Editing Sun (section)

== Composition ==
{{See also|Molecules in stars}}
The Sun consists mainly of the elements [[hydrogen]] and [[helium]]. At this time in the Sun's life, they account for 74.9% and 23.8%, respectively, of the mass of the Sun in the photosphere.<ref name=lodders>{{cite journal |doi=10.1086/375492 |last=Lodders |first=Katharina |author-link=Katharina Lodders |date=10 July 2003 |title=Solar System Abundances and Condensation Temperatures of the Elements |journal=The Astrophysical Journal |volume=591 |issue=2 |pages=1220–1247 |url=http://weft.astro.washington.edu/courses/astro557/LODDERS.pdf |bibcode=2003ApJ...591.1220L |access-date=1 September 2015 |archive-url=https://web.archive.org/web/20151107043527/http://weft.astro.washington.edu/courses/astro557/LODDERS.pdf |archive-date=7 November 2015 |url-status=dead |citeseerx=10.1.1.666.9351 |s2cid=42498829}}<br />{{Cite journal |last=Lodders |first=K. |author-link=Katharina Lodders |title=Abundances and Condensation Temperatures of the Elements |url=http://www.lpi.usra.edu/meetings/metsoc2003/pdf/5272.pdf |journal=[[Meteoritics & Planetary Science]] |volume=38 |issue=suppl |page=5272 |date=2003 |bibcode=2003M&PSA..38.5272L |access-date=3 August 2008 |archive-date=13 May 2011 |archive-url=https://web.archive.org/web/20110513163004/http://www.lpi.usra.edu/meetings/metsoc2003/pdf/5272.pdf |url-status=live}}</ref> All heavier elements, called ''[[metallicity|metals]]'' in astronomy, account for less than 2% of the mass, with [[oxygen]] (roughly 1% of the Sun's mass), [[carbon]] (0.3%), [[neon]] (0.2%), and [[iron]] (0.2%) being the most abundant.<ref name=hkt2004>{{Cite book |last1=Hansen |first1=C. J. |last2=Kawaler |first2=S. A. |last3=Trimble |first3=V. |title=Stellar Interiors: Physical Principles, Structure, and Evolution |pages=19–20 |edition=2nd |publisher=[[Springer Science+Business Media|Springer]] |date=2004 |isbn=978-0-387-20089-7}}</ref>

The Sun's original chemical composition was inherited from the [[interstellar medium]] out of which it formed. Originally it would have been about 71.1% hydrogen, 27.4% helium, and 1.5% heavier elements.<ref name=lodders /> The hydrogen and most of the helium in the Sun would have been produced by [[Big Bang nucleosynthesis]] in the first 20 minutes of the universe, and the heavier elements were [[stellar nucleosynthesis|produced by previous generations of stars]] before the Sun was formed, and spread into the interstellar medium during the [[stellar evolution|final stages of stellar life]] and by events such as [[supernova]]e.<ref name="hkt2004_78">{{Cite book |last1=Hansen |first1=C. J. |title=Stellar Interiors: Physical Principles, Structure, and Evolution |last2=Kawaler |first2=S. A. |last3=Trimble |first3=V. |year=2004 |publisher=[[Springer Science+Business Media|Springer]] |isbn=978-0-387-20089-7 |edition=2nd |pages=77–78}}</ref>

Since the Sun formed, the main fusion process has involved fusing hydrogen into helium. Over the past 4.6&nbsp;billion years, the amount of helium and its location within the Sun has gradually changed. The proportion of helium within the core has increased from about 24% to about 60% due to fusion, and some of the helium and heavy elements have settled from the photosphere toward the centre of the Sun because of [[gravity]]. The proportions of heavier elements are unchanged. [[Heat transfer|Heat is transferred]] outward from the Sun's core by radiation rather than by convection (see [[#Radiative zone|Radiative zone]] below), so the fusion products are not lifted outward by heat; they remain in the core,<ref name=hkt2004_9.2.3>{{Cite book |last1=Hansen |first1=C. J. |last2=Kawaler |first2=S. A. |last3=Trimble |first3=V. |title=Stellar Interiors: Physical Principles, Structure, and Evolution |pages=§&nbsp;9.2.3 |no-pp=yes |edition=2nd |publisher=[[Springer Science+Business Media|Springer]] |year=2004 |isbn=978-0-387-20089-7}}</ref> and gradually an inner core of helium has begun to form that cannot be fused because presently the Sun's core is not hot or dense enough to fuse helium. In the current photosphere, the helium fraction is reduced, and the [[metallicity]] is only 84% of what it was in the [[Protostar|protostellar]] phase (before nuclear fusion in the core started). In the future, helium will continue to accumulate in the core, and in about 5&nbsp;billion years this gradual build-up will eventually cause the Sun to exit the [[main sequence]] and become a [[red giant]].<ref>{{cite journal |last=Iben |first=Icko Jnr. |title=Stellar Evolution. II. The Evolution of a 3 M<sub>☉</sub> Star from the Main Sequence Through Core Helium Burning |journal=The Astrophysical Journal |volume=142 |page=1447 |date=November 1965 |doi=10.1086/148429 |bibcode=1965ApJ...142.1447I}}</ref>

The chemical composition of the photosphere is normally considered representative of the composition of the primordial Solar System.<ref name="aller1968">{{Cite journal |last=Aller |first=L. H. |title=The chemical composition of the Sun and the solar system |journal=Proceedings of the Astronomical Society of Australia |volume=1 |issue=4 |page=133 |date=1968 |bibcode=1968PASA....1..133A |doi=10.1017/S1323358000011048 |s2cid=119759834 |doi-access=free}}</ref> Typically, the solar heavy-element abundances described above are measured both by using [[astronomical spectroscopy|spectroscopy]] of the Sun's photosphere and by measuring abundances in [[meteorites]] that have never been heated to melting temperatures. These meteorites are thought to retain the composition of the protostellar Sun and are thus not affected by the settling of heavy elements. The two methods generally agree well.<ref name="basu2008">{{Cite journal |last1=Basu |first1=S. |last2=Antia |first2=H. M. |year=2008 |title=Helioseismology and Solar Abundances |journal=[[Physics Reports]] |volume=457 |issue=5–6 |pages=217–283 |arxiv=0711.4590 |bibcode=2008PhR...457..217B |doi=10.1016/j.physrep.2007.12.002 |s2cid=119302796}}</ref>