Supernova

see Supernova (disambiguation).

SN 1994D(bright spot on the lower left), atype Ia supernovain theNGC 4526galaxy

A supernova is an astronomical event that occurs during the last stellar evolutionarystages of a massive star's life, whose dramatic and catastrophic destruction is marked by one final titanic explosion. For a short time, this causes the sudden appearance of a 'new' bright star, before slowly fading from sight over several weeks or months.

Only three Milky Way naked-eye supernova events have been observed during the last thousand years, though many have been telescopically seen in other galaxies. 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.^[1] Statistical observations of supernovae in other galaxies suggest they should occur on average about three times every century in the Milky Way, and that any galactic supernova would almost certainly be observable in modern astronomical equipment.^[2]

Supernovae are more energetic than novae. InLatin, 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.^[3] It is pronounced /ˌsuːpərnoʊvə/ with the pluralsupernovae /ˌsuːpərnoʊviː/ or supernovas(abbreviated SN, plural SNe after "supernovae").

During maximum brightness, the total equivalent radiant energies produced by supernovae may briefly outshine an entire output of a typical galaxy and emit energies equal to that created over the lifetime of anysolar-like star.^[4] Such extreme catastrophes may also expel much, if not all, of its stellar material away from the star,^[5] at velocities up to 30,000 km/s or 10% of the speed of light. This drives an expanding and fast-movingshock wave^[6] into the surrounding interstellar medium, and in turn, sweeping up an expanding shell of gas and dust, which is observed as a supernova remnant. Supernovae create, fuse and eject the bulk of the chemical elements produced by nucleosynthesis.^[7] Supernovae play asignificant role in enriching the interstellar medium with the heavier atomic masschemical elements.^[8] Furthermore, the expanding shock waves from supernova explosions can trigger the formation of new stars.^[9][10] 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.^[11] They are also potentially strong galactic sources ofgravitational waves.^[12]

Theoretical studies of many supernovae indicate that most are triggered by one of two basic mechanisms: the sudden re-ignition ofnuclear fusion in a degenerate star or the sudden gravitational collapse of a massive star's core. In the first instance, a degeneratewhite dwarf may accumulate sufficient material from a binary companion, either through accretion or via a merger, to raise its core temperature enough to trigger runawaynuclear fusion, completely disrupting the star. In the second case, the core of a massive starmay 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.

Due to the wide range of astrophysical consequences of these events, astronomers now deem supernovae research, across the fields of stellar and galactic evolution, as an especially important area for investigation.