Editing Supernova (section)

{{Short description|Astrophysical phenomenon}}
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{{Use dmy dates|date=February 2023}}
[[File:SN1994D.jpg|thumb|right|upright=1.3|[[SN 1994D]] (bright spot on the lower left), a [[type Ia supernova]] within its host galaxy, [[NGC 4526]]]]
A '''supernova''' ({{plural form}}: '''supernovae''' or '''supernovas''') is a powerful and luminous [[explosion]] of a [[star]]. A supernova occurs during the last [[stellar evolution|evolutionary stages]] of a [[massive star]], or when a [[white dwarf]] is triggered into runaway [[nuclear fusion]]. The original object, called the ''progenitor'', either collapses to a [[neutron star]] or [[black hole]], or is completely destroyed to form a [[diffuse nebula]]. The peak optical [[luminosity]] of a supernova can be comparable to that of an entire [[galaxy]] before fading over several weeks or months.

The last supernova directly observed in the [[Milky Way]] was [[Kepler's Supernova]] in 1604, appearing not long after [[Tycho's Supernova]] in 1572, both of which were visible to the [[naked eye]]. The [[supernova remnant|remnants]] of more recent supernovae have been found, and observations of supernovae in other galaxies suggest they occur in the Milky Way on average about three times every century. A supernova in the Milky Way would almost certainly be observable through modern astronomical telescopes. 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 galaxy]] of the Milky Way.

Theoretical studies indicate that most supernovae are triggered by one of two basic mechanisms: the sudden re-ignition of nuclear fusion in a [[white dwarf]], or the sudden [[gravitational collapse]] of a massive star's [[stellar core|core]].
* In the re-ignition of a white dwarf, the object's temperature is raised enough to trigger [[Thermal runaway|runaway]] nuclear fusion, completely disrupting the star. Possible causes are an accumulation of material from a [[Binary star|binary companion]] through [[Accretion (astrophysics)|accretion]], or by a [[stellar merger]].
* In the case of a massive star's sudden implosion, the core of a [[stellar evolution#Massive stars|massive star]] will undergo sudden collapse once it is unable to produce sufficient energy from fusion to counteract the star's own gravity, which must happen once the star begins [[alpha process|fusing iron]], but may happen during an earlier stage of [[metallicity|metal]] fusion.

Supernovae can expel several [[solar mass]]es of material at speeds up to several percent of the [[speed of light]]. 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 are a major source of [[chemical element|elements]] in the interstellar medium from [[oxygen]] to [[rubidium]]. The expanding shock waves of supernovae can trigger the [[star formation|formation of new stars]]. Supernovae are a major source of [[cosmic ray]]s. They might also produce [[gravitational wave]]s.