Editing Sun (section)

== General characteristics ==
[[File:Celestial body size comparison horizontal.png|thumb|upright=1.8|Size comparison of the Sun, all eight [[Planets of the solar system|planets of]] the [[Solar System]], other bodies, and some larger stars. The Sun is 1.4 million kilometres (4.643 [[light-second]]s) wide, about 109 times [[List of Solar System objects by size|wider than]] Earth, or four times the [[Lunar distance]], and contains 99.86% of all Solar System [[Solar mass|mass]].]]

The Sun is a [[G-type main-sequence star]] that makes up about 99.86% of the mass of the Solar System.<ref name="Woolfson00">{{Cite journal |last=Woolfson |first=M. |date=2000 |title=The origin and evolution of the solar system |url=http://inis.jinr.ru/sl/vol1/_djvu/P_Physics/Woolfson%20M.M.%20Origin%20and%20evolution%20of%20the%20solar%20system%20(IOP)(425s).pdf |url-status=live |journal=[[Astronomy & Geophysics]] |volume=41 |issue=1 |page=12 |bibcode=2000A&G....41a..12W |doi=10.1046/j.1468-4004.2000.00012.x |archive-url=https://web.archive.org/web/20200711133403/http://inis.jinr.ru/sl/vol1/_djvu/P_Physics/Woolfson%20M.M.%20Origin%20and%20evolution%20of%20the%20solar%20system%20(IOP)(425s).pdf |archive-date=11 July 2020 |access-date=12 April 2020 |doi-access=free}}</ref><!-- There are several estimations on the mass of the Solar System; for further information please have a look at the talk page. --> It has an [[absolute magnitude]] of +4.83, estimated to be brighter than about 85% of the stars in the [[Milky Way]], most of which are [[red dwarf]]s.<ref>{{Cite news |last=Than |first=K. |date=2006 |title=Astronomers Had it Wrong: Most Stars are Single |publisher=Space.com |url=http://www.space.com/scienceastronomy/060130_mm_single_stars.html |url-status=live |access-date=1 August 2007 |archive-url=https://web.archive.org/web/20101221093125/http://www.space.com/scienceastronomy/060130_mm_single_stars.html |archive-date=21 December 2010}}</ref><ref>{{Cite journal |last=Lada |first=C. J. |year=2006 |title=Stellar multiplicity and the initial mass function: Most stars are single |journal=[[Astrophysical Journal Letters]] |volume=640 |issue=1 |pages=L63–L66 |arxiv=astro-ph/0601375 |bibcode=2006ApJ...640L..63L |doi=10.1086/503158 |s2cid=8400400}}</ref> It is more massive than 95% of the stars within {{cvt|7|pc|ly}}.<ref>{{cite journal |last1=Robles |first1=José A. |last2=Lineweaver |first2=Charles H. |last3=Grether |first3=Daniel |last4=Flynn |first4=Chris |last5=Egan |first5=Chas A. |last6=Pracy |first6=Michael B. |last7=Holmberg |first7=Johan |last8=Gardner |first8=Esko |title=A Comprehensive Comparison of the Sun to Other Stars: Searching for Self-Selection Effects |journal=The Astrophysical Journal |date=September 2008 |volume=684 |issue=1 |pages=691–706 |doi=10.1086/589985 |arxiv=0805.2962 |bibcode=2008ApJ...684..691R |hdl=1885/34434 |url=https://iopscience.iop.org/article/10.1086/589985/fulltext/73840.text.html |access-date=24 May 2024}}</ref>
The Sun is a [[Population I stars|Population I]], or heavy-element-rich,{{efn|name=heavy elements}} star.<ref name="zeilik">{{Cite book |last1=Zeilik |first1=M. A. |title=Introductory Astronomy & Astrophysics |last2=Gregory |first2=S. A. |year=1998 |publisher=Saunders College Publishing |isbn=978-0-03-006228-5 |edition=4th |page=322}}</ref> Its formation approximately 4.6&nbsp;billion years ago may have been triggered by shockwaves from one or more nearby [[supernova]]e.<ref name="Connelly2012">{{Cite journal |last1=Connelly |first1=James N. |last2=Bizzarro |first2=Martin |last3=Krot |first3=Alexander N. |last4=Nordlund |first4=Åke |last5=Wielandt |first5=Daniel |last6=Ivanova |first6=Marina A. |date=2 November 2012 |title=The Absolute Chronology and Thermal Processing of Solids in the Solar Protoplanetary Disk |journal=[[Science (journal)|Science]] |volume=338 |issue=6107 |pages=651–655 |bibcode=2012Sci...338..651C |doi=10.1126/science.1226919 |pmid=23118187 |s2cid=21965292}}</ref><ref name="Falk">{{Cite journal |last1=Falk |first1=S. W. |last2=Lattmer |first2=J. M. |last3=Margolis |first3=S. H. |year=1977 |title=Are supernovae sources of presolar grains? |journal=[[Nature (journal)|Nature]] |volume=270 |issue=5639 |pages=700–701 |bibcode=1977Natur.270..700F |doi=10.1038/270700a0 |s2cid=4240932}}</ref> This is suggested by a high [[Abundance of the chemical elements|abundance]] of heavy elements in the Solar System, such as [[gold]] and [[uranium]], relative to the abundances of these elements in so-called [[Population II]], heavy-element-poor, stars. The heavy elements could most plausibly have been produced by [[endothermic]] nuclear reactions during a supernova, or by [[Nuclear transmutation|transmutation]] through [[neutron absorption]] within a massive second-generation star.<ref name="zeilik" />

