory_, vol. iii. p. 212.
Footnote 976:
_Phil. Trans._ vol. xcii. p. 522.
Footnote 977:
_Trans. Royal Dublin Society_, vol. ii. p. 93.
Footnote 978:
_Celestial Photographs_, vol. ii. p. 127.
Footnote 979:
_Phil. Trans._ vol. cxxiii. p. 402.
Footnote 980:
_Ibid._ vol. cxl. plate xxxvii. fig. 11, p. 513.
Footnote 981:
_Phil. Trans._ vol. cli. p. 721.
Footnote 982:
_Publ. Lick Observatory_, vol. iii. p. 203.
Footnote 983:
_Monthly Notices_, vol. lvi. p. 379.
Footnote 984:
_Astr. Journ._ vol. ii. p. 141.
Footnote 985:
_Publ. Lick Observatory_, vol. ii. p. 159.
Footnote 986:
_Astr. Nach._ No. 3017; Burnham, _Monthly Notices_, vol. lii. pp. 33,
449.
Footnote 987:
_Ibid._ vol. xlviii. p. 388.
Footnote 988:
_Monthly Notices_, vol. lii. p. 40; _Publ. Lick Observatory_, vol. ii.
p. 165.
Footnote 989:
_Ibid._ vol. iii. p. 217.
Footnote 990:
_Astr. Nach._ No. 1885.
Footnote 991:
_Publ. Lick Observatory_, vol. iii. pp. 212, 217 (Keeler).
Footnote 992:
This assertion must be qualified if Miss Cannon’s observation of the
line λ 5007 in certain members of the Wolf-Rayet family be
substantiated. _Harvard Annals_, vol. xxviii. p. 141.
Footnote 993:
Wilsing and Scheiner, _Astr. Nach._ No. 3805.
Footnote 994:
Campbell, _Astr. and Astrophysics_, vol. xiii. p. 494.
Footnote 995:
A formula connecting these three lines into a series has been
published by Mr. E. F. J. Love (_Monthly Notices_ vol. lxii. p. 524),
but their conformity to it may be purely accidental.
Footnote 996:
Ferry’s experiments at Upsala in 1898 went to show that C gains in
relative strength with _diminishing_ pressure (_Physical Review_, vol.
vii. p. 6).
Footnote 997:
Campbell, _Astr. and Astrophysics_, vol. xiii. p. 496.
Footnote 998:
_Publ. Lick Observatory_, vol. iii. p. 209.
Footnote 999:
_Astr. and Astrophysics_, vol. xiii. p. 495.
Footnote 1000:
_Ibid._ vol. xii. pp. 52, 55.
Footnote 1001:
_Astr. Nach._ Nos. 2292, 2293, 2309.
Footnote 1002:
_Publ. Lick Observatory_, vol. iii. p. 214.
Footnote 1003:
_Astr. and Astrophysics_, vol. xiii. p. 498.
Footnote 1004:
_Ibid._ vol. xii. p. 55.
Footnote 1005:
_Astr. Nach._ No. 2353; _Copernicus_, vol. i. p. 2.
Footnote 1006:
_Monthly Notices_, vol. xxxiv. p. 71.
Footnote 1007:
Keeler, _Publ. Lick Observatory_, vol. iii. p. 214.
Footnote 1008:
_Monthly Notices_, vol. lii. pp. 45, 46.
Footnote 1009:
_Memoirs Royal Astr. Society_, vol. xxiii. p. 61.
Footnote 1010:
_Monthly Notices_, vol. lii. p. 31.
Footnote 1011:
_Publ. Lick Observatory_, vol. iii. p. 211.
Footnote 1012:
_Astr. and Astrophysics_, vol. xii. p. 52.
Footnote 1013:
_Observatory_, vol. v. p. 26.
Footnote 1014:
D’Arrest, _Leipzig Abhandl._ Bd. iii. p. 308, 1857.
