nly 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