The Sun is by far the [[List of brightest natural objects in the sky|brightest object in the Earth's sky]], with an [[apparent magnitude]] of −26.74.<ref>{{Cite journal |last=Burton |first=W. B. |date=1986 |title=Stellar parameters |journal=[[Space Science Reviews]] |volume=43 |issue=3–4 |pages=244–250 |doi=10.1007/BF00190626 |s2cid=189796439}}</ref><ref>{{Cite journal |last1=Bessell |first1=M. S. |last2=Castelli |first2=F. |last3=Plez |first3=B. |date=1998 |title=Model atmospheres broad-band colors, bolometric corrections and temperature calibrations for O–M stars |journal=[[Astronomy and Astrophysics]] |volume=333 |pages=231–250 |bibcode=1998A&A...333..231B}}</ref> This is about 13&nbsp;billion times brighter than the next brightest star, [[Sirius]], which has an apparent magnitude of −1.46.<ref name="Hoffleit1991">{{cite book |chapter=HR 2491 |title=Bright Star Catalogue |edition=5th Revised |last1=Hoffleit |first1=D. |author-link1=Dorrit Hoffleit |display-authors=etal |date=1991 |publisher=[[Centre de Données astronomiques de Strasbourg|CDS]] |url=http://vizier.u-strasbg.fr/viz-bin/VizieR-S?HR%202491 |bibcode=1991bsc..book.....H}}</ref>

{{convert|1|AU|e6km e6mi|lk=in|spell=In|disp=x|abbr=off| (about |)}} is defined as the mean distance between the centres of the Sun and the Earth. The instantaneous distance varies by about {{±}}{{convert|2.5|e6km|abbr=none}} as Earth moves from [[perihelion]] around 3 January to [[aphelion]] around 4 July.<ref name="USNO">{{Cite web |date=31 January 2008 |title=Equinoxes, Solstices, Perihelion, and Aphelion, 2000–2020 |url=http://aa.usno.navy.mil/data/docs/EarthSeasons.php |url-status=dead |archive-url=https://web.archive.org/web/20071013000301/http://aa.usno.navy.mil/data/docs/EarthSeasons.php |archive-date=13 October 2007 |access-date=17 July 2009 |publisher=[[US Naval Observatory]]}}</ref> At its average distance, light travels from the Sun's horizon to Earth's horizon in about 8 minutes and 20 seconds,<ref>{{Cite web |last=Cain |first=Fraser |date=15 April 2013 |title=How long does it take sunlight to reach the Earth? |url=https://phys.org/news/2013-04-sunlight-earth.html |url-status=live |archive-url=https://web.archive.org/web/20220302095547/https://phys.org/news/2013-04-sunlight-earth.html |archive-date=2 March 2022 |access-date=2 March 2022 |website=phys.org}}</ref> while light from the closest points of the Sun and Earth takes about two seconds less. The energy of this [[sunlight]] supports almost all life<ref group="lower-alpha">[[Hydrothermal vent communities]] live so deep under the sea that they have no access to sunlight. Bacteria instead use sulfur compounds as an energy source, via [[chemosynthesis]].</ref> on Earth by [[photosynthesis]],<ref>{{cite web |title=The Sun's Energy: An Essential Part of the Earth System |url=https://scied.ucar.edu/learning-zone/earth-system/energy-from-sun |website=Center for Science Education |access-date=24 May 2024}}</ref> and drives [[Earth's climate]] and weather.<ref>{{cite web |title=The Sun's Influence on Climate |url=https://press.princeton.edu/books/hardcover/9780691153834/the-suns-influence-on-climate |date=23 June 2015 |publisher=Princeton University Press |access-date=24 May 2024}}</ref>