Footnote 1015:
Wilsing, _Astr. Nach._ Nos. 3190, 3261.
Footnote 1016:
Kleiber, _Astr. Nach._ No. 3037.
Footnote 1017:
_Astroph. Journ._ vol. x. p. 197.
Footnote 1018:
_Astr. Jahrbuch_, 1788, p. 242; _Phil. Trans._ vol. lxxv. p. 263.
Footnote 1019:
_Monthly Notices_, vol. xxxvi. p. 66.
Footnote 1020:
_Ibid._ vol. xlviii. p. 387.
Footnote 1021:
_Phil. Trans._ vol. cxxxiv. plate xix. fig. 29.
Footnote 1022:
_Nature_, vol. xliii. p. 420.
Footnote 1023:
_Astr. Jahrbuch_, 1803, p. 106.
Footnote 1024:
_Trans. Royal Dublin Society_, vol. ii. p. 152.
Footnote 1025:
_Astr. Nach._ No. 1018.
Footnote 1026:
_Nova Acta Societatis R. Upsaliensis_, ser. iii. vol. ix. p. 99, 1874.
Footnote 1027:
_Astr. Nach._ No. 2186.
Footnote 1028:
_Astr. Journ._ No. 200.
Footnote 1029:
_Potsdam Publ._ No. 14, p. 35.
Footnote 1030:
_Astr. Nach._ No. 2800.
Footnote 1031:
_Astroph. Journ._ vol. x. p. 199.
Footnote 1032:
_Astr. Nach._ No. 3200.
Footnote 1033:
_Monthly Notices_, vol. lx. p. 245.
Footnote 1034:
_Ibid._ p. 257.
Footnote 1035:
_Astr. and Astrophysics_, vol. xii. pp. 52, 55.
Footnote 1036:
_Ibid._ vol. xiii. p. 497.
Footnote 1037:
_Trans. Royal Dublin Society_, vol. ii. p. 156.
Footnote 1038:
_Monthly Notices_, vol. lii. p. 44.
Footnote 1039:
_Celestial Photographs_, vol. ii. p. 133.
Footnote 1040:
_Publ. Pacific Society_, vol. xi. p. 180.
Footnote 1041:
_Knowledge_, vol. xxiii. p. 250.
Footnote 1042:
_Memoirs Royal Astr. Society_, vol. xxxvi. p. 47.
Footnote 1043:
_Monthly Notices_, vol. lii. p. 38.
Footnote 1044:
_Cape Results_, p. 114.
Footnote 1045:
Swift, _Pop. Ast._ December 1897, p. 426.
Footnote 1046:
_Phil. Trans._ vols. cxl. p. 513; cli. p. 716.
Footnote 1047:
_Memoirs Royal Astr. Society_, vol. xxiii. p. 60.
Footnote 1048:
_Celestial Photographs_, vol. ii. p. 35.
Footnote 1049:
_Cape Results_, p. 90, plate vi. fig. 7.
Footnote 1050:
_Potsdam Publ._ No. 14, p. 37.
Footnote 1051:
_Phil. Trans._ vol. cli. plate xxx. fig. 40.
Footnote 1052:
_Monthly Notices_, vol. lii. p. 46.
Footnote 1053:
_Publ. Lick Observatory_, vol. iii. p. 215.
Footnote 1054:
_Astr. and Astrophysics_, vol. xiii. p. 499.
Footnote 1055:
_English Mechanic_, vol. xlii. p. 311.
Footnote 1056:
_Harvard Annals_, vol. xxxiii. p. 146.
Footnote 1057:
_Publ. Pacific Society_, No. 70, p. 201, 1899.
Footnote 1058:
Scheiner, _Astr. Nach._ No. 3086; Wilsing, _ibid._ No. 3261.
Footnote 1059:
_Monthly Notices_, vol. xlviii. p. 393.
Footnote 1060:
_Knowledge_, vol. xv. p. 149.
