Double Stars.

Whenever you have a chance of looking at the heavens through a telescope, you should ask to be shown what is called a double star.  There are many stars in the heavens which present no remarkable appearance to the unaided eye, but which a good telescope at once shows to be of quite a complex nature.  These are what we call double stars, in which two quite distinct stars are placed so close together that the unaided eye is unable to separate them.  Under the magnifying power of the telescope, however, they are seen to be distinct.  In order to give some notion of what these objects are like, I shall briefly describe three of them.  The first lies in that best known constellation, the Great Bear.  If you look at his tail, which consists of three stars, you will see that near the middle one of the three a small star is situated; we call this little star Alcor, but it is the brighter one near Alcor to which I specially call your attention.  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.