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Array ( [0] => {{Short description|Star at the center of the Solar System}} [1] => {{Other uses|Sun (disambiguation)|The Sun (disambiguation)}} [2] => {{Featured article}} [3] => {{Pp-semi-indef}} [4] => {{Pp-move}} [5] => {{Use American English|date=August 2019}} [6] => {{Use dmy dates|date=August 2019}} [7] => {{CS1 config|mode=cs1}} [8] => {{Infobox [9] => | title = Sun [10] => | image = [[File:The Sun in white light.jpg|300px|frameless|alt=White glowing ball with black sunspots]] [11] => | caption = The Sun, captured through a clear [[solar filter]] [12] => | labelstyle = background: inherit; [13] => | label1 = Names [14] => | data1 = Sun, [[Sol (Roman mythology)|Sol]], [[Sun (Germanic mythology)|Sól]], [[Helios]] [15] => | label2 = Adjectives [16] => | data2 = Solar [17] => | label3 = [[Planet symbols#Sun|Symbol]] [18] => | data3 = [[File:Sun symbol (bold).svg|24px|alt=Circle with dot in the middle]] [19] => | header4 = Observation data [20] => | label5 = {{longitem|Mean distance from Earth}} [21] => | data5 = {{val|1|ul=AU}}
{{Convert|149600000|km|mi|abbr=on|disp=br}}
8 min 19 s, [[speed of light|light speed]]{{Cite journal |last1=Pitjeva |first1=E. V. |last2=Standish |first2=E. M. |date=2009 |title=Proposals for the masses of the three largest asteroids, the Moon–Earth mass ratio and the Astronomical Unit |url=https://zenodo.org/record/1000691 |url-status=live |journal=[[Celestial Mechanics and Dynamical Astronomy]] |language=en |volume=103 |issue=4 |pages=365–372 |bibcode=2009CeMDA.103..365P |doi=10.1007/s10569-009-9203-8 |issn=1572-9478 |s2cid=121374703 |archive-url=https://web.archive.org/web/20190709062657/https://zenodo.org/record/1000691 |archive-date=9 July 2019 |access-date=13 July 2019}} [22] => | label6 = {{longitem|[[Apparent magnitude|Visual brightness]]}} [23] => | data6 = −26.74 (''V''){{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 |url-status=live |archive-url=https://web.archive.org/web/20100715200549/http://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html |archive-date=15 July 2010 |access-date=12 August 2013 |publisher=[[NASA Goddard Space Flight Center]]}} [24] => | label7 = {{longitem|[[Absolute magnitude]]}} [25] => | data7 = 4.83 [26] => | label8 = {{longitem|[[Stellar classification|Spectral classification]]}} [27] => | data8 = G2V{{Cite book |last=Zombeck |first=Martin V. |url=http://ads.harvard.edu/books/hsaa/ |title=Handbook of Space Astronomy and Astrophysics 2nd edition |date=1990 |publisher=[[Cambridge University Press]] |access-date=13 January 2016 |archive-url=https://web.archive.org/web/20210203012304/http://ads.harvard.edu/books/hsaa/ |archive-date=3 February 2021 |url-status=live}} [28] => | label9 = [[Metallicity]] [29] => | data9 = ''Z'' = 0.0122{{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 |s2cid=123824232 |doi-access=free}} [30] => | label10 = [[Angular size]] [31] => | data10 = 0.527–0.545°{{Cite web |title=Eclipse 99: Frequently Asked Questions |url=http://education.gsfc.nasa.gov/eclipse/pages/faq.html |url-status=dead |archive-url=https://web.archive.org/web/20100527142627/http://education.gsfc.nasa.gov/eclipse/pages/faq.html |archive-date=27 May 2010 |access-date=24 October 2010 |publisher=NASA}} [32] => | header11 = Orbital characteristics [33] => | label12 = {{longitem|Mean distance from [[Milky Way]] core}} [34] => | data12 = 24,000 to 28,000 [[light-year]]s [35] => | label13 = [[Galactic year|Galactic period]] [36] => | data13 = 225–250 million [[julian year (astronomy)|years]] [37] => | label14 = [[Velocity]] [38] => | data14 = {{indented plainlist| [39] => *{{Convert|251|km/s|mi/s|abbr=on}}
orbit [[Galactic Center]] [40] => *{{Convert|20|km/s|mi/s|abbr=on}}
to stellar neighborhood [41] => *{{Convert|370|km/s|mi/s|abbr=on}}
to [[cosmic microwave background]]{{Cite journal |last=Hinshaw |first=G. |display-authors=etal |year=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 |s2cid=3629998}} [42] => }} [43] => | label15 = [[Axial tilt|Obliquity]] [44] => | data15 = {{unbulleted list| [45] => |7.25° ([[ecliptic]]) [46] => |67.23° ([[galactic plane]]) [47] => }} [48] => | label16 = {{longitem|[[Right ascension]] North pole}} [49] => | data16 = 286.13° (286° 7′ 48″) [50] => | label17 = {{longitem|[[Declination]] of North pole}} [51] => | data17 = +63.87° (63° 52′ 12"N) [52] => | label18 = {{longitem|Sidereal [[Solar rotation|rotation period]]}} [53] => | data18 = {{unbulleted list [54] => |25.05 days (equator) [55] => |34.4 days (poles) [56] => }} [57] => | label19 = {{longitem|Equatorial rotation velocity}} [58] => | data19 = {{val|1.997|u=km/s}}{{Cite web |title=Solar System Exploration: Planets: Sun: Facts & Figures |url=http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sun&Display=Facts&System=Metric |url-status=dead |archive-url=https://web.archive.org/web/20080102034758/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sun&Display=Facts&System=Metric |archive-date=2 January 2008 |publisher=NASA}} [59] => | header20 = Physical characteristics [60] => | label21 = {{longitem|Equatorial [[radius]]}} [61] => | data21 = {{Convert|696300|km|mi|abbr=on|disp=br}}{{Cite arXiv |eprint=1510.07674 |class=astro-ph.SR |title=IAU 2015 Resolution B3 on Recommended Nominal Conversion Constants for Selected Solar and Planetary Properties |year=2015 | display-authors=3 |last1=Mamajek |first1=E. E. |last2=Prsa |first2=A. |last3=Torres |first3=G. |last4=Harmanec |first4=P. |last5=Asplund |first5=M. |last6=Bennett |first6=P. D. |last7=Capitaine |first7=N. |last8=Christensen-Dalsgaard |first8=J. |last9=Depagne |first9=E. |last10=Folkner |first10=W. M. |last11=Haberreiter |first11=M. |last12=Hekker |first12=S. |last13=Hilton |first13=J. L. |last14=Kostov |first14=V. |last15=Kurtz |first15=D. W. |last16=Laskar |first16=J. |last17=Mason |first17=B. D. |last18=Milone |first18=E. F. |last19=Montgomery |first19=M. M. |last20=Richards |first20=M. T. |last21=Schou |first21=J. |last22=Stewart |first22=S. G. }}{{Citation |last1=Emilio |first1=Marcelo |title=Measuring the Solar Radius from Space during the 2003 and 2006 Mercury Transits |journal=The Astrophysical Journal |volume=750 |issue=2 |page=135 |year=2012 |arxiv=1203.4898 |bibcode=2012ApJ...750..135E |doi=10.1088/0004-637X/750/2/135 |s2cid=119255559 |last2=Kuhn |first2=Jeff R. |last3=Bush |first3=Rock I. |last4=Scholl |first4=Isabelle F.}}
{{val|109|u=× [[Earth radius|Earth radii]]}} [62] => | label23 = [[Flattening]] [63] => | data23 = 0.00005 [64] => | label24 = [[Surface area]] [65] => | data24 = {{Convert|6.09|e12km2|e12mi2|abbr=on|disp=br}}
{{nowrap|{{val|fmt=commas|12000}}}} × Earth [66] => | label25 = [[Volume]] [67] => | data25 = {{unbulleted list [68] => |{{val|1.412|e=18|u=km3}} [69] => |{{val|fmt=commas|1300000}} × Earth [70] => }} [71] => | label26 = [[Mass]] [72] => | data26 = {{unbulleted list [73] => |{{val|1.9885|e=30|u=kg}} [74] => |{{val|4.3839|e=30|u=lbs}} [75] => |{{val|fmt=commas|332950|u=[[Earth mass|Earths]]}} [76] => }} [77] => | label27 = Average [[density]] [78] => | data27 = {{convert|1.408|g/cm3|lb/in3|abbr=on|disp=br}}
{{val|0.255}} × Earth [79] => | label28 = Age [80] => | data28 = 4.6 billion years{{Cite journal |last1=Bonanno |first1=A. |last2=Schlattl |first2=H. |last3=Paternò |first3=L. |year=2002 |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 |s2cid=119436299}}{{Cite journal |last1=Connelly |first1=JN |last2=Bizzarro |first2=M |last3=Krot |first3=AN |last4=Nordlund |first4=Å |last5=Wielandt |first5=D |last6=Ivanova |first6=MA |date=2 November 2012 |title=The Absolute Chronology and Thermal Processing of Solids in the Solar Protoplanetary Disk |journal=Science |volume=338 |issue=6107 |pages=651–655 |bibcode=2012Sci...338..651C |doi=10.1126/science.1226919 |pmid=23118187 |s2cid=21965292}}{{Registration required}} [81] => | label29 = {{longitem|Equatorial [[surface gravity]]}} [82] => | data29 = {{convert|274|m/s2|ft/s2|abbr=on|disp=br}}
28 × Earth [83] => | label30 = {{longitem|[[Moment of inertia factor]]}} [84] => | data30 = ≈{{val|0.070}} [85] => | label31 = {{longitem|Surface [[escape velocity]]}} [86] => | data31 = {{val|617.7|u=km/s}}
55 × Earth [87] => | label32 = Temperature [88] => | data32 = {{unbulleted list| [89] => |15,700,000 [[Kelvin|K]] (center) [90] => |5,772 K ([[photosphere]]) [91] => |5,000,000 K ([[Solar corona|corona]]) [92] => }} [93] => | label33 = [[Luminosity]] [94] => | data33 = {{unbulleted list| [95] => |{{val|3.828|e=26|ul=W}} [96] => |{{val|3.75|e=28|u=[[lumen (unit)|lm]]}} [97] => |{{val|98|u=lm/W}} [[Luminous efficacy|efficacy]] [98] => }} [99] => | label34 = [[Color index|Color]] (B-V) [100] => | data34 = 0.656David F. Gray (1992), ''[http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1992PASP..104.1035G&db_key=AST&data_type=HTML&format=&high=44b52c369023103 The Inferred Color Index of the Sun] {{Webarchive|url=https://web.archive.org/web/20170705114048/http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1992PASP..104.1035G&db_key=AST&data_type=HTML&format=&high=44b52c369023103 |date=5 July 2017 }}'', Publications of the Astronomical Society of the Pacific, vol. 104, no. 681, pp. 1035–1038 (November 1992). [101] => | label35 = Mean [[radiance]] [102] => | data35 = {{val|2.009|e=7|u=W·m⁻²·sr⁻¹}} [103] => | label37 = {{longitem|[[Photosphere]] composition by mass}} [104] => | data37 = {{unbulleted list| [105] => | 73.46% [[hydrogen]] [106] => | 24.85% [[helium]] [107] => | 0.77% [[oxygen]] [108] => | 0.29% [[carbon]] [109] => | 0.16% [[iron]] [110] => | 0.12% [[neon]] [111] => | 0.09% [[nitrogen]] [112] => | 0.07% [[silicon]] [113] => | 0.05% [[magnesium]] [114] => | 0.04% [[sulfur]]{{Cite web |title=The Sun's Vital Statistics |url=http://solar-center.stanford.edu/vitalstats.html |url-status=live |archive-url=https://www.webcitation.org/6BOkQXma3?url=http://solar-center.stanford.edu/vitalstats.html |archive-date=14 October 2012 |access-date=29 July 2008 |publisher=Stanford Solar Center}} 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 |access-date=12 July 2017 |archive-date=30 July 2021 |archive-url=https://web.archive.org/web/20210730024856/https://history.nasa.gov/SP-402/contents.htm |url-status=live }} [115] => }} [116] => }} [117] => [118] => The '''Sun''' is the [[star]] at the [[Barycenter (astronomy)|center]] of the [[Solar System]]. It is a massive, hot [[ball (mathematics)|ball]] of [[plasma (physics)|plasma]], inflated and heated by [[energy]] produced by [[nuclear fusion]] reactions at its core. Part of this energy is emitted from its [[Photosphere|surface]] as [[light|visible light]], [[ultraviolet]], and [[infrared]] radiation, providing most of the energy for [[life]] on [[Earth]]. The Sun has been an [[The Sun in culture|object of veneration]] in many cultures. It has been a central subject for astronomical research since [[Ancient history|antiquity]]. [119] => [120] => The Sun orbits the [[Galactic Center]] at a distance of 24,000 to 28,000 [[light-year]]s.{{cite journal |last1=Francis |first1=Charles |last2=Anderson |first2=Erik |date=June 2014 |title=Two estimates of the distance to the Galactic Centre |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=441 |issue=2 |pages=1105–1114 |arxiv=1309.2629 |bibcode=2014MNRAS.441.1105F |doi=10.1093/mnras/stu631 |s2cid=119235554}} From Earth, it is on average {{val|1|ul=AU}} ({{val|1.496|e=8|u=km}}) or about 8 [[light-minute]]s away. [[Solar radius|Its diameter]] is about {{val|1391400|u=km|fmt=commas}} ({{val|864600|u=mi|fmt=commas}}; {{val|4.64}} [[light-second|LS]]), 109 times that of Earth. [[solar mass|Its mass]] is about 330,000 times that of Earth, making up about 99.86% of the total mass of the Solar System.{{Cite journal |last=Woolfson |first=M. |date=2000 |title=The origin and evolution of the solar system |url=http://inis.jinr.ru/sl/vol1/_djvu/P_Physics/Woolfson%20M.M.%20Origin%20and%20evolution%20of%20the%20solar%20system%20(IOP)(425s).pdf |url-status=live |journal=[[Astronomy & Geophysics]] |volume=41 |issue=1 |page=12 |bibcode=2000A&G....41a..12W |doi=10.1046/j.1468-4004.2000.00012.x |archive-url=https://web.archive.org/web/20200711133403/http://inis.jinr.ru/sl/vol1/_djvu/P_Physics/Woolfson%20M.M.%20Origin%20and%20evolution%20of%20the%20solar%20system%20(IOP)(425s).pdf |archive-date=11 July 2020 |access-date=12 April 2020 |doi-access=free}} Roughly 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]].{{Cite journal |last1=Basu |first1=S. |last2=Antia |first2=H.M. |year=2008 |title=Helioseismology and Solar Abundances |journal=[[Physics Reports]] |volume=457 |issue=5–6 |pages=217–283 |arxiv=0711.4590 |bibcode=2008PhR...457..217B |doi=10.1016/j.physrep.2007.12.002 |s2cid=119302796}} [121] => [122] => The Sun is a [[G-type main-sequence star]] (G2V), informally called a [[G-type main-sequence star|yellow dwarf]], though its light is actually white. It formed approximately 4.6 billionAll numbers in this article are [[short scale]]. One billion is 10⁹, or 1,000,000,000.{{Cite journal |last1=Connelly |first1=James N. |last2=Bizzarro |first2=Martin |last3=Krot |first3=Alexander N. |last4=Nordlund |first4=Åke |last5=Wielandt |first5=Daniel |last6=Ivanova |first6=Marina A. |date=2 November 2012 |title=The Absolute Chronology and Thermal Processing of Solids in the Solar Protoplanetary Disk |journal=[[Science (journal)|Science]] |volume=338 |issue=6107 |pages=651–655 |bibcode=2012Sci...338..651C |doi=10.1126/science.1226919 |pmid=23118187 |s2cid=21965292}} 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]]. [123] => [124] => Every second, the Sun's core fuses about 600 billion [[kilograms]] (kg) of hydrogen into helium and converts 4 billion kg of [[mass–energy equivalence|matter into energy]]. Far in the future, when [[hydrogen fusion]] in the Sun's core diminishes to the point where the Sun is no longer in [[hydrostatic equilibrium]], its core will undergo a marked increase in density and temperature which will cause its outer layers to expand, eventually transforming the Sun into a [[red giant]]. This