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.