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In astrophysics, silicon burning is a very brief sequence of nuclear fusion reactions that occur in massive stars with a minimum of about 8–11 solar masses. Silicon burning is the final stage of fusion for massive stars that have run out of the fuels that power them for their long lives in the ''main sequence'' on the Hertzsprung-Russell diagram. It follows the previous stages of hydrogen, helium, carbon, neon and oxygen burning processes. Silicon burning begins when gravitational contraction raises the star’s core temperature to 2.7–3.5 billion kelvins (GK). The exact temperature depends on mass. When a star has completed the silicon-burning phase, no further fusion is possible. The star catastrophically collapses and may explode in what is known as a Type II supernova. ==Nuclear fusion sequence and silicon photodisintegration== After a star completes the oxygen burning process, its core is composed primarily of silicon and sulfur.〔Woosley SE, Arnett WD, Clayton DD, "Hydrostatic oxygen burning in stars II. oxygen burning at balanced power", Astrophys. J. 175, 731 (1972)〕 If it has sufficiently high mass, it further contracts until its core reaches temperatures in the range of 2.7–3.5 GK (230–300 keV). At these temperatures, silicon and other elements can photodisintegrate, emitting a proton or alpha particle.〔 Silicon burning proceeds by photo disintegration rearrangement,〔Donald D. Clayton, ''Principles of stellar evolution and nucleosynthesis'', Chapter 7 (Uiversity of Chicago Press 1983)〕 which creates new elements by adding one of these freed alpha particles〔 (the equivalent of a helium nucleus) per capture step in the following sequence (photoejection of alphas not shown): : The silicon-burning sequence lasts about one day before being struck by the shock wave that was launched by the core collapse. Burning then becomes much more rapid at the elevated temperature and stops only when the rearrangement chain has been converted to nickel-56 or is stopped by supernova ejection and cooling. The star can no longer release energy via nuclear fusion because a nucleus with 56 nucleons has the lowest mass per nucleon of all the elements in the alpha process sequence. Only minutes are available for the nickel-56 to decay within the core of a massive star, and only seconds if in the ejecta. The star has run out of nuclear fuel and within minutes its core begins to contract. During this phase of the contraction, the potential energy of gravitational contraction heats the interior to 5 GK (430 keV) and this opposes and delays the contraction. However, since no additional heat energy can be generated via new fusion reactions, the final unopposed contraction rapidly accelerates into a collapse lasting only a few seconds. The central portion of the star is now crushed into either a neutron star or, if the star is massive enough, a black hole. The outer layers of the star are blown off in an explosion known as a Type II supernova that lasts days to months. The supernova explosion releases a large burst of neutrons, which may synthesize in about one second roughly half of the supply of elements in the universe that are heavier than iron, via a rapid neutron-capture sequence known as the ''r-process'' (where the “r” stands for rapid neutron capture). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Silicon-burning process」の詳細全文を読む スポンサード リンク
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