process will make the Sun large enough to render Earth uninhabitable approximately five billion years from the present. Subsequently, the Sun will shed its outer layers and become a dense type of cooling star (a [[white dwarf]]), and no longer produce energy by fusion, but it will still glow and give off heat from its previous fusion for trillions of years. After that it is theorized to become a super dense [[black dwarf]], giving off no more energy. [125] => [126] => == Etymology == [127] => The English word ''sun'' developed from [[Old English]] {{lang|ang|sunne}}. Cognates appear in other [[Germanic languages]], including [[West Frisian language|West Frisian]] {{lang|fy|sinne}}, [[Dutch language|Dutch]] {{lang|nl|zon}}, [[Low German]] {{lang|nds|Sünn}}, [[Standard German]] {{lang|de|Sonne}}, [[Bavarian language|Bavarian]] {{lang|bar|Sunna}}, [[Old Norse]] {{lang|non|sunna}}, and [[Gothic language|Gothic]] {{lang|got|sunnō}}. All these words stem from [[Proto-Germanic]] {{lang|gem-x-proto|*sunnōn}}.{{Cite book |last=Barnhart |first=R.K. |title=The Barnhart Concise Dictionary of Etymology |date=1995 |publisher=[[HarperCollins]] |isbn=978-0-06-270084-1 |page=776}}Vladimir Orel (2003) [https://archive.org/details/Orel-AHandbookOfGermanicEtymology/mode/2up/search/sun ''A Handbook of Germanic Etymology''], Brill This is ultimately related to the word for ''sun'' in other branches of the [[Indo-European language]] family, though in most cases a [[nominative]] stem with an ''l'' is found, rather than the [[genitive]] stem in ''n'', as for example in [[Latin]] {{lang|la|sōl}}, [[ancient Greek]] {{lang|grc|ἥλιος}} ({{transliteration|grc|hēlios}}), [[Welsh language|Welsh]] {{lang|cy|haul}} and [[Czech language|Czech]] {{lang|cs|slunce}}, as well as (with *l > ''r'') Sanskrit {{lang|sa|स्वर}} ({{transliteration|sa|svár}}) and [[Persian language|Persian]] {{lang|fa|خور}} ({{transliteration|fa|xvar}}). Indeed, the ''l''-stem survived in Proto-Germanic as well, as {{lang|gem-x-proto|*sōwelan}}, which gave rise to Gothic {{lang|got|sauil}} (alongside {{lang|got|sunnō}}) and Old Norse prosaic {{lang|non|sól}} (alongside poetic {{lang|non|sunna}}), and through it the words for ''sun'' in the modern Scandinavian languages: [[Swedish language|Swedish]] and [[Danish language|Danish]] {{lang|sv|sol}}, [[Icelandic language|Icelandic]] {{lang|is|sól}}, etc. [128] => [129] => The principal adjectives for the Sun in English are ''sunny'' for sunlight and, in technical contexts, ''solar'' ({{IPAc-en|ˈ|s|oʊ|l|ər}}),{{OED|solar}} from Latin {{lang|la|sol}}{{cite book |last1=Little |first1=William |title=Oxford Universal Dictionary on Historical Principles |last2=Fowler |first2=H.W. |last3=Coulson |first3=J. |year=1955 |edition=3rd |chapter=Sol |asin=B000QS3QVQ |chapter-url=https://archive.org/details/oxforduniversald07litt |chapter-url-access=registration}}—the latter found in terms such as ''solar day'', ''[[solar eclipse]]'' and ''Solar System''. From the Greek {{transliteration|grc|helios}} comes the rare adjective ''heliac'' ({{IPAc-en|ˈ|h|iː|l|i|æ|k}}).{{OED|heliac}} In English, the Greek and Latin words occur in poetry as personifications of the Sun, [[Helios]] ({{IPAc-en|ˈ|h|iː|l|i|ə|s}}) and [[Sol (Roman mythology)|Sol]] ({{IPAc-en|'|s|ɒ|l}}),{{Cite encyclopedia |title=Helios |encyclopedia=[[Lexico]] UK English Dictionary |publisher=[[Oxford University Press]] |url=http://www.lexico.com/definition/Helios |archive-url=https://web.archive.org/web/20200327234645/https://www.lexico.com/definition/helios |archive-date=27 March 2020 |url-status=dead}}{{OED|Sol}} while in science fiction ''Sol'' may be used to distinguish the Sun from other stars. The term ''[[Sol (day on Mars)|sol]]'' with a lower-case ''s'' is used by planetary astronomers for the duration of a [[solar day]] on another planet such as [[Mars]].{{Cite web |date=15 November 2006 |title=Opportunity's View, Sol 959 (Vertical) |url=http://www.nasa.gov/mission_pages/mer/images/pia01892.html |url-status=live |archive-url=https://web.archive.org/web/20121022155351/http://www.nasa.gov/mission_pages/mer/images/pia01892.html |archive-date=22 October 2012 |access-date=1 August 2007 |publisher=NASA}} [130] => [131] => The English [[Names of the days of the week|weekday name]] ''[[Sunday]]'' stems from Old English {{lang|ang|Sunnandæg}} "sun's day", a [[Interpretatio germanica|Germanic interpretation]] of the Latin phrase {{lang|la|[[Dies (deity)|diēs]] sōlis}}, itself a translation of the ancient Greek {{lang|grc|ἡμέρα ἡλίου}} ({{transliteration|grc|[[Hemera|hēmera]] hēliou}}) 'day of the sun'.