What is the shape of our universe, and what are its dimensions? This is
a tremendous question to ask. It is like asking an intelligent insect,
living on a single leaf in the midst of a great Brazilian forest, to say
what is the shape and size of the forest. Yet man's ingenuity has proved
equal to giving an answer even to this question, and by a method exactly
similar to that which would be adopted by the insect. Suppose, for
instance, that the forest was shaped as an elongated oval, and the
insect lived on a tree near the centre of the oval. If the trees were
approximately equally spaced from one another they would appear much
denser along the length of the oval than across its width. This is the
simple consideration that has guided astronomers in determining the
shape of our stellar universe. There is one direction in the heavens
along which the stars appear denser than in the directions at right
angles to it. That direction is the direction in which we look towards
the Milky Way. If we count the number of stars visible all over the
heavens, we find they become more and more numerous as we approach the
Milky Way. As we go farther and farther from the Milky Way the stars
thin out until they reach a maximum sparseness in directions at right
angles to the plane of the Milky Way. We may consider the Milky Way to
form, as it were, the equator of our system, and the line at right
angles to point to the north and south poles.
Our system, in fact, is shaped something like a lens, and our sun is
situated near the centre of this lens. In the remoter part of this lens,
near its edge, or possibly outside it altogether, lies the great series
of star clouds which make up the Milky Way. All the stars are in motion
within this system, but the very remarkable discovery has been made that
these motions are not entirely random. The great majority of the stars
whose motions can be measured fall into two groups drifting past one
another in opposite directions. The velocity of one stream relative to
the other is about twenty-five miles per second. The stars forming these
two groups are thoroughly well mixed; it is not a case of an inner
stream going one way and an outer stream the other. But there are not
quite as many stars going one way as the other. For every two stars in
one stream there are three in the other. Now, as we have said, some
eminent astronomers hold that the spiral nebulæ are universes like our
own, and if we look at the two photographs (Figs. 25 and 26) we see that
these spirals present features which, in the light of what we have just
said about our system, are very remarkable. The nebula in Coma Berenices
is a spiral edge-on to us, and we see that it has precisely the
lens-shaped middle and the general flattened shape that we have found in
our own system. The nebula in Canes Venatici is a spiral facing towards
us, and its shape irresistibly suggests motions along the spiral arms.
This motion, whether it is towards or away from the central, lens-shaped
portion, would cause a double streaming motion in that central portion
of the kind we have found in our own system. Again, and altogether apart
from these considerations, there are good reasons for supposing our
Milky Way to possess a double-armed spiral structure. And the great
patches of dark absorbing matter which are known to exist in the Milky
Way (see Fig. 22) would give very much the mottled appearance we notice
in the arms (which we see edge-on) of the nebula in Coma Berenices. The
hypothesis, therefore, that our universe is a spiral nebula has much to
be said for it. If it be accepted it greatly increases our estimate of
the size of the material universe. For our central, lens-shaped system
is calculated to extend towards the Milky Way for more than twenty
thousand times a million million miles, and about a third of this
distance towards what we have called the poles. If, as we suppose, each
spiral nebula is an independent stellar universe comparable in size with
our own, then, since there are hundreds of thousands of spiral nebulæ,
we see that the size of the whole material universe is indeed beyond our
comprehension.
[Illustration: _Photo: Mount Wilson Observatory._