THE STELLAR UNIVERSE--_continued_
It is very interesting to consider the proper motions of stars with
reference to such an isolated stellar system as has been pictured in the
previous chapter. These proper motions are so minute as a rule, that we
are quite unable to determine whether the stars which show them are
moving along in straight lines, or in orbits of immense extent. It
would, in fact, take thousands of years of careful observation to
determine whether the paths in question showed any degree of curving. In
the case of the more distant stars, the accurate observations which have
been conducted during the last hundred years have not so far revealed
any proper motions with regard to them; but one cannot escape the
conclusion that these stars move as the others do.
If space outside our stellar system is infinite in extent, and if all
the stars within that system are moving unchecked in every conceivable
direction, the result must happen that after immense ages these stars
will have drawn apart to such a distance from each other, that the
system will have entirely disintegrated, and will cease to exist as a
connected whole. Eventually, indeed, as Professor Newcomb points out,
the stars will have separated so far from each other that each will be
left by itself in the midst of a black and starless sky. If, however, a
certain proportion of stars have a speed sufficiently slow, they will
tend under mutual attraction to be brought to rest by collisions, or
forced to move in orbits around each other. But those stars which move
at excessive speeds, such, for instance, as 1830 Groombridge, or the
star in the southern constellation of Pictor, seem utterly incapable of
being held back in their courses by even the entire gravitative force of
our stellar system acting as a whole. These stars must, therefore, move
eventually right through the system and pass out again into the empty
spaces beyond. Add to this; certain investigations, made into the speed
of 1830 Groombridge, furnish a remarkable result. It is calculated,
indeed, that had this star been _falling through infinite space for
ever_, pulled towards us by the combined gravitative force of our entire
system of stars, it could not have gathered up anything like the speed
with which it is at present moving. No force, therefore, which we can
conjure out of our visible universe, seems powerful enough either to
have impressed upon this runaway star the motion which it now has, or to
stay it in its wild course. What an astounding condition of things!
Speculations like this call up a suspicion that there may yet exist
other universes, other centres of force, notwithstanding the apparent
solitude of our stellar system in space. It will be recollected that the
idea of this isolation is founded upon such facts as, that the heavens
do not blaze with light, and that the stars gradually appear to thin out
as we penetrate the system with increasing telescopic power. But
perchance there is something which hinders us from seeing out into space
beyond our cluster of stars; which prevents light, in fact, from
reaching us from other possible systems scattered through the depths
beyond. It has, indeed, been suggested by Mr. Gore[38] that the
light-transmitting ether may be after all merely a kind of "atmosphere"
of the stars; and that it may, therefore, thin off and cease a little
beyond the confines of our stellar system, just as the air thins off and
practically ceases at a comparatively short distance from the earth. A
clashing together of solid bodies outside our atmosphere could plainly
send us no sound, for there is no air extending the whole way to bear to
our ears the vibrations thus set up; so light emitted from any body
lying beyond our system of stars, would not be able to come to us if the
ether, whose function it is to convey the rays of light, ceased at or
near the confines of that system.
Perchance we have in this suggestion the key to the mystery of how our
sun and the other stellar bodies maintain their functions of temperature
and illumination. The radiations of heat and light arriving at the
limits of this ether, and unable to pass any further, may be thrown back
again into the system in some altered form of energy.
But these, at best, are mere airy and fascinating speculations. We have,
indeed, no evidence whatever that the luminiferous ether ceases at the
boundary of the stellar system. If, therefore, it extends outwards
infinitely in every direction, and if it has no absorbing or weakening
effect on the vibrations which it transmits, we cannot escape from the
conclusion that practically all the rays of light ever emitted by all
the stars must chase one another eternally through the never-ending
abysses of space.
[38] _Planetary and Stellar Studies_, by John Ellard Gore, F.R.A.S.,
M.R.I.A., London, 1888.