How many stars are there? A glance at a photograph of star-clouds will
tell at once that it is quite impossible to count them. The fine
photograph reproduced in Figure 2 represents a very small patch of that
pale-white belt, the Milky Way, which spans the sky at night. It is true
that this is a particularly rich area of the Milky Way, but the entire
belt of light has been resolved in this way into masses or clouds of
stars. Astronomers have counted the stars in typical districts here and
there, and from these partial counts we get some idea of the total
number of stars. There are estimated to be between two and three
thousand million stars.
Yet these stars are separated by inconceivable distances from each
other, and it is one of the greatest triumphs of modern astronomy to
have mastered, so far, the scale of the universe. For several centuries
astronomers have known the relative distances from each other of the sun
and the planets. If they could discover the actual distance of any one
planet from any other, they could at once tell all the distances within
the Solar System.
The sun is, on the latest measurements, at an average distance of
92,830,000 miles from the earth, for as the orbit of the earth is not a
true circle, this distance varies. This means that in six months from
now the earth will be right at the opposite side of its path round the
sun, or 185,000,000 miles away from where it is now. Viewed or
photographed from two positions so wide apart, the nearest stars show a
tiny "shift" against the background of the most distant stars, and that
is enough for the mathematician. He can calculate the distance of any
star near enough to show this "shift." We have found that the nearest
star to the earth, a recently discovered star, is twenty-five trillion
miles away. Only thirty stars are known to be within a hundred trillion
miles of us.
This way of measuring does not, however, take us very far away in the
heavens. There are only a few hundred stars within five hundred trillion
miles of the earth, and at that distance the "shift" of a star against
the background (parallax, the astronomer calls it) is so minute that
figures are very uncertain. At this point the astronomer takes up a new
method. He learns the different types of stars, and then he is able to
deduce more or less accurately the distance of a star of a known type
from its faintness. He, of course, has instruments for gauging their
light. As a result of twenty years work in this field, it is now known
that the more distant stars of the Milky Way are at least a hundred
thousand trillion (100,000,000,000,000,000) miles away from the sun.
Our sun is in a more or less central region of the universe, or a few
hundred trillion miles from the actual centre. The remainder of the
stars, which are all outside our Solar System, are spread out,
apparently, in an enormous disc-like collection, so vast that even a ray
of light, which travels at the rate of 186,000 miles a second, would
take 50,000 years to travel from one end of it to the other. This, then
is what we call our universe.
Are there other Universes?
Why do we say "our universe"? Why not _the_ universe? It is now believed
by many of our most distinguished astronomers that our colossal family
of stars is only one of many universes. By a universe an astronomer
means any collection of stars which are close enough to control each
other's movements by gravitation; and it is clear that there might be
many universes, in this sense, separated from each other by profound
abysses of space. Probably there are.
For a long time we have been familiar with certain strange objects in
the heavens which are called "spiral nebulæ" (Fig 4). We shall see at a
later stage what a nebula is, and we shall see that some astronomers
regard these spiral nebulæ as worlds "in the making." But some of the
most eminent astronomers believe that they are separate
universes--"island-universes" they call them--or great collections of
millions of stars like our universe. There are certain peculiarities in
the structure of the Milky Way which lead these astronomers to think
that our universe may be a spiral nebula, and that the other spiral
nebulæ are "other universes."
[Illustration: _Photo: Harvard College Observatory._