Page:Astronomy for Everybody.djvu/316

sible until recent times. Even within the last century little more could be said than that they were shining bodies whose nature was to us a mystery. At the present time we may define the stars as immense globes of matter, generally millions of times the size of the earth, so intensely hot that they shine by their own light, and so massive that they may continue to give light and heat for unknown millions of years without cooling off. What we have said of the sun probably applies in a greater or less degree to the great majority of the stars. It is true that we cannot study their surfaces because, even in the most powerful telescopes, they appear as mere points of light. But the analogy with our sun and with other heavenly bodies leads us to believe that each of them revolves on its axis as the sun does, and that, could we see it at the proper distance, it would present much the same appearance as our sun. We have abundant evidence that rotation is the order of nature in the case of all the heavenly bodies. In the few cases where it is possible to decide whether a star does or does not rotate, the question has been answered in the affirmative.

There are innumerable differences of detail among the stars. Indeed it would seem that no two are exactly alike in their physical constitution, any more than two men are alike in their personal appearance and make-up. In the chapter on the sun we tried to give an idea of the enormous temperature of that body, which far exceeds any degree of heat we can produce on the earth. We have good reason to believe that, while the stars differ widely in temperature, the great majority of them are