Editing Stellar nucleosynthesis (section)

===Hydrogen fusion===
{{Redirect|Hydrogen burning|the combustion of hydrogen gas|Hydrogen#Combustion}}
{{Main|Proton–proton chain reaction|CNO cycle|Deuterium fusion}}
{{multiple image
| align     = right
| direction = vertical
| width     = 300
| image1    = Fusion in the Sun.svg
| caption1  = '''Proton–proton chain reaction'''
| image2    = CNO Cycle.svg
| caption2  = '''CNO-I cycle'''<br />The helium nucleus is released at the top-left step.
}}
Hydrogen fusion (nuclear fusion of four protons to form a [[helium-4]] nucleus<ref name=jones2009/>) is the dominant process that generates energy in the cores of [[main sequence|main-sequence]] stars. It is also called "hydrogen burning", which should not be confused with the [[Chemical reaction|chemical]] [[hydrogen#Combustion|combustion of hydrogen]] in an [[oxidizing]] atmosphere. There are two predominant processes by which stellar hydrogen fusion occurs: [[proton–proton chain]] and the carbon–nitrogen–oxygen (CNO) cycle. Ninety percent of all stars, with the exception of [[white dwarfs]], are fusing hydrogen by these two processes.<ref>Seeds, M. A., ''Foundations of Astronomy'' ([[Belmont, California|Belmont, CA]]: [[Cengage|Wadsworth Publishing Company]], 1986), p. 245.</ref>{{rp|245}}

In the cores of lower-mass main-sequence stars such as the [[Sun]], the dominant energy production process is the [[proton–proton chain reaction]]. This creates a helium-4 nucleus through a sequence of reactions that begin with the fusion of two protons to form a [[deuterium]] nucleus (one proton plus one neutron) along with an ejected positron and neutrino.<ref name=bohm_vitense1992/> In each complete fusion cycle, the proton–proton chain reaction releases about 26.2&nbsp;MeV.<ref name=bohm_vitense1992/> Proton-proton chain with a dependence of approximately T{{sup|4}}, meaning the reaction cycle is highly sensitive to temperature; a 10% rise of temperature would increase energy production by this method by 46%, hence, this hydrogen fusion process can occur in up to a third of the star's radius and occupy half the star's mass. For stars above 35% of the Sun's mass,<ref name=aaa496_3_787/> the [[energy flux]] toward the surface is sufficiently low and energy transfer from the core region remains by [[radiative heat transfer]], rather than by [[Convection (heat transfer)|convective heat transfer]].<ref name=deloore_doom1992/> As a result, there is little mixing of fresh hydrogen into the core or fusion products outward.

In higher-mass stars, the dominant energy production process is the [[CNO cycle]], which is a [[catalytic cycle]] that uses nuclei of carbon, nitrogen and oxygen as intermediaries and in the end produces a helium nucleus as with the proton–proton chain.<ref name=bohm_vitense1992/> During a complete CNO cycle, 25.0&nbsp;MeV of energy is released. The difference in energy production of this cycle, compared to the proton–proton chain reaction, is accounted for by the energy lost through [[neutrino]] emission.<ref name=bohm_vitense1992/> CNO cycle is highly sensitive to temperature, with rates proportional to the 16th to 20th power of the temperature; a 10% increase in temperature would result in a 350% increase in energy production. About 90% of the CNO cycle energy generation occurs within the inner 15% of the star's mass, hence it is strongly concentrated at the core.<ref name=jeffrey2010/> This results in such an intense outward energy flux that [[convective]] energy transfer becomes more important than does [[radiative transfer]]. As a result, the core region becomes a [[convection zone]], which stirs the hydrogen fusion region and keeps it well mixed with the surrounding proton-rich region.<ref name=karttunen_oja2007/> This core convection occurs in stars where the CNO cycle contributes more than 20% of the total energy. As the star ages and the core temperature increases, the region occupied by the convection zone slowly shrinks from 20% of the mass down to the inner 8% of the mass.<ref name=jeffrey2010/> The Sun produces on the order of 1% of its energy from the CNO cycle.<ref>{{Cite web|title=Neutrinos yield first experimental evidence of catalyzed fusion dominant in many stars|url=https://phys.org/news/2020-11-neutrinos-yield-experimental-evidence-catalyzed.html|access-date=2020-11-26|website=phys.org|language=en}}</ref>{{efn|In the [https://www.nature.com/articles/s41586-020-2934-0 November 2020] issue of [[Nature (journal)|''Nature'']], particle physicist Andrea Pocar points out, "Confirmation of CNO burning in our sun, where it operates at only one percent, reinforces our confidence that we understand how stars work."}}<ref>[[Gregory Robert Choppin|Choppin, G. R.]], [[Jan-Olov Liljenzin|Liljenzin, J.-O.]], [[Jan Rydberg|Rydberg, J.]], & [[:sv:Christian Ekberg|Ekberg, C.]], ''Radiochemistry and Nuclear Chemistry'' (Cambridge, MA: [[Academic Press]], 2013), [https://books.google.com/books?id=CN88gBPtiucC&pg=PA357&redir_esc=y#v=onepage&q&f=false p. 357].</ref>{{rp|357}}<ref>{{Cite journal|last1=Agostini|first1=M.|last2=Altenmüller|first2=K.|last3=Appel|first3=S.|last4=Atroshchenko|first4=V.|last5=Bagdasarian|first5=Z.|last6=Basilico|first6=D.|last7=Bellini|first7=G.|last8=Benziger|first8=J.|last9=Biondi|first9=R.|last10=Bravo|first10=D.|last11=Caccianiga|first11=B.|date=25 November 2020|title=Experimental evidence of neutrinos produced in the CNO fusion cycle in the Sun|url=https://www.nature.com/articles/s41586-020-2934-0|journal=Nature|language=en|volume=587|issue=7835|pages=577–582|doi=10.1038/s41586-020-2934-0|pmid=33239797|issn=1476-4687|arxiv=2006.15115|bibcode=2020Natur.587..577B|s2cid=227174644}}</ref>{{efn|"This result therefore paves the way toward a direct measurement of the solar metallicity using CNO neutrinos. Our findings quantify the relative contribution of CNO fusion in the Sun to be of the order of 1 per cent."—M. Agostini, et al.}}

The type of hydrogen fusion process that dominates in a star is determined by the temperature dependency differences between the two reactions. The proton–proton chain reaction starts at temperatures about {{val|4|e=6|ul=K}},<ref name=reid_hawley2005/> making it the dominant fusion mechanism in smaller stars. A self-maintaining CNO chain requires a higher temperature of approximately {{val|1.6|e=7|u=K}}, but thereafter it increases more rapidly in efficiency as the temperature rises, than does the proton–proton reaction.<ref name=salaris_cassini2005/> Above approximately {{val|1.7|e=7|u=K}}, the CNO cycle becomes the dominant source of energy. This temperature is achieved in the cores of main-sequence stars with at least 1.3 times the mass of the [[Sun]].<ref name=apj701_1_837/> The Sun itself has a core temperature of about {{val|1.57|e=7|u=K}}.<ref>Wolf, E. L., ''Physics and Technology of Sustainable Energy'' ([[Oxford]], [[Oxford University Press]], 2018), [https://books.google.com/books?id=BP9eDwAAQBAJ&pg=PA5&redir_esc=y#v=onepage&q&f=false p. 5].</ref>{{rp|5}} As a main-sequence star ages, the core temperature will rise, resulting in a steadily increasing contribution from its CNO cycle.<ref name=jeffrey2010/>