{{cite book |last=Barnhart |first=R.K. |date=1995 |title=The Barnhart Concise Dictionary of Etymology |page=778 |publisher=[[HarperCollins]] |isbn=978-0-06-270084-1}} The [[astronomical symbol]] for the Sun is a circle with a center dot, [[File:Sun symbol (fixed width).svg|16px|☉]]. It is used for such units as ''M''_☉ ([[Solar mass]]), ''R''_☉ ([[Solar radius]]) and ''L''_☉ ([[Solar luminosity]]). [132] => [133] => == General characteristics == [134] => The Sun is a [[G-type main-sequence star]] that makes up 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.{{Cite news |last=Than |first=K. |date=2006 |title=Astronomers Had it Wrong: Most Stars are Single |publisher=Space.com |url=http://www.space.com/scienceastronomy/060130_mm_single_stars.html |url-status=live |access-date=1 August 2007 |archive-url=https://web.archive.org/web/20101221093125/http://www.space.com/scienceastronomy/060130_mm_single_stars.html |archive-date=21 December 2010}}{{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 |s2cid=8400400}} The Sun is a [[Population I stars|Population I]], or heavy-element-rich,{{efn|name=heavy elements}} star.{{Cite book |last1=Zeilik |first1=M.A. |title=Introductory Astronomy & Astrophysics |last2=Gregory |first2=S.A. |date=1998 |publisher=Saunders College Publishing |isbn=978-0-03-006228-5 |edition=4th |page=322}} Its formation may have been triggered by shockwaves from one or more nearby [[supernova]]e.{{Cite journal |last1=Falk |first1=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 |s2cid=4240932}} 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. [135] => [136] => The Sun is by far the [[List of brightest natural objects in the sky|brightest object in the Earth's sky]], with an [[apparent magnitude]] of −26.74.{{Cite journal |last=Burton |first=W.B. |date=1986 |title=Stellar parameters |journal=[[Space Science Reviews]] |volume=43 |issue=3–4 |pages=244–250 |doi=10.1007/BF00190626 |s2cid=189796439}}{{Cite journal |last1=Bessell |first1=M.S. |last2=Castelli |first2=F. |last3=Plez |first3=B. |date=1998 |title=Model atmospheres broad-band colors, bolometric corrections and temperature calibrations for O–M stars |journal=[[Astronomy and Astrophysics]] |volume=333 |pages=231–250 |bibcode=1998A&A...333..231B}} This is about 13 billion times brighter than the next brightest star, [[Sirius]], which has an apparent magnitude of −1.46. [137] => [138] => {{convert|1|AU|e6km e6mi|lk=in|spell=In|disp=x|abbr=off| (about |)}} is defined as the mean distance between the centres of the Sun and the Earth. The instantaneous distance varies by about ± 2.5 million km or 1.55 million miles as Earth moves from [[perihelion]] on ~ January 3rd to [[aphelion]] on ~ July 4th.{{Cite web |date=31 January 2008 |title=Equinoxes, Solstices, Perihelion, and Aphelion, 2000–2020 |url=http://aa.usno.navy.mil/data/docs/EarthSeasons.php |url-status=live |archive-url=https://web.archive.org/web/20071013000301/http://aa.usno.navy.mil/data/docs/EarthSeasons.php |archive-date=13 October 2007 |access-date=17 July 2009 |publisher=[[US Naval Observatory]]}} At its average distance, light travels from the Sun's horizon to Earth's horizon in about 8 minutes and 20 seconds,{{Cite web |last=Cain |first=Fraser |date=15 April 2013 |title=How long does it take sunlight to reach the Earth? |url=https://phys.org/news/2013-04-sunlight-earth.html |url-status=live |archive-url=https://web.archive.org/web/20220302095547/https://phys.org/news/2013-04-sunlight-earth.html |archive-date=2 March 2022 |access-date=2 March 2022 |website=phys.org |language=en}} 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[[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]]. on Earth by [[photosynthesis]],{{Cite book |last=Simon |first=A. |url=https://books.google.com/books?id=1gXImRmz7u8C&q=bacteria+that+live+with+out+the+sun&pg=PA26 |title=The Real Science Behind the X-Files : Microbes, meteorites, and mutants |date=2001 |publisher=[[Simon & Schuster]] |isbn=978-0-684-85618-6 |pages=25–27 |access-date=3 November 2020 |archive-url=https://web.archive.org/web/20210417061644/https://books.google.com/books?id=1gXImRmz7u8C&q=bacteria+that+live+with+out+the+sun&pg=PA26 |archive-date=17 April 2021 |url-status=live}} and drives [[Earth's climate]] and weather. [139] => [140] => The Sun does not have a definite boundary, but its density decreases exponentially with increasing height above the [[photosphere]].{{Cite book |last1=Beer |first1=J. |title=Cosmogenic Radionuclides: Theory and Applications in the Terrestrial and Space Environments |last2=McCracken |first2=K. |last3=von Steiger |first3=R. |date=2012 |publisher=[[Springer Science+Business Media]] |isbn=978-3-642-14651-0 |page=41}} For the purpose of measurement, 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.