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.