Editing Sun (section)

=== Core ===
{{Main|Solar core}}
The core of the Sun extends from the centre to about 20–25% of the solar radius.<ref name="Garcia2007">{{Cite journal |last=García |first=R. |date=2007 |title=Tracking solar gravity modes: the dynamics of the solar core |journal=[[Science (journal)|Science]] |volume=316 |issue=5831 |pages=1591–1593 |bibcode=2007Sci...316.1591G |doi=10.1126/science.1140598 |pmid=17478682 |s2cid=35285705 |display-authors=etal}}</ref> It has a density of up to {{val|150|u=g|up=cm3}}<ref name="Basu">{{Cite journal |last1=Basu |first1=Sarbani |last2=Chaplin |first2=William J. |last3=Elsworth |first3=Yvonne |last4=New |first4=Roger |last5=Serenelli |first5=Aldo M. |year=2009 |title=Fresh insights on the structure of the solar core |journal=[[The Astrophysical Journal]] |volume=699 |issue=2 |pages=1403–1417 |arxiv=0905.0651 |bibcode=2009ApJ...699.1403B |doi=10.1088/0004-637X/699/2/1403 |s2cid=11044272}}</ref><ref name="NASA1">{{Cite web |date=18 January 2007 |title=NASA/Marshall Solar Physics |url=http://solarscience.msfc.nasa.gov/interior.shtml |url-status=live |archive-url=https://web.archive.org/web/20190329081742/https://solarscience.msfc.nasa.gov/interior.shtml |archive-date=29 March 2019 |access-date=11 July 2009 |publisher=[[Marshall Space Flight Center]]}}</ref> (about 150 times the density of water) and a temperature of close to 15.7&nbsp;million [[kelvin]] (K).<ref name="NASA1" /> By contrast, the Sun's surface temperature is about {{val|5,800|u=K}}. Recent analysis of [[Solar and Heliospheric Observatory|SOHO]] mission data favours the idea that the core is rotating faster than the radiative zone outside it.<ref name="Garcia2007" /> Through most of the Sun's life, energy has been produced by nuclear fusion in the core region through the [[proton–proton chain]]; this process converts hydrogen into helium.<ref>{{Cite conference |last=Broggini |first=C. |date=2003 |title=Physics in Collision, Proceedings of the XXIII International Conference: Nuclear Processes at Solar Energy |url=http://www.slac.stanford.edu/econf/C030626 |conference=XXIII Physics in Collisions Conference |location=Zeuthen, Germany |page=21 |arxiv=astro-ph/0308537 |bibcode=2003phco.conf...21B |archive-url=https://web.archive.org/web/20170421113407/http://www.slac.stanford.edu/econf/C030626/ |archive-date=21 April 2017 |access-date=12 August 2013 |url-status=live}}</ref> Currently, 0.8% of the energy generated in the Sun comes from another sequence of fusion reactions called the [[CNO cycle]]; the proportion coming from the CNO cycle is expected to increase as the Sun becomes older and more luminous.<ref name="jpcs271_1_012031">{{Cite journal |last1=Goupil |first1=M. J. |last2=Lebreton |first2=Y. |last3=Marques |first3=J. P. |last4=Samadi |first4=R. |last5=Baudin |first5=F. |date=2011 |title=Open issues in probing interiors of solar-like oscillating main sequence stars 1. From the Sun to nearly suns |journal=[[Journal of Physics: Conference Series]] |volume=271 |issue=1 |page=012031 |arxiv=1102.0247 |bibcode=2011JPhCS.271a2031G |doi=10.1088/1742-6596/271/1/012031 |s2cid=4776237}}</ref><ref>{{Cite journal |last=The Borexino Collaboration |date=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 (journal)|Nature]] |volume=587 |issue=? |pages=577–582 |arxiv=2006.15115 |bibcode=2020Natur.587..577B |doi=10.1038/s41586-020-2934-0 |pmid=33239797 |s2cid=227174644 |access-date=26 November 2020 |archive-date=27 November 2020 |archive-url=https://web.archive.org/web/20201127093809/https://www.nature.com/articles/s41586-020-2934-0 |url-status=live}}</ref>