{{Cite book |last=Phillips |first=K.J.H. |title=Guide to the Sun |date=1995 |publisher=[[Cambridge University Press]] |isbn=978-0-521-39788-9 |page=73}} By this measure, the Sun is a near-perfect sphere with an [[oblateness]] estimated at 9 millionths,{{Cite journal |last1=Godier |first1=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 |url-status=dead |journal=[[Astronomy and Astrophysics]] |volume=355 |pages=365–374 |bibcode=2000A&A...355..365G |archive-url=https://web.archive.org/web/20110510022519/http://aa.springer.de/papers/0355001/2300365.pdf |archive-date=10 May 2011 |access-date=22 February 2006}}{{Cite news |date=2 October 2008 |title=How Round is the Sun? |publisher=NASA |url=https://science.nasa.gov/science-news/science-at-nasa/2008/02oct_oblatesun/ |url-status=live |access-date=7 March 2011 |archive-url=https://web.archive.org/web/20190329081811/https://science.nasa.gov/science-news/science-at-nasa/2008/02oct_oblatesun |archive-date=29 March 2019}}{{Cite news |date=6 February 2011 |title=First Ever STEREO Images of the Entire Sun |publisher=NASA |url=https://science.nasa.gov/science-news/science-at-nasa/2011/06feb_fullsun/ |url-status=live |access-date=7 March 2011 |archive-url=https://web.archive.org/web/20110308024941/http://science.nasa.gov/science-news/science-at-nasa/2011/06feb_fullsun/ |archive-date=8 March 2011}} which means that its polar diameter differs from its equatorial diameter by only {{convert|10|km|mi|sp=us}}.{{Cite web |last=Jones |first=G. |date=16 August 2012 |title=Sun is the most perfect sphere ever observed in nature |url=https://www.theguardian.com/science/2012/aug/16/sun-perfect-sphere-nature |url-status=live |archive-url=https://web.archive.org/web/20140303022045/http://www.theguardian.com/science/2012/aug/16/sun-perfect-sphere-nature |archive-date=3 March 2014 |access-date=19 August 2013 |website=[[The Guardian]]}} The tidal effect of the planets is weak and does not significantly affect the shape of the Sun.{{Cite book |last=Schutz |first=B.F. |title=Gravity from the ground up |date=2003 |publisher=[[Cambridge University Press]] |isbn=978-0-521-45506-0 |pages=98–99}} 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.{{Cite book |last=Phillips |first=K.J.H. |title=Guide to the Sun |date=1995 |publisher=[[Cambridge University Press]] |isbn=978-0-521-39788-9 |pages=78–79}} Viewed from a vantage point above its north pole, the Sun rotates [[counterclockwise]] around its axis of spin.{{efn|name=rotation}}{{Cite web |title=The Anticlockwise Solar System |url=https://www.spaceacademy.net.au/library/notes/anticlok.htm |url-status=live |archive-url=https://web.archive.org/web/20200807081832/https://www.spaceacademy.net.au/library/notes/anticlok.htm |archive-date=7 August 2020 |access-date=2 July 2020 |publisher=Australian Space Academy}} [141] => [142] => == Composition == [143] => {{See also|Molecules in stars}} [144] => The Sun consists mainly of the elements [[hydrogen]] and [[helium]]. At this time in the Sun's life, they account for 74.9% and 23.8%, respectively, of the mass of the Sun in the photosphere.{{cite journal |doi=10.1086/375492 |last=Lodders |first=Katharina|author-link=Katharina Lodders |date=10 July 2003 |title=Solar System Abundances and Condensation Temperatures of the Elements |journal=The Astrophysical Journal |volume=591 |issue=2 |pages=1220–1247 |url=http://weft.astro.washington.edu/courses/astro557/LODDERS.pdf |bibcode=2003ApJ...591.1220L |access-date=1 September 2015 |archive-url=https://web.archive.org/web/20151107043527/http://weft.astro.washington.edu/courses/astro557/LODDERS.pdf |archive-date=7 November 2015 |url-status=dead |citeseerx=10.1.1.666.9351 |s2cid=42498829 }}
{{Cite journal |last=Lodders |first=K. |author-link=Katharina Lodders|title=Abundances and Condensation Temperatures of the Elements |url=http://www.lpi.usra.edu/meetings/metsoc2003/pdf/5272.pdf |journal=[[Meteoritics & Planetary Science]] |volume=38 |issue=suppl |page=5272 |date=2003 |bibcode=2003M&PSA..38.5272L |access-date=3 August 2008 |archive-date=13 May 2011 |archive-url=https://web.archive.org/web/20110513163004/http://www.lpi.usra.edu/meetings/metsoc2003/pdf/5272.pdf |url-status=live }} 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.{{Cite book |last1=Hansen |first1=C.J. |last2=Kawaler |first2=S.A. |last3=Trimble |first3=V. |title=Stellar Interiors: Physical Principles, Structure, and Evolution |pages=19–20 |edition=2nd |publisher=[[Springer Science+Business Media|Springer]] |date=2004 |isbn=978-0-387-20089-7}} [145] => [146] => In solar research it is more common to express the abundance of each element in dex, which is a scaled logarithmic unit.

