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 of
constellations, 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.
Let us now look at another double star of a different kind. This time
it is in the constellation of Gemini. The heavenly twins are called
Castor and Pollux. Of these, Castor is a very beautiful double star,
consisting of two bright points, a great deal closer together than were
those in the Great Bear; consequently a better telescope is required
for the purpose of showing them separately. Castor has been watched
for many years, and it can be seen that one of these stars is slowly
revolving around the other; but it takes a very long time, amounting
to hundreds of years, for a complete circuit to be accomplished. This
seems very astonishing, but when you remember how exceedingly far
Castor is, you will perceive that that pair of stars which appear so
close together that it requires a telescope to show them apart must
indeed be separated by hundreds of millions of miles. Let us try to
conceive our own system transformed into a double star. If we took
our outermost planet--Neptune--and enlarged him a good deal, and then
heated him sufficiently to make him glow like a sun, he would still
continue to revolve round our sun at the same distance, and thus a
double star would be produced. An inhabitant of Castor who turned his
telescope towards us would be able to see the sun as a star. He would
not, of course, be able to see the earth, but he might see Neptune like
another small star close to the sun. If generations of astronomers in
Castor continued their observations of our system, they would find a
binary star, of which one component took a century and a half to go
round the other. Need we then be surprised that when we look at Castor
we observe movements that seem very slow?
There is often so much diffused light about the bright stars seen in
a telescope, and so much twinkling in some states of the atmosphere,
that stars appear to dance about in rather a puzzling fashion,
especially to one who is not accustomed to astronomical observations.
I remember hearing how a gentleman once came to visit an observatory.
The astronomer showed him Castor through a powerful telescope as a fine
specimen of a double star, and then, by way of improving his little
lesson, the astronomer mentioned that one of these stars was revolving
around the other. “Oh, yes,” said the visitor, “I saw them going round
and round in the telescope.” He would, however, have had to wait for a
few centuries with his eye to the instrument before he would have been
entitled to make this assertion.
Double stars also frequently delight us by giving beautifully
contrasted colors. I dare say you have often noticed the red and the
green lights that are used on railways in the signal lamps. Imagine one
of those red and one of those green lights away far up in the sky and
placed close together, then you would have some idea of the appearance
that a colored double star presents, though, perhaps, I should add that
the hues in the heavenly bodies are not so vividly different as are
those which our railway people find necessary. There is a particularly
beautiful double star of this kind in the constellation of the Swan.
You could make an imitation of it by boring two holes, with a red-hot
needle, in a piece of card, and then covering one of these holes with a
small bit of the topaz-colored gelatine with which Christmas crackers
are made. The other star is to be similarly colored with blue gelatine.
A slide made on this principle placed in the lantern gives a very good
representation of these two stars on the screen. There are many other
colored doubles besides this one; and, indeed, it is noteworthy that
we hardly ever find a blue or a green star by itself in the sky; it is
always as a member of one of these pairs.