EVOLUTION OF STARS
Evolution of Stars This illustration depicts the stages in the lives of two stars. Stars form from a nebula (upper left), which consists of dust particles and hydrogen gas. Gravity pulls this material together into globules, which gradually expand as they convert their constituent hydrogen into helium during nuclear reactions. At its time of death, the center star explodes in a supernova, leaving behind a pulsar (blue object to the left of the supernova), while the core of the other star collapses, forming a planetary nebula (lower right).