Supernovae are a major source of elements in the interstellar medium from oxygen to rubidium. This drives an expanding shock wave into the surrounding interstellar medium, sweeping up an expanding shell of gas and dust observed as a supernova remnant. Supernovae can expel several solar masses of material at velocities up to several percent of the speed of light. While some observed supernovae are more complex than these two simplified theories, the astrophysical mechanics are established and accepted by the astronomical community. In the massive star case, the core of a massive star may undergo sudden collapse once it is unable to produce sufficient energy from fusion to counteract the star's own gravity. Possible causes are an accumulation of material from a binary companion through accretion, or a stellar merger. In the first class of events, the object's temperature is raised enough to trigger runaway nuclear fusion, completely disrupting the star. Theoretical studies indicate that most supernovae are triggered by one of two basic mechanisms: the sudden re-ignition of nuclear fusion in a degenerate star such as a white dwarf, or the sudden gravitational collapse of a massive star's core. The most recent naked-eye supernova was SN 1987A, which was the explosion of a blue supergiant star in the Large Magellanic Cloud, a satellite of the Milky Way. These supernovae would almost certainly be observable with modern astronomical telescopes. Observations of supernovae in other galaxies suggest they occur in the Milky Way on average about three times every century.
The last supernova to be directly observed in the Milky Way was Kepler's Supernova in 1604, but the remnants of more recent supernovae have been found. The word supernova was coined by Walter Baade and Fritz Zwicky in 1929. Adding the prefix "super-" distinguishes supernovae from ordinary novae, which are far less luminous. In Latin, nova means "new", referring astronomically to what appears to be a temporary new bright star. Supernovae are more energetic than novae. The peak optical luminosity of a supernova can be comparable to that of an entire galaxy before fading over several weeks or months. The original object, called the progenitor, either collapses to a neutron star or black hole, or is completely destroyed. This transient astronomical event occurs during the last evolutionary stages of a massive star or when a white dwarf is triggered into runaway nuclear fusion.
It has the plural form supernovae /- v iː/ or supernovas, and is abbreviated SN or SNe.
SN 1994D (bright spot on the lower left), a type Ia supernova within its host galaxy, NGC 4526Ī supernova is a powerful and luminous explosion of a star.