THE STELLAR UNIVERSE
The stars appear fairly evenly distributed all around us, except in one
portion of the sky where they seem very crowded, and so give one an
impression of being very distant. This portion, known as the Milky Way,
stretches, as we have already said, in the form of a broad band right
round the entire heavens. In those regions of the sky most distant from
the Milky Way the stars appear to be thinly sown, but become more and
more closely massed together as the Milky Way is approached.
This apparent distribution of the stars in space has given rise to a
theory which was much favoured by Sir William Herschel, and which is
usually credited to him, although it was really suggested by one Thomas
Wright of Durham in 1750; that is to say, some thirty years or more
before Herschel propounded it. According to this, which is known as the
"Disc" or "Grindstone" Theory, the stars are considered as arranged in
space somewhat in the form of a thick disc, or grindstone, close to the
_central_ parts of which our solar system is situated.[35] Thus we
should see a greater number of stars when we looked out through the
_length_ of such a disc in any direction, than when we looked out
through its _breadth_. This theory was, for a time, supposed to account
quite reasonably for the Milky Way, and for the gradual increase in the
number of stars in its vicinity.
It is quite impossible to verify directly such a theory, for we know the
actual distance of only about forty-three stars. We are unable,
therefore, definitely to assure ourselves whether, as the grindstone
theory presupposes, the stellar universe actually reaches out very much
further from us in the direction of the Milky Way than in the other
parts of the sky. The theory is clearly founded upon the supposition
that the stars are more or less equal in size, and are scattered through
space at fairly regular distances from each other.
Brightness, therefore, had been taken as implying nearness to us, and
faintness great distance. But we know to-day that this is not the case,
and that the stars around us are, on the other hand, of various degrees
of brightness and of all orders of size. Some of the faint stars--for
instance, the galloping star in Pictor--are indeed nearer to us than
many of the brighter ones. Sirius, on the other hand, is twice as far
off from us as [a] Centauri, and yet it is very much brighter; while
Canopus, which in brightness is second only to Sirius out of the whole
sky, is too far off for its distance to be ascertained! It must be
remembered that no parallax had yet been found for any star in the days
of Herschel, and so his estimations of stellar distances were
necessarily of a very circumstantial kind. He did not, however, continue
always to build upon such uncertain ground; but, after some further
examination of the Milky Way, he gave up his idea that the stars were
equally disposed in space, and eventually abandoned the grindstone
theory.
Since we have no means of satisfactorily testing the matter, through
finding out the various distances from us at which the stars are really
placed, one might just as well go to the other extreme, and assume that
the thickening of stars in the region of the Milky Way is not an effect
of perspective at all, but that the stars in that part of the sky are
actually more crowded together than elsewhere--a thing which astronomers
now believe to be the case. Looked at in this way, the shape of the
stellar universe might be that of a globe-shaped aggregation of stars,
in which the individuals are set at fairly regular distances from each
other; the whole being closely encircled by a belt of densely packed
stars. It must, however, be allowed that the gradual increase in the
number of stars towards the Milky Way appears a strong argument in
favour of the grindstone theory; yet the belt theory, as above detailed,
seems to meet with more acceptance.
There is, in fact, one marked circumstance which is remarkably difficult
of explanation by means of the grindstone theory. This is the existence
of vacant spaces--holes, so to speak, in the groundwork of the Milky
Way. For instance, there is a cleft running for a good distance along
its length, and there is also a starless gap in its southern portion. It
seems rather improbable that such a great number of stars could have
arranged themselves so conveniently, as to give us a clear view right
out into empty space through such a system in its greatest thickness;
as if, in fact, holes had been bored, and clefts made, from the boundary
of the disc clean up to where our solar system lies. Sir John Herschel
long ago drew attention to this point very forcibly. It is plain that
such vacant spaces can, on the other hand, be more simply explained as
mere holes in a belt; and the best authorities maintain that the
appearance of the Milky Way confirms a view of this kind.
Whichever theory be indeed the correct one, it appears at any rate that
the stars do not stretch out in every direction to an infinite distance;
but that _the stellar system is of limited extent_, and has in fact a
boundary.
