The sum of centuries of speculation on the probable course of evolution in stars is discussed by one of the world's greatest astronomers, with a full report of his own conclusions, How long stars exist, the relation of their luminosity to their mass, the evolution of a star in relation to the main sequence, the significance of rotation, are among the crucial problems considered. While the discussion is replete with technical detail, sufficient background is included to enable the amateur astronomer or anyone with scientific training to follow the argument. Originally published in 1950. The Princeton Legacy Library uses the latest print-on-demand technology to again make available previously out-of-print books from the distinguished backlist of Princeton University Press. These editions preserve the original texts of these important books while presenting them in durable paperback and hardcover editions. The goal of the Princeton Legacy Library is to vastly increase access to the rich scholarly heritage found in the thousands of books published by Princeton University Press since its founding in 1905.