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{{About|the star}}
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{{Infobox
| bodystyle   = border-collapse:collapse
|   title     = The Sun [[Image:Sun symbol.svg|25px]]
|   image     = [[File:Sun white.jpg|290px]]
| caption     = Sun with [[sunspot]]s and [[limb darkening]] as seen in [[visible light]] with solar filter.
| headerstyle = background:#FCC857
|  labelstyle = padding:2px
|   datastyle = padding:2px
<!-- section break, rows 1-19-->
| header1  = Observation data
|  label2  = Mean distance<br>from [[Earth]]
|   data2  = 1 [[astronomical unit|au]] ≈ {{val|1.496|e=8|u=km}}<br>8&nbsp;min 19&nbsp;s at [[speed of light|light speed]]
|  label3  = [[Apparent magnitude|Visual brightness]] (''V'')
|   data3  = −26.74<ref name=nssdc>{{cite web|last=Williams|first=D. R. |date=1 July 2013 |title=Sun Fact Sheet |url=http://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html |publisher=[[NASA Goddard Space Flight Center]] |accessdate=12 August 2013}}</ref>
|  label4  = [[Absolute magnitude]]
|   data4  = 4.83<ref name=nssdc />
|  label5  = [[Spectral classification]]
|   data5  = G2V<ref>{{cite book|last=Zombeck|first=Martin V.|date=1990|title=Handbook of Space Astronomy and Astrophysics 2nd edition|publisher=[[Cambridge University Press]]|url=http://ads.harvard.edu/books/hsaa/}}</ref>
|  label6  = [[Metallicity]]
|   data6  = ''Z'' = 0.0122<ref>{{cite journal |last1=Asplund |first1=M. |last2=Grevesse |first2=N. |last3=Sauval |first3=A. J. |date=2006 |title=The new solar abundances – Part I: the observations |journal=[[Communications in Asteroseismology]] |volume=147 |pages=76–79 |bibcode=2006CoAst.147...76A |doi=10.1553/cia147s76}}</ref>
|  label7  = [[Angular size]]
|   data7  = 31.6–32.7 [[minutes of arc]]<ref>{{cite web|title=Eclipse 99: Frequently Asked Questions |url=http://education.gsfc.nasa.gov/eclipse/pages/faq.html |publisher=[[NASA]] |accessdate=24 October 2010 |deadurl=yes |archiveurl=https://web.archive.org/web/20100527142627/http://education.gsfc.nasa.gov/eclipse/pages/faq.html |archivedate=27 May 2010 |df= }}</ref>
|  label8  = Adjectives
|   data8  = Solar
| header10 = [[Orbit]]al characteristics
|  label11 = Mean distance<br>from [[Milky Way]] core
|   data11 = ≈ {{val|2.7|e=17|u=km}}<br>{{nowrap|{{val|fmt=commas|27200|ul=light-years}}}}
|  label12 = [[Galactic year|Galactic period]]
|   data12 = (2.25–2.50){{e|8}} [[julian year (astronomy)|yr]]
|  label13 = [[Velocity]]
|   data13 = ≈ {{val|220|u=km/s}} (orbit around the center of the Milky Way) <br>≈ {{val|20|u=km/s}} (relative to average velocity of other stars in stellar neighborhood) <br>≈ {{val|370|u=km/s}}<ref>{{cite journal |last=Hinshaw |first=G. |display-authors=etal |date=2009 |title=Five-year Wilkinson Microwave Anisotropy Probe observations: data processing, sky maps, and basic results |journal=[[The Astrophysical Journal Supplement Series]] |volume=180 |issue=2 |pages=225–245 |arxiv=0803.0732 |bibcode=2009ApJS..180..225H |doi=10.1088/0067-0049/180/2/225}}</ref> (relative to the [[Cosmic microwave background radiation#CMBR dipole anisotropy|cosmic microwave background]])
<!-- section break, rows 20-39 -->
| header20 = Physical characteristics
|  label21  = Equatorial [[radius]]
|   data21  = [[Solar radius|695,700]]&nbsp;km<ref name=IAU2015resB3>{{citation | first1=E.E. | last1=Mamajek | first2=A. | last2=Prsa | first3=G. | last3=Torres | first4=al. | last4=et | title=IAU 2015 Resolution B3 on Recommended Nominal Conversion Constants for Selected Solar and Planetary Properties  | arxiv=1510.07674}}</ref><br> {{val|109|u=R_Earth}}<ref name=sse/>
|  label22  = Equatorial [[circumference]]
|   data22  = {{val|4.379|e=6|u=km}}<ref name=sse/><br>109 × Earth<ref name=sse>{{cite web |title=Solar System Exploration: Planets: Sun: Facts & Figures |url=http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sun&Display=Facts&System=Metric |archiveurl=https://web.archive.org/web/20080102034758/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sun&Display=Facts&System=Metric |archivedate=2 January 2008 |publisher=[[NASA]]
}}</ref>
|  label23  = [[Flattening]]
|   data23  = {{val|9|e=-6}}
|  label24  = [[Surface area]]
|   data24  = {{val|6.09|e=12|u=km2}}<ref name=sse/><br>{{nowrap|{{val|fmt=commas|12000}}}} × Earth<ref name=sse/>
|  label25  = [[Volume]]
|   data25  = {{val|1.41|e=18|u=km3}}<ref name=sse/><!-- NASA source has "1.412 x 10^18 km^3", which is 1.412 × 10^27 m^3 (basic arithmetic, also verified using the formula for volume of a sphere), but the Sun is not a sphere, and their radius is off a bit from what we have above, so we need to at least round a bit--><br>{{nowrap|{{val|fmt=commas|1300000}}}} × Earth
|  label26  = [[Mass]]
|   data26  = {{val|1.98855|.00025|e=30|u=kg}}<ref name=nssdc/><br>{{nowrap|{{val|fmt=commas|333000|u=Earth mass}}}}<ref name=nssdc/><!-- NASA Sun Fact Sheet states 333,000, a figure coherent with data already present in en.wiki -->
|  label27  = Average [[density]]
|   data27  = {{val|1.408|u=g/cm3}}<ref name=nssdc/><ref name=sse/><ref>{{cite web |last=Ko |first=M. |date=1999 |title=Density of the Sun |url=http://hypertextbook.com/facts/1999/MayKo.shtml |editor=Elert, G. |work=The Physics Factbook}}</ref><br>{{val|0.255}} × Earth<ref name=nssdc/><ref name=sse/>
|  label28  = Center [[density]] (modeled)
|   data28  = {{val|162.2|u=g/cm3}}<ref name=nssdc/><br>{{val|12.4}} × Earth
|  label29  = Equatorial [[surface gravity]]
|   data29  = {{val|274.0|u=m/s2}}<ref name=nssdc/><br>{{val|27.94|u=[[g-force|''g'']]}}<br>{{nowrap|{{val|fmt=commas|27542.29|u=''[[cgs]]''}}}}<br>28 × Earth<ref name=sse/>
|  label30  = [[Moment of inertia factor]]
|   data30  = {{val|0.070}}<ref name=nssdc /> (estimate)
|  label31  = [[Escape velocity]]<br>(from the surface)
|   data31  = {{val|617.7|u=km/s}}<ref name=sse/><br>55 × Earth<ref name=sse/>
|  label32  = Temperature
|   data32  = Center (modeled): {{val|1.57|e=7|u=K}}<ref name=nssdc/><br>[[Photosphere]] (effective): {{nowrap|{{val|fmt=commas|5772|ul=K}}}}<ref name=nssdc/><br> [[Corona]]: ≈ {{val|5|e=6|u=K}}
|  label33  = [[Luminosity]] (L<sub>sol</sub>)
|   data33  = {{val|3.828|e=26|ul=W}}<ref name=nssdc/><br>≈ {{val|3.75|e=28|u=[[lumen (unit)|lm]]}}<br>≈ {{val|98|u=lm/W}} [[Luminous efficacy|efficacy]]
|  label34  = Mean [[radiance]]&nbsp;(I<sub>sol</sub>)
|   data34  = {{val|2.009|e=7|u=W·m<sup>−2</sup>·sr<sup>−1</sup>}}
|  label35  = Age
|   data35  = ≈ 4.6 billion years<ref name="Bonanno">{{Cite journal |last=Bonanno |first=A. |last2=Schlattl |first2=H. |last3=Paternò |first3=L. |date=2008 |title=The age of the Sun and the relativistic corrections in the EOS |journal=[[Astronomy and Astrophysics]] |volume=390 |issue=3 |pages=1115–1118 |arxiv=astro-ph/0204331 |bibcode=2002A&A...390.1115B |doi=10.1051/0004-6361:20020749 |ref=harv}}</ref><ref>{{Cite journal|url=//www.sciencemag.org/content/338/6107/651.full|title=The Absolute Chronology and Thermal Processing of Solids in the Solar Protoplanetary Disk|date=2 November 2012|accessdate=17 March 2014|doi=10.1126/science.1226919 |journal=Science|volume=338 |issue= 6107 |pages=651–655|bibcode = 2012Sci...338..651C |pmid=23118187 | last1 = Connelly | first1 = JN | last2 = Bizzarro | first2 = M | last3 = Krot | first3 = AN | last4 = Nordlund | first4 = Å | last5 = Wielandt | first5 = D | last6 = Ivanova | first6 = MA}}{{Registration required}}</ref>
<!-- section break, rows 40-49 -->
| header40  = [[Rotation]] characteristics
|  label41  = [[Axial tilt|Obliquity]]
|   data41  = 7.25°<ref name=nssdc/><br>(to the [[ecliptic]])<br>67.23°<br>(to the [[galactic plane]])
|  label42  = [[Right ascension]]<br>of North pole<ref name="iau-iag">{{cite web |last1=Seidelmann |first1=P. K. |display-authors=etal |title=Report Of The IAU/IAG Working Group On Cartographic Coordinates And Rotational Elements Of The Planets And Satellites: 2000 |url=http://www.hnsky.org/iau-iag.htm |date=2000 |accessdate=22 March 2006}}</ref>
|   data42  = 286.13°<br>{{nowrap|19 h 4 min 30 s}}
|  label43  = [[Declination]]<br>of North pole
|   data43  = +63.87°<br>63° 52' North
|  label44  = Sidereal [[Solar rotation|rotation period]] <br>(at equator)
|   data44  = 25.05 d<ref name=nssdc/>
|  label45  = (at 16° latitude)
|   data45  = 25.38 d<ref name=nssdc/><br>{{nowrap|25 d 9 h 7 min 12 s}}<ref name="iau-iag"/>
|  label46  = (at poles)
|   data46  = 34.4 d<ref name=nssdc/><!-- derived from T = ( 14.37 - 2.33 sin^2 L - 1.56 sin^4 L ) °/day, L = 90° -->
|  label47  = Rotation velocity<br>(at equator)
|   data47  = {{val|7.189|e=3|u=km/h}}<ref name="sse"/><!-- Derived from NASA source: equatorial circumference of 4,379,000 kilometres divided by sidereal rotation period of 609.12 hours; maybe this kind of basic calculation could be done in some generic template code? -->
<!-- section break, rows 50-69 -->
| header50  = [[photosphere|Photospheric]] composition (by mass)
|  label51  = [[Hydrogen]]
|   data51  = 73.46%<ref>{{cite web |title=The Sun's Vital Statistics |url=http://solar-center.stanford.edu/vitalstats.html |publisher=[[Stanford Solar Center]] |accessdate=29 July 2008}} Citing {{cite book |last=Eddy |first=J. |date=1979 |title=A New Sun: The Solar Results From Skylab |url=https://history.nasa.gov/SP-402/contents.htm |page=37 |publisher=[[NASA]] |id=NASA SP-402}}</ref>
|  label52  = [[Helium]]
|   data52  = 24.85%
|  label53  = [[Oxygen]]
|   data53  = 0.77%
|  label54  = [[Carbon]]
|   data54  = 0.29%
|  label55  = [[Iron]]
|   data55  = 0.16%
|  label56  = [[Neon]]
|   data56  = 0.12%
|  label57  = [[Nitrogen]]
|   data57  = 0.09%
|  label58  = [[Silicon]]
|   data58  = 0.07%
|  label59  = [[Magnesium]]
|   data59  = 0.05%
|  label60  = [[Sulfur]]
|   data60  = 0.04%
}}

The '''Sun''' is the [[star]] at the center of the [[Solar System]].<!-- Please don't change "the" to "our"&nbsp;— there is only one "Solar System", and thus "the" is correct. See Talk page for this article and Solar System. --> It is a nearly perfect sphere of hot [[plasma (physics)|plasma]],<ref>
{{Cite news
 |url=https://science.nasa.gov/science-news/science-at-nasa/2008/02oct_oblatesun/
 |title=How Round is the Sun?
 |publisher=NASA
 |date=2 October 2008
 |accessdate=7 March 2011
}}</ref><ref>
{{Cite news
 |url=https://science.nasa.gov/science-news/science-at-nasa/2011/06feb_fullsun/
 |title=First Ever STEREO Images of the Entire Sun
 |publisher=NASA
 |date=6 February 2011
 |accessdate=7 March 2011
}}</ref> with internal [[convection|convective]] motion that generates a [[magnetic field]] via a [[Solar dynamo|dynamo process]].<ref name="doi10.1146/annurev-astro-081913-040012">{{Cite journal | doi = 10.1146/annurev-astro-081913-040012| title = Solar Dynamo Theory| journal = Annual Review of Astronomy and Astrophysics| volume = 52| pages = 251–290| year = 2014| last1 = Charbonneau | first1 = P. |bibcode = 2014ARA&A..52..251C }}</ref> It is by far the most important source of [[energy]] for [[life]] on [[Earth]]. Its diameter is about 109 times that of Earth, and [[Solar mass|its mass]] is about 330,000 times that of Earth, accounting for about 99.86% of the total 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
 |journal=[[Astronomy & Geophysics]]
 |volume=41 |issue=1 |page=12
 |bibcode=2000A&G....41a..12W
 |doi=10.1046/j.1468-4004.2000.00012.x
 |ref=harv
}}</ref>
<!-- There are several estimations on the mass of the Solar System; for further information please have a look at the talk page. -->About three quarters of the Sun's mass consists of [[hydrogen]] (~73%); the rest is mostly [[helium]] (~25%), with much smaller quantities of heavier elements, including [[oxygen]], [[carbon]], [[neon]], and [[iron]].<ref name=basu2008>
{{Cite journal
 |last=Basu |first=S.
 |last2=Antia |first2=H. M.
 |date=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
 |ref=harv
}}</ref>

The Sun is a [[G-type main-sequence star]] (G2V) based on its [[stellar classification|spectral class]]. As such, it is informally referred to as a yellow dwarf. It formed approximately 4.6 billion<ref group=lower-alpha name=short>All numbers in this article are short scale. One billion is 10<sup>9</sup>, or 1,000,000,000.</ref><ref name="Bonanno" /><ref name="Connelly2012">{{cite journal |title=The Absolute Chronology and Thermal Processing of Solids in the Solar Protoplanetary Disk |journal=[[Science (journal)|Science]] |first1=James N. |last1=Connelly |first2=Martin |last2=Bizzarro |first3=Alexander N. |last3=Krot |first4=Åke |last4=Nordlund |first5=Daniel |last5=Wielandt |first6=Marina A. |last6=Ivanova |volume=338 |issue=6107 |pages=651–655 |date=2 November 2012 |doi=10.1126/science.1226919 |bibcode=2012Sci...338..651C |pmid=23118187}}</ref> years ago from the [[gravitational collapse]] of matter within a region of a large [[molecular cloud]]. Most of this matter gathered in the center, whereas the rest flattened into an orbiting disk that [[formation and evolution of the Solar System|became the Solar System]]. The central mass became so hot and dense that it eventually initiated [[nuclear fusion]] in its [[solar core|core]]. It is thought that almost all stars [[Star formation|form by this process]].

The Sun is roughly middle-aged; it has not changed dramatically for more than four billion<ref group=lower-alpha name=short /> years, and will remain fairly stable for more than another five billion years. After [[hydrogen fusion]] in its core has diminished to the point at which it is no longer in [[hydrostatic equilibrium]], the core of the Sun will experience a marked increase in density and temperature while its outer layers expand to eventually become a [[red giant]]. It is calculated that the Sun will become sufficiently large to engulf the current orbits of [[Mercury (planet)|Mercury]] and [[Venus]], and render [[Earth]] uninhabitable.

The enormous effect of the Sun on Earth has been recognized since [[prehistoric times]], and the Sun has been [[The Sun in culture|regarded by some cultures]] as a [[solar deity|deity]]. The [[Synodic day|synodic]] rotation of Earth and its orbit around the Sun are the basis of the [[solar calendar]], which is the predominant [[calendar]] in use today.

