A violent stellar explosion that can shine as brightly as an entire galaxy of billions of normal stars. Astronomers divide supernovae into two groups: Type I and Type II. Type I supernovae most likely 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. Type II supernovae are the final stage in the evolution of stars that are 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.
The explosion resulting when a massive star explodes violently, becoming temporarily brighter than a thousand stars. Because supernovae are so bright, they can be seen from very far away, and thus are useful as standard candles. For more about supernovae, check out the distant supernova search.
A star that, due to accretion of matter from a companion star (Type Ia) or exhaustion of its own fuel supply (Types Ib, Ic, and II), can no longer support itself against gravity and thus collapses, throwing off its outer layers in a burst of energy that outshines an entire galaxy.