The
sharpest eye would never suspect that it was composed of two stars
placed close together. Even a small telescope will, however, show this
to be the case, and this is the easiest and the first observation that
a young astronomer should make when beginning to turn a telescope to
the heavens. Of course you will not imagine that I mean Alcor to be
the second component of the double star; it is the bright star near
Alcor which is the double. Here are two marbles, and these marbles are
fastened an inch apart. You can see them, of course, to be separate;
but if the pair were moved further and further away, then you would
soon not be able to distinguish between them, though the actual
distance between the marbles had not altered. Look at these two wax
tapers which are now lighted; the little flames are an inch apart. You
would have to view them from a station a third of a mile away if the
distance between the two flames were to appear the same as that
between the two components of this double star. Your eye would never
be able to discriminate between two lights only an inch apart at so
great a distance; a telescope would, however, enable you to do so, and
this is the reason why we have to use telescopes to show us double
stars.

You might look at that double star year after year throughout the
course of a long life without finding any appreciable change in the
relative positions of its components. But we know that there is no
such thing as rest in the universe; even if you could balance a body
so as to leave it for a moment at rest, it would not stay there, for
the simple reason that all the bodies round it in every direction are
pulling at it, and it is certain that the pull in one direction will
preponderate, so that move it must. Especially is this true in the
case of two suns like those forming a double star. Placed
comparatively near each other they could not remain permanently in
that position; they must gradually draw together and come into
collision with an awful crash. There is only one way by which such a
disaster could be averted. That is by making one of these stars
revolve around the other just as the earth revolves around the sun, or
the moon revolves around the earth. Some motion must, therefore, be
going on in every genuine double star, whether we have been able to
see that motion or not.

Let us now look at another double star of a different kind. This time
it is in the constellation of Gemini. The heave