==Name and etymology==
The English proper name ''Sun'' developed from [[Old English]] ''sunne'' and may be related to ''south''. Cognates to English ''sun'' appear in other [[Germanic languages]], including [[Old Frisian]] ''sunne'', ''sonne'', [[Old Saxon]] ''sunna'', [[Middle Dutch]] ''sonne'', modern [[Dutch language|Dutch]] ''zon'', [[Old High German]] ''sunna'', modern German ''Sonne'', [[Old Norse]] ''sunna'', and [[Gothic language|Gothic]] ''sunnō''. All Germanic terms for the Sun stem from [[Proto-Germanic]] *''sunnōn''.<ref name=BARNHART776>
{{cite book
 |last=Barnhart |first=R. K.
 |date=1995
 |title=The Barnhart Concise Dictionary of Etymology
 |page=776
 |publisher=[[HarperCollins]]
 |isbn=0-06-270084-7
}}</ref><ref name=MALLORY129>
{{cite book
 |last=Mallory |first=J. P.
 |date=1989
 |title=In Search of the Indo-Europeans: Language, Archaeology and Myth
 |page=129
 |publisher=[[Thames & Hudson]]
 |isbn=0-500-27616-1
}}</ref>

The English weekday name ''Sunday'' stems from Old English (''Sunnandæg''; "Sun's day", from before 700) and is ultimately a result of a [[Interpretatio germanica|Germanic interpretation]] of Latin ''dies solis'', itself a translation of the Greek ἡμέρα ἡλίου (''hēméra hēlíou'').<ref name="BARNHART778">
{{cite book
 |last=Barnhart |first=R. K.
 |date=1995
 |title=The Barnhart Concise Dictionary of Etymology
 |page=778
 |publisher=[[HarperCollins]]
 |isbn=0-06-270084-7
}}</ref> The Latin name for the Sun, ''Sol'', is not common in general English language use; the adjectival form is the related word ''solar''.<ref>
{{cite book
 |last1=Little |first1=W
 |last2=Fowler |first2=H. W.
 |last3=Coulson |first3=J.
 |chapter=Sol
 |title=Oxford Universal Dictionary on Historical Principles
 |edition=3rd
 |asin=B000QS3QVQ
}}</ref><ref>
{{cite web
 |title=Sol
 |url=http://www.merriam-webster.com/dictionary/Sol
 |publisher=[[Merriam-Webster]]
 |accessdate=19 July 2009
}}</ref> The term ''sol'' is also used by planetary astronomers to refer to the duration of a [[solar day]] on another planet, such as [[Mars]].<ref>
{{cite web
 |date=15 November 2006
 |title=Opportunity's View, Sol 959 (Vertical)
 |url=http://www.nasa.gov/mission_pages/mer/images/pia01892.html
 |publisher=[[NASA]]
 |accessdate=1 August 2007
}}</ref> A mean [[Earth]] solar day is approximately 24 hours, whereas a mean Martian 'sol' is 24 hours, 39 minutes, and 35.244 seconds.<ref>
{{cite web
 |last=Allison |first=M.
 |last2=Schmunk |first2=R.
 |date=8 August 2012
 |title=Technical Notes on Mars Solar Time as Adopted by the Mars24 Sunclock
 |url=http://www.giss.nasa.gov/tools/mars24/help/notes.html
 |publisher=[[NASA]]/[[GISS]]
 |accessdate=16 September 2012
}}</ref>

===Religious aspects===

{{main article |Solar deity}}
Solar deities and Sun worship can be found throughout most of recorded history in various forms, including the Egyptian [[Ra]], the Hindu [[Surya]], the Japanese [[Amaterasu]], the Germanic [[Sól (sun)|Sól]], and the Aztec [[Tonatiuh]], among others.

From at least the [[4th Dynasty]] of [[Ancient Egypt]], the Sun was worshipped as the [[Ra|god Ra]], portrayed as a falcon-headed divinity surmounted by the solar disk, and surrounded by a serpent. In the [[New Kingdom of Egypt|New Empire]] period, the Sun became identified with the [[dung beetle]], whose spherical ball of dung was identified with the Sun. In the form of the Sun disc [[Aten]], the Sun had a brief resurgence during the [[Amarna Period]] when it again became the preeminent, if not only, divinity for the [[Pharaoh]] [[Akhenaton]].<ref>{{cite book|last1=Teeter|first1=Emily|title=Religion and Ritual in Ancient Egypt|date=2011|publisher=Cambridge University Press|location=New York|isbn=9780521848558}}</ref><ref>{{cite book|last1=Frankfort|first1=Henri|title=Ancient Egyptian Religion: an Interpretation|date=2011|publisher=Dover Publications|isbn=0486411389}}</ref>

The Sun is viewed as a goddess in [[Germanic paganism]], [[Sól (sun)|Sól/Sunna]].<ref name=MALLORY129/> Scholars theorize that the Sun, as a Germanic goddess, may represent an extension of an earlier [[Proto-Indo-Europeans|Proto-Indo-European]] Sun deity because of [[Indo-European languages|Indo-European linguistic]] connections between Old Norse ''Sól'', [[Sanskrit]] ''[[Surya]]'', [[Gaulish language|Gaulish]] ''[[Sulis]]'', [[Lithuanian language|Lithuanian]] ''[[Saulė]]'', and [[Slavic languages|Slavic]] ''Solntse''.<ref name=MALLORY129/>

In ancient Roman culture, [[Sunday]] was the day of the Sun god. It was adopted as the [[Sabbath]] day by Christians who did not have a Jewish background. The symbol of light was a pagan device adopted by Christians, and perhaps the most important one that did not come from Jewish traditions. In paganism, the Sun was a source of life, giving warmth and illumination to mankind. It was the center of a popular cult among Romans, who would stand at dawn to catch the first rays of sunshine as they prayed. The celebration of the [[winter solstice]] (which influenced Christmas) was part of the Roman cult of the unconquered Sun ([[Sol Invictus]]). Christian churches were built with an orientation so that the congregation faced toward the sunrise in the East.<ref>{{cite book|author=Owen Chadwick|title=A History of Christianity|url=https://books.google.com/books?id=qugouOh3KjMC&pg=PA22|year=1998|publisher=St. Martin's Press|page=22}}</ref>

==Characteristics==

The Sun is a [[G-type main-sequence star]] that comprises about 99.86% of the mass of the Solar System. The Sun 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
 |accessdate=1 August 2007
}}</ref><ref>{{Cite journal
 |last=Lada |first=C. J.
 |date=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
 |ref=harv
}}</ref>
The Sun is a [[Population I stars|Population I]], or heavy-element-rich,{{efn|name=heavy elements}} star.<ref name=zeilik>
{{Cite book
 |last=Zeilik |first=M. A.
 |last2=Gregory |first2=S. A.
 |date=1998
 |title=Introductory Astronomy & Astrophysics
 |edition=4th |page=322
 |publisher=[[Saunders College Publishing]]
 |isbn=0-03-006228-4
}}</ref> The formation of the Sun may have been triggered by shockwaves from one or more nearby [[supernova]]e.<ref name="Falk">
{{Cite journal
 |last=Falk |first=S. W.
 |last2=Lattmer |first2=J. M.
 |last3=Margolis |first3=S. H.
 |date=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
 |ref=harv
}}</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 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
 |ref=harv
}}</ref><ref>{{Cite journal
 |last=Bessell |first=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=harv
}}</ref> This is about 13 billion times brighter than the next brightest star, [[Sirius]], which has an apparent magnitude of −1.46. The mean distance of the Sun's center to Earth's center is approximately {{convert|1|AU|km mi|lk=in|disp=x| (about |)}}, though the distance varies as Earth moves from [[perihelion]] in January to [[aphelion]] in 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
 |publisher=[[US Naval Observatory]]
 |accessdate=17 July 2009
}}</ref> At this average distance, light travels from the Sun's horizon to Earth's horizon in about 8 minutes and 19 seconds, 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 name="Simon2001">{{Cite book
 |last=Simon |first=A.
 |title=The Real Science Behind the X-Files : Microbes, meteorites, and mutants
 |url=https://books.google.com/?id=1gXImRmz7u8C&pg=PA26&dq=bacteria+that+live+with+out+the+sun
 |pages=25–27
 |publisher=[[Simon & Schuster]]
 |date=2001
 |isbn=0-684-85618-2
}}</ref> and drives [[Earth's climate]] and weather.

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
 |last=Beer |first=J.
 |last2=McCracken |first2=K.
 |last3=von Steiger |first3=R.
 |date=2012
 |title=Cosmogenic Radionuclides: Theory and Applications in the Terrestrial and Space Environments
 |page=41
 |publisher=[[Springer Science+Business Media]]
 |isbn=978-3-642-14651-0
}}</ref> For the purpose of measurement, however, the Sun's radius is considered to be the distance from its center 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.
 |date=1995
 |title=Guide to the Sun
 |page=73
 |publisher=[[Cambridge University Press]]
 |isbn=978-0-521-39788-9
}}</ref> By this measure, the Sun is a near-perfect sphere with an [[oblateness]] estimated at about 9 millionths,<ref name="Godier">{{Cite journal
 |last=Godier |first=S.
 |last2=Rozelot |first2=J.-P.
 |date=2000
 |title=The solar oblateness and its relationship with the structure of the tachocline and of the Sun's subsurface
 |url=http://aa.springer.de/papers/0355001/2300365.pdf
 |journal=[[Astronomy and Astrophysics]]
 |volume=355 |pages=365–374
 |bibcode=2000A&A...355..365G
 |ref=harv
}}</ref> which means that its polar diameter differs from its equatorial diameter by only {{convert|10|km|mi}}.<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
 |work=[[The Guardian]]
 |accessdate=19 August 2013
}}</ref>
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.
 |date=2003
 |title=Gravity from the ground up
 |pages=98–99
 |publisher=[[Cambridge University Press]]
 |isbn=978-0-521-45506-0
}}</ref> 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.
 |date=1995
 |title=Guide to the Sun
 |pages=78–79
 |publisher=[[Cambridge University Press]]
 |isbn=978-0-521-39788-9
}}</ref>

==Sunlight==
{{Main article|Sunlight}}

The [[solar constant]] is the amount of power that the Sun deposits per unit area that is directly exposed to sunlight. The solar constant is equal to approximately {{val|1368|u=W/m2|fmt=commas}} (watts per square meter) at a distance of one [[astronomical unit]] (AU) from the Sun (that is, on or near Earth).<ref name=TSI>{{cite web|title=Construction of a Composite Total Solar Irradiance (TSI) Time Series from 1978 to present |url=http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant|accessdate = 5 October 2005}}</ref> Sunlight on the surface of Earth is [[attenuation (electromagnetic radiation)|attenuated]] by Earth's atmosphere, so that less power arrives at the surface (closer to {{val|1000|u=W/m2|fmt=commas}}) in clear conditions when the Sun is near the [[zenith]].<ref name=El-Sharkawi2005>{{Cite book|last=El-Sharkawi|first=Mohamed A.|title=Electric energy|date=2005|publisher=CRC Press|isbn=978-0-8493-3078-0|pages=87–88}}</ref> Sunlight at the top of Earth's atmosphere is composed (by total energy) of about 50% infrared light, 40% visible light, and 10% ultraviolet light.<ref name="Solar radiation">[http://curry.eas.gatech.edu/Courses/6140/ency/Chapter3/Ency_Atmos/Radiation_Solar.pdf Solar radiation]</ref> The atmosphere in particular filters out over 70% of solar ultraviolet, especially at the shorter wavelengths.<ref>{{cite web|url=http://rredc.nrel.gov/solar/spectra/am1.5/ |title=Reference Solar Spectral Irradiance:  Air Mass 1.5|accessdate=12 November 2009}}</ref> Solar [[ultraviolet radiation]] ionizes Earth's dayside upper atmosphere, creating the electrically conducting [[ionosphere]].<ref name=Phillips1995>
{{Cite book
 |last=Phillips |first=K. J. H.
 |date=1995
 |title=Guide to the Sun
 |pages=14–15, 34–38
 |publisher=[[Cambridge University Press]]
 |isbn=978-0-521-39788-9
}}</ref>

The Sun's color is white, with a [[CIE 1931 color space|CIE]] color-space index near (0.3, 0.3), when viewed from space or when the Sun is high in the sky. When measuring all the photons emitted, the Sun is actually emitting more photons in the green portion of the spectrum than any other.<ref>{{cite web|url=http://www.universetoday.com/18689/color-of-the-sun/|title=What Color is the Sun?|publisher=Universe Today|access-date=23 May 2016}}</ref><ref>{{cite web|url=http://solar-center.stanford.edu/SID/activities/GreenSun.html|title=What Color is the Sun?|publisher=[[Stanford University|Stanford]] Solar Center|access-date=23 May 2016}}</ref> When the Sun is low in the sky, [[Diffuse sky radiation|atmospheric scattering]] renders the Sun yellow, red, orange, or magenta. Despite its typical whiteness, most people mentally picture the Sun as yellow; the reasons for this are the subject of debate.<ref name="yellow sun paradox">
{{Cite journal
 |last=Wilk |first=S. R.
 |date=2009
 |title=The Yellow Sun Paradox
 |url=http://www.osa-opn.org/Content/ViewFile.aspx?id=11147
 |journal=[[Optics & Photonics News]]
 |pages=12–13
 |ref=harv
}}</ref>
The Sun is a [[G-type main-sequence star|G2V]] star, with ''G2'' indicating its [[effective temperature|surface temperature]] of approximately 5,778&nbsp;K (5,505&nbsp;°C, 9,941&nbsp;°F), and ''V'' that it, like most stars, is a [[main sequence|main-sequence]] star.<ref name=Phillips1995-47>
{{Cite book
 |last=Phillips|first=K. J. H.
 |date=1995
 |title=Guide to the Sun
 |pages=47–53
 |publisher=[[Cambridge University Press]]
 |isbn=978-0-521-39788-9
}}</ref><ref>{{cite news|title=Dr Karl's Great Moments In Science: Lazy Sun is less energetic than compost |url=http://www.abc.net.au/science/articles/2012/04/17/3478276.htm|accessdate=25 February 2014|newspaper=[[Australian Broadcasting Corporation]]|date=17 April 2012|author=Karl S. Kruszelnicki|quote="Every second, the Sun burns 620 million tonnes of hydrogen..."}}</ref> The average [[luminance]] of the Sun is about 1.88&nbsp;[[giga]]&nbsp;[[candela per square metre]], but as viewed through Earth's atmosphere, this is lowered to about 1.44&nbsp;Gcd/m<sup>2</sup>.{{efn|1=1.88&nbsp;Gcd/m<sup>2</sup> is calculated from the solar illuminance of {{val|128000|u=lux}} (see [[sunlight]]) times the square of the distance to the center of the Sun, divided by the cross sectional area of the Sun. 1.44&nbsp;Gcd/m<sup>2</sup> is calculated using {{val|98000|u=lux}}.}} However, the luminance is not constant across the disk of the Sun ([[limb darkening]]).

==Composition==

{{see also|Molecules in stars}}

The Sun is composed primarily of the [[chemical element]]s [[hydrogen]] and [[helium]]; they account for 74.9% and 23.8% of the mass of the Sun in the photosphere, respectively.<ref name=lodders>
{{cite journal| doi = 10.1086/375492| last = Lodders| first = Katharina| date = July 10, 2003| title = Solar System Abundances and Condensation Temperatures of the Elements| journal = The Astrophysical Journal| publisher = The American Astronomical Society| volume = 591| issue = 2| pages = 1220–1247| url = http://weft.astro.washington.edu/courses/astro557/LODDERS.pdf| format = PDF| bibcode = 2003ApJ...591.1220L| ref = harv}}<br>{{Cite journal
 |last=Lodders |first=K.
 |title=Abundances and Condensation Temperatures of the Elements
 |url=http://www.lpi.usra.edu/meetings/metsoc2003/pdf/5272.pdf
 |format=PDF|journal=[[Meteoritics & Planetary Science]]
 |volume=38 |issue=suppl. |page=5272
 |date=2003
 |bibcode=2003M&PSA..38.5272L
 |ref=harv
}}</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
 |last=Hansen |first=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=0-387-20089-4
}}</ref>

The Sun inherited its chemical composition from the [[interstellar medium]] out of which it formed. The hydrogen and helium in the Sun were produced by [[Big Bang nucleosynthesis]], and the heavier elements were produced by [[stellar nucleosynthesis]] in generations of stars that completed their [[stellar evolution]] and returned their material to the interstellar medium before the formation of the Sun.<ref name=hkt2004_78>{{Cite book
 |last=Hansen |first=C.J. |last2=Kawaler |first2=S.A. |last3=Trimble |first3=V.
 |title=Stellar Interiors: Physical Principles, Structure, and Evolution
 |pages=77–78
 |edition=2nd
 |publisher=[[Springer Science+Business Media|Springer]]
 |date=2004
 |isbn=0-387-20089-4
}}</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 |page=133
 |date=1968
 |bibcode=1968PASAu...1..133A
 |ref=harv
}}</ref> However, since the Sun formed, some of the helium and heavy elements have gravitationally settled from the photosphere. Therefore, in today's 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). The protostellar Sun's composition is believed to have been 71.1% hydrogen, 27.4% helium, and 1.5% heavier elements.<ref name=lodders/>

Today, nuclear fusion in the Sun's core has modified the composition by converting hydrogen into helium, so the innermost portion of the Sun is now roughly 60% helium, with the abundance of heavier elements unchanged. Because heat is transferred from the Sun's core by radiation rather than by convection (see [[#Radiative zone|Radiative zone]] below), none of the fusion products from the core have risen to the photosphere.<ref name=hkt2004_9.2.3>{{Cite book
 |last=Hansen |first=C.J. |last2=Kawaler |first2=S.A. |last3=Trimble |first3=V.
 |title=Stellar Interiors: Physical Principles, Structure, and Evolution
 |pages=§ 9.2.3 |nopp=yes
 |edition=2nd
 |publisher=[[Springer Science+Business Media|Springer]]
 |date=2004
 |isbn=0-387-20089-4
}}</ref>

