A supernova (plural: supernovae or supernovas) is a stellar explosion that creates an extremely luminous object. A supernova usually causes a burst of radiation that may briefly outshine its entire host galaxy before fading from view over several weeks or months. During this relatively short time interval, the supernova radiates as much energy as the Sun would emit over about 10 billion years. The explosion expels much or all of a star's material at a velocity of up to a tenth the speed of light, driving a shock wave into the surrounding interstellar gas. This shock wave sweeps up an expanding shell of gas and dust called a supernova remnant.
Several types of supernovae exist that may be triggered in one of two ways, involving either turning off or suddenly turning on the production of energy through nuclear fusion. The core of a massive star may cease to generate energy from nuclear fusion and undergo sudden gravitational collapse into a neutron star or black hole, releasing gravitational potential energy that heats and expels the star's outer layers. Alternatively, a white dwarf star may accumulate sufficient material from a stellar companion (usually through accretion, rarely via a merger) to raise its core temperature enough to ignite carbon fusion, at which point it undergoes runaway nuclear fusion, completely disrupting it. In accretional cases this occurs as the white dwarf approaches the Chandrasekhar limit of roughly 1.38 times the mass of the Sun. This latter type of supernova is distinct from a much smaller surface thermonuclear explosion on a white dwarf, which is called a nova. Solitary stars with a mass below approximately 8 solar masses, such as the Sun itself, evolve into white dwarfs without ever becoming supernovae.
On average, supernovae occur about once every 50 years in a galaxy the size of the Milky Way and play a significant role in enriching the interstellar medium with heavy elements. Furthermore, the expanding shock waves from supernova explosions can trigger the formation of new stars.
Nova (plural novae) means "new" in Latin, referring to what appears to be a very bright new star shining in the celestial sphere; the prefix "super" distinguishes supernovae from ordinary novae, which also involve a star increasing in brightness, though to a lesser extent and through a different mechanism. According to Merriam-Webster's Collegiate Dictionary, the word supernova was first used in print in 1926.