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.