The reactive core zone of "hydrogen burning", where hydrogen is converted into helium, is starting to surround an inner core of "helium ash". This development will continue and will eventually cause the Sun to leave the [[main sequence]], to become a [[red giant]].<ref>Iben, I Jnr (1965) "Stellar Evolution. II. The Evolution of a 3 M_{sun} Star from the Main Sequence Through Core Helium Burning". (Astrophysical Journal, vol. 142, p.1447)</ref>

The solar heavy-element abundances described above are typically measured both 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 settling of heavy elements. The two methods generally agree well.<ref name=basu2008 />

===Singly ionized iron-group elements===
In the 1970s, much research focused on the abundances of [[iron group|iron-group]] elements in the Sun.<ref name="biemont1978">
{{Cite journal
 |last=Biemont |first=E.
 |date=1978
 |title=Abundances of singly ionized elements of the iron group in the Sun
 |journal=[[Monthly Notices of the Royal Astronomical Society]]
 |volume=184 |pages=683–694
 |bibcode=1978MNRAS.184..683B
 |ref=harv
 |doi=10.1093/mnras/184.4.683
}}</ref><ref>Ross and Aller 1976, Withbroe 1976, Hauge and Engvold 1977, cited in Biemont 1978.</ref> Although significant research was done, until 1978 it was difficult to determine the abundances of some iron-group elements (e.g. [[cobalt]] and [[manganese]]) via [[spectrography]] because of their [[hyperfine structure]]s.<ref name="biemont1978"/>

The first largely complete set of [[oscillator strength]]s of singly ionized iron-group elements were made available in the 1960s,<ref>Corliss and Bozman (1962 cited in Biemont 1978) and Warner (1967 cited in Biemont 1978)</ref> and these were subsequently improved.<ref>Smith (1976 cited in Biemont 1978)</ref> In 1978, the abundances of singly ionized elements of the iron group were derived.<ref name="biemont1978"/>

===Isotopic composition===
Various authors have considered the existence of a gradient in the [[isotope|isotopic]] compositions of solar and planetary [[noble gas]]es,<ref>Signer and Suess 1963; Manuel 1967; Marti 1969; Kuroda and Manuel 1970; Srinivasan and Manuel 1971, all cited in Manuel and Hwaung 1983</ref> e.g. correlations between isotopic compositions of [[neon]] and [[xenon]] in the Sun and on the planets.<ref>Kuroda and Manuel 1970 cited in Manuel and Hwaung 1983:7</ref>

Prior to 1983, it was thought that the whole Sun has the same composition as the solar atmosphere.<ref name="manuel1983">
{{Cite journal
 |last=Manuel |first=O. K.
 |last2=Hwaung |first2=G.
 |date=1983
 |title=Solar abundances of the elements
 |journal=[[Meteoritics (journal)|Meteoritics]]
 |volume=18 |issue=3 |pages=209–222
 |bibcode=1983Metic..18..209M
 |doi=10.1111/j.1945-5100.1983.tb00822.x
 |ref=harv
}}</ref> In 1983, it was claimed that it was [[fractionation]] in the Sun itself that caused the isotopic-composition relationship between the planetary and solar-wind-implanted noble gases.<ref name="manuel1983"/>

==Structure and energy production==

===Core===
{{Main article|Solar core}}

[[File:Sun poster.svg|thumb|x250px|The structure of the Sun]]

The [[Solar core|core]] of the Sun extends from the center 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
 |ref=harv
|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=S.
 |display-authors=etal
 |date=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
}}</ref><ref name=NASA1>
{{cite web
 |date=18 January 2007
 |title=NASA/Marshall Solar Physics
 |url=http://solarscience.msfc.nasa.gov/interior.shtml
 |publisher=[[Marshall Space Flight Center]]
 |accessdate=11 July 2009
}}</ref> (about 150 times the density of water) and a temperature of close to 15.7 million [[kelvin]]s (K).<ref name=NASA1/> By contrast, the Sun's surface temperature is approximately 5,800&nbsp;K. Recent analysis of [[Solar and Heliospheric Observatory|SOHO]] mission data favors a faster rotation rate in the core than in the radiative zone above.<ref name="Garcia2007"/> Through most of the Sun's life, energy has been produced by [[nuclear fusion]] in the core region through a series of steps called the [[Proton-proton chain reaction|p–p (proton–proton) chain]]; this process converts [[hydrogen]] into [[helium]].<ref>
{{Cite conference
 |conference=XXIII Physics in Collisions Conference
 |location=Zeuthen, Germany
 |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
 |page=21
 |arxiv=astro-ph/0308537
 |bibcode=2003phco.conf...21B
 |ref=harv
}}</ref> Only 0.8% of the energy generated in the Sun comes from the [[CNO cycle]], though this proportion is expected to increase as the Sun becomes older.<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
}}</ref>

The core is the only region in the Sun that produces an appreciable amount of [[thermal energy]] through fusion; 99% of the power is generated within 24% of the Sun's radius, and by 30% of the radius, fusion has stopped nearly entirely. The remainder of the Sun is heated by this energy as it is transferred outwards through many successive layers, finally to the solar photosphere where it escapes into space as sunlight or the [[kinetic energy]] of particles.<ref name=Phillips1995-47/><ref name=Zirker2002-15>
{{Cite book
 |last=Zirker|first=J. B.
 |date=2002
 |title=Journey from the Center of the Sun
 |pages=15–34
 |publisher=[[Princeton University Press]]
 |isbn=978-0-691-05781-1
}}</ref>

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 6.2{{e|11}} kg/s.<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.
 |date=1982
 |title=The Physical Universe: An Introduction to Astronomy
 |page=102
 |publisher=[[University Science Books]]
 |isbn=0-935702-05-9
}}</ref> so the Sun releases energy at the mass–energy conversion rate of 4.26 million metric tons per second (which requires 600 metric megatons of hydrogen <ref>{{cite web |title=Ask Us: Sun |url=https://helios.gsfc.nasa.gov/qa_sun.html |work=Cosmicopia |publisher=NASA |date=2012 |accessdate=13 July 2017}}</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 [[Trinitrotoluene|TNT]] per second. Theoretical models of the Sun's interior indicate a power density of approximately 276.5 W/m<sup>3</sup>,<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
 |publisher=Contemporary Physics Education Project
 |accessdate=30 August 2011
 |archiveurl=http://webarchive.loc.gov/all/20011129122524/http%3A//fusedweb%2Ellnl%2Egov/cpep/chart_pages/5%2Eplasmas/sunlayers%2Ehtml |archivedate= 29 November 2001 }}</ref> a value that more nearly approximates that of reptile metabolism or a compost pile<ref>{{cite web|url=http://www.abc.net.au/science/articles/2012/04/17/3478276.htm|title=Lazy Sun is less energetic than compost|date=17 April 2012|publisher=}}</ref> than of a thermonuclear bomb.{{efn|name=power production density}}

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 |page=102
 |journal=[[AIP Conference Proceedings]]
 |arxiv=astro-ph/9405040
 |bibcode=1995AIPC..320..102H
 |doi=10.1063/1.47009
 |ref=harv
}}</ref><ref>
{{cite web
 |last=Myers|first=S. T.
 |date=18 February 1999
 |title=Lecture 11 – Stellar Structure I: Hydrostatic Equilibrium
 |work=Introduction to Astrophysics II
 |accessdate=15 July 2009
 |url=http://www.aoc.nrao.edu/~smyers/courses/astro12/L11.html
}}</ref>

===Radiative zone===
{{main article|Radiative zone}}
From the core out to about 0.7 solar radii, [[thermal radiation]] is the primary means of energy transfer.<ref name="autogenerated1">
{{cite web
 |url=http://mynasa.nasa.gov/worldbook/sun_worldbook.html
 |publisher=NASA|title=Sun
 |work=World Book at NASA
 |accessdate=10 October 2012
 |archiveurl = https://web.archive.org/web/20130510142009/http://mynasa.nasa.gov/worldbook/sun_worldbook.html |archivedate=2013-05-10}}</ref> The temperature drops from approximately 7 million to 2 million kelvins with increasing distance from the core.<ref name=NASA1/> This [[temperature gradient]] is less than the value of the [[adiabatic lapse rate]] and hence cannot drive convection, which explains why the transfer of energy through this zone is by [[radiation]] instead of thermal [[convection]].<ref name=NASA1/> [[Ions]] of [[hydrogen]] and [[helium]] emit [[photons]], which travel only a brief distance before being reabsorbed by other ions.<ref name="autogenerated1"/> The density drops a hundredfold (from 20 g/cm<sup>3</sup> to 0.2 g/cm<sup>3</sup>) from 0.25 solar radii to the 0.7 radii, the top of the radiative zone.<ref name="autogenerated1"/><!-- http://adsabs.harvard.edu/abs/2008SoPh..251..101M -->

===Tachocline===
{{main article|Tachocline}}

The radiative zone and the convective zone are separated by a transition layer, the [[tachocline]]. This is a region where the sharp regime change between the uniform rotation of the radiative zone and the differential rotation of the convection zone results in a large [[shear (fluid)|shear]] between the two—a condition where successive horizontal layers slide past one another.<ref>
{{Cite book
 |last=Tobias |first=S. M.
 |date=2005
 |chapter=The solar tachocline: Formation, stability and its role in the solar dynamo
 |url=https://books.google.com/?id=PLNwoJ6qFoEC&pg=PA193
 |pages=193–235
 |editor=A. M. Soward
 |display-editors=etal
 |title=Fluid Dynamics and Dynamos in Astrophysics and Geophysics
 |publisher=[[CRC Press]]
 |isbn=978-0-8493-3355-2
}}</ref> Presently, it is hypothesized (see [[Solar dynamo]]) that a magnetic dynamo within this layer generates the Sun's [[magnetic field]].<ref name=NASA1/>

===Convective zone===
{{main article|Convection zone}}
The Sun's convection zone extends from 0.7 solar radii (200,000&nbsp;km) to near the surface. In this layer, the solar plasma is not dense enough or hot enough to transfer the heat energy of the interior outward via radiation. Instead, the density of the plasma is low enough to allow convective currents to develop and move the Sun's energy outward towards its surface. Material heated at the tachocline picks up heat and expands, thereby reducing its density and allowing it to rise. As a result, an orderly motion of the mass develops into [[thermal|thermal cells]] that carry the majority of the heat outward to the Sun's photosphere above. Once the material diffusively and radiatively cools just beneath the photospheric surface, its density increases, and it sinks to the base of the convection zone, where it again picks up heat from the top of the radiative zone and the convective cycle continues. At the photosphere, the temperature has dropped to 5,700 K and the density to only 0.2 g/m<sup>3</sup> (about 1/6,000 the density of air at sea level).<ref name=NASA1/>

The thermal columns of the convection zone form an imprint on the surface of the Sun giving it a granular appearance  called the [[granule (solar physics)|solar granulation]] at the smallest scale and [[supergranulation]] at larger scales. Turbulent convection in this outer part of the solar interior sustains "small-scale" dynamo action over the near-surface volume of the Sun.<ref name=NASA1/> The Sun's thermal columns are [[Bénard cells]] and take the shape of hexagonal prisms.<ref>
{{Cite book
 |last=Mullan |first=D. J
 |date=2000
 |chapter=Solar Physics: From the Deep Interior to the Hot Corona
 |url=https://books.google.com/?id=rk5fxs55_OkC&pg=PA22
 |page=22
 |editor=Page, D.
 |editor2=Hirsch, J.G.
 |title=From the Sun to the Great Attractor
 |publisher=[[Springer Science+Business Media|Springer]]
 |isbn=978-3-540-41064-5
}}</ref>

===Photosphere===
[[File:EffectiveTemperature 300dpi e.png|thumb|The [[effective temperature]], or [[black body]] temperature, of the Sun (5,777 K) is the temperature a black body of the same size must have to yield the same total emissive power.]]
{{Main article|Photosphere}}
The visible surface of the Sun, the photosphere, is the layer below which the Sun becomes [[opacity (optics)|opaque]] to visible light.<ref name=Abhyankar1977/> Above the photosphere visible sunlight is free to propagate into space, and almost all of its energy escapes the Sun entirely. The change in opacity is due to the decreasing amount of [[Hydrogen anion|H<sup>−</sup> ions]], which absorb visible light easily.<ref name=Abhyankar1977/> Conversely, the visible light we see is produced as electrons react with [[hydrogen]] atoms to produce H<sup>−</sup> ions.<ref name="Gibson">
{{Cite book
 |last=Gibson |first=E. G.
 |date=1973
 |title=The Quiet Sun
 |publisher=[[NASA]]
 |asin=B0006C7RS0
}}</ref><ref name="Shu">
{{Cite book
 |last=Shu |first=F. H.
 |title=The Physics of Astrophysics
 |volume=1
 |publisher=[[University Science Books]]
 |date=1991
 |isbn=0-935702-64-4
}}</ref>
The photosphere is tens to hundreds of kilometers thick, and is slightly less opaque than air on Earth. Because the upper part of the photosphere is cooler than the lower part, an image of the Sun appears brighter in the center than on the edge or ''limb'' of the solar disk, in a phenomenon known as [[limb darkening]].<ref name=Abhyankar1977/> The spectrum of sunlight has approximately the spectrum of a [[black-body]] radiating at about 6,000 [[kelvin|K]], interspersed with atomic [[absorption line]]s from the tenuous layers above the photosphere. The photosphere has a particle density of ~10<sup>23</sup>&nbsp;m<sup>−3</sup> (about 0.37% of the particle number per volume of [[Earth's atmosphere]] at sea level). The photosphere is not fully ionized—the extent of ionization is about 3%, leaving almost all of the hydrogen in atomic form.<ref>
{{cite journal
 |last1=Rast|first1=M.
 |last2=Nordlund |first2=Å.
 |last3=Stein |first3=R.
 |last4=Toomre |first4=J.
 |date=1993
 |title=Ionization Effects in Three-Dimensional Solar Granulation Simulations
 |journal=[[The Astrophysical Journal Letters]]
 |volume=408 |issue=1 |page=L53–L56
 |bibcode=1993ApJ...408L..53R
 |doi=10.1086/186829
}}</ref>

During early studies of the [[optical spectrum]] of the photosphere, some absorption lines were found that did not correspond to any [[chemical element]]s then known on Earth. In 1868, [[Norman Lockyer]] hypothesized that these absorption lines were caused by a new element that he dubbed ''[[helium]]'', after the Greek Sun god [[Helios]]. Twenty-five years later, helium was isolated on Earth.<ref name="Lockyer">
{{cite web
 |last=Parnel |first=C.
 |title=Discovery of Helium
 |url=http://www-solar.mcs.st-andrews.ac.uk/~clare/Lockyer/helium.html
 |publisher=[[University of St Andrews]]
 |accessdate=22 March 2006
}}</ref>

===Atmosphere===
{{See also|Corona|Coronal loop}}
[[File:Solar eclipse 1999 4 NR.jpg|thumb|right|During a total [[solar eclipse]], the solar [[corona]] can be seen with the naked eye, during the brief period of totality.]]

During a total [[solar eclipse]], when the disk of the Sun is covered by that of the Moon, parts of the Sun's surrounding atmosphere can be seen. It is composed of four distinct parts: the [[chromosphere]], the [[solar transition region|transition region]], the [[corona]] and the [[heliosphere]].

The coolest layer of the Sun is a temperature minimum region extending to about {{val|500|u=km}} above the photosphere, and has a temperature of about {{val|4100|ul=K|fmt=commas}}.<ref name=Abhyankar1977>
{{Cite journal
 |last=Abhyankar |first=K. D.
 |date=1977
 |title=A Survey of the Solar Atmospheric Models
 |url=http://prints.iiap.res.in/handle/2248/510
 |journal=[[Bulletin of the Astronomical Society of India]]
 |volume=5 |pages=40–44
 |bibcode=1977BASI....5...40A
 |ref=harv
}}</ref> This part of the Sun is cool enough to allow the existence of simple molecules such as [[carbon monoxide]] and water, which can be detected via their absorption spectra.<ref name=Solanki1994>
{{Cite journal
 |last=Solanki |first=S. K.
 |last2=Livingston |first2=W.
 |last3=Ayres |first3=T.
 |date=1994
 |title=New Light on the Heart of Darkness of the Solar Chromosphere
 |journal=[[Science (journal)|Science]]
 |pmid=17748350
 |volume=263 |issue=5143 |pages=64–66
 |bibcode=1994Sci...263...64S
 |doi=10.1126/science.263.5143.64
 |ref=harv
}}</ref>

The chromosphere, transition region, and corona are much hotter than the surface of the Sun.<ref name=Abhyankar1977/> The reason is not well understood, but evidence suggests that [[Alfvén wave]]s may have enough energy to heat the corona.<ref>
{{Cite journal
 |last=De Pontieu |first=B.
 |date=2007
 |title=Chromospheric Alfvénic Waves Strong Enough to Power the Solar Wind
 |journal=[[Science (journal)|Science]]
 |volume=318 |issue=5856 |pages=1574–77
 |bibcode=2007Sci...318.1574D
 |doi=10.1126/science.1151747
 |pmid=18063784
 |ref=harv
|display-authors=etal}}</ref>

Above the temperature minimum layer is a layer about {{val|2000|u=km|fmt=commas}} thick, dominated by a spectrum of emission and absorption lines.<ref name=Abhyankar1977/> It is called the ''chromosphere'' from the Greek root ''chroma'', meaning color, because the chromosphere is visible as a colored flash at the beginning and end of total [[solar eclipse]]s.<ref name="autogenerated1"/> The temperature of the chromosphere increases gradually with altitude, ranging up to around {{val|20000|u=K|fmt=commas}} near the top.<ref name=Abhyankar1977/> In the upper part of the chromosphere [[helium]] becomes partially [[ionization|ionized]].<ref name=Hansteen1997>
{{Cite journal
 |last=Hansteen |first=V. H.
 |last2=Leer |first2=E.
 |last3=Holzer |first3=T. E.
 |date=1997
 |title=The role of helium in the outer solar atmosphere
 |journal=[[The Astrophysical Journal]]
 |volume=482 |issue=1 |pages=498–509
 |bibcode=1997ApJ...482..498H
 |doi=10.1086/304111
 |ref=harv
}}</ref>

[[File:171879main LimbFlareJan12 lg.jpg|thumb|left|350px|Taken by [[Hinode]]'s Solar Optical Telescope on 12 January 2007, this image of the Sun reveals the filamentary nature of the plasma connecting regions of different magnetic polarity.]]

