Inastrophysics,silicon burning is a very brief[1] sequence ofnuclear fusion reactions that occur in massivestars 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 themain sequence on theHertzsprung–Russell diagram. It follows the previous stages ofhydrogen,helium,carbon,neon andoxygen burning processes.
Silicon burning begins when gravitational contraction raises the star's core temperature to 2.7–3.5 billion kelvin (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 aType II supernova.
After a star completes theoxygen-burning process, its core is composed primarily of silicon and sulfur.[2][3] If it has sufficiently high mass, it further contracts until its core reaches temperatures in the range of 2.7–3.5 GK (230–300keV). At these temperatures, silicon and other elements canphotodisintegrate, emitting a proton or an alpha particle.[2] Silicon burning proceeds by photodisintegration rearrangement,[4] which creates new elements by thealpha process, adding one of these freed alpha particles[2] (the equivalent of a helium nucleus) per capture step in the following sequence (photoejection of alphas not shown):
| 28 14Si | + | 4 2He | → | 32 16S |
| 32 16S | + | 4 2He | → | 36 18Ar |
| 36 18Ar | + | 4 2He | → | 40 20Ca |
| 40 20Ca | + | 4 2He | → | 44 22Ti |
| 44 22Ti | + | 4 2He | → | 48 24Cr |
| 48 24Cr | + | 4 2He | → | 52 26Fe |
| 52 26Fe | + | 4 2He | → | 56 28Ni |
The chain could theoretically continue, as adding further alphas continues to be exothermic all the way totin-100.[5] However, the steps after nickel-56 are much less exothermic and the temperature is so high thatphotodisintegration prevents further progress.[2]
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. Thenickel-56 decays first tocobalt-56 and then toiron-56, with half-lives of 6 and 77 days respectively, but this happens later, because only minutes are available within the core of a massive star. The star has run out of nuclear fuel and within minutes its core begins to contract.[citation needed]
During this phase of the contraction, the potential energy of gravitational contraction heats the interior to 5 GK (430 keV), which opposes and delays the contraction.[6] 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.[7] The central portion of the star is now crushed into a neutron core with the temperature soaring further to 100 GK (8.6 MeV)[8] that quickly cools down[9] into aneutron star if the mass of the star is below 20 M☉.[7] Between 20 M☉ and 40–50 M☉, fallback of the material will make the neutron core collapse further into ablack hole.[10] The outer layers of the star are blown off in an explosion known as aType IIsupernova 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 ther-process (where the "r" stands for "rapid" neutron capture).[citation needed]