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Proton–proton chain
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{{Short description|One of the fusion reactions by which stars convert hydrogen to helium}} {{Redirect|pp 1|other uses|Pp1 (disambiguation){{!}}Pp1}} [[File:Nuclear energy generation.svg|right|upright=1.5|thumb|[[Logarithm]] of the relative energy output (ε) of [[Proton–proton chain reaction|proton–proton]] (PP), [[CNO cycle|CNO]] and [[Triple-alpha process|Triple-α]] fusion processes at different temperatures (T). The dashed line shows the combined energy generation of the PP and CNO processes within a star. At the Sun's core temperature of 15.5 million K the PP process is dominant. The PP process and the CNO process are equal at around 20 MK.<ref name= Adelberger/>]] [[File:Fusion in the Sun.svg|lang=en|thumb|250px|Scheme of the proton–proton branch I reaction]] The '''proton–proton chain''', also commonly referred to as the '''{{nowrap|p–p}} chain''', is one of two known sets of [[nuclear fusion]] reactions by which [[star]]s convert [[hydrogen]] to [[helium]]. It dominates in stars with masses less than or equal to that of the [[Sun]],<ref>{{cite web|url=http://csep10.phys.utk.edu/astr162/lect/energy/ppchain.html|title=The Proton–Proton Chain|website=Astronomy 162: Stars, Galaxies, and Cosmology|archive-url=https://web.archive.org/web/20160620155744/http://csep10.phys.utk.edu/astr162/lect/energy/ppchain.html|archive-date=2016-06-20|url-status=dead|access-date=2018-07-30}}</ref> whereas the [[CNO cycle]], the other known reaction, is suggested by theoretical models to dominate in stars with masses greater than about 1.3 [[solar mass]]es.<ref>{{cite book|first1=Maurizio|last1=Salaris|first2=Santi|last2=Cassisi|title=Evolution of Stars and Stellar Populations|publisher=[[John Wiley and Sons]]|year=2005|isbn=0-470-09220-3|pages=119–121|url=https://books.google.com/books?id=p4ojTNkcFx8C&pg=PA119}}</ref> In general, proton–proton fusion can occur only if the [[kinetic energy]] ([[temperature]]) of the protons is high enough to overcome their mutual [[Coulomb repulsion|electrostatic repulsion]].<ref>[[Ishfaq Ahmad]], ''The Nucleus'', '''1''': 42, 59, (1971), The Proton type-nuclear fission reaction.</ref> In the Sun, [[Deuterium|deuteron]]-producing events are rare. [[Diproton]]s are the much more common result of proton–proton reactions within the star, and diprotons almost immediately decay back into two protons. Since the conversion of hydrogen to helium is slow, the complete conversion of the hydrogen initially in the [[solar core|core of the Sun]] is calculated to take more than ten billion years.<ref>Kenneth S. Krane, ''Introductory Nuclear Physics'', Wiley, 1987, p. 537.</ref> Although sometimes called the "proton–proton chain reaction", it is not a [[chain reaction]] in the normal sense. In most nuclear reactions, a chain reaction designates a reaction that produces a product, such as neutrons given off during [[nuclear fission|fission]], that quickly induces another such reaction. The proton–proton chain is, like a [[decay chain]], a series of reactions. The product of one reaction is the starting material of the next reaction. There are two main chains leading from hydrogen to helium in the Sun. One chain has five reactions, the other chain has six.
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