Footnote 1061:
_Potsdam Publ._ No. 14, p. 38.
Footnote 1062:
_Publ. Lick Observatory_, vol. iii. p. 213; _Astroph. Journ._ vol. x.
p. 195.
Footnote 1063:
_Astr. Nach._ No. 3086.
Footnote 1064:
_Phil. Trans._ vol. cxl. p. 514, fig. 15.
Footnote 1065:
_Abhandl. Leipziger Akad._ Bd. iii. p. 321; _Astr. Nach._ No. 1885.
Footnote 1066:
_Memoirs Royal Astr. Society_, vol. xxiii. p. 61.
Footnote 1067:
_Les Étoiles_, t. ii. p. 16.
Footnote 1068:
_Monthly Notices_, vol. xxviii. p. 154.
Footnote 1069:
_Ibid._ vol. lii. p. 35.
Footnote 1070:
_Ibid._ vol. lx. p. 256.
Footnote 1071:
Comas Solà, _Astr. Nach._ No. 3751.
Footnote 1072:
_Knowledge_, vol. xii. p. 147.
Footnote 1073:
_Ibid._ vol. xx. p. 193.
Footnote 1074:
_Astr. Nach._ No. 3027.
Footnote 1075:
_Harvard Annals_, vol. xxxii. p. 66.
Footnote 1076:
_Astr. and Astrophysics_, vol. xiii. p. 813.
Footnote 1077:
_Harvard Annals_, vol. xxxii. p. 77.
Footnote 1078:
_Astr. Nach._ No. 3513.
Footnote 1079:
_Jahrbuch der Erfindungen_, 1898, p. 101; _Potsdam Publ._ Bd. xi.
Footnote 1080:
_Harvard Annals_, vol. xxxii. p. 80.
Footnote 1081:
Vogel, _Sitzungsberichte_, Berlin, 13 März 1902.
Footnote 1082:
_Astroph. Journ._ vol. x. p. 29.
Footnote 1083:
_Astr. Nach._ No. 3476.
Footnote 1084:
_Publ. Lick Observatory_, vol. iii. p. 224.
Footnote 1085:
_Astroph. Journ._ vol. ix. p. 312.
Footnote 1086:
_Astroph. Journ._ vol. viii. p. 32; _Astr. Nach._ No. 3471.
Footnote 1087:
_Astroph. Journ._ vol. ix. p. 133.
Footnote 1088:
_Ibid._ p. 140.
Footnote 1089:
_Publ. Pacific Society_, vol. v. p. 207.
Footnote 1090:
_Astr. Nach._ No. 3476.
Footnote 1091:
_Proc. Royal Society_, vol. lviii. p. 255.
Footnote 1092:
_Proc. Royal Society_, vol. xlvi. p. 41.
Footnote 1093:
_Astr. and Astrophysics_, vol. xiii. p. 478.
Footnote 1094:
_Washington Observations_, 1878, App. i.
Footnote 1095:
_Publ. Leander M‘Cormick Observatory_, vol. i. pt. vii. p. 274.
Footnote 1096:
_Mémoires de l’Acad. de St. Pétersbourg_, t. v. No. 4, p. 115.
Footnote 1097:
_Publ. Leander M‘Cormick Observatory_, vol. i. p. 333.
Footnote 1098:
_Knowledge_, vol. xvii. p. 133.
Footnote 1099:
Copied in _Knowledge_, vol. xvi. p. 69.
Footnote 1100:
_Phil. Trans._ 1786, p. 494; 1789, p. 247; 1811, p. 289; quoted by
Holden, _American Journ. of Science_, vol. xiv. p. 434, 1877.
Footnote 1101:
_Memoirs Royal Astr. Society_, vol. iii. p. 63.
Footnote 1102:
Reproduced in _Astroph. Journ._ vol. xi. p. 325.