\mathrm{A}(e) = 12+\log_{10}(\frac{ne}{n\mathrm{H}})

; 'e' is the element in question and nH is number of hydrogen atoms. By definition hydrogen has an abundance of 12, the helium abundance varies between about 10.3 and 10.5 depending on the phase of the solar cycle;{{Cite journal |last1=Alterman |first1=Benjamin L. |last2=Kasper |first2=Justin C. |last3=Leamon |first3=Robert J. |last4=McIntosh |first4=Scott W. |date=April 2021 |title=Solar wind helium abundance heralds solar cycle onset |journal=Solar Physics |volume=296 |issue=4 |pages=67 |arxiv=2006.04669 |bibcode=2021SoPh..296...67A |doi=10.1007/s11207-021-01801-9 |s2cid=233738140}} carbon is 8.47, neon is 8.29, oxygen is 7.69{{Cite journal |last1=Pietrow |first1=A. G. M. |last2=Hoppe |first2=R. |last3=Bergemann |first3=M. |last4=Calvo |first4=F. |year=2023 |title=Solar oxygen abundance using SST/CRISP center-to-limb observations of the O I 7772 Å line |journal=Astronomy & Astrophysics |volume=672 |issue=4 |pages=L6 |arxiv=2304.01048 |bibcode=2023A&A...672L...6P |doi=10.1051/0004-6361/202346387 |s2cid=257912497}} and iron is 7.62. [147] => [148] => The Sun's original chemical composition was inherited from the [[interstellar medium]] out of which it formed. Originally it would have been about 71.1% hydrogen, 27.4% helium, and 1.5% heavier elements. The hydrogen and most of the helium in the Sun would have been produced by [[Big Bang nucleosynthesis]] in the first 20 minutes of the universe, and the heavier elements were [[stellar nucleosynthesis|produced by previous generations of stars]] before the Sun was formed, and spread into the interstellar medium during the [[stellar evolution|final stages of stellar life]] and by events such as [[supernova]]e.{{Cite book |last1=Hansen |first1=C.J. |title=Stellar Interiors: Physical Principles, Structure, and Evolution |last2=Kawaler |first2=S.A. |last3=Trimble |first3=V. |date=2004 |publisher=[[Springer Science+Business Media|Springer]] |isbn=978-0-387-20089-7 |edition=2nd |pages=77–78}} [149] => [150] => Since the Sun formed, the main fusion process has involved fusing hydrogen into helium. Over the past 4.6 billion years, the amount of helium and its location within the Sun has gradually changed. Within the core, the proportion of helium has increased from about 24% to about 60% due to fusion, and some of the helium and heavy elements have settled from the photosphere toward the center of the Sun because of [[gravity]]. The proportions of heavier elements are unchanged. [[Heat transfer|Heat is transferred]] outward from the Sun's core by radiation rather than by convection (see [[#Radiative zone|Radiative zone]] below), so the fusion products are not lifted outward by heat; they remain in the core{{Cite book |last1=Hansen |first1=C.J. |last2=Kawaler |first2=S.A. |last3=Trimble |first3=V. |title=Stellar Interiors: Physical Principles, Structure, and Evolution |pages=§ 9.2.3 |no-pp=yes |edition=2nd |publisher=[[Springer Science+Business Media|Springer]] |date=2004 |isbn=978-0-387-20089-7}} and gradually an inner core of helium has begun to form that cannot be fused because presently the Sun's core is not hot or dense enough to fuse helium. In the current photosphere, the helium fraction is reduced, and the [[metallicity]] is only 84% of what it was in the [[Protostar|protostellar]] phase (before nuclear fusion in the core started). In the future, helium will continue to accumulate in the core, and in about 5 billion years this gradual build-up will eventually cause the Sun to exit the [[main sequence]] and become a [[red giant]].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) [151] => [152] => The chemical composition of the photosphere is normally considered representative of the composition of the primordial Solar System.{{Cite journal |last=Aller |first=L.H. |title=The chemical composition of the Sun and the solar system |journal=Proceedings of the Astronomical Society of Australia |volume=1 |issue=4 |page=133 |date=1968 |bibcode=1968PASA....1..133A|doi=10.1017/S1323358000011048|s2cid=119759834 |doi-access=free }} 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 the settling of heavy elements. The two methods generally agree well. [153] => [154] => == Structure and fusion == [155] => {{Main|Standard solar model}} [156] => [[File:Sun poster.svg|thumb|upright=2.25|Illustration of the Sun's structure, in false color for contrast]] [157] => [158] => === Core === [159] => {{Main|Solar core}} [160] => The core of the Sun extends from the center to about 20–25% of the solar radius.