The Astrophysics of Variable Stars
Stars experience stages of birth, growth, middle and old age and finally death. Astronomers talk of these stages as progressive stellar evolution, although it bears no resemblance to the biological theory of evolution proposed by Darwin. They begin with nebulae and generally end with a nebula, whether it be a planetary nebula or the expanding mass of a supernova explosion. Stars are the only entities in our universe that follow the strict rule of evolution, slow change with time, but they always remain stars, of course, for the majority of their lives.