A supernova is a violent stellar explosion that can shine as brightly as an entire galaxy of billions of normal stars. Astronomers divide supernovae into two primary groups: Type Ia and Type II. Type Ia supernovae form as a white dwarf "steals" hot gas from a companion star. If enough gas piles up on the surface of the white dwarf, a runaway thermonuclear explosion blasts the star to bits, leaving nothing behind. These are the brightest supernovae, and can be used to measure the distances to other galaxies. A Type II supernova is the final stage in the evolution of a star that is at least eight times as massive as the Sun. Such a star reaches a point where it can no longer produce nuclear energy in its core. Without the outward pressure created by this energy, gravity wins out and causes the star's core to collapse to form a neutron star or black hole. The star's outer layers "rebound" violently, blasting into space at several percent of the speed of light.