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A supernova is a stellar explosion which appears to result in the creation of a new star upon the celestial sphere. ("Nova" is Latin for "new"). The "super" prefix distinguishes this from a nova, which also involves a star increasing in brightness, though to a lesser extent and through a different mechanism. Supernovae involve the expulsion of a star's outer layers; filling the surrounding space with hydrogen and helium (along with other elements); the debris eventually forms clouds of dust and gas. When the explosion of a supernova compresses nearby clouds (the results of nearby explosions), such compression can form a solar nebula.
Supernovae can release several times joules of energy. This has resulted in the foe being the standard unit of energy in supernova research.
As part of the attempt to understand supernova explosions, astronomers have classified them according to the lines of different chemical elements that appear in their spectra.
The first element for division is the presence or absence of a line from hydrogen. If a supernova's spectrum does not contain a hydrogen line, it is classified type I, otherwise type II.
Among those groups, there are subdivisions according to the presence of other lines.
Type Ia supernovae don't have helium, and present a silicon line.
They are generally thought to be caused by the explosion of a white dwarf, at or close to the Chandrasekhar limit.
One possibility is that the white dwarf was orbiting a moderately massive star. The dwarf pulls matter from its companion to the point that it reaches the Chandrasekhar limit. The dwarf collapses into a neutron star or black hole, and the collapse causes the remaining carbon and oxygen atoms in it to fuse. This fusion produces a shockwave, and the dwarf is blown to bits. This is different from the mechanism of a nova in which the white dwarf doesn't reach the Chandrasekhar limit and collapse, but merely ignites nuclear fusion in the matter it has accreted on its surface.
The increase in luminosity is given by energy liberated by the explosion, and the rather long time it takes to decline is fueled by radioactive cobalt decaying into iron. One useful characteristic of Type IA supernovae is that they are very bright and appear to all have the same intristic brightness and hence have been used as standard candles to determine cosmological distances. That no clear explanation has been given as to why type Ia's should have the same brightness is something that most astronomers find deeply troubling.
Type Ib and Ic do not have the silicon line and are believed to correspond to stars ending their lives (as type II), but they would have lost their hydrogen before, thus the H lines don't appear on their spectra. Type Ib supernovae are thought to be the result of a Wolf-Rayet star collapsing.
Type II results when a very massive star's core begins fusing iron, which uses energy instead of liberating it. When the mass of the iron core reaches the Chandrasekhar limit (this takes only a matter of days), it decays spontaneously into neutrons and collapses. A tremendous burst of neutrinos is produced, removing energy from the star. Through a process that is not well understood some of the energy liberated in the neutrino burst is transferred to the outer layers of the star. When the shock wave reaches the surface of the star several hours later, there is a massive increase in brightness. The core of the star may become a neutron star or a black hole, depending on its mass, although because of the lack of understanding of the processes of supernova collapse, it is unknown what the cutoff mass is.
Type II supernovae can further be divided into type II-P and II-L. Type II-P should a "plateau" in their light curve while II-L's should a "linear" decrease in their light curve. This is believed to result from differences in the envelope of the stars. II-P's have a large hydrogen envelope that traps energy released in the form of gamma rays and releases it slowly, while II-L's are believed to have much smaller envelopes converting less of the gamma ray energy into visible light.
The decay of the light curve in Type-II supernova follows the decay of nickel-57.
Some exceptionally large stars may instead produce a "hypernova" when they die, a theoretical type of explosion. In the hypernova mechanism, the core of the star collapses directly into a black hole and two extremely energetic jets of plasma are emitted from its rotational poles at nearly light speed. These jets emit intense gamma rays, and are one of many candidate explanations for gamma ray bursts.
Supernova discoveries are reported to the IAU,
which sends out a circular with the name it assigns to it.
The name is formed by the year of discovery, and a one- or
two-letter designation. The first 26 supernovae of the year
get a letter from A to Z. After Z, they start with aa, ab,
and so on.
Supernovae often leave behind supernova remnants; the study of these objects has helped to increase our knowledge of supernovae.
Supernovae tend to enrich the surrounding interstellar medium with metals (that for astronomers, are all the elements after helium). Thus, each stellar generation has a slightly different composition, going from an almost pure mixture of hydrogen and helium to a more metal-rich composition. The different chemical abundances have important influences on the star's life, and may decisively influence the possibility of having planets orbiting it.
See also:
Classification
Naming of Supernovae
Notable supernovae
The 1604 supernova was used by Galileo as evidence against the Aristotelian dogma of his period, that the heavens never changed.Role of supernovae on stellar evolution
