The article discusses white dwarf stars and supersoft sources of low energy x-rays. Since the 1930s astronomers have known that ordinary stars shine because of nuclear fusion deep in their interior. Recently, however, researchers have discovered a new class of stars in which the nuclear fusion takes place not in the deep interior but in the outer layers just below the surface. These stars appear to be white dwarfs--dense, burned out stars that have exhausted their nuclear fuel-in orbit around ordinary stars. The dwarfs steal hydrogen gas from their companions, accumulate it on their surface and resume fusion. The result is a torrent of x-rays with a distinctive "soft" range of wavelengths; such stars are known as luminous supersoft x-ray sources As the dwarfs gain weight, they eventually grow unstable, at which point they can collapse into an even denser neutron star or explode. The disruption of white dwarfs has long been considered as the cause of one sort of supernova explosion, called type 1a. With the discovery of the supersoft sources, observers have identified for the first time a class of star system that can detonate in its way. Type 1a supernovae have become important as bright "standard candles" for measuring distances to faraway galaxies and thereby the pace of cosmic expansion. Much of the lingering uncertainty in estimates of the age and the expansion rate of the universe is connected to astronomers' ignorance of what gives rise to these supernovae. Supersoft sources may be one of the long-sought missing links. Some theoretical physicists suggested that the supersoft sources were white dwarfs that gave off x-rays as gas crashed onto their surface--much as hard x-ray sources result from the accretion of matter onto a neutron star or into a black hole.