In the first place, Science has no grounds for supposing that light is
in any way absorbed or destroyed merely by its passage through the
"ether," that imponderable medium which is believed to transmit the
luminous radiations through space. This of course is tantamount to
saying that all the direct light from all the stars should reach us,
excepting that little which is absorbed in its passage through our own
atmosphere. If stars, and stars, and stars existed in every direction
outwards without end, it can be proved mathematically that in such
circumstances there could not remain the tiniest space in the sky
without a star to fill it, and that therefore the heavens would always
blaze with light, and the night would be as bright as the noonday.[36]
How very far indeed this is from being the case, may be gathered from an
estimate which has been made of the general amount of light which we
receive from the stars. According to this estimate the sky is considered
as more or less dark, the combined illumination sent to us by all the
stars being only about the one-hundreth part of what we get from the
full moon.[37]
Secondly, it has been suggested that although light may not suffer any
extinction or diminution from the ether itself, still a great deal of
illumination may be prevented from reaching us through myriads of
extinguished suns, or dark meteoric matter lying about in space. The
idea of such extinguished suns, dark stars in fact, seems however to be
merely founded upon the sole instance of the invisible companion of
Algol; but, as we have seen, there is no proof whatever that it is a
dark body. Again, some astronomers have thought that the dark holes in
the Milky Way, "Coal Sacks," as they are called, are due to masses of
cool, or partially cooled matter, which cuts off the light of the stars
beyond. The most remarkable of these holes is one in the neighbourhood
of the Southern Cross, known as the "Coal Sack in Crux." But Mr. Gore
thinks that the cause of the holes is to be sought for rather in what
Sir William Herschel termed "clustering power," _i.e._ a tendency on the
part of stars to accumulate in certain places, thus leaving others
vacant; and the fact that globular and other clusters are to be found
very near to such holes certainly seems corroborative of this theory. In
summing up the whole question, Professor Newcomb maintains that there
does not appear any evidence of the light from the Milky Way stars,
which are apparently the furthest bodies we see, being intercepted by
dark bodies or dark matter. As far as our telescopes can penetrate, he
holds that we see the stars _just as they are_.
Also, if there did exist an infinite number of stars, one would expect
to find evidence in some direction of an overpoweringly great
force,--the centre of gravity of all these bodies.
It is noticed, too, that although the stars increase in number with
decrease in magnitude, so that as we descend in the scale we find three
times as many stars in each magnitude as in the one immediately above
it, yet this progression does not go on after a while. There is, in
fact, a rapid falling off in numbers below the twelfth magnitude; which
looks as if, at a certain distance from us, the stellar universe were
beginning to _thin out_.
Again, it is estimated, by Mr. Gore and others, that only about 100
millions of stars are to be seen in the whole of the sky with the best
optical aids. This shows well the limited extent of the stellar system,
for the number is not really great. For instance, there are from fifteen
to sixteen times as many persons alive upon the earth at this moment!
Last of all, there appears to be strong photographic evidence that our
sidereal system is limited in extent. Two photographs taken by the late
Dr. Isaac Roberts of a region rich in stellar objects in the
constellation of Cygnus, clearly show what has been so eloquently called
the "darkness behind the stars." One of these photographs was taken in
1895, and the other in 1898. On both occasions the state of the
atmosphere was practically the same, and the sensitiveness of the films
was of the same degree. The exposure in the first case was only one
hour; in the second it was about two hours and a half. And yet both
photographs show _exactly the same stars, even down to the faintest_.
From this one would gather that the region in question, which is one of
the most thickly star-strewn in the Milky Way, is _penetrable right
through_ with the means at our command. Dr. Roberts himself in
commenting upon the matter drew attention to the fact, that many
astronomers seemed to have tacitly adopted the assumption that the stars
extend indefinitely through space.
From considerations such as these the foremost astronomical authorities
of our time consider themselves justified in believing that the
collection of stars around us is _finite_; and that although our best
telescopes may not yet be powerful enough to penetrate to the final
stars, still the rapid decrease in numbers as space is sounded with
increasing telescopic power, points strongly to the conclusion that the
boundaries of the stellar system may not lie very far beyond the
uttermost to which we can at present see.
Is it possible then to make an estimate of the extent of this stellar
system?