Footnote 1103:
_Popular Astronomy_, vol. i. p. 251.
Footnote 1104:
_Publ. Lick Observatory_, vol. iii. p. 205.
Footnote 1105:
_Astroph. Journ._ vol. ii. p. 162.
Footnote 1106:
_Phil. Trans._ vol. cxxiii. p. 461.
Footnote 1107:
_Cape Results_, p. 10.
Footnote 1108:
_Sidereal Messenger_, vol. iv. p. 38.
Footnote 1109:
_Celestial Photographs_, vol. i. p. 101.
Footnote 1110:
_Amer. Journ. of Science_, vol. xi. p. 341, third series.
Footnote 1111:
_Monthly Notices_, vol. xlvii. p. 420.
Footnote 1112:
_Forty-Seventh Annual Report._
Footnote 1113:
_Harvard Annals_, vol. xxvi. p. 206.
Footnote 1114:
_Astroph. Journ._ vol. ii. p. 350.
Footnote 1115:
_Astr. Nach._ No. 3082.
Footnote 1116:
_Ibid._ No. 3157.
Footnote 1117:
_Ibid._ No. 3301; _Knowledge_, vol. xix. p. 205.
Footnote 1118:
_Knowledge_, vol. xvii. p. 17.
Footnote 1119:
_Ibid._ vol. xiv. pp. 188, 230; _Observatory_, vol. xiv. p. 301;
_Astr. Nach._ No. 3048.
Footnote 1120:
_Knowledge_, vol. xxii. p. 132.
Footnote 1121:
_Astr. and Astrophysics_, vol. xi. p. 528; Wolf, _Astr. Nach._ No.
3130.
Footnote 1122:
_Celestial Photographs_, vol. i. p. 113.
Footnote 1123:
_Astr. Nach._ No. 3738.
Footnote 1124:
_Cape Results_, p. 20.
Footnote 1125:
_Ibid._ p. 24.
Footnote 1126:
_Conn. des Temps pour 1784_, p. 229.
Footnote 1127:
Smyth, _Celestial Cycle_, p. 145 (ed. 1881).
Footnote 1128:
_Phil. Trans._ vols. cxxxiv. p. 322, cli. p. 715; _Trans. Royal Dublin
Soc._ vol. ii. p. 47.
Footnote 1129:
_Celestial Photographs_, vol. i. p. 52.
Footnote 1130:
_Harvard Annals_, vol. xiii. pp. 64, 66.
Footnote 1131:
_Phil. Trans._ vol. cli. p. 713, plate xxv. fig. 6.
Footnote 1132:
_Comptes Rendus_, t. cxxvi. p. 1191.
Footnote 1133:
_Phil. Trans._ vol. cli. p. 713; _Trans. Royal Dublin Society_, vol.
ii. p. 32.
Footnote 1134:
Webb, _Celestial Objects_, vol. ii. p. 188 (5th edition).
Footnote 1135:
_Trans. Royal Dublin Society_, vol. ii. p. 51.
Footnote 1136:
_Knowledge_, vol. xviii. p. 253.
Footnote 1137:
Smyth, _Celestial Cycle_, p. 361.
Footnote 1138:
_Harvard Annals_, vol. xxxiii. p. 144.
Footnote 1139:
J. Herschel, _Phil. Trans._ vol. cxxiii. p. 434; Rosse, _Trans. Royal
Dublin Society_, vol. ii. p. 122.
Footnote 1140:
_Celestial Photographs_, vol. i. p. 81.
Footnote 1141:
Smyth, _Celestial Cycle_, p. 472; Webb, _Celestial Objects_, vol. ii.
p. 137.
Footnote 1142:
_Phil. Trans._ vol. clvi. p. 390; _Harvard Annals_, vol. xxxiii. p.
144.
Footnote 1143:
_Trans. Royal Dublin Society_, vol. ii. p. 150.
Footnote 1144:
_Ibid._ p. 123.
