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.