Above the chromosphere, in a thin (about 200&nbsp;km) [[solar transition region|transition region]], the temperature rises rapidly from around 20,000 [[kelvin|K]] in the upper chromosphere to coronal temperatures closer to 1,000,000 [[kelvin|K]].<ref name=Erdelyi2007/> The temperature increase is facilitated by the full ionization of helium in the transition region, which significantly reduces radiative cooling of the plasma.<ref name=Hansteen1997/> The transition region does not occur at a well-defined altitude. Rather, it forms a kind of [[Halo (optical phenomenon)|nimbus]] around chromospheric features such as [[Spicule (solar physics)|spicules]] and [[Solar filament|filaments]], and is in constant, chaotic motion.<ref name="autogenerated1"/> The transition region is not easily visible from Earth's surface, but is readily observable from [[outer space|space]] by instruments sensitive to the [[extreme ultraviolet]] portion of the [[electromagnetic spectrum|spectrum]].<ref name=Dwivedi2006>
{{Cite journal
 |last=Dwivedi |first=B. N.
 |date=2006
 |title=Our ultraviolet Sun
 |url=http://www.iisc.ernet.in/currsci/sep102006/587.pdf
 |journal=[[Current Science]]
 |volume=91|issue=5|pages=587–595
 |ref=harv
}}</ref>

The [[corona]] is the next layer of the Sun. The low corona, near the surface of the Sun, has a particle density around 10<sup>15</sup>&nbsp;m<sup>−3</sup> to 10<sup>16</sup>&nbsp;m<sup>−3</sup>.<ref name=Hansteen1997/>{{efn|name=particle density}} The average temperature of the corona and solar wind is about 1,000,000–2,000,000 K; however, in the hottest regions it is 8,000,000–20,000,000 K.<ref name=Erdelyi2007/> Although no complete theory yet exists to account for the temperature of the corona, at least some of its heat is known to be from [[magnetic reconnection]].<ref name=Erdelyi2007/><ref name=Russell2001>
{{Cite book
 |last=Russell |first=C. T.
 |date=2001
 |chapter=Solar wind and interplanetary magnetic filed: A tutorial
 |url=http://www-ssc.igpp.ucla.edu/personnel/russell/papers/SolWindTutorial.pdf
 |pages=73–88
 |editor=Song, Paul
 |editor2=Singer, Howard J.
 |editor3=Siscoe, George L.
 |title=Space Weather (Geophysical Monograph)
 |publisher=[[American Geophysical Union]]
 |isbn=978-0-87590-984-4
}}</ref>
The corona is the extended atmosphere of the Sun, which has a volume much larger than the volume enclosed by the Sun's photosphere. A flow of plasma outward from the Sun into interplanetary space is the [[solar wind]].<ref name=Russell2001/>

The [[heliosphere]], the tenuous outermost atmosphere of the Sun, is filled with the solar wind plasma. This outermost layer of the Sun is defined to begin at the distance where the flow of the [[solar wind]] becomes ''superalfvénic''—that is, where the flow becomes faster than the speed of [[Alfvén wave]]s,<ref>
{{Cite book
 |first=Emslie |last=A. G
 |first2=Miller |last2=J. A.
 |date=2003
 |chapter=Particle Acceleration
 |chapterurl=https://books.google.com/books?id=W_oZYFplXX0C&pg=PA275
 |editor=Dwivedi, B. N.
 |title=Dynamic Sun
 |page=275
 |publisher=[[Cambridge University Press]]
 |isbn=978-0-521-81057-9
}}</ref> at approximately 20 solar radii (0.1 AU).
Turbulence and dynamic forces in the heliosphere cannot affect the shape of the solar corona within, because the information can only travel at the speed of Alfvén waves. The solar wind travels outward continuously through the heliosphere,<ref>{{cite web
 |date=22 April 2003 
 |title=A Star with two North Poles 
 |url=https://science.nasa.gov/headlines/y2003/22apr_currentsheet.htm 
 |work=Science @ NASA 
 |publisher=[[NASA]] 
 |deadurl=yes 
 |archiveurl=https://web.archive.org/web/20090718014855/https://science.nasa.gov/headlines/y2003/22apr_currentsheet.htm 
 |archivedate=18 July 2009 
 |df= 
}}</ref><ref>{{Cite journal
 |last=Riley 
 |first=P. 
 |last2=Linker 
 |first2=J. A. 
 |last3=Mikić 
 |first3=Z. 
 |date=2002 
 |title=Modeling the heliospheric current sheet: Solar cycle variations 
 |url=http://ulysses.jpl.nasa.gov/science/monthly_highlights/2002-July-2001JA000299.pdf 
 |journal=[[Journal of Geophysical Research]] 
 |volume=107 
 |issue=A7 
 |pages=SSH 8–1 
 |bibcode=2002JGRA..107.1136R 
 |doi=10.1029/2001JA000299 
 |id=CiteID 1136 
 |deadurl=yes 
 |archiveurl=https://web.archive.org/web/20090814052347/http://ulysses.jpl.nasa.gov/science/monthly_highlights/2002-July-2001JA000299.pdf 
 |archivedate=14 August 2009 
 |df= 
}}</ref> forming the solar magnetic field into a [[Parker spiral|spiral]] shape,<ref name=Russell2001/> until it impacts the [[Heliopause (astronomy)|heliopause]] more than 50 [[Astronomical unit|AU]] from the Sun. In December 2004, the [[Voyager 1]] probe passed through a shock front that is thought to be part of the heliopause.<ref>
{{cite press
 |date=2005
 |title=The Distortion of the Heliosphere: Our Interstellar Magnetic Compass
 |url=http://www.spaceref.com/news/viewpr.html?pid=16394
 |publisher=[[European Space Agency]]
 |accessdate=22 March 2006
}}</ref> In late 2012 Voyager 1 recorded a marked increase in [[cosmic ray]] collisions and a sharp drop in lower energy particles from the solar wind, which suggested that the probe had passed through the heliopause and entered the [[interstellar medium]].<ref>{{cite book|url=https://books.google.co.uk/books?id=JxauCQAAQBAJ&pg=PA163|title=The Cosmic Compendium: Interstellar Travel|pages=163–4|author=Anderson, Rupert W.|year=2015}}</ref>

===Photons and neutrinos===
{{see also|solar irradiance}}

High-energy [[gamma ray|gamma-ray]] photons initially released with fusion reactions in the core are almost immediately absorbed by the solar plasma of the radiative zone, usually after traveling only a few millimeters. Re-emission happens in a random direction and usually at a slightly lower energy. With this sequence of emissions and absorptions, it takes a long time for radiation to reach the Sun's surface. Estimates of the photon travel time range between 10,000 and 170,000&nbsp;years.<ref name="NASA">
{{cite web
 |date=2007
 |title=Ancient sunlight
 |url=http://sunearthday.nasa.gov/2007/locations/ttt_sunlight.php
 |work=Technology Through Time
 |publisher=[[NASA]]
 |issue=50
 |accessdate=24 June 2009
 |ref=harv
}}</ref> In contrast, it takes only 2.3 seconds for the [[neutrino]]s, which account for about 2% of the total energy production of the Sun, to reach the surface. Because energy transport in the Sun is a process that involves photons in thermodynamic equilibrium with matter, the time scale of energy transport in the Sun is longer, on the order of 30,000,000 years. This is the time it would take the Sun to return to a stable state, if the rate of energy generation in its core were suddenly changed.<ref>
{{Cite journal
 |last=Stix |first=M.
 |date=2003
 |title=On the time scale of energy transport in the sun
 |url=http://www.springerlink.com/content/l256u14247171u67/
 |journal=[[Solar Physics (journal)|Solar Physics]]
 |volume=212 |issue=1 |pages=3–6
 |bibcode=2003SoPh..212....3S
 |doi=10.1023/A:1022952621810
}}</ref>

Neutrinos are also released by the fusion reactions in the core, but, unlike photons, they rarely interact with matter, so almost all are able to escape the Sun immediately. For many years measurements of the number of neutrinos produced in the Sun were [[Solar neutrino problem|lower than theories predicted]] by a factor of 3. This discrepancy was resolved in 2001 through the discovery of the effects of [[neutrino oscillation]]: the Sun emits the number of neutrinos predicted by the [[theory]], but neutrino detectors were missing {{frac|2|3}} of them because the neutrinos had changed [[flavor (particle physics)|flavor]] by the time they were detected.<ref name="Schlattl">
{{Cite journal
 |last=Schlattl |first=H.
 |date=2001
 |title=Three-flavor oscillation solutions for the solar neutrino problem
 |journal=[[Physical Review D]]
 |volume=64 |issue=1 |page=013009
 |arxiv=hep-ph/0102063
 |bibcode=2001PhRvD..64a3009S
 |doi=10.1103/PhysRevD.64.013009
 |ref=harv
 }}</ref>

==Magnetism and activity==

===Magnetic field===

{{See also|Stellar magnetic field|Sunspots|List of solar cycles|Solar phenomena}}

[[File:172197main NASA Flare Gband lg-withouttext.jpg|thumb|Visible light photograph of sunspot, 13 December 2006]]

{{Multiple image
| direction = vertical
| width     = 350
| image1    = Sunspot butterfly diagram.svg
| image2    = Sunspot area variation.svg
| caption2  = [[Solar cycle#Sunspots|Butterfly diagram]] showing paired sunspot pattern. Graph is of sunspot area.
}}

[[File:Sun - August 1, 2010.jpg|thumb|In this false-color ultraviolet image, the Sun shows a C3-class solar flare (white area on upper left), a solar tsunami (wave-like structure, upper right) and multiple filaments of [[plasma (physics)|plasma]] following a magnetic field, rising from the stellar surface.]]
[[File:Heliospheric-current-sheet.gif|thumb|right|The [[heliospheric current sheet]] extends to the outer reaches of the Solar System, and results from the influence of the Sun's rotating magnetic field on the [[Plasma (physics)|plasma]] in the [[interplanetary medium]].<ref>
{{cite web
 |date=2006
 |title=The Mean Magnetic Field of the Sun
 |url=http://wso.stanford.edu/#MeanField
 |publisher=[[Wilcox Solar Observatory]]
 |accessdate=1 August 2007
}}</ref>]]

The Sun has a [[magnetic field]] that varies across the surface of the Sun. Its polar field is {{convert|1|-|2|G|sigfig=1|lk=on}}, whereas the field is typically {{convert|3000|G|sigfig=1}} in features on the Sun called [[sunspot]]s and {{convert|10|-|100|G|sigfig=1}} in [[solar prominence]]s.<ref name= nssdc />

The magnetic field also varies in time and location. The quasi-periodic 11-year [[solar cycle]] is the most prominent variation in which the number and size of sunspots waxes and wanes.<ref name="doi10.1146/annurev-astro-081913-040012" /><ref name="Zirker2002-119">{{Cite book
 |last=Zirker |first=J. B.
 |date=2002
 |title=Journey from the Center of the Sun
 |pages=119–120
 |publisher=[[Princeton University Press]]
 |isbn=978-0-691-05781-1
}}</ref><ref name="Lang">{{Cite book
 |last=Lang |first=Kenneth R.
 |date=2008
 |title=The Sun from Space
 |page=75
 |publisher=[[Springer-Verlag]]
 |ISBN=978-3540769521
}}</ref>

Sunspots are visible as dark patches on the Sun's [[photosphere]], and correspond to concentrations of magnetic field where the [[convection|convective transport]] of heat is inhibited from the solar interior to the surface. As a result, sunspots are slightly cooler than the surrounding photosphere, and, so, they appear dark. At a typical [[solar minimum]], few sunspots are visible, and occasionally none can be seen at all. Those that do appear are at high solar latitudes. As the solar cycle progresses towards its [[solar maximum|maximum]], sunspots tend form closer to the solar equator, a phenomenon known as [[Spörer's law]]. The largest sunspots can be tens of thousands of kilometers across.<ref name="Sunspot2001">
{{cite web
 |date=30 March 2001
 |title=The Largest Sunspot in Ten Years
 |url=http://www.gsfc.nasa.gov/gsfc/spacesci/solarexp/sunspot.htm
 |publisher=[[Goddard Space Flight Center]]
 |accessdate=10 July 2009
 |archiveurl = https://web.archive.org/web/20070823050403/http://www.gsfc.nasa.gov/gsfc/spacesci/solarexp/sunspot.htm
 |archivedate = 23 August 2007
}}</ref>

An 11-year sunspot cycle is half of a 22-year [[Babcock Model|Babcock]]–Leighton [[solar dynamo|dynamo]] cycle, which corresponds to an oscillatory exchange of energy between [[toroidal and poloidal]] solar magnetic fields. At [[solar maximum|solar-cycle maximum]], the external poloidal dipolar magnetic field is near its dynamo-cycle minimum strength, but an internal [[Toroidal and poloidal|toroidal]] quadrupolar field, generated through differential rotation within the tachocline, is near its maximum strength. At this point in the dynamo cycle, buoyant upwelling within the convective zone forces emergence of toroidal magnetic field through the photosphere, giving rise to pairs of sunspots, roughly aligned east–west and having footprints with opposite magnetic polarities. The magnetic polarity of sunspot pairs alternates every solar cycle, a phenomenon known as the Hale cycle.<ref>{{Cite journal | last1 = Hale | first1 = G. E. | last2 = Ellerman | first2 = F. | last3 = Nicholson | first3 = S. B. | last4 = Joy | first4 = A. H. | title = The Magnetic Polarity of Sun-Spots | journal = The Astrophysical Journal | volume = 49 | page = 153 | year = 1919 | doi = 10.1086/142452|bibcode = 1919ApJ....49..153H }}</ref><ref name="solarcycle">
{{cite web
 |date=4 January 2008
 |title=NASA Satellites Capture Start of New Solar Cycle
 |publisher=[[PhysOrg]]
 |url=http://www.physorg.com/news119271347.html
 |accessdate=10 July 2009
}}</ref>

During the solar cycle's declining phase, energy shifts from the internal toroidal magnetic field to the external poloidal field, and sunspots diminish in number and size. At [[solar minimum|solar-cycle minimum]], the toroidal field is, correspondingly, at minimum strength, sunspots are relatively rare, and the poloidal field is at its maximum strength. With the rise of the next 11-year sunspot cycle, differential rotation shifts magnetic energy back from the poloidal to the toroidal field, but with a polarity that is opposite to the previous cycle. The process carries on continuously, and in an idealized, simplified scenario, each 11-year sunspot cycle corresponds to a change, then, in the overall polarity of the Sun's large-scale magnetic field.<ref>
{{Cite news
 |date=16 February 2001
 |title=Sun flips magnetic field
 |url=http://edition.cnn.com/2001/TECH/space/02/16/sun.flips/
 |work=[[CNN]]
 |accessdate=11 July 2009
}}</ref><ref>
{{cite web
 |last=Phillips 
 |first=T. 
 |date=15 February 2001 
 |title=The Sun Does a Flip 
 |url=https://science.nasa.gov/headlines/y2001/ast15feb_1.htm 
 |publisher=[[NASA]] 
 |accessdate=11 July 2009 
 |deadurl=yes 
 |archiveurl=https://web.archive.org/web/20090512121817/https://science.nasa.gov/headlines/y2001/ast15feb_1.htm 
 |archivedate=12 May 2009 
 |df= 
}}</ref>

The solar magnetic field extends well beyond the Sun itself. The electrically conducting solar wind plasma carries the Sun's magnetic field into space, forming what is called the [[interplanetary magnetic field]].<ref name=Russell2001/> In an approximation known as ideal [[magnetohydrodynamics]], plasma particles only move along the magnetic field lines. As a result, the outward-flowing solar wind stretches the interplanetary magnetic field outward, forcing it into a roughly radial structure. For a simple dipolar solar magnetic field, with opposite hemispherical polarities on either side of the solar magnetic equator, a thin [[heliospheric current sheet|current sheet]] is formed in the solar wind.<ref name=Russell2001/> At great distances, the rotation of the Sun twists the dipolar magnetic field and corresponding current sheet into an [[Archimedean spiral]] structure called the [[Parker spiral]].<ref name=Russell2001/> The interplanetary magnetic field is much stronger than the dipole component of the solar magnetic field. The Sun's dipole magnetic field of 50–400&nbsp;[[tesla (unit)|μT]] (at the photosphere) reduces with the inverse-cube of the distance to about 0.1&nbsp;nT at the distance of Earth. However, according to spacecraft observations the interplanetary field at Earth's location is around 5&nbsp;nT, about a hundred&nbsp;times greater.<ref name=Wang2003>
{{Cite journal
 |last=Wang |first=Y.-M.
 |last2=Sheeley |first2=N. R.
 |date=2003
 |title=Modeling the Sun's Large-Scale Magnetic Field during the Maunder Minimum
 |journal=[[The Astrophysical Journal]]
 |volume=591 |issue=2 |pages=1248–56
 |bibcode=2003ApJ...591.1248W
 |doi=10.1086/375449
 |ref=harv
}}</ref> The difference is due to magnetic fields generated by electrical currents in the plasma surrounding the Sun.