Footnote 1145:
_Celestial Photographs_, vol. ii. p. 129.
Footnote 1146:
_Sidereal Messenger_, vol. iv. p. 4.
Footnote 1147:
Dreyer, _Memoirs Royal Astr. Society_, vol. xlix. p. 215.
Footnote 1148:
_Monthly Notices_, vol. lii. p. 455.
Footnote 1149:
_Astr. Nach._ No. 1391.
Footnote 1150:
_Ibid._ No. 1366.
Footnote 1151:
_Lick Publications_, vol. ii. p. 176.
Footnote 1152:
_Monthly Notices_, vol. lvi. p. 66.
Footnote 1153:
_Monthly Notices_, vol. lix. p. 372.
Footnote 1154:
_Ibid._ vol. lx. p. 424.
Footnote 1155:
_Astr. Nach._ No. 1689; Barnard, _Monthly Notices_, vol. lv. p. 442.
Footnote 1156:
_Monthly Notices_, vol. l. p. 440.
Footnote 1157:
_Astr. Nach._ No. 2212.
Footnote 1158:
_Monthly Notices_, vol. lv. p. 451.
Footnote 1159:
Quoted by Bigourdan, _Comptes Rendus_, t. cxii. p. 471.
Footnote 1160:
_Astr. Nach._ Nos. 3030, 3167, 3168.
Footnote 1161:
_Publications of the Lick Observatory_, vol. ii. p. 172.
Footnote 1162:
_Astr. Nach._ No. 3097; _Monthly Notices_, vol. lv. p. 452.
Footnote 1163:
_Astr. Nach._ No. 3097.
Footnote 1164:
_Observatory_, vol. xv. p. 104.
Footnote 1165:
Denning, _Astr. Nach._ No. 3111.
Footnote 1166:
_Astr. and Astrophysics_, vol. xi. p. 566.
Footnote 1167:
_Monthly Notices_, vol. xxxviii. p. 104; _Astr. Nach._ No. 2293; _V.
J. S. Astr. Ges._ Jahrgang xiv. p. 167.
Footnote 1168:
_Publ. Lick Observatory_, vol. ii. p. 172.
Footnote 1169:
_Memoirs Royal Astr. Society_, vol. xlix. p. 213.
Footnote 1170:
_Publ. Lick Observatory_, vol. ii. p. 176.
Footnote 1171:
Ranyard, _Knowledge_, vol. xv. p. 132.
Footnote 1172:
_Knowledge_, vol. xv. p. 191.
Footnote 1173:
_Phil. Trans._ vol. clviii. p. 72.
Footnote 1174:
_Knowledge_, vol. xix. p. 39.
Footnote 1175:
Clerk Maxwell, _Theory of Heat_, 10th ed. p. 245.
Footnote 1176:
_Cardiff Address_, p. 21.
Footnote 1177:
E. S. Ferry, _Physical Review_, vol. vii. p. 6; Lewis, _Astroph.
Journ._ vol. x. p. 141.
Footnote 1178:
Maunder, _Knowledge_, vol. xix. p. 286.
Footnote 1179:
Barnard, _Monthly Notices_, vol. l. p. 310.
Footnote 1180:
_Ibid._ p. 314; _Astr. and Astrophysics_, vol. xiii. p. 179.
Footnote 1181:
_Astroph. Journ._, vol. ix. p. 157.
Footnote 1182:
Ranyard, _Knowledge_, vol. xiv. p. 112.
Footnote 1183:
Barnard, _Report Harvard Conference_, Aug. 1898, p. 19.
Footnote 1184:
Burns, _Knowledge_, vol. xxii. p. 227.
Footnote 1185:
_Ibid._ vol. xix. p. 205.
Footnote 1186:
_Annals of Cape Observatory_, vol. iii. p. 22 (Introduction).
Footnote 1187:
_Knowledge_, vol. xiv. p. 51.
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