User:FT2/scc
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Stellar core collapse occurs when electron degeneracy and energy-generating fusion processes at the core of a massive star are unable to resist the inward gravitational forces affecting a star, causing the star's core to implode.[2] The implosion is cataclysmic. Depending upon the cause and the star's pre-collapse structure, the results may include release of massive amounts of energy or subatomic particles, production of elements beyond iron, and lead to a supernova, neutron star, or black hole.
Four main mechanisms are known to cause core collapse in stars: - failure of fusion, when the fuel cycle is unable to provide sufficient energy to counter the star's own gravity, accretion of material-typically by a white dwarf-from other objects, leading to sufficient mass for collapse, degeneracy of the core followed by electron capture,[3] and quantum fluctuations due to pair production in very massive stars causing the star to briefly lose supporting photon pressure. Two further possible modes of collapse are hypothesized, one based upon collapse halted by degeneracy pressure of quarks or smaller particles, and one in which external tidal forces in some binary star systems could cause compression and collapse. Both have been studied but the result is as yet unclear. The exact mode of collapse and the resulting products depends upon factors such as mass, rotation, presence of stable or unstable companion stars, and metallicity (the degree to which the star contains elements heavier than helium).
Scientific models of core collapse are still being refined, but ignoring rotational effects it is broadly believed that the key modes of collapse for a single star are as follows: Under about 8 solar masses a star does not have sufficient mass for core collapse. Around 8-10 solar masses the core does not initially collapse but forms a strongly degenerate oxygen-neon-magnesium core that eventually collapses due to electron capture. Above 10 solar masses a nickel-iron core forms that collapses once the final fusion processāsilicon burningāends, giving rise to a Type Ib and Ic ("stripped core-collapse") or type II supernova. Above around 25 solar masses and having solar or less metallicity, the same core collapse takes place but the resulting supernova is weak - the debris is drawn back into the neutron star remnant and the additional mass is sufficient to trigger further collapse into a black hole by fallback (see: Tolman-Oppenheimer-Volkoff limit). Most stars above about 40 solar masses and having solar or less metallicity collapse directly into a black hole.
In nickel-iron core collapse (the best known and first recognized type), a star possesses the mass needed to fuse elements that have an atomic mass greater than hydrogen and helium, albeit at increasingly high temperatures and pressure, and for increasingly shorter periods of time. The star fuses increasingly higher mass elements, starting with hydrogen and then helium, until finally a core of iron and nickel is produced. Fusion of iron or nickel produces no net energy, so further fusion is unable to take place. When the mass of the inert core exceeds the Chandrasekhar limit of about 1.44 solar masses, and with insufficient fusion pressure, electron degeneracy alone is no longer sufficient to counter gravity. A cataclysmic implosion takes place within seconds, in which the outer core reaches an inward velocity of up to 23% of the speed of light and the inner core reaches temperatures of up to 100 billion kelvin. The result of this collapse depends upon the type of star involved; in some cases the explosion (known as a "supernova") briefly outshines a galaxy, creates elements beyond iron that cannot ordinarily be created by stellar processes, and releases more energy than our sun will over its entire life. Because of the underlying mechanism, the resulting variable star for these is also described as a core-collapse supernova. Core collapse of a less massive inert neon-oxygen-magnesium core (recognized in 1979) occurs in a similar way but is triggered by loss of support via electron capture.
Two other types of core collapse mechanism are known to exist. Some white dwarfs that are insufficiently massive for core collapse may subsequently gain enough extra mass from external sources to trigger core collapse, leading to collapse, sudden heating, and a type Ia supernova. Also some very massive but low metallicity stars become unstable due to pair production and may blow themselves apart upon core collapse[citation needed] in a pair-instability supernova, leaving no remnant.