On November 11, 1572, Danish astronomer and nobleman Tycho Brahe saw a new star in the constellation Cassiopeia, blazing as bright as Jupiter. In many ways, it was the birth of modern astronomy--a shining disproof of the belief that the heavens were fixed and unchanging. Such new stars have not ceased to surprise. Some 400 years later astronomers realized that they briefly outshine billions of ordinary stars and must therefore be spectacular explosions. In 1934 Fritz Zwicky of the California Institute of Technology coined the name supernovae for them. Quite apart from being among the most dramatic events known to science, supernovae play a special role in the universe and in the work of astronomers: seeding space with heavy elements, regulating galaxy formation and evolution, even serving as markers of cosmic expansion.
Zwicky and his colleague Walter Baade speculated that the explosive energy comes from gravity. Their idea was that a normal star implodes until its core reaches the density of an atomic nucleus. Like a crystal vase falling onto a concrete floor, the collapsing material releases enough gravitational potential energy to blow the rest of the star apart. An alternative emerged in 1960, when Fred Hoyle of the University of Cambridge and Willy Fowler of Caltech conceived of the explosions as giant nuclear bombs. When a sunlike star exhausts its hydrogen fuel and then its helium, it turns to its carbon and oxygen. Not only can the fusion of these elements release a titanic pulse of energy, it produces radioactive nickel 56, whose gradual decay would account for the months-long afterglow of the initial explosion.