===Variation in activity===
[[File:Solar-cycle-data.png|thumb|right|Measurements of solar cycle variation during the last 30 years]]

The Sun's magnetic field leads to many effects that are collectively called [[solar variation|solar activity]]. [[Solar flares]] and [[coronal mass ejections|coronal-mass ejections]] tend to occur at sunspot groups. Slowly changing high-speed streams of [[solar wind]] are emitted from [[coronal holes]] at the photospheric surface. Both coronal-mass ejections and high-speed streams of solar wind carry plasma and [[interplanetary magnetic field]] outward into the Solar System.<ref name=Zirker2002>
{{Cite book
 |last=Zirker |first=J. B.
 |date=2002
 |title=Journey from the Center of the Sun
 |pages=120–127
 |publisher=[[Princeton University Press]]
 |isbn=978-0-691-05781-1
}}</ref> The effects of solar activity on Earth include [[aurora (astronomy)|auroras]] at moderate to high latitudes and the disruption of radio communications and [[electric power]]. Solar activity is thought to have played a large role in the [[formation and evolution of the Solar System]].

With solar-cycle modulation of sunspot number comes a corresponding modulation of [[space weather]] conditions, including those surrounding Earth where technological systems can be affected.

===Long-term change===

Long-term secular change in sunspot number is thought, by some scientists, to be correlated with long-term change in solar irradiance,<ref>
{{cite journal
 |last=Willson |first=R. C.
 |last2=Hudson |first2=H. S.
 |date=1991
 |title=The Sun's luminosity over a complete solar cycle
 |journal=[[Nature (journal)|Nature]]
 |volume=351
 |issue=6321 |pages=42–4
 |doi=10.1038/351042a0
|bibcode = 1991Natur.351...42W }}</ref> which, in turn, might influence Earth's long-term climate.<ref>{{cite journal|authorlink=John A. Eddy|last=Eddy|first=John A.|title=The Maunder Minimum|journal=[[Science (journal)|Science]]|volume=192|issue=4245|pages=1189–1202|date=June 1976|pmid=17771739|doi=10.1126/science.192.4245.1189|jstor=17425839|bibcode=1976Sci...192.1189E}}</ref>
For example, in the 17th century, the solar cycle appeared to have stopped entirely for several decades; few sunspots were observed during a period known as the [[Maunder minimum]]. This coincided in time with the era of the [[Little Ice Age]], when Europe experienced unusually cold temperatures.<ref name="Lean">
{{Cite journal
 |last=Lean |first=J.
 |last2=Skumanich |first2=A.
 |last3=White |first3=O.
 |date=1992
 |title=Estimating the Sun's radiative output during the Maunder Minimum
 |journal=[[Geophysical Research Letters]]
 |volume=19 |issue=15 |pages=1591–1594
 |doi=10.1029/92GL01578
 |ref=harv |bibcode=1992GeoRL..19.1591L
}}</ref> Earlier extended minima have been discovered through analysis of [[tree ring]]s and appear to have coincided with lower-than-average global temperatures.<ref>
{{Cite book
 |last=Mackay |first=R. M.
 |last2=Khalil |first2=M. A. K
 |chapter=Greenhouse gases and global warming
 |url= https://books.google.com/?id=tQBS3bAX8fUC&pg=PA1&dq=solar+minimum+dendochronology
 |editor=Singh, S. N.
 |date=2000
 |title=Trace Gas Emissions and Plants
 |pages=1–28
 |publisher=[[Springer (publisher)|Springer]]
 |isbn=978-0-7923-6545-7
}}</ref>

A recent theory claims that there are magnetic instabilities in the core of the Sun that cause fluctuations with periods of either 41,000 or 100,000 years. These could provide a better explanation of the [[ice age]]s than the [[Milankovitch cycles]].<ref>
{{Cite journal
 |last=Ehrlich |first=R.
 |title=Solar Resonant Diffusion Waves as a Driver of Terrestrial Climate Change
 |journal=[[Journal of Atmospheric and Solar-Terrestrial Physics]]
 |volume=69 |issue=7 |pages=759–766
 |date=2007
 |doi=10.1016/j.jastp.2007.01.005
 |arxiv=astro-ph/0701117
 |ref=harv
|bibcode = 2007JASTP..69..759E }}</ref><ref>
{{Cite journal
 |last=Clark |first=S.
 |title=Sun's fickle heart may leave us cold
 |url=http://www.newscientist.com/article/mg19325884.500-suns-fickle-heart-may-leave-us-cold.html
 |journal=[[New Scientist]]
 |issue=2588 |page=12
 |date=2007
 |doi=10.1016/S0262-4079(07)60196-1
 |volume=193
 |ref=harv
}}</ref>

==Life phases==
{{Main article|Formation and evolution of the Solar System|Stellar evolution}}

The Sun today is roughly halfway through the most stable part of its life. It has not changed dramatically for over four billion<ref group=lower-alpha name=short /> years, and will remain fairly stable for more than five billion more. However, after hydrogen fusion in its core has stopped, the Sun will undergo severe changes, both internally and externally.

===Formation===
The Sun formed about 4.6 billion years ago from the collapse of part of a giant [[molecular cloud]] that consisted mostly of hydrogen and helium and that probably gave birth to many other stars.<!-- We would say 4.57, but there may be uncertainty; for example, http://arxiv.org/pdf/1507.05847.pdf seems to suggest 4.587 rather than 4.567 --><ref name=Zirker2002-7>
{{Cite book|last=Zirker|first=Jack B.|title=Journey from the Center of the Sun|date=2002|publisher=[[Princeton University Press]]|isbn=978-0-691-05781-1|pages=7–8}}
</ref> This age is estimated using [[computer simulation|computer models]] of [[stellar evolution]] and through [[nucleocosmochronology]].<ref name="Bonanno"/> The result is consistent with the [[radiometric dating|radiometric date]] of the oldest Solar System material, at 4.567 billion years ago.<ref>
{{Cite journal
 |last=Amelin |first=Y. |last2=Krot |first2=A. |last3=Hutcheon |first3=I.
 |last4=Ulyanov |first4=A.
 |title=Lead isotopic ages of chondrules and calcium-aluminum-rich inclusions
 |journal=[[Science (journal)|Science]]
 |volume=297 |issue=5587 |pages=1678–1683
 |date=2002
 |doi=10.1126/science.1073950
 |pmid=12215641
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|bibcode = 2002Sci...297.1678A }}</ref><ref name="nature436">
{{Cite journal
 |last=Baker |first=J. |last2=Bizzarro |first2=M. |last3=Wittig |first3=N.
 |last4=Connelly |first4=J. |last5=Haack |first5=H.
 |title=Early planetesimal melting from an age of 4.5662 Gyr for differentiated meteorites
 |journal=[[Nature (journal)|Nature]]
 |volume=436
 |issue=7054|pages=1127–1131
 |date=2005
 |pmid=16121173
 |doi=10.1038/nature03882
 |ref=harv
|bibcode = 2005Natur.436.1127B }}</ref> Studies of ancient [[meteorite]]s reveal traces of stable daughter nuclei of short-lived isotopes, such as [[iron-60]], that form only in exploding, short-lived stars. This indicates that one or more supernovae must have occurred near the location where the Sun formed. A [[shock wave]] from a nearby supernova would have triggered the formation of the Sun by compressing the matter within the molecular cloud and causing certain regions to collapse under their own gravity.<ref>{{Cite journal| last1 = Williams | first1 = J.| title = The astrophysical environment of the solar birthplace| journal = Contemporary Physics| volume = 51| issue = 5| pages = 381–396| year = 2010| doi = 10.1080/00107511003764725|bibcode = 2010ConPh..51..381W |arxiv = 1008.2973 }}</ref> As one fragment of the cloud collapsed it also began to rotate because of [[conservation of angular momentum]] and heat up with the increasing pressure. Much of the mass became concentrated in the center, whereas the rest flattened out into a disk that would become the planets and other Solar System bodies. Gravity and pressure within the core of the cloud generated a lot of heat as it accreted more matter from the surrounding disk, eventually triggering [[stellar nucleosynthesis|nuclear fusion]]. Thus, the Sun was born.

===Main sequence===
[[File:Solar evolution (English).svg|right|thumb|320px|Evolution of the Sun's [[Solar luminosity|luminosity]], [[Solar radius|radius]] and [[effective temperature]] compared to the present Sun. After Ribas (2010)<ref name=ribas2010>{{Cite journal | last=Ribas | first=Ignasi |title=Proceedings of the IAU Symposium 264 'Solar and Stellar Variability – Impact on Earth and Planets': The Sun and stars as the primary energy input in planetary atmospheres| volume=264 | pages=3–18 |date=February 2010 | doi=10.1017/S1743921309992298 | bibcode=2010IAUS..264....3R | journal=Proceedings of the International Astronomical Union |arxiv = 0911.4872 }}</ref>]]
The Sun is about halfway through its [[main sequence|main-sequence]] stage, during which nuclear fusion reactions in its core fuse hydrogen into helium. Each second, more than four million [[tonne]]s of matter are converted into energy within the Sun's core, producing [[neutrino]]s and [[solar radiation]]. At this rate, the Sun has so far converted around 100 times the mass of Earth into energy, about 0.03% of the total mass of the Sun. The Sun will spend a total of approximately 10 [[1000000000 (number)|billion]] years as a main-sequence star.<ref>
{{Cite book
 |last=Goldsmith |first=D. |last2=Owen |first2=T.
 |title=The search for life in the universe
 |url=https://books.google.com/?id=Q17NmHY6wloC&pg=PA96
 |page=96
 |publisher=[[University Science Books]]
 |date=2001
 |isbn=978-1-891389-16-0
}}</ref> The Sun is gradually becoming hotter during its time on the main sequence, because the helium atoms in the core occupy less volume than the [[hydrogen atom]]s that were fused. The core is therefore shrinking, allowing the outer layers of the Sun to move closer to the centre and experience a stronger gravitational force, according to the [[inverse-square law]]. This stronger force increases the pressure on the core, which is resisted by a gradual increase in the rate at which fusion occurs. This process speeds up as the core gradually becomes denser. It is estimated that the Sun has become 30% brighter in the last 4.5 billion years.<ref>{{cite web|url=http://faculty.wcas.northwestern.edu/~infocom/The%20Website/evolution.html|title=The Sun's Evolution|publisher=}}</ref> At present, it is increasing in brightness by about 1% every 100 million years.<ref>{{cite web|url=http://news.sciencemag.org/climate/2014/01/earth-wont-die-soon-thought|title=Earth Won't Die as Soon as Thought|date=22 January 2014|publisher=}}</ref>

===After core hydrogen exhaustion===
<!-- [[End of the Sun]] redirects to this section, please fix that if renaming this section. Thanks! -->
[[File:Sun red giant.svg|thumb|301px|left|The size of the current Sun (now in the [[main sequence]]) compared to its estimated size during its red-giant phase in the future]]
The Sun does not have enough mass to explode as a [[supernova]]. Instead it will exit the [[main sequence]] in approximately 5 billion years and start to turn into a [[red giant]].<ref>{{cite web|author1=Nola Taylor Redd|title=Red Giant Stars: Facts, Definition & the Future of the Sun|url=http://www.space.com/22471-red-giant-stars.html|website=space.com|accessdate=20 February 2016}}</ref><ref name=schroder>{{Cite journal | last1 = Schröder | first1 = K. -P. | last2 = Connon Smith | first2 = R. | doi = 10.1111/j.1365-2966.2008.13022.x | title = Distant future of the Sun and Earth revisited | journal = Monthly Notices of the Royal Astronomical Society | volume = 386 | pages = 155–163 | year = 2008 | pmid =  | pmc = |arxiv = 0801.4031 |bibcode = 2008MNRAS.386..155S }}</ref> As a red giant, the Sun will grow so large that it will engulf Mercury, Venus, and probably Earth.<ref name=schroder /><ref name=sackmann>{{Cite journal | last1 = Boothroyd | first1 = A. I. | last2 = Sackmann | first2 = I. ‐J. | doi = 10.1086/306546 | title = The CNO Isotopes: Deep Circulation in Red Giants and First and Second Dredge‐up | journal = The Astrophysical Journal | volume = 510 | pages = 232–250 | year = 1999 | pmid =  | pmc = |bibcode = 1999ApJ...510..232B }}</ref>

Even before it becomes a red giant, the luminosity of the Sun will have nearly doubled, and Earth will receive as much sunlight as Venus receives today. Once the core hydrogen is exhausted in 5.4 billion years, the Sun will expand into a [[subgiant]] phase and slowly double in size over about half a billion years. It will then expand more rapidly over about half a billion years until it is over two hundred times larger than today and a couple of thousand times more luminous. This then starts the [[red giant branch|red-giant-branch]] phase where the Sun will spend around a billion years and lose around a third of its mass.<ref name=schroder/>

[[File:Evolution of a Sun-like star.svg|300px|right|thumb|Evolution of a Sun-like star. The track of a one solar mass star on the [[Hertzsprung–Russell diagram]] is shown from the main sequence to the post-asymptotic-giant-branch stage.]]
After the red-giant branch the Sun has approximately 120 million years of active life left, but much happens. First, the core, full of [[degenerate matter|degenerate]] helium ignites violently in the [[helium flash]], where it is estimated that 6% of the core, itself 40% of the Sun's mass, will be converted into carbon within a matter of minutes through the [[triple-alpha process]].<ref>{{cite web|url=http://faculty.wcas.northwestern.edu/~infocom/The%20Website/end.html|title=The End Of The Sun|publisher=}}</ref> The Sun then shrinks to around 10 times its current size and 50 times the luminosity, with a temperature a little lower than today. It will then have reached the [[red clump]] or [[horizontal branch]], but a star of the Sun's mass does not evolve blueward along the horizontal branch. Instead, it just becomes moderately larger and more luminous over about 100 million years as it continues to burn helium in the core.<ref name=schroder/>

When the helium is exhausted, the Sun will repeat the expansion it followed when the hydrogen in the core was exhausted, except that this time it all happens faster, and the Sun becomes larger and more luminous. This is the [[asymptotic giant branch|asymptotic-giant-branch]] phase, and the Sun is alternately burning hydrogen in a shell or helium in a deeper shell. After about 20 million years on the early asymptotic giant branch, the Sun becomes increasingly unstable, with rapid mass loss and [[thermal pulse]]s that increase the size and luminosity for a few hundred years every 100,000 years or so. The thermal pulses become larger each time, with the later pulses pushing the luminosity to as much as 5,000 times the current level and the radius to over 1 AU.<ref name=agb>{{Cite journal | last1 = Vassiliadis | first1 = E. | last2 = Wood | first2 = P. R. | doi = 10.1086/173033 | title = Evolution of low- and intermediate-mass stars to the end of the asymptotic giant branch with mass loss | journal = The Astrophysical Journal | volume = 413 | page = 641 | year = 1993 | pmid =  | pmc = |bibcode = 1993ApJ...413..641V }}</ref> According to a 2008 model, Earth's orbit is shrinking due to [[tidal forces]] (and, eventually, drag from the lower [[chromosphere]]), so that it will be engulfed by the Sun near the tip of the red giant branch phase, 1 and 3.8 million years after Mercury and Venus have respectively suffered the same fate. Models vary depending on the rate and timing of mass loss. Models that have higher mass loss on the red-giant branch produce smaller, less luminous stars at the tip of the asymptotic giant branch, perhaps only 2,000 times the luminosity and less than 200 times the radius.<ref name=schroder/> For the Sun, four thermal pulses are predicted before it completely loses its outer envelope and starts to make a [[planetary nebula]]. By the end of that phase – lasting approximately 500,000 years – the Sun will only have about half of its current mass.

