A supernova is an exploding star that can reach a luminosity of well over 100 million times that of the Sun. A supernova's brightness rises and falls rapidly over the course of about a month, then fades slowly over months and years. There are two broad classes of supernovae: those that get their energy from a sudden burst of fusion energy and those whose energy comes from gravitational collapse. In practice, these are distinguished on the basis of their different light curves and spectral characteristics. The type Ia supernovae used as standard candles in measurements of the expansion rate of the universe are thought to arise from the explosion of white dwarf stars in a binary system. As the white dwarf draws matter from its companion star, its carbon core reaches the temperature and density at which it can ignite and fuse explosively in a nuclear flame to iron. This violent explosion destroys the star, and creates about half a solar mass of radioactive isotopes that power the bright peak of the light curve.