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.