The post-asymptotic-giant-branch evolution is even faster. The luminosity stays approximately constant as the temperature increases, with the ejected half of the Sun's mass becoming ionised into a [[planetary nebula]] as the exposed core reaches 30,000 K. The final naked core, a [[white dwarf]], will have a temperature of over 100,000 K, and contain an estimated 54.05% of the Sun's present day mass.<ref name=schroder/> The planetary nebula will disperse in about 10,000 years, but the white dwarf will survive for trillions of years before fading to a hypothetical [[black dwarf]].<ref name=bloecker1>{{bibcode|1995A&A...297..727B}}</ref><ref name=bloecker2>{{bibcode|1995A&A...299..755B}}</ref>

==Motion and location==

===Orbit in Milky Way===

[[File:236084main MilkyWay-full-annotated.jpg|thumb|right|Illustration of the Milky Way, showing the location of the Sun]]

The Sun lies close to the inner rim of the [[Milky Way]]'s [[Orion Arm]], in the [[Local Interstellar Cloud]] or the [[Gould Belt]], at a distance of 7.5–8.5 [[Kiloparsec|kpc]] (25,000–28,000 light-years) from the [[Galactic Center]].<ref>[http://interstellar.jpl.nasa.gov/interstellar/probe/introduction/neighborhood.html, Our Local Galactic Neighborhood, NASA] {{webarchive |url=https://web.archive.org/web/20151107044627/http://interstellar.jpl.nasa.gov/interstellar/probe/introduction/neighborhood.html |date=7 November 2015 }}</ref><ref>{{cite web|url=http://www.centauri-dreams.org/?p=14203|title=Into the Interstellar Void|work=Centauri Dreams}}</ref>
<ref name="distance1">
{{Cite journal
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}}</ref><ref name="distance2">
{{Cite journal
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 |title=A Geometric Determination of the Distance to the Galactic Center
 |journal=[[Astrophysical Journal]]
 |volume=597 |issue=2 |pages=L121–L124
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|arxiv = astro-ph/0306220 |display-authors=etal}}</ref><ref name="distance3">
{{Cite journal
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 |title=First results from SPIFFI. I: The Galactic Center
 |url=http://www2011.mpe.mpg.de/SPIFFI/preprints/first_result_an1.pdf
 |format=PDF|journal=[[Astronomische Nachrichten]]
 |volume=325 |issue=2 |pages=120–123
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|display-authors=etal}}</ref><ref name="eisenhaueretal2005">
{{Cite journal
 |last=Eisenhauer |first=F.
 |title=SINFONI in the Galactic Center: Young Stars and Infrared Flares in the Central Light-Month
 |journal=[[Astrophysical Journal]]
 |volume = 628 |issue=1 |pages=246–259
 |date=2005
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|arxiv = astro-ph/0502129 |display-authors=etal}}</ref>
The Sun is contained within the [[Local Bubble]], a space of rarefied hot gas, possibly produced by the supernova remnant [[Geminga]].<ref>{{Cite journal|last1=Gehrels|first1=Neil|last2=Chen|first2=Wan|date= 25 February 1993 |title=The Geminga supernova as a possible cause of the local interstellar bubble|journal=Nature|volume=361|issue=6414|pages=706–707|doi=10.1038/361704a0|last3=Mereghetti |first3=S. |ref=harv|bibcode = 1993Natur.361..704B }}</ref> The distance between the local arm and the next arm out, the [[Perseus Arm]], is about 6,500 light-years.<ref name="fn9">
{{cite press
 |last=English |first=J.
 |title=Exposing the Stuff Between the Stars
 |url = http://www.ras.ucalgary.ca/CGPS/press/aas00/pr/pr_14012000/pr_14012000map1.html
 |publisher=Hubble News Desk
 |date=2000
 |accessdate = 10 May 2007
}}</ref> The Sun, and thus the Solar System, is found in what scientists call the [[galactic habitable zone]].
The ''Apex of the Sun's Way'', or the [[solar apex]], is the direction that the Sun travels relative to other nearby stars. This motion is towards a point in the constellation [[Hercules (constellation)|Hercules]], near the star [[Vega]]. Of the 50 [[Nearest stars|nearest stellar systems]] within 17 light-years from Earth (the closest being the red dwarf [[Proxima Centauri]] at approximately 4.2 light-years), the Sun ranks fourth in mass.<ref>{{Cite journal|last=Adams |first=F. C. |last2=Graves |first2=G. |last3=Laughlin |first3=G. J. M. |date=2004 |title=Red Dwarfs and the End of the Main Sequence |url=http://redalyc.uaemex.mx/pdf/571/57102211.pdf |journal=[[Revista Mexicana de Astronomía y Astrofísica]] |volume=22 |pages=46–49 |bibcode=2004RMxAC..22...46A |ref=harv |deadurl=yes |archiveurl=https://web.archive.org/web/20110726103734/http://redalyc.uaemex.mx/pdf/571/57102211.pdf |archivedate=26 July 2011 }}</ref>

The Sun orbits the center of the Milky Way, and it is presently moving in the direction of constellation of [[Cygnus (constellation)|Cygnus]]. The Sun's orbit around the Milky Way is  roughly elliptical with orbital perturbations due to the non-uniform mass distribution in Milky Way, such as that in and between the galactic spiral arms. In addition, the Sun oscillates up and down relative to the galactic plane approximately 2.7 times per orbit.<ref>{{cite book|last1=Moore|first1=Patrick|last2=Rees|first2=Robin|title=Patrick Moore's Data Book of Astronomy|date=16 January 2014|publisher=Cambridge University Press|location=[[Cambridge]]|isbn=1139495224|ref=harv}}</ref> It has been argued that the Sun's passage through the higher density spiral arms often coincides with [[mass extinction]]s on Earth, perhaps due to increased [[impact events]].<ref name="extinction">{{Cite journal |last=Gillman |first=M. |last2=Erenler |first2=H. |title=The galactic cycle of extinction |journal=[[International Journal of Astrobiology]] |volume=7 | issue = 1 | pages=17–26 |date=2008 |doi=10.1017/S1473550408004047 |ref=harv |bibcode=2008IJAsB...7...17G}}</ref> It takes the Solar System about 225–250 million years to complete one orbit through the Milky Way (a ''[[galactic year]]''),<ref name="fn10">
{{cite web |last=Leong |first=S. |title=Period of the Sun's Orbit around the Galaxy (Cosmic Year) |url=http://hypertextbook.com/facts/2002/StacyLeong.shtml |work=The Physics Factbook |date=2002 |accessdate=10 May 2007}}</ref> so it is thought to have completed 20–25 orbits during the lifetime of the Sun. The [[orbital speed]] of the Solar System about the center of the Milky Way is approximately 251&nbsp;km/s (156&nbsp;mi/s).<ref name="space.newscientist.com">{{Cite journal |last=Croswell |first=K. |date=2008 |title=Milky Way keeps tight grip on its neighbor |url=http://www.newscientist.com/article/dn12652-milky-way-keeps-a-light-grip-on-speedy-neighbours.html#.VQ7JD46WnCY |journal=[[New Scientist]] |volume=199 |issue=2669 |page=8 |doi=10.1016/S0262-4079(08)62026-6 |ref=harv}}</ref> At this speed, it takes around 1,190 years for the Solar System to travel a distance of 1 light-year, or 7 days to travel 1 [[Astronomical unit|AU]].<ref>{{Cite book |last=Garlick |first=M.A. |title=The Story of the Solar System |page=46 |publisher=[[Cambridge University Press]] |date=2002 |isbn=0-521-80336-5}}</ref>

The Milky Way is moving with respect to the [[cosmic microwave background radiation]] (CMB) in the direction of the constellation [[Hydra (constellation)|Hydra]] with a speed of 550&nbsp;km/s, and the Sun's resultant velocity with respect to the CMB is about 370&nbsp;km/s in the direction of [[Crater (constellation)|Crater]] or [[Leo (constellation)|Leo]].<ref>{{Cite journal
 |last=Kogut |first=A.
 |date=1993
 |title=Dipole Anisotropy in the COBE Differential Microwave Radiometers First-Year Sky Maps
 |journal=[[Astrophysical Journal]]
 |volume=419 |page=1
 |arxiv=astro-ph/9312056
 |doi=10.1086/173453
|bibcode = 1993ApJ...419....1K |display-authors=etal}}</ref>

==Theoretical problems==
[[File:Map of the full sun.jpg|thumb|Map of the full Sun by STEREO and [[Solar Dynamics Observatory|SDO]] spacecraft]]

===Coronal heating problem===
{{Main article|Corona}}
The temperature of the photosphere is approximately 6,000&nbsp;K, whereas the temperature of the corona reaches 1,000,000–2,000,000&nbsp;K.<ref name=Erdelyi2007/> The high temperature of the corona shows that it is heated by something other than direct [[heat conduction]] from the photosphere.<ref name=Russell2001/>

It is thought that the energy necessary to heat the corona is provided by turbulent motion in the convection zone below the photosphere, and two main mechanisms have been proposed to explain coronal heating.<ref name=Erdelyi2007/> The first is [[wave]] heating, in which sound, gravitational or magnetohydrodynamic waves are produced by turbulence in the convection zone.<ref name=Erdelyi2007/> These waves travel upward and dissipate in the corona, depositing their energy in the ambient matter in the form of heat.<ref name="Alfven">{{Cite journal |last=Alfvén |first=H. |title=Magneto-hydrodynamic waves, and the heating of the solar corona |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=107 |issue=2 |pages=211–219 |date=1947 |bibcode=1947MNRAS.107..211A |ref=harv |doi=10.1093/mnras/107.2.211}}</ref> The other is [[magnetic field|magnetic]] heating, in which magnetic energy is continuously built up by photospheric motion and released through [[magnetic reconnection]] in the form of large [[solar flare]]s and myriad similar but smaller events—[[nanoflares]].<ref name="Parker2">{{Cite journal |last=Parker |first=E.N. |title=Nanoflares and the solar X-ray corona |journal=[[Astrophysical Journal]] |volume=330 |issue=1 |page=474 |date=1988 |doi=10.1086/166485 |bibcode=1988ApJ...330..474P |ref=harv}}</ref>

Currently, it is unclear whether waves are an efficient heating mechanism. All waves except [[Alfvén wave]]s have been found to dissipate or refract before reaching the corona.<ref name="Sturrock">{{Cite journal |last=Sturrock |first=P.A. |last2=Uchida |first2=Y. |title=Coronal heating by stochastic magnetic pumping |journal=[[Astrophysical Journal]] |volume=246 |issue=1 |page=331 |date=1981 |doi=10.1086/158926 |bibcode=1981ApJ...246..331S |ref=harv}}</ref> In addition, Alfvén waves do not easily dissipate in the corona. Current research focus has therefore shifted towards flare heating mechanisms.<ref name=Erdelyi2007>{{Cite journal|last=Erdèlyi|first=R.|last2=Ballai|first2=I.|title=Heating of the solar and stellar coronae: a review |date=2007 |journal=Astron. Nachr. |volume=328 |issue=8 |pages=726–733 |doi=10.1002/asna.200710803 |ref=harv |bibcode=2007AN....328..726E}}</ref>

===Faint young Sun problem===
{{Main article|Faint young Sun paradox}}

Theoretical models of the Sun's development suggest that 3.8 to 2.5 billion years ago, during the [[Archean|Archean eon]], the Sun was only about 75% as bright as it is today. Such a weak star would not have been able to sustain liquid water on Earth's surface, and thus life should not have been able to develop. However, the geological record demonstrates that Earth has remained at a fairly constant temperature throughout its history, and that the young Earth was somewhat warmer than it is today. One theory among scientists is that the atmosphere of the young Earth contained much larger quantities of [[greenhouse gas]]es (such as [[carbon dioxide]], [[methane]]) than are present today, which trapped enough heat to compensate for the smaller amount of [[solar energy]] reaching it.<ref name="Kasting">
{{Cite journal
 |last=Kasting |first=J.F.
 |last2=Ackerman |first2=T.P.
 |title=Climatic Consequences of Very High Carbon Dioxide Levels in the Earth's Early Atmosphere
 |journal=[[Science (journal)|Science]]
 |volume=234 |issue=4782 |pages=1383–1385
 |date=1986
 |doi=10.1126/science.11539665
 |pmid=11539665
 |ref=harv
}}</ref>

However, examination of Archaean sediments appears inconsistent with the hypothesis of high greenhouse concentrations. Instead, the moderate temperature range may be explained by a lower surface [[albedo]] brought about by less continental area and the "lack of biologically induced cloud condensation nuclei". This would have led to increased absorption of solar energy, thereby compensating for the lower solar output.<ref name = "Rosing">{{cite journal
 |author1=Rosing, Minik T. |author2=Bird, Dennis K. |author3=Sleep, Norman H. |author4=Bjerrum, Christian J. | title=No climate paradox under the faint early Sun
 | journal=Nature | volume=464
 | issue=7289 | pages=744–747
 | date=April 1, 2010
 | pmid=20360739 | doi=10.1038/nature08955 |bibcode = 2010Natur.464..744R }}</ref>

==History of observation==

The enormous effect of the Sun on Earth has been recognized since [[prehistoric times]], and the Sun has been [[solar deity|regarded by some cultures]] as a [[deity]].

===Early understanding===
[[File:Solvognen DO-6865 2000.jpg|thumb|right|The [[Trundholm sun chariot]] pulled by a horse is a sculpture believed to be illustrating an important part of [[Nordic Bronze Age]] mythology. The sculpture is probably from around 1350 [[Anno Domini|BC]]. It is displayed at the [[National Museum of Denmark]].]]
{{See also|The Sun in culture}}
The Sun has been an object of veneration in many cultures throughout human history. Humanity's most fundamental understanding of the Sun is as the luminous disk in the [[sky]], whose presence above the [[horizon]] creates day and whose absence causes night. In many prehistoric and ancient cultures, the Sun was thought to be a [[solar deity]] or other [[supernatural]] entity. [[Sun worship|Worship of the Sun]] was central to civilizations such as the [[ancient Egypt]]ians, the [[Inca]] of South America and the [[Aztec]]s of what is now [[Mexico]]. In religions such as [[Hinduism]], the Sun is still considered a god. Many ancient monuments were constructed with solar phenomena in mind; for example, stone [[megalith]]s accurately mark the summer or winter [[solstice]] (some of the most prominent megaliths are located in [[Nabta Playa]], [[Egypt]]; [[Mnajdra]], Malta and at [[Stonehenge]], England); [[Newgrange]], a prehistoric human-built mount in Ireland, was designed to detect the winter solstice; the pyramid of [[El Castillo, Chichen Itza|El Castillo]] at [[Chichén Itzá]] in Mexico is designed to cast shadows in the shape of serpents climbing the [[pyramid]] at the vernal and autumnal [[equinox]]es.

The Egyptians portrayed the god [[Ra]] as being carried across the sky in a solar barque, accompanied by lesser gods, and to the Greeks, he was [[Helios]], carried by a chariot drawn by fiery horses. From the reign of [[Elagabalus]] in the [[Decline of the Roman Empire|late Roman Empire]] the Sun's birthday was a holiday celebrated as [[Sol Invictus]] (literally "Unconquered Sun") soon after the winter solstice, which may have been an antecedent to Christmas. Regarding the [[fixed star]]s, the Sun appears from Earth to revolve once a year along the [[ecliptic]] through the [[zodiac]], and so Greek astronomers categorized it as one of the seven [[classical planets|planets]] (Greek ''planetes'', "wanderer"); the naming of the [[Names of the days of the week|days of the weeks]] after the seven planets dates to the [[Roman Empire|Roman era]].<ref name=oed>{{cite web| url= http://www.oxforddictionaries.com/definition/american_english/planet|publisher = Oxford Dictionaries| title = Planet| accessdate=22 March 2015|date=December 2007}}</ref><ref name=almagest>{{Cite journal|first=Bernard R.|last=Goldstein|title=Saving the phenomena : the background to Ptolemy's planetary theory| journal=Journal for the History of Astronomy|volume=28|issue=1|date=1997|pages=1–12|location=Cambridge (UK) |bibcode=1997JHA....28....1G|ref=harv}}</ref><ref>{{Cite book|title=Ptolemy's Almagest|author= Ptolemy|last2=Toomer|first2=G. J.|publisher=Princeton University Press|date=1998|isbn=978-0-691-00260-6}}</ref>

===Development of scientific understanding===

In the early first millennium BC, [[Babylonian astronomy|Babylonian astronomers]] observed that the Sun's motion along the ecliptic is not uniform, though they did not know why; it is today known that this is due to the movement of [[Earth]] in an [[elliptic orbit]] around the Sun, with Earth moving faster when it is nearer to the Sun at [[Apsis|perihelion]] and moving slower when it is farther away at [[Apsis|aphelion]].<ref>{{Cite book|title=Babylon to Voyager and beyond: a history of planetary astronomy|first=David|last=Leverington|publisher=[[Cambridge University Press]]|date=2003|isbn=0-521-80840-5|pages=6–7|ref=harv|postscript=<!--None-->}}</ref>

One of the first people to offer a scientific or philosophical explanation for the Sun was the [[Ancient Greece|Greek]] [[philosopher]] [[Anaxagoras]]. He reasoned that it was not the [[chariot]] of [[Helios]], but instead a giant flaming ball of metal even larger than the land of the [[Peloponnese|Peloponnesus]] and that the [[Moon]] reflected the light of the Sun.<ref>
{{Cite journal
 |last=Sider |first=D.
 |title=Anaxagoras on the Size of the Sun
 |jstor=269068
 |journal=[[Classical Philology (journal)|Classical Philologys]]
 |volume=68 |issue=2 |pages=128–129
 |date=1973
 |doi=10.1086/365951
 |ref=harv
}}</ref> For teaching this [[heresy]], he was imprisoned by the authorities and [[capital punishment|sentenced to death]], though he was later released through the intervention of [[Pericles]]. [[Eratosthenes]] estimated the distance between Earth and the Sun in the 3rd century BC as "of stadia [[myriad]]s 400 and 80000", the translation of which is ambiguous, implying either 4,080,000 [[Stadion (unit)|stadia]] (755,000&nbsp;km) or 804,000,000 stadia (148 to 153 million kilometers or 0.99 to 1.02 AU); the latter value is correct to within a few percent. In the 1st century AD, [[Ptolemy]] estimated the distance as 1,210 times [[Earth radius|the radius of Earth]], approximately {{convert|{{#expr:1.210*6.371round2}}|e6km|AU|sp=us}}.<ref>
{{Cite journal
 |last=Goldstein |first=B.R.
 |title=The Arabic Version of Ptolemy's Planetary Hypotheses
 |journal=[[Transactions of the American Philosophical Society]]
 |volume=57 |issue=4 |pages=9–12
 |date=1967
 |doi=10.2307/1006040
 |ref=harv
 |jstor=1006040
}}</ref>

The theory that the Sun is the center around which the planets orbit was first proposed by the ancient Greek [[Aristarchus of Samos]] in the 3rd century BC, and later adopted by [[Seleucus of Seleucia]] (see [[Heliocentrism]]). This view was developed in a more detailed [[mathematical model]] of a heliocentric system in the 16th century by [[Nicolaus Copernicus]].

