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[[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
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>
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