Whatever estimates we may attempt to form cannot however be regarded as
at all exact, for we know the actual distances of such a very few only
of the nearest of the stars. But our knowledge of the distances even of
these few, permits us to assume that the stars close around us may be
situated, on an average, at about eight light-years from each other; and
that this holds good of the stellar spaces, with the exception of the
encircling girdle of the Milky Way, where the stars seem actually to be
more closely packed together. This girdle further appears to contain the
greater number of the stars. Arguing along these lines, Professor
Newcomb reaches the conclusion that the farthest stellar bodies which we
see are situated at about between 3000 and 4000 light-years from us.
Starting our inquiry from another direction, we can try to form an
estimate by considering the question of proper motions.
It will be noticed that such motions do not depend entirely upon the
actual speed of the stars themselves, but that some of the apparent
movement arises indirectly from the speed of our own sun. The part in a
proper motion which can be ascribed to the movement of our solar system
through space is clearly a displacement in the nature of a parallax--Sir
William Herschel called it "_Systematic_ Parallax"; so that knowing the
distance which we move over in a certain lapse of time, we are able to
hazard a guess at the distances of a good many of the stars. An inquiry
upon such lines must needs be very rough, and is plainly based upon the
assumption that the stars whose distances we attempt to estimate are
moving at an average speed much like that of our own sun, and that they
are not "runaway stars" of the 1830 Groombridge order. Be that as it
may, the results arrived at by Professor Newcomb from this method of
reasoning are curiously enough very much on a par with those founded on
the few parallaxes which we are really certain about; with the exception
that they point to somewhat closer intervals between the individual
stars, and so tend to narrow down our previous estimate of the extent of
the stellar system.
Thus far we get, and no farther. Our solar system appears to lie
somewhere near the centre of a great collection of stars, separated each
one from the other, on an average, by some 40 billions of miles; the
whole being arranged in the form of a mighty globular cluster. Light
from the nearest of these stars takes some four years to come to us. It
takes about 1000 times as long to reach us from the confines of the
system. This globe of stars is wrapt around closely by a stellar girdle,
the individual stars in which are set together more densely than those
in the globe itself. The entire arrangement appears to be constructed
upon a very regular plan. Here and there, as Professor Newcomb points
out, the aspect of the heavens differs in small detail; but generally it
may be laid down that the opposite portions of the sky, whether in the
Milky Way itself, or in those regions distant from it, show a marked
degree of symmetry. The proper motions of stars in corresponding
portions of the sky reveal the same kind of harmony, a harmony which may
even be extended to the various colours of the stars. The stellar
system, which we see disposed all around us, appears in fine to bear all
the marks of an _organised whole_.
The older astronomers, to take Sir William Herschel as an example,
supposed some of the nebulæ to be distant "universes." Sir William was
led to this conclusion by the idea he had formed that, when his
telescopes failed to show the separate stars of which he imagined these
objects to be composed, he must put down the failure to their stupendous
distance from us. For instance, he thought the Orion Nebula, which is
now known to be made up of glowing gas, to be an external stellar
system. Later on, however, he changed his mind upon this point, and came
to the conclusion that "shining fluid" would better account both for
this nebula, and for others which his telescopes had failed to separate
into component stars.
The old ideas with regard to external systems and distant universes have
been shelved as a consequence of recent research. All known clusters and
nebulæ are now firmly believed to lie _within_ our stellar system.
This view of the universe of stars as a sort of island in the
immensities, does not, however, give us the least idea about the actual
extent of space itself. Whether what is called space is really infinite,
that is to say, stretches out unendingly in every direction, or whether
it has eventually a boundary somewhere, are alike questions which the
human mind seems utterly unable to picture to itself.
[35] The Ptolemaic idea dies hard!
[36] Even the Milky Way itself is far from being a blaze of light, which
shows that the stars composing it do not extend outwards indefinitely.
[37] Mr. Gore has recently made some remarkable deductions, with regard
to the amount of light which we get from the stars. He considers that
most of this light comes from stars below the sixth magnitude; and
consequently, if all the stars visible to the naked eye were to be
blotted out, the glow of the night sky would remain practically the same
as it is at present. Going to the other end of the scale, he thinks also
that the combined light which we get from all the stars below the
seventeenth magnitude is so very small, that it may be neglected in such
an estimation. He finds, indeed, that if there are stars so low as the
twentieth magnitude, one hundred millions of them would only be equal in
brightness to a single first-magnitude star like Vega. On the other
hand, it is possible that the light of the sky at night is not entirely
due to starlight, but that some of it may be caused by phosphorescent
glow.