Observations of sunspots were recorded during the [[Han Dynasty]] (206 BC–AD 220) by [[Chinese astronomy|Chinese astronomers]], who maintained records of these observations for centuries. [[Averroes]] also provided a description of sunspots in the 12th century.<ref>
{{cite book |last=Ead |first=Hamed A. |title=Averroes As A Physician |publisher=[[University of Cairo]]}}</ref> The invention of the [[telescope]] in the early 17th century permitted detailed observations of [[sunspot]]s by [[Thomas Harriot]], [[Galileo Galilei]] and other astronomers. Galileo posited that sunspots were on the surface of the Sun rather than small objects passing between Earth and the Sun.<ref>
{{cite web
 |title=Galileo Galilei (1564–1642)
 |url=http://www.bbc.co.uk/history/historic_figures/galilei_galileo.shtml
 |publisher=BBC
 |accessdate=22 March 2006
}}</ref>

[[Astronomy in medieval Islam|Arabic astronomical contributions]] include [[Muhammad ibn Jābir al-Harrānī al-Battānī|Albatenius]]' discovery that the direction of the Sun's [[apogee]] (the place in the Sun's orbit against the fixed stars where it seems to be moving slowest) is changing.<ref>''A short History of scientific ideas to 1900'', C. Singer, Oxford University Press, 1959, p. 151.</ref> (In modern heliocentric terms, this is caused by a gradual motion of the aphelion of the ''Earth's'' orbit). [[Ibn Yunus]] observed more than 10,000 entries for the Sun's position for many years using a large [[astrolabe]].<ref>The Arabian Science, C. Ronan, pp. 201–244 in ''The Cambridge Illustrated History of the World's Science'', Cambridge University Press, 1983; at pp. 213–214.</ref>

[[File:Sun-bonatti.png|thumb|Sol, the Sun, from a 1550 edition of [[Guido Bonatti]]'s ''Liber astronomiae''.]]
From an observation of a  [[transit of Venus]] in 1032, the Persian astronomer and polymath [[Avicenna]] concluded that Venus is closer to Earth than the Sun.<ref name=Goldstein>{{Cite journal|title=Theory and Observation in Medieval Astronomy|first=Bernard R.|last=Goldstein|journal=[[Isis (journal)|Isis]]|volume=63|issue=1|date=March 1972|publisher=[[University of Chicago Press]]|pages=39–47 [44]|doi=10.1086/350839|ref=harv}}</ref> In 1672 [[Giovanni Cassini]] and [[Jean Richer]] determined the distance to [[Mars]] and were thereby able to calculate the distance to the Sun.

In 1666, [[Isaac Newton]] observed the Sun's light using a [[prism (optics)|prism]], and showed that it is made up of light of many colors.<ref>
{{cite web
 |title=Sir Isaac Newton (1643–1727)
 |url=http://www.bbc.co.uk/history/historic_figures/newton_isaac.shtml
 |publisher=BBC
 |accessdate=22 March 2006
}}</ref> In 1800, [[William Herschel]] discovered [[infrared]] radiation beyond the red part of the solar spectrum.<ref>
{{cite web
 |title=Herschel Discovers Infrared Light 
 |url=http://coolcosmos.ipac.caltech.edu/cosmic_classroom/classroom_activities/herschel_bio.html 
 |publisher=Cool Cosmos 
 |accessdate=22 March 2006 
 |deadurl=yes 
 |archiveurl=https://web.archive.org/web/20120225094516/http://coolcosmos.ipac.caltech.edu/cosmic_classroom/classroom_activities/herschel_bio.html 
 |archivedate=25 February 2012 
 |df= 
}}</ref> The 19th century saw advancement in spectroscopic studies of the Sun; [[Joseph von Fraunhofer]] recorded more than 600 [[absorption lines]] in the spectrum, the strongest of which are still often referred to as [[Fraunhofer lines]]. In the early years of the modern scientific era, the source of the Sun's energy was a significant puzzle. [[Lord Kelvin]] suggested that the Sun is a gradually cooling liquid body that is radiating an internal store of heat.<ref name=kelvin>
{{Cite journal
 |last=Thomson |first=W.
 |title=On the Age of the Sun's Heat
 |url=http://zapatopi.net/kelvin/papers/on_the_age_of_the_suns_heat.html
 |journal=[[Macmillan's Magazine]]
 |date=1862
 |volume=5 |pages=388–393
 |ref=harv
}}</ref> Kelvin and [[Hermann von Helmholtz]] then proposed a [[Kelvin–Helmholtz mechanism|gravitational contraction]] mechanism to explain the energy output, but the resulting age estimate was only 20 million years, well short of the time span of at least 300 million years suggested by some geological discoveries of that time.<ref name=kelvin /><ref>{{cite journal|year=2000|title=Kelvin's age of the Earth paradox revisited|journal=[[Journal of Geophysical Research]]|volume=105|issue=B6|pages=13155–13158|bibcode=2000JGR...10513155S|doi=10.1029/2000JB900028|last1=Stacey|first1=Frank D.}}</ref><!-- In XIX century, before discovery of radionuclear dating, there was no reason to suggest that Earth exists for as long as 4 billion years. --> In 1890 [[Joseph Norman Lockyer|Joseph Lockyer]], who discovered helium in the solar spectrum, proposed a meteoritic hypothesis for the formation and evolution of the Sun.<ref>
{{Cite book
 |last=Lockyer |first=J.N.
 |title=The meteoritic hypothesis; a statement of the results of a spectroscopic inquiry into the origin of cosmical systems
 |publisher=[[Macmillan and Co.|Macmillan and Co]]
 |date=1890
 |bibcode=1890mhsr.book.....L
}}</ref>

Not until 1904 was a documented solution offered. [[Ernest Rutherford]] suggested that the Sun's output could be maintained by an internal source of heat, and suggested [[radioactive decay]] as the source.<ref>
{{cite web
 |last=Darden |first=L.
 |title=The Nature of Scientific Inquiry
 |url=http://www.philosophy.umd.edu/Faculty/LDarden/sciinq/
 |date=1998
}}</ref> However, it would be [[Albert Einstein]] who would provide the essential clue to the source of the Sun's energy output with his [[mass-energy equivalence]] relation {{nowrap|''E'' {{=}} ''mc''<sup>2</sup>}}.<ref>{{Cite book|last = Hawking |first = S. W. |author-link = Stephen Hawking |date = 2001 |title = The Universe in a Nutshell |publisher = Bantam Books |isbn = 0-553-80202-X}}</ref> In 1920, Sir [[Arthur Eddington]] proposed that the pressures and temperatures at the core of the Sun could produce a nuclear fusion reaction that merged hydrogen (protons) into helium nuclei, resulting in a production of energy from the net change in mass.<ref>
{{cite web
 |title=Studying the stars, testing relativity: Sir Arthur Eddington
 |url=http://www.esa.int/esaSC/SEMDYPXO4HD_index_0.html
 |work=Space Science
 |publisher=[[European Space Agency]]
 |date=2005
 |accessdate=1 August 2007
}}</ref> The preponderance of hydrogen in the Sun was confirmed in 1925 by [[Cecilia Payne-Gaposchkin|Cecilia Payne]] using the [[ionization]] theory developed by [[Meghnad Saha]], an Indian physicist. The theoretical concept of fusion was developed in the 1930s by the astrophysicists [[Subrahmanyan Chandrasekhar]] and [[Hans Bethe]]. Hans Bethe calculated the details of the two main energy-producing nuclear reactions that power the Sun.<ref name="Bethe">
{{Cite journal
 |last=Bethe |first=H.
 |title=On the Formation of Deuterons by Proton Combination
 |journal=[[Physical Review]]
 |volume=54 |issue=10 |pages=862–862
 |date=1938
 |doi=10.1103/PhysRev.54.862.2
 |last2=Critchfield
 |first2=C.
 |ref=harv
|bibcode = 1938PhRv...54Q.862B }}</ref><ref name="Bethe2">
{{Cite journal
 |last=Bethe |first=H.
 |title=Energy Production in Stars
 |journal=[[Physical Review]]
 |volume=55 |issue=1 |pages=434–456
 |date=1939
 |doi=10.1103/PhysRev.55.434
 |ref=harv
|bibcode=1939PhRv...55..434B
}}</ref> In 1957, [[Margaret Burbidge]], [[Geoffrey Burbidge]], [[William Alfred Fowler|William Fowler]] and [[Fred Hoyle]] showed that most of the elements in the universe have been [[nucleosynthesis|synthesized]] by nuclear reactions inside stars, some like the Sun.<ref>
{{Cite journal
 |first=E.M. |last=Burbidge
 |first2=G.R. |last2=Burbidge
 |first3=W.A. |last3=Fowler
 |first4=F. |last4=Hoyle
 |title=Synthesis of the Elements in Stars
 |journal=[[Reviews of Modern Physics]]
 |volume=29 |issue=4 |pages=547–650
 |date=1957
 |doi=10.1103/RevModPhys.29.547
 |bibcode=1957RvMP...29..547B
 |ref=harv
}}</ref>

===Solar space missions===
{{see also|Solar observatory}}
[[File:Sunspots and Solar Flares.jpg|thumb|The Sun giving out a large [[geomagnetic storm]] on 1:29 pm, EST, 13 March 2012]]
[[File:Moon transit of sun large.ogv|thumb|left|A lunar transit of the Sun captured during calibration of STEREO B's ultraviolet imaging cameras<ref>{{cite web
 |last=Phillips 
 |first=T. 
 |title=Stereo Eclipse 
 |url=https://science.nasa.gov/headlines/y2007/12mar_stereoeclipse.htm 
 |work=Science@NASA 
 |publisher=[[NASA]] 
 |date=2007 
 |accessdate=19 June 2008 
 |deadurl=yes 
 |archiveurl=https://web.archive.org/web/20080610082213/https://science.nasa.gov/headlines/y2007/12mar_stereoeclipse.htm 
 |archivedate=10 June 2008 
 |df= 
}}</ref>]]
The first satellites designed to observe the Sun were [[NASA]]'s [[Pioneer program|Pioneers]] 5, 6, 7, 8 and 9, which were launched between 1959 and 1968. These probes orbited the Sun at a distance similar to that of Earth, and made the first detailed measurements of the solar wind and the solar magnetic field. [[Pioneer 9]] operated for a particularly long time, transmitting data until May 1983.<ref>{{cite web
 |last=Wade |first=M.
 |title=Pioneer 6-7-8-9-E
 |url=http://www.astronautix.com/craft/pio6789e.htm
 |date=2008
 |publisher=[[Encyclopedia Astronautica]]
 |accessdate=22 March 2006
}}</ref><ref>{{cite web
 |title=Solar System Exploration: Missions: By Target: Our Solar System: Past: Pioneer 9
 |url=http://solarsystem.nasa.gov/missions/profile.cfm?MCode=Pioneer_09
 |publisher=[[NASA]]
 |accessdate=30 October 2010
 |quote=NASA maintained contact with Pioneer 9 until May 1983
}}</ref>

In the 1970s, two [[Helios probes|Helios]] spacecraft and the [[Skylab]] [[Apollo Telescope Mount]] provided scientists with significant new data on solar wind and the solar corona. The Helios 1 and 2 probes were U.S.–German collaborations that studied the solar wind from an orbit carrying the spacecraft inside [[Mercury (planet)|Mercury]]'s orbit at [[perihelion]].<ref name=Burlaga2001/> The Skylab space station, launched by NASA in 1973, included a solar [[observatory]] module called the Apollo Telescope Mount that was operated by astronauts resident on the station.<ref name=Dwivedi2006/> Skylab made the first time-resolved observations of the solar transition region and of ultraviolet emissions from the solar corona.<ref name=Dwivedi2006/> Discoveries included the first observations of [[coronal mass ejection]]s, then called "coronal transients", and of [[coronal hole]]s, now known to be intimately associated with the [[solar wind]].<ref name=Burlaga2001>{{Cite journal|last=Burlaga|first=L.F.|title=Magnetic Fields and plasmas in the inner heliosphere: Helios results|date=2001|journal=Planetary and Space Science|volume=49|issue=14–15|pages=1619–27|doi=10.1016/S0032-0633(01)00098-8|ref=harv|bibcode=2001P&SS...49.1619B}}</ref>

In 1980, the [[Solar Maximum Mission]] was launched by [[NASA]]. This spacecraft was designed to observe [[gamma ray]]s, [[X-ray]]s and [[Ultraviolet|UV]] radiation from [[solar flare]]s during a time of high solar activity and [[Sun#External links|solar luminosity]]. Just a few months after launch, however, an electronics failure caused the probe to go into standby mode, and it spent the next three years in this inactive state. In 1984 [[Space Shuttle Challenger|Space Shuttle ''Challenger'']] mission [[STS-41C]] retrieved the satellite and repaired its electronics before re-releasing it into orbit. The Solar Maximum Mission subsequently acquired thousands of images of the solar corona before [[Atmospheric reentry|re-entering]] Earth's atmosphere in June 1989.<ref>
{{cite web |last=Burkepile |first=C.J.
 |title=Solar Maximum Mission Overview
 |url=http://web.hao.ucar.edu/public/research/svosa/smm/smm_mission.html
 |date=1998
 |accessdate=22 March 2006
| archiveurl = https://web.archive.org/web/20060405183758/http://web.hao.ucar.edu/public/research/svosa/smm/smm_mission.html| archivedate = 5 April 2006}}</ref>

Launched in 1991, Japan's [[Yohkoh]] (''Sunbeam'') satellite observed solar flares at X-ray wavelengths. Mission data allowed scientists to identify several different types of flares, and demonstrated that the corona away from regions of peak activity was much more dynamic and active than had previously been supposed. Yohkoh observed an entire solar cycle but went into standby mode when an [[solar eclipse|annular eclipse]] in 2001 caused it to lose its lock on the Sun. It was destroyed by atmospheric re-entry in 2005.<ref>
{{cite press
 |title=Result of Re-entry of the Solar X-ray Observatory "Yohkoh" (SOLAR-A) to the Earth's Atmosphere
 |url=http://www.jaxa.jp/press/2005/09/20050913_yohkoh_e.html
 |publisher=[[Japan Aerospace Exploration Agency]]
 |date=2005
 |accessdate=22 March 2006
}}</ref>

One of the most important solar missions to date has been the [[Solar and Heliospheric Observatory]], jointly built by the [[European Space Agency]] and [[NASA]] and launched on 2 December 1995.<ref name=Dwivedi2006/> Originally intended to serve a two-year mission, a mission extension through 2012 was approved in October 2009.<ref name=sohoext>{{cite web| date = 7 October 2009|url = http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=45685|title = Mission extensions approved for science missions|work = ESA Science and Technology|accessdate = 16 February 2010}}</ref> It has proven so useful that a follow-on mission, the [[Solar Dynamics Observatory]] (SDO), was launched in February 2010.<ref name=sdolaunch>{{cite web| date = 11 February 2010|url = http://www.nasa.gov/home/hqnews/2010/feb/HQ_10-040_SDO_launch.html|title = NASA Successfully Launches a New Eye on the Sun|work = NASA Press Release Archives|accessdate = 16 February 2010}}</ref> Situated at the [[Lagrangian point]] between Earth and the Sun (at which the gravitational pull from both is equal), SOHO has provided a constant view of the Sun at many wavelengths since its launch.<ref name=Dwivedi2006/> Besides its direct solar observation, SOHO has enabled the discovery of a large number of [[comet]]s, mostly tiny [[sungrazing comet]]s that incinerate as they pass the Sun.<ref>
{{cite web
 |title=Sungrazing Comets
 |url=http://sungrazer.nrl.navy.mil/
 |publisher=[[Large Angle and Spectrometric Coronagraph|LASCO]] ([[US Naval Research Laboratory]])
 |accessdate=19 March 2009
}}</ref>
[[File:Giant prominence on the sun erupted.jpg|thumb|300px|A solar prominence erupts in August 2012, as captured by SDO]]

All these satellites have observed the Sun from the plane of the ecliptic, and so have only observed its equatorial regions in detail. The [[Ulysses probe]] was launched in 1990 to study the Sun's polar regions. It first travelled to [[Jupiter]], to "slingshot" into an orbit that would take it far above the plane of the ecliptic. Once Ulysses was in its scheduled orbit, it began observing the solar wind and magnetic field strength at high solar latitudes, finding that the solar wind from high latitudes was moving at about 750&nbsp;km/s, which was slower than expected, and that there were large magnetic waves emerging from high latitudes that scattered galactic [[cosmic ray]]s.<ref>
{{cite web
 |author=[[Jet Propulsion Laboratory|JPL]]/[[California Institute of Technology|CALTECH]] 
 |title=Ulysses: Primary Mission Results 
 |url=http://ulysses.jpl.nasa.gov/science/mission_primary.html 
 |publisher=[[NASA]] 
 |date=2005 
 |accessdate=22 March 2006 
 |deadurl=yes 
 |archiveurl=https://web.archive.org/web/20060106150819/http://ulysses.jpl.nasa.gov/science/mission_primary.html 
 |archivedate=6 January 2006 
 |df= 
}}</ref>

