ASTRONOMERS like to scare their hearers from time to time, like the old-fashioned nurse telling children about ogres, with pictures of the end of the world. It may be burnt up by the exploding sun, or frozen by the sun’s extinction (though the latter is less probable); it may lose all its air by radiation into space, and chemical absorption into the earth. But, at the close, the audience is reassured that their fear of its happening in a million years is baseless—that the earth has at least ten million years of existence ahead—and they go away relieved.