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A supernova is a type of 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, it can induce their gravitational collapse to form new stars, and enrich those new stars in heavy elements.
Supernovae can release several times <math>10^{44}<math> joules of energy, roughly equivalent to the output of the Sun over its entire lifetime.
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. See 'Optical Spectra of Supernovae' by Filippenko () for a good description of the classes.
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 and the shape of its light curve.
Type Ia supernovae lack helium and present a silicon absorption line in their spectra near peak light. The most commonly accepted theory of these type of supernovae is that they are the result of a carbon-oxygen white dwarf accreting matter from a nearby companion star, typically a red giant, until it reaches the Chandrasekhar limit. The increase in pressure from the resultant collapse of the star ignites carbon fusion in the star's core. This in turns causes the star to explode violently and to release a shockwave in which matter is typically ejected at speeds on the order of 10,000 km/s. The energy released in the explosion also causes an extreme increase in luminosity.
The theory of these type of supernovae is similar to that of novae, in which a white dwarf accretes matter more slowly and does not reach the Chandrasekhar limit. In the case of a nova, the infalling matter causes a fusion reaction of material near its surface but does not cause the star to collapse.
Type Ia supernovae have a characteristic light curve (graph of luminosity as a function of time after the explosion). Near the time of maximum luminosity, the spectrum contains lines of intermediate-mass elements from oxygen to calcium; these are the main constituents of the outer layers of the star. Months after the explosion, when the outer layers have expanded to the point of transparency, the spectrum is dominated by light emitted by material near the core of the star: heavy elements synthesized during the explosion, most prominently iron-group elements. The radioactive decay of Nickel-56 through Cobalt-56 to Iron-56 produces high-energy photons which dominate the energy output of the ejecta at intermediate to late times.
Unlike the other types of supernove, Type Ia supernovae are generally found in all types of galaxies, including ellipticals. They show no preference for regions of current star formation.
The similarity in the shapes of the luminosity profiles of all known Type Ia supernovae has led to their use as a standard candle in extragalactic astronomy. The cause of this similarity in the luminosity curve is still an open question mark.
The Type Ia supernova releases the highest amounts of energy amongst all known classifications of supernovae. The farthest single object ever detected in the universe (galaxies or globular clusters don't count) was a Type Ia supernova located billions of light-years away.
The early spectra of Types Ib and Ic do not show lines of hydrogen, nor the strong silicon absorption feature near 6150 Angstroms. These events, like supernovae of Type II, are probably massive stars running out of fuel at their centers; however, the progenitors of Types Ib and Ic have lost most of their envelopes due to strong stellar winds or interaction with a companion. Type Ib supernovae are thought to be the result of a Wolf-Rayet star collapsing.
A supernovae of Type II results when the core of a massive star (more than 12 solar masses), having progressed through a sequence of burning (fusing) the lighter elements H, He, C, Ne, O and Si, starts forming a core of iron (Fe). This is the end of the chain of nuclear fusion reactions which liberate energy, since Fe is the most stable nucleus, so the iron just accumulates. Although the first stage of hydrogen burning may have lasted millions of years, the last stage of silicon burning may take only hours, producing iron.
The Fe core is under huge gravitational pressure, and since there is no fusion and cannot be supported by ordinary gas pressure, it is supported by electron degeneracy pressure, the electrons pushing against other electrons. When it builds up to the Chandrasekhar limit at which electron degeneracy pressure cannot sustain it, the iron core collapses until the neutrons press against each other. This is the core collapse. The collapsing core produces high energy gamma rays, which decompose some iron nuclei into 13 He plus 4 neutrons, a reaction which absorbs energy and so accelerates the collapse. At very high densities, electrons and protons combine into neutrons and neutrinos. This is called "electron capture". The neutrinos escape the core, carrying energy and further accelerating the collapse, which proceeds in milliseconds as the core detaches from the outer layers of the star and reaches the density of nuclear matter. At this point the collapse is stopped and actually bounces a little, creating a shock wave which slams into the collapsing outer layers of the star. A "proto-neutron star" begins to form at the core, though if it is massive enough, it will continue collapsing to form a black hole.
Currently it is not understood how this shockwave completes the process of pressuring the outer layer of the star. This is the "supernova problem", and various models suggest different ways that the huge amounts of energy present are formed into a single explosion. One such model is the Convective overturn model, which suggests that convection, either from neutrinos from below, or infalling matter from above, completes the process of destroying the progenitor star. Heavier elements than iron are formed during this explosion by neutron capture, and from the pressure of the neutrinos pressing into the boundary of the "neutrinosphere", seeding the surrounding space with a cloud of gas and dust which is richer in heavy elements than the one the star originally formed from.
Neutrino physics, which is not well understood, is crucial to the understanding of this process, since half of the energy of the core collapse is radiated out in the form of neutrinos. The other crucial area of investigation is the "hydrology" of the plasma that makes up the dying star, how it behaves during the core collapse determines when and how the "shockwave" forms and when and how it "stalls" and is re-energized.
The remaining core of the star may become a neutron star or a black hole, depending on its mass, although because the processes of supernova collapse are poorly understood, it is unknown what the cutoff mass is.
Type II supernovae can be further classified based on the shape of their light curves into Type II-P and Type II-L. Type II-P reach a "plateau" in their light curve while II-L's have a "linear" decrease in their light curve ("linear" in magnitude versus time, or exponential in luminosity versus time). 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.
One can also sub-divide supernovaed of Type II based on their spectra. While most Type II supernova show very broad emission lines which indicate expansion velocities of many thousands of km/s, some have relatively narrow features which may be produced by the interaction of the ejecta with circumstellar material; these are called Type IIn, where the "n" stands for "narrow". A few supernovae, such as SN 1987K and 1993J, appear to change types: they show lines of hydrogen at early times, but, over a period of weeks to months, become dominated by lines of helium. The term "Type IIb" is used to describe the combination of features normally associated with Types II and Ib. These are likely massive stars which have lost most, but not all, of their hydrogen envelopes. As the ejecta expand, the hydrogen layer quickly becomes optically thin and reveals the deeper layers.
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.
The 1604 supernova was used by Galileo as evidence against the Aristotelian dogma of his period, that the heavens never changed.
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.
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