Elemental abundances in the photosphere are well known from [[astronomical spectroscopy|spectroscopic]] studies, but the composition of the interior of the Sun is more poorly understood. A [[solar wind]] sample return mission, [[Genesis (spacecraft)|Genesis]], was designed to allow astronomers to directly measure the composition of solar material.<ref>
{{Cite journal
 |last=Calaway |first=M.J.
 |title=Genesis capturing the Sun: Solar wind irradiation at Lagrange 1
 |journal=[[Nuclear Instruments and Methods in Physics Research B]]
 |volume=267 |issue=7 |pages=1101–1108
 |date=2009
 |doi=10.1016/j.nimb.2009.01.132
 |last2=Stansbery
 |first2=Eileen K.
 |last3=Keller
 |first3=Lindsay P.
 |ref=harv
|bibcode = 2009NIMPB.267.1101C }}</ref>

The [[STEREO|Solar Terrestrial Relations Observatory]] (STEREO) mission was launched in October 2006. Two identical spacecraft were launched into orbits that cause them to (respectively) pull further ahead of and fall gradually behind Earth. This enables [[stereoscopic]] imaging of the Sun and solar phenomena, such as [[coronal mass ejections]].<ref name=inst>{{cite web| date = 8 March 2006|url = http://www.nasa.gov/mission_pages/stereo/spacecraft/index.html|title = STEREO Spacecraft & Instruments|work = NASA Missions|accessdate = 30 May 2006}}</ref><ref>{{Cite journal| title= Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI)|last = Howard | first = R. A.|last2 = Moses | first2 = J. D.| last3 = Socker | first3 = D. G.| last4 = Dere | first4 = K. P.| last5 = Cook | first5 = J. W.|journal=Advances in Space Research|volume= 29|issue= 12|pages=2017–2026|date= 2002| ref= harv |bibcode=2008SSRv..136...67H |doi=10.1007/s11214-008-9341-4
}}</ref>

The [[Indian Space Research Organisation]] has scheduled the launch of a 100&nbsp;kg satellite named [[Aditya (spacecraft)|Aditya]] for 2017–18. Its main instrument will be a [[coronagraph]] for studying the dynamics of the Solar corona.<ref>{{cite web |url= http://timesofindia.indiatimes.com/india/Aditya-1-launch-delayed-to-2015-16/articleshow/16326842.cms|title= Aditya 1 launch delayed to 2015–16| first = Srinivas | last = Laxman| last2 = Rhik Kundu | first2 = TNN |date=9 September 2012|work= [[The Times of India]]|publisher= [[Bennett, Coleman & Co. Ltd.]]}}</ref>

==Observation and effects==
[[File:Anatomy of a Sunset-2.jpg|thumb|right|300px|During certain atmospheric conditions, the Sun becomes clearly visible to the naked eye, and can be observed without stress to the eyes. Click on this photo to see the full cycle of a [[sunset]], as observed from the high plains of the [[Mojave Desert]].]]
[[File:STS-134 EVA4 view to the Russian Orbital Segment.jpg|thumb|right|The Sun, as seen from low Earth orbit overlooking the [[International Space Station]]. This sunlight is not filtered by the lower atmosphere, which blocks much of the solar spectrum]]
The brightness of the Sun can cause pain from looking at it with the [[naked eye]]; however, doing so for brief periods is not hazardous for normal non-dilated eyes.<ref>

{{Cite journal
 |first=T.J. |last=White |first2=M.A. |last2=Mainster |first3=P.W. |last3=Wilson
 |first4=J.H. |last4=Tips
 |title=Chorioretinal temperature increases from solar observation
 |journal=[[Bulletin of Mathematical Biophysics]]
 |volume=33 |issue=1 |pages=1–17
 |date=1971
 |doi=10.1007/BF02476660
 |ref=harv
}}</ref><ref>
{{Cite journal
 |first=M.O.M. |last=Tso |first2=F.G. |last2=La Piana
 |title=The Human Fovea After Sungazing
 |journal=[[Transactions of the American Academy of Ophthalmology and Otolaryngology]]
 |date=1975
 |volume=79 |pages=OP788–95
 |pmid=1209815
 |issue=6
 |ref=harv
}}</ref> Looking directly at the Sun causes [[phosphene]] visual artifacts and temporary partial blindness. It also delivers about 4&nbsp;milliwatts of sunlight to the retina, slightly heating it and potentially causing damage in eyes that cannot respond properly to the brightness.<ref>
{{Cite journal
 |last=Hope-Ross |first=M.W.
 |title=Ultrastructural findings in solar retinopathy
 |journal=[[Eye (journal)|Eye]]
 |volume=7
 |issue=4 |date=1993
 |doi=10.1038/eye.1993.7
 |pmid=8325420
 |last2=Mahon
 |first2=GJ
 |last3=Gardiner
 |first3=TA
 |last4=Archer
 |first4=DB
 |ref=harv
 |pages=29–33
}}</ref><ref>
{{Cite journal
 |title=Solar Retinopathy from Sun-Gazing Under Influence of LSD
 |last=Schatz |first=H. |last2=Mendelblatt |first2=F.
 |journal=[[British Journal of Ophthalmology]]
 |volume=57 |issue=4 |date=1973
 |doi=10.1136/bjo.57.4.270
 |pmid=4707624
 |ref=harv |pmc=1214879
 |pages=270–3
}}</ref> [[ultraviolet|UV]] exposure gradually yellows the lens of the eye over a period of years, and is thought to contribute to the formation of [[cataracts]], but this depends on general exposure to solar UV, and not whether one looks directly at the Sun.<ref>
{{cite web
 |last=Chou |first=B.R.
 |title=Eye Safety During Solar Eclipses
 |url=http://eclipse.gsfc.nasa.gov/SEhelp/safety2.html
 |date=2005
}} "''While environmental exposure to UV radiation is known to contribute to the accelerated aging of the outer layers of the eye and the development of cataracts, the concern over improper viewing of the Sun during an eclipse is for the development of "eclipse blindness" or retinal burns.''"</ref> Long-duration viewing of the direct Sun with the naked eye can begin to cause UV-induced, sunburn-like lesions on the retina after about 100 seconds, particularly under conditions where the UV light from the Sun is intense and well focused;<ref>
{{Cite journal
 |first=W.T. Jr. |last=Ham |first2=H.A. |last2=Mueller |first3=D.H. |last3=Sliney
 |journal=[[Nature (journal)|Nature]]
 |title=Retinal sensitivity to damage from short wavelength light
 |volume=260
 |issue=5547 |pages=153–155
 |date=1976
 |doi=10.1038/260153a0
 |ref=harv
|bibcode = 1976Natur.260..153H }}</ref><ref>
{{Cite book
 |first=W.T. Jr. |last=Ham |first2=H.A. |last2=Mueller |first3=J.J. Jr. |last3=Ruffolo
 |first4=D. III|last4=Guerry
 |chapter=Solar Retinopathy as a function of Wavelength: its Significance for Protective Eyewear
 |title=The Effects of Constant Light on Visual Processes
 |editor=Williams, T.P.
 |editor2=Baker, B.N.
 |publisher=[[Plenum Press]]
 |pages=319–346
 |date=1980
 |isbn=0-306-40328-5
}}</ref> conditions are worsened by young eyes or new lens implants (which admit more UV than aging natural eyes), Sun angles near the zenith, and observing locations at high altitude.

Viewing the Sun through light-concentrating [[optics]] such as [[binoculars]] may result in permanent damage to the retina without an appropriate filter that blocks UV and substantially dims the sunlight. When using an attenuating filter to view the Sun, the viewer is cautioned to use a filter specifically designed for that use. Some improvised filters that pass UV or [[infrared|IR]] rays, can actually harm the eye at high brightness levels.<ref>
{{Cite book
 |first=T. |last=Kardos
 |title=Earth science
 |url=https://books.google.com/?id=xI6EDV_PRr4C&pg=PT102
 |page=87
 |publisher= [[J.W. Walch]]
 |date=2003
 |isbn=978-0-8251-4500-1
}}</ref>
[[Herschel wedge]]s, also called Solar Diagonals, are effective and inexpensive for small telescopes. The sunlight that is destined for the eyepiece is reflected from an unsilvered surface of a piece of glass. Only a very small fraction of the incident light is reflected. The rest passes through the glass and leaves the instrument. If the glass breaks because of the heat, no light at all is reflected, making the device fail-safe. Simple filters made of darkened glass allow the full intensity of sunlight to pass through if they break, endangering the observer's eyesight. Unfiltered binoculars can deliver hundreds of times as much energy as using the naked eye, possibly causing immediate damage. It is claimed that even brief glances at the midday Sun through an unfiltered telescope can cause permanent damage.<ref name=Macdonald>{{cite book|last=Macdonald|first=Lee|date=2012|title=How to Observe the Sun Safely|publisher=Springer Science + Business Media|place=New York|chapter=2. Equipment for Observing the Sun|page=17|doi=10.1007/978-1-4614-3825-0_2|quote=NEVER LOOK DIRECTLY AT THE SUN THROUGH ANY FORM OF OPTICAL EQUIPMENT, EVEN FOR AN INSTANT. A brief glimpse of the Sun through a telescope is enough to cause permanent eye damage, or even blindness. Even looking at the Sun with the naked eye for more than a second or two is not safe. Do not assume that it is safe to look at the Sun through a filter, no matter how dark the filter appears to be.}}</ref>
<!-- reference is useful but doesn't support the claim here. -- <ref name="Marsh">
{{Cite journal
 |last=Marsh |first=J.C.D.
 |title=Observing the Sun in Safety
 |journal=[[Journal of the British Astronomical Association]]
 |volume=92 |issue=6 |page=257
 |date=1982
 |bibcode=1982JBAA...92..257M
 |ref=harv
}}</ref>-->

[[File:Doppelsonne Halo Echzell Hessen 12-08-2012.jpg|thumb|left|[[Halo (optical phenomenon)|Halo]] with [[sun dog]]s]]
Partial [[solar eclipse]]s are hazardous to view because the eye's [[pupil]] is not adapted to the unusually high visual contrast: the pupil dilates according to the total amount of light in the field of view, ''not'' by the brightest object in the field. During partial eclipses most sunlight is blocked by the [[Moon]] passing in front of the Sun, but the uncovered parts of the photosphere have the same [[surface brightness]] as during a normal day. In the overall gloom, the pupil expands from ~2&nbsp;mm to ~6&nbsp;mm, and each retinal cell exposed to the solar image receives up to ten times more light than it would looking at the non-eclipsed Sun. This can damage or kill those cells, resulting in small permanent blind spots for the viewer.<ref name="Espenak">
{{cite web
 |last=Espenak |first=Fred
 |title=Eye Safety During Solar Eclipses
 |url=http://eclipse.gsfc.nasa.gov/SEhelp/safety.html
 |publisher=[[NASA]]
 |date=26 April 1996
}}</ref> The hazard is insidious for inexperienced observers and for children, because there is no perception of pain: it is not immediately obvious that one's vision is being destroyed.

[[File:Actual Sunrise.jpeg|300px|thumb|right|A sunrise]]

During [[sunrise]] and [[sunset]], sunlight is attenuated because of [[Rayleigh scattering]] and [[Mie theory|Mie scattering]] from a particularly long passage through Earth's atmosphere,<ref name=Haber2005>{{Cite journal|last=Haber|first=Jorg| last2 = Magnor | first2 = Marcus| last3 = Seidel | first3 = Hans-Peter|title=Physically based Simulation of Twilight Phenomena|date=2005|journal=ACM Transactions on Graphics|volume=24|issue=4|pages=1353–1373|doi=10.1145/1095878.1095884|citeseerx = 10.1.1.67.2567|ref=harv}}</ref> and the Sun is sometimes faint enough to be viewed comfortably with the naked eye or safely with optics (provided there is no risk of bright sunlight suddenly appearing through a break between clouds). Hazy conditions, atmospheric dust, and high humidity contribute to this atmospheric attenuation.<ref>{{Cite journal|title=Diurnal asymmetries in global radiation| first = I. G. | last = Piggin|journal=Springer|date=1972|volume=20|issue=1|doi=10.1007/BF02243313|pages=41–48|ref=harv|bibcode = 1972AMGBB..20...41P }}</ref>

An [[optical phenomenon]], known as a [[green flash]], can sometimes be seen shortly after sunset or before sunrise. The flash is caused by light from the Sun just below the horizon being [[refraction|bent]] (usually through a [[temperature inversion]]) towards the observer. Light of shorter wavelengths (violet, blue, green) is bent more than that of longer wavelengths (yellow, orange, red) but the violet and blue light is [[Rayleigh scattering|scattered]] more, leaving light that is perceived as green.<ref>
{{cite web
 |title=The Green Flash
 |url=http://www.bbc.co.uk/weather/features/understanding/greenflash.shtml
 |publisher=BBC
 |accessdate=10 August 2008
 |archiveurl=https://web.archive.org/web/20081216135504/http://www.bbc.co.uk/weather/features/understanding/greenflash.shtml
 |archivedate=16 December 2008
}}</ref>

[[Ultraviolet]] light from the Sun has [[antiseptic]] properties and can be used to sanitize tools and water. It also causes [[sunburn]], and has other biological effects such as the production of [[vitamin D]] and [[sun tanning]].  Ultraviolet light is strongly attenuated by Earth's [[ozone layer]], so that the amount of UV varies greatly with [[latitude]] and has been partially responsible for many biological adaptations, including variations in [[human skin color]] in different regions of the globe.<ref>
{{Cite journal
 |last=Barsh |first=G.S.
 |title=What Controls Variation in Human Skin Color?
 |journal=[[PLoS Biology]]
 |volume=1
 |issue=1 |page=e7
 |date=2003
 |pmid=14551921
 |pmc=212702
 |doi=10.1371/journal.pbio.0000027
 |ref=harv
}}</ref>

==Planetary system==

{{main article |Solar System}}

The Sun has eight known planets. This includes four [[terrestrial planets]] ([[Mercury (planet)|Mercury]], [[Venus]], [[Earth]], and [[Mars]]), two [[gas giants]] ([[Jupiter]] and [[Saturn]]), and two [[ice giants]] ([[Uranus]] and [[Neptune]]). The Solar System also has at least five [[dwarf planets]], an [[asteroid belt]], numerous [[comets]], and a large number of icy bodies which lie beyond the orbit of Neptune.

==See also==
{{Wikipedia books |1=The Sun}}
{{cmn|30em|
* [[Advanced Composition Explorer]]
* [[Antisolar point]]
* [[List of brightest stars]]
* [[Solar energy]]
* [[Sun dogs]]
* [[Sun path]]
* [[Sun-Earth Day]]
* [[Sunday]]
* [[Sungazing]]
* [[Timeline of the far future]]
}}
{{Portal bar|Star|Solar System}}

==Notes==
{{notes
| notes =
{{efn
| name = heavy elements
| In [[astronomy|astronomical sciences]], the term ''heavy elements'' (or ''metals'') refers to all [[chemical element|elements]] except hydrogen and helium.
}}
{{efn
| name = power production density
| A 50 kg adult human has a volume of about 0.05 m<sup>3</sup>, which corresponds to 13.8 watts, at the volumetric power of the solar center. This is 285 kcal/day, about 10% of the actual average caloric intake and output for humans in non-stressful conditions.
}}
{{efn
| name = particle density
| Earth's atmosphere near sea level has a particle density of about 2{{e|25}}&nbsp;m<sup>−3</sup>.
}}
}}

==References==
{{reflist|30em}}

==Further reading==
* {{Cite book|last=Cohen |first=Richard |date=2010 |title=Chasing the Sun: the Epic Story of the Star that Gives us Life |publisher=[[Simon & Schuster]]|isbn=1-4000-6875-4}}
* {{Cite journal|last=Thompson |first=M. J. |date=2004 |title=Solar interior: Helioseismology and the Sun's interior |journal=[[Astronomy & Geophysics]] |volume=45 |issue=4 |pages=21–25 }}
* [http://www.scholarpedia.org/article/Solar_activity Solar Activity] [[Scholarpedia]] Hugh Hudson 3(3):3967. {{doi|10.4249/scholarpedia.3967}}

==External links==
{{Sister project links|Sun}}
* [http://sohowww.nascom.nasa.gov/ Nasa SOHO (Solar & Heliospheric Observatory) satellite]
* [http://www.nso.edu/ National Solar Observatory]
* [http://www.astronomycast.com/astronomy/episode-30-the-sun-spots-and-all/ Astronomy Cast: The Sun]
* [http://www.boston.com/bigpicture/2008/10/the_sun.html A collection of spectacular images of the Sun from various institutions] (''[[The Boston Globe]]'')
* [http://www.acrim.com/ Satellite observations of solar luminosity]
* [http://www.suntrek.org/ Sun|Trek, an educational website about the Sun]
* [https://web.archive.org/web/20050518081349/http://www.solarphysics.kva.se/ The Swedish 1-meter Solar Telescope, SST]
* [http://alienworlds.glam.ac.uk/sunStructure.html An animated explanation of the structure of the Sun] (University of Glamorgan)
* [https://www.youtube.com/watch?v=qpMRtvFD8ek&hl=fr Animation – The Future of the Sun]
* [https://web.archive.org/web/20100315111135/https://science.nasa.gov/headlines/y2010/12mar_conveyorbelt.htm Solar Conveyor Belt Speeds Up]&nbsp;– NASA&nbsp;– images, link to report on Science
* [https://www.youtube.com/watch?v=w-41gAPmUG0&feature=youtube_gdata&ab_channel=NASAGoddard NASA 5-year timelapse video of the Sun]
* [https://www.youtube.com/watch?v=6tmbeLTHC_0&ab_channel=NASAGoddard Sun in Ultra High Definition] NASA 11/1/2015

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