The Sun does not have a definite boundary, but its density decreases exponentially with increasing height above the [[photosphere]].<ref name="Beer et al, 2012-41">{{Cite book |last1=Beer |first1=J. |title=Cosmogenic Radionuclides: Theory and Applications in the Terrestrial and Space Environments |url=https://books.google.com/books?id=zKA0tZg0HwEC&pg=PA41 |last2=McCracken |first2=K. |last3=von Steiger |first3=R. |date=2012 |publisher=Springer |isbn=978-3-642-14651-0 |page=41}}</ref> For the purpose of measurement, the Sun's radius is considered to be the distance from its centre to the edge of the photosphere, the apparent visible surface of the Sun.<ref name="Phillips1995-73">{{Cite book |last=Phillips |first=K. J. H. |title=Guide to the Sun |date=1995 |publisher=[[Cambridge University Press]] |isbn=978-0-521-39788-9 |url=https://books.google.com/books?id=idwBChjVP0gC&pg=PA73 |page=73}}</ref>
The roundness of the Sun is the relative difference between its radius at its equator, <math>R_\textrm{eq}</math>, and at its pole, <math>R_\textrm{pol}</math>, called the [[oblateness]],<ref name=Meftah-2015>{{Cite journal |last1=Meftah |first1=M. |last2=Irbah |first2=A. |last3=Hauchecorne |first3=A. |last4=Corbard |first4=T. |last5=Turck-Chièze |first5=S. |last6=Hochedez |first6=J.-F. |last7=Boumier |first7=P. |last8=Chevalier |first8=A. |last9=Dewitte |first9=S. |last10=Mekaoui |first10=S. |last11=Salabert |first11=D. |date=March 2015 |title=On the Determination and Constancy of the Solar Oblateness |url=http://link.springer.com/10.1007/s11207-015-0655-6 |journal=Solar Physics |language=en |volume=290 |issue=3 |pages=673–687 |doi=10.1007/s11207-015-0655-6 |bibcode=2015SoPh..290..673M |issn=0038-0938}}</ref>

<math display="block">\Delta_\odot = (R_\textrm{eq} -R_\textrm{pol})/R_\textrm{pol}.</math>

The value is difficult to measure. Atmospheric distortion means the measurement must be done on satellites; the value is very small meaning very precise technique is needed.<ref name=GoughOverview2012>{{Cite journal |last=Gough |first=Douglas |date=28 September 2012 |title=How Oblate Is the Sun? |url=https://www.science.org/doi/10.1126/science.1226988 |journal=Science |language=en |volume=337 |issue=6102 |pages=1611–1612 |doi=10.1126/science.1226988 |pmid=23019636 |bibcode=2012Sci...337.1611G |issn=0036-8075}}</ref>

The oblateness was once proposed to be sufficient to explain the [[perihelion precession of Mercury]] but Einstein proposed that [[general relativity]] could explain the precession using a spherical Sun.<ref name=GoughOverview2012/> When high precision measurements of the oblateness became available via the [[Solar Dynamics Observatory]]<ref>{{Cite journal |last1=Kuhn |first1=J. R. |last2=Bush |first2=R. |last3=Emilio |first3=M. |last4=Scholl |first4=I. F. |date=28 September 2012 |title=The Precise Solar Shape and Its Variability |url=https://www.science.org/doi/10.1126/science.1223231 |journal=Science |language=en |volume=337 |issue=6102 |pages=1638–1640 |doi=10.1126/science.1223231 |pmid=22903522 |bibcode=2012Sci...337.1638K |issn=0036-8075}}</ref> and the
[[Picard (satellite)|Picard]] satellite<ref name=Meftah-2015/> the measured value was even smaller than expected,<ref name=GoughOverview2012/> 8.2 x 10<sup>−6</sup>, or 8 parts per million.
These measurements determined the Sun to be the natural object closest to a perfect sphere ever observed.<ref name="perfect sphere">{{Cite web |last=Jones |first=G. |date=16 August 2012 |title=Sun is the most perfect sphere ever observed in nature |url=https://www.theguardian.com/science/2012/aug/16/sun-perfect-sphere-nature |url-status=live |archive-url=https://web.archive.org/web/20140303022045/http://www.theguardian.com/science/2012/aug/16/sun-perfect-sphere-nature |archive-date=3 March 2014 |access-date=19 August 2013 |website=The Guardian}}</ref> The oblateness value remains constant independent of solar irradiation changes.<ref name=Meftah-2015/> The tidal effect of the planets is weak and does not significantly affect the shape of the Sun.<ref name="Schutz2003">{{Cite book |last=Schutz |first=B. F. |title=Gravity from the ground up |url=https://books.google.com/books?id=P_T0xxhDcsIC&pg=PA98 |year=2003 |publisher=[[Cambridge University Press]] |isbn=978-0-521-45506-0 |pages=98–99}}</ref>

