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Supernovas (aka Supernovae plural) are astronomical events that occurs during the last stellar evolutionary stages of a massive star's life, whose dramatic and catastrophic destruction is marked by one final titanic explosion. This causes the sudden appearance of a "new" bright star, before slowly fading from sight over several weeks or months.
Hubble Space Telescope-Image of Supernova 1994D (SN1994D) in galaxy NGC 4526 (SN 1994D is the bright spot on the lower left). image: wikipedia/NASA
Supernovae are more energetic than novae. In Latin, nova means "new", referring astronomically to what appears to be a temporary new bright star. Adding the prefix "super-" distinguishes supernovae from ordinary novae, which are far less luminous. The word supernova was coined by Walter Baade and Fritz Zwicky in 1931. It is pronounced /ˌsuːpərnoʊvə/ with the plural supernovae /ˌsuːpərnoʊviː/ or supernovas (abbreviated SN, plural SNe after "supernovae").
How often do they occur?
Only three Milky Way naked-eye supernova events have been observed during the last thousand years, though many have been seen in other galaxies using telescopes. The most recent directly observed supernova in the Milky Way was Kepler's Supernova in 1604, but remnants of two more recent supernovae have been found retrospectively.
Statistical observations of supernovae in other galaxies suggest they occur on average about three times every century in the Milky Way, and that any galactic supernova would almost certainly be observable with modern astronomical telescopes.
How fast they expel matter
Supernovae may expel much, if not all, of the material away from a star, at velocities up to 30,000 km/s or 10% of the speed of light. This drives an expanding and fast-moving shock wave into the surrounding interstellar medium, and in turn, sweeping up an expanding shell of gas and dust, which is observed as a supernova remnant.
They are atomic builders
Supernovae create, fuse and eject the bulk of the chemical elements produced by nucleosynthesis.
The Crab Nebula is a pulsar wind nebula associated with the 1054 supernova. image: Hubble STCi/wikipedia
Supernovae play a significant role in enriching the interstellar medium with the heavier atomic mass chemical elements. Furthermore, the expanding shock waves from supernovae can trigger the formation of new stars.
Supernova remnants are expected to accelerate a large fraction of galactic primary cosmic rays, but direct evidence for cosmic ray production was found only in a few of them so far. They are also potentially strong galactic sources of gravitational waves.
How do they occur?
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 or the sudden gravitational collapse of a massive star's core.
- In the first instance, a degenerate white dwarf may accumulate sufficient material from a binary companion, either through accretion or via a merger, to raise its core temperature enough to trigger runaway nuclear fusion, completely disrupting the star.
- In the second case, the core of a massive star may undergo sudden gravitational collapse, releasing gravitational potential energy as a supernova. While some observed supernovae are more complex than these two simplified theories, the astrophysical collapse mechanics have been established and accepted by most astronomers for some time.
Although no supernova has been observed in the Milky Way since Kepler's Star of 1604 (SN 1604), supernova remnants indicate that on average the event occurs about three times every century in the Milky Way. They play a significant role in enriching the interstellar medium with higher mass elements. Furthermore, the expanding shock waves from supernova explosions can trigger the formation of new stars.
Source adapted from: Supernova. (2017, February 23). In Wikipedia, The Free Encyclopedia. Retrieved 21:48, February 23, 2017, from https://en.wikipedia.org/w/index.php?title=Supernova&oldid=766983951