The core is the only region of the Sun that produces an appreciable amount of [[thermal energy]] through fusion; 99% of the Sun's power is generated in the innermost 24% of its radius, and almost no fusion occurs beyond 30% of the radius. The rest of the Sun is heated by this energy as it is transferred outward through many successive layers, finally to the solar photosphere where it escapes into space through radiation (photons) or advection (massive particles).<ref name="Phillips1995-47">{{Cite book |last=Phillips |first=K. J. H. |title=Guide to the Sun |year=1995 |publisher=[[Cambridge University Press]] |isbn=978-0-521-39788-9 |url=https://books.google.com/books?id=idwBChjVP0gC&pg=PA47 |pages=47–53}}</ref><ref name=Zirker2002-15>{{Cite book |last=Zirker |first=J. B. |date=2002 |title=Journey from the Center of the Sun |pages=[https://archive.org/details/journeyfromcente0000zirk/page/15 15–34] |publisher=[[Princeton University Press]] |isbn=978-0-691-05781-1 |url=https://archive.org/details/journeyfromcente0000zirk/page/15}}</ref>
[[File:Proton-proton reaction chain.svg|thumb|Illustration of a proton-proton reaction chain, from hydrogen forming [[deuterium]], [[helium-3]], and regular [[helium-4]]|alt=circles and arrows showing protons combining in a series of fusion reactions yielding helium-3 which breaks down tow helium-4]]
The proton–proton chain occurs around {{val|9.2|e=37}} times each second in the core, converting about 3.7{{e|38}} protons into [[alpha particle]]s (helium nuclei) every second (out of a total of ~8.9{{e|56}} free protons in the Sun), or about {{val|6.2|e=11|u=kg|up=s}}. However, each proton (on average) takes around 9 billion years to fuse with another using the PP chain.<ref name="Phillips1995-47" /> Fusing four free [[proton]]s (hydrogen nuclei) into a single alpha particle (helium nucleus) releases around 0.7% of the fused mass as energy,<ref>{{Cite book |last=Shu |first=F. H. |url=https://archive.org/details/physicaluniverse00shuf/page/102 |title=The Physical Universe: An Introduction to Astronomy |year=1982 |publisher=University Science Books |isbn=978-0-935702-05-7 |page=[https://archive.org/details/physicaluniverse00shuf/page/102 102]}}</ref> so the Sun releases energy at the mass–energy conversion rate of 4.26&nbsp;billion kg/s (which requires 600 billion kg of hydrogen<ref>{{Cite web |year=2012 |title=Ask Us: Sun |url=https://helios.gsfc.nasa.gov/qa_sun.html |url-status=dead |archive-url=https://web.archive.org/web/20180903223810/https://helios.gsfc.nasa.gov/qa_sun.html |archive-date=3 September 2018 |access-date=13 July 2017 |website=Cosmicopia |publisher=NASA}}</ref>), for 384.6&nbsp;[[Yotta-|yottawatts]] ({{val|3.846|e=26|u=W}}),<ref name="nssdc" /> or 9.192{{e|10}}&nbsp;[[TNT equivalent|megatons of TNT]] per second. The large power output of the Sun is mainly due to the huge size and density of its core (compared to Earth and objects on Earth), with only a fairly small amount of power being generated per [[cubic metre]]. Theoretical models of the Sun's interior indicate a maximum power density, or energy production, of approximately 276.5 [[watt]]s per cubic metre at the centre of the core,<ref>{{Cite web |last=Cohen |first=H. |date=9 November 1998 |title=Table of temperatures, power densities, luminosities by radius in the Sun |url=http://fusedweb.llnl.gov/CPEP/Chart_Pages/5.Plasmas/Sunlayers.html |archive-url=http://webarchive.loc.gov/all/20011129122524/http%3A//fusedweb%2Ellnl%2Egov/cpep/chart_pages/5%2Eplasmas/sunlayers%2Ehtml |archive-date=29 November 2001 |access-date=30 August 2011 |publisher=Contemporary Physics Education Project}}</ref> which, according to [[Karl Kruszelnicki]], is about the same power density inside a [[compost pile]].<ref>{{Cite web |date=17 April 2012 |title=Lazy Sun is less energetic than compost |url=http://www.abc.net.au/science/articles/2012/04/17/3478276.htm |url-status=live |archive-url=https://web.archive.org/web/20140306123113/http://www.abc.net.au/science/articles/2012/04/17/3478276.htm |archive-date=6 March 2014 |access-date=25 February 2014 |publisher=Australian Broadcasting Corporation}}</ref>

The fusion rate in the core is in a self-correcting equilibrium: a slightly higher rate of fusion would cause the core to heat up more and [[thermal expansion|expand]] slightly against the weight of the outer layers, reducing the density and hence the fusion rate and correcting the [[Perturbation (astronomy)|perturbation]]; and a slightly lower rate would cause the core to cool and shrink slightly, increasing the density and increasing the fusion rate and again reverting it to its present rate.<ref>{{Cite journal |last1=Haubold |first1=H. J. |last2=Mathai |first2=A. M. |date=1994 |title=Solar Nuclear Energy Generation & The Chlorine Solar Neutrino Experiment |volume=320 |issue=1994 |pages=102–116 |journal=[[AIP Conference Proceedings]] |arxiv=astro-ph/9405040 |bibcode=1995AIPC..320..102H |doi=10.1063/1.47009 |citeseerx=10.1.1.254.6033 |s2cid=14622069}}</ref><ref>{{Cite web |last=Myers |first=S. T. |date=18 February 1999 |title=Lecture 11 – Stellar Structure I: Hydrostatic Equilibrium |url=http://www.aoc.nrao.edu/~smyers/courses/astro12/L11.html |url-status=live |archive-url=https://web.archive.org/web/20110512180052/http://www.aoc.nrao.edu/~smyers/courses/astro12/L11.html |archive-date=12 May 2011 |access-date=15 July 2009 |website=Introduction to Astrophysics II}}</ref>