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In astrophysics, silicon burning is a very brief sequence of
nuclear fusion Nuclear fusion is a reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles ( neutrons or protons). The difference in mass between the reactants and products is manife ...
reactions that occur in massive stars with a minimum of about 8–11 solar masses.
Silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ta ...
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 Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
,
helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
,
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...
, neon and
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
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 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. 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
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). At these temperatures, silicon and other elements can photodisintegrate, emitting a proton or an alpha particle. Silicon burning proceeds by photodisintegration rearrangement,Donald D. Clayton, ''Principles of stellar evolution and nucleosynthesis'', Chapter 7 (University of Chicago Press 1983) which creates new elements by the
alpha process The alpha process, also known as the alpha ladder, is one of two classes of nuclear fusion reactions by which stars convert helium into heavier elements, the other being the triple-alpha process. The triple-alpha process consumes only helium, a ...
, 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): : Although the chain could theoretically continue, steps after nickel-56 are much less exothermic and the temperature is so high that
photodisintegration Photodisintegration (also called phototransmutation, or a photonuclear reaction) is a nuclear process in which an atomic nucleus absorbs a high-energy gamma ray, enters an excited state, and immediately decays by emitting a subatomic particle. The ...
prevents further progress. 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 nickel-56 decays in a few days or weeks first to cobalt-56 and then to iron-56, 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. 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 A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. w ...
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).


Binding energy

This graph shows the binding energy per nucleon of various nuclides. The binding energy is the difference between the energy of free protons and neutrons and the energy of the nuclide. If the product or products of a reaction have higher binding energy per nucleon than the reactant or reactants, then the reaction is exothermic (releases energy) and can go forward, though this is valid only for reactions that do not change the number of protons or neutrons (no
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reactions). As can be seen, light nuclides such as deuterium or helium release large amounts of energy (a big increase in binding energy) when combined to form heavier elements—the process of fusion. Conversely, heavy elements such as uranium release energy when broken into lighter elements—the process of nuclear fission. In stars, rapid nucleosynthesis proceeds by adding helium nuclei (alpha particles) to heavier nuclei. As mentioned above, this process ends around atomic mass 56. Decay of nickel-56 explains the large amount of iron-56 seen in metallic meteorites and the cores of rocky planets.


See also

* Alpha nuclide *
Alpha process The alpha process, also known as the alpha ladder, is one of two classes of nuclear fusion reactions by which stars convert helium into heavier elements, the other being the triple-alpha process. The triple-alpha process consumes only helium, a ...
* Stellar evolution * Supernova nucleosynthesis * Neutron capture: ** p-process ** ''r''-process ** ''s''-process


References


External links


''Stellar Evolution: The Life and Death of Our Luminous Neighbors,'' by Arthur Holland and Mark Williams of the University of Michigan


*

'' b
Tufts University
*

'' by G. Hermann *Arnett, W. D.
Advanced evolution of massive stars. VII – Silicon burning
/ Astrophysical Journal Supplement Series, vol. 35, Oct. 1977, p. 145–159. * {{Nuclear processes Nucleosynthesis