More than a hundred years ago the great German philosopher Kant suggested that our sun, our earth, and all the heavenly bodies might have begun as gases, and the astronomer Laplace taught this as the most likely history of their formation. After a few years, however, when powerful telescopes showed that many of the nebulæ were only clusters of very minute stars, astronomers thought that Laplace's teaching had been wrong. But now the spectroscope has revealed to us glowing gas actually filling large spaces in the sky, and every year accurate observations and experiments tell us more and more about these marvellous distant mists. Some day, though perhaps not while you or I are here to know it, Orion's nebula, with its glowing gas and minute star-dust, may give some clue to the early history of the heavenly bodies; and for this reason I wish you to recognise and ponder over it, as I have often done, when it shines down on the rugged moor in the stillness of a clear frosty winter's night.
But we must pass on for, while I have been talking, the whole sky has become bespangled with hundreds of stars. That glorious one to the west, which you can find by following (Fig. 54) a curved line upwards from Betelgeux, is the beautiful red star Aldebaran or the hindmost; so called by the Arabs, because he drives before him that well-known cluster, the Pleiades, which we reach by continuing the curve westwards and upwards. Stop to look at this cluster through your telescopes, for it will delight you; even with the naked eye you can count from six to ten stars in it, and an opera-glass will show about thirty, though they are so scattered you will have to move the glass about to find them. Yet though my telescope shows a great many more, you cannot even count all the chief ones through it, for in powerful telescopes more than 600 stars have been seen in the single cluster! while a photograph taken by Mr. Roberts shows also four lovely patches of nebula.
And now from the Pleiades let us pass on directly overhead to the beautiful star Capella, which once was red but now is blue, and drop down gently to the south-east, where Castor and Pollux, the two most prominent stars in the constellation "Gemini" or the twins, show brilliantly against the black sky. Pause here a moment, for I want to tell you something about Castor, the one nearest to Capella. If you look at Castor through your telescopes, some of you may possibly guess that it is really two stars, but you will have to look through mine to see it clearly. These two stars have been watched carefully for many years, and there is now no doubt that one of them is moving slowly round the other. Such stars as these are called "binary," to distinguish them from stars that merely _appear_ double because they stand nearly in a line one behind the other in the heavens, although they may be millions of miles apart. But "binary" stars are actually moving in one system, and revolve round each other as our earth moves round the sun.