===Rotation===
{{main|Solar rotation}}
The Sun rotates faster at its equator than at its [[poles of astronomical bodies|poles]]. This [[Solar rotation|differential rotation]] is caused by [[convection|convective motion]] due to heat transport and the [[Coriolis effect|Coriolis force]] due to the Sun's rotation. In a frame of reference defined by the stars, the rotational period is approximately 25.6 days at the equator and 33.5 days at the poles. Viewed from Earth as it orbits the Sun, the apparent rotational period of the Sun at its equator is about 28 days.<ref name="Phillips1995-78">{{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=PA78 |pages=78–79}}</ref> Viewed from a vantage point above its north pole, the Sun rotates [[counterclockwise]] around its axis of spin.{{efn|name=rotation}}<ref name="spaceacademy">{{Cite web |title=The Anticlockwise Solar System |url=https://www.spaceacademy.net.au/library/notes/anticlok.htm |url-status=live |archive-url=https://web.archive.org/web/20200807081832/https://www.spaceacademy.net.au/library/notes/anticlok.htm |archive-date=7 August 2020 |access-date=2 July 2020 |publisher=Australian Space Academy}}</ref>

A survey of [[solar analog]]ues suggests the early Sun was rotating up to ten times faster than it does today. This would have made the surface much more active, with greater X-ray and UV emission. [[Sunspot]]s would have covered 5–30% of the surface.<ref>{{cite conference |title=The Sun in time: age, rotation, and magnetic activity of the Sun and solar-type stars and effects on hosted planets |last1=Guinan |first1=Edward F. |last2=Engle |first2=Scott G. |conference=The Ages of Stars, Proceedings of the International Astronomical Union, IAU Symposium |volume=258 |pages=395–408 |date=June 2009 |doi=10.1017/S1743921309032050 |arxiv=0903.4148 |bibcode=2009IAUS..258..395G}}</ref> The rotation rate was gradually slowed by [[Magnetic braking (astronomy)|magnetic braking]], as the Sun's magnetic field interacted with the outflowing [[solar wind]].<ref>{{cite journal |title=Magnetic Braking of Sun-like and Low-mass Stars: Dependence on Coronal Temperature |last1=Pantolmos |first1=George |last2=Matt |first2=Sean P. |journal=The Astrophysical Journal |volume=849 |issue=2 |at=id. 83 |date=November 2017 |doi=10.3847/1538-4357/aa9061 |doi-access=free |arxiv=1710.01340 |bibcode=2017ApJ...849...83P}}</ref> A vestige of this rapid primordial rotation still survives at the Sun's core, which rotates at a rate of once per week; four times the mean surface rotation rate.<ref>{{cite journal |title=Asymptotic g modes: Evidence for a rapid rotation of the solar core |last1=Fossat |first1=E. |last2=Boumier |first2=P. |last3=Corbard |first3=T. |last4=Provost |first4=J. |last5=Salabert |first5=D. |last6=Schmider |first6=F. X. |last7=Gabriel |first7=A. H. |last8=Grec |first8=G. |last9=Renaud |first9=C. | last10=Robillot | first10=J. M. |last11=Roca-Cortés |first11=T. |last12=Turck-Chièze |first12=S. |last13=Ulrich |first13=R. K. |last14=Lazrek |first14=M. |journal=Astronomy & Astrophysics |volume=604 |at=id. A40 |date=August 2017 |doi=10.1051/0004-6361/201730460 |arxiv=1708.00259 |bibcode=2017A&A...604A..40F}}</ref><ref>{{cite web |title=ESA, NASA's SOHO Reveals Rapidly Rotating Solar Core |first=Susannah |last=Darling |date=1 August 2017 |publisher=[[NASA]] |url=https://www.nasa.gov/science-research/heliophysics/esa-nasas-soho-reveals-rapidly-rotating-solar-core/ |access-date=31 May 2024}}</ref>