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