Cosmos The Cosmic Journey

SUPERNOVA

A supernova is a stellar explosion that briefly outshines an entire galaxy, radiating as much energy as the Sun or any ordinary star is expected to emit over its entire life span, before fading from view over several weeks or months.The extremely luminous burst ofradiation expels much or all of a star's material at a velocity of up to 30,000 km/s(10% of the speed of light), driving a shock wave into the surrounding interstellar medium. This shock wave sweeps up an expanding shell of gas and dust called asupernova remnant. A great proportion of primary cosmic rays comes from supernovae.

Supernovae are more energetic than a nova.Nova 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 are far less luminous. The wordsupernova was coined by Walter Baade andFritz Zwicky in 1931.It is pronounced/ˌsuːpərnoʊvə/ with the plural supernovae/ˌsuːpərnoʊviː/ or supernovas (abbreviatedSN, plural SNe after "supernovae").

Supernovae can be triggered in one of two ways: by the sudden re-ignition of nuclear fusion in a degenerate star; or by thegravitational collapse of the core of a massive star. In the first case, a degeneratewhite dwarf may accumulate sufficient material from a companion, either throughaccretion or via a merger, to raise its core temperature, ignite carbon fusion, and triggerrunaway nuclear fusion, completely disrupting the star. In the second case, the core of a massive star may undergo suddengravitational collapse, releasing gravitational potential energy that can create a supernova explosion.

The last directly observed supernova in theMilky Way was Kepler's Star of 1604 (SN 1604); remnants of two more recent supernovae have been found retrospectively. Nevertheless, observations in other galaxies indicate that supernovae occur on average about three times every century in the Milky Way.They play a significant role in enriching the interstellar medium with highermass elements. Furthermore, the expanding shock waves from supernova explosions can trigger the formation of new stars.
Supernova remnants

The Crab Nebula, an example of asupernova remnant

A supernova occurs when a high-mass star reaches the end of its life. When nuclear fusion in the core of the star stops, the star collapses. The gas falling inward either rebounds or gets so strongly heated that it expands outwards from the core, thus causing the star to explode.The expanding shell of gas forms a supernova remnant, a special diffuse nebula.Although much of the optical and X-ray emission from supernova remnants originates from ionized gas, a great amount of the radio emission is a form of non-thermal emission calledsynchrotron emission. This emission originates from high-velocity electronsoscillating within magnetic fields.