{{Cite journal |last=García |first=R. |date=2007 |title=Tracking solar gravity modes: the dynamics of the solar core |journal=[[Science (journal)|Science]] |volume=316 |issue=5831 |pages=1591–1593 |bibcode=2007Sci...316.1591G |doi=10.1126/science.1140598 |pmid=17478682 |s2cid=35285705|display-authors=etal }} It has a density of up to {{val|150|u=g|up=cm3}}{{Cite journal |last1=Basu |first1=S. |display-authors=etal |year=2009 |title=Fresh insights on the structure of the solar core |journal=[[The Astrophysical Journal]] |volume=699 |issue=2 |pages=1403–1417 |arxiv=0905.0651 |bibcode=2009ApJ...699.1403B |doi=10.1088/0004-637X/699/2/1403|s2cid=11044272 }}{{Cite web |date=18 January 2007 |title=NASA/Marshall Solar Physics |url=http://solarscience.msfc.nasa.gov/interior.shtml |url-status=live |archive-url=https://web.archive.org/web/20190329081742/https://solarscience.msfc.nasa.gov/interior.shtml |archive-date=29 March 2019 |access-date=11 July 2009 |publisher=[[Marshall Space Flight Center]]}} (about 150 times the density of water) and a temperature of close to 15.7 million [[kelvin]] (K). By contrast, the Sun's surface temperature is about {{val|5,800|u=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. Through most of the Sun's life, energy has been produced by nuclear fusion in the core region through the [[proton–proton chain]]; this process converts hydrogen into helium.{{Cite conference |last=Broggini |first=C. |date=2003 |title=Physics in Collision, Proceedings of the XXIII International Conference: Nuclear Processes at Solar Energy |url=http://www.slac.stanford.edu/econf/C030626 |conference=XXIII Physics in Collisions Conference |location=Zeuthen, Germany |page=21 |arxiv=astro-ph/0308537 |bibcode=2003phco.conf...21B |archive-url=https://web.archive.org/web/20170421113407/http://www.slac.stanford.edu/econf/C030626/ |archive-date=21 April 2017 |access-date=12 August 2013 |url-status=live}} Currently, only 0.8% of the energy generated in the Sun comes from another sequence of fusion reactions called the [[CNO cycle]], though this proportion is expected to increase as the Sun becomes older and more luminous.{{Cite journal |last1=Goupil |first1=M.J. |last2=Lebreton |first2=Y. |last3=Marques |first3=J.P. |last4=Samadi |first4=R. |last5=Baudin |first5=F. |date=2011 |title=Open issues in probing interiors of solar-like oscillating main sequence stars 1. From the Sun to nearly suns |journal=[[Journal of Physics: Conference Series]] |volume=271 |issue=1 |page=012031 |arxiv=1102.0247 |bibcode=2011JPhCS.271a2031G |doi=10.1088/1742-6596/271/1/012031|s2cid=4776237 }}{{Cite journal |last=The Borexino Collaboration |date=2020 |title=Experimental evidence of neutrinos produced in the CNO fusion cycle in the Sun |url=https://www.nature.com/articles/s41586-020-2934-0 |journal=[[Nature (journal)|Nature]] |volume=587 |issue=? |pages=577–582 |arxiv=2006.15115 |bibcode=2020Natur.587..577B |doi=10.1038/s41586-020-2934-0 |pmid=33239797 |s2cid=227174644 |access-date=26 November 2020 |archive-date=27 November 2020 |archive-url=https://web.archive.org/web/20201127093809/https://www.nature.com/articles/s41586-020-2934-0 |url-status=live }} [161] => [162] => The core is the only region of 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 rest of the Sun is heated by this energy as it is transferred outward through many successive layers, finally to the solar photosphere where it escapes into space through radiation (photons) or advection (massive particles).{{Cite book |last=Phillips |first=K.J.H. |title=Guide to the Sun |date=1995 |publisher=[[Cambridge University Press]] |isbn=978-0-521-39788-9 |pages=47–53}}{{Cite book |last=Zirker |first=J.B. |date=2002 |title=Journey from the Center of the Sun |pages=[https://archive.org/details/journeyfromcente0000zirk/page/15 15–34] |publisher=[[Princeton University Press]] |isbn=978-0-691-05781-1 |url=https://archive.org/details/journeyfromcente0000zirk/page/15 }} [163] => [[File:Proton-proton reaction chain.svg|thumb|Illustration of a proton-proton reaction chain, from hydrogen forming [[deuterium]], [[helium-3]], and regular [[helium-4]]|left]] [164] => The proton–proton chain occurs around {{val|9.2|e=37}} times each second in the core, converting about 3.7{{e|38}} protons into [[alpha particle]]s (helium nuclei) every second (out of a total of ~8.9{{e|56}} free protons in the Sun), or about {{val|6.2|e=11|u=kg|up=s}}. However, each proton (on average) takes around 9 billion years to fuse with another using the PP chain. Fusing four free [[proton]]s (hydrogen nuclei) into a single alpha particle (helium nucleus) releases around 0.7% of the fused mass as energy,{{Cite book |last=Shu |first=F.H. |url=https://archive.org/details/physicaluniverse00shuf/page/102 |title=The Physical Universe: An Introduction to Astronomy |date=1982 |publisher=University Science Books |isbn=978-0-935702-05-7 |page=[https://archive.org/details/physicaluniverse00shuf/page/102 102]}} so the Sun releases energy at the mass–energy conversion rate of 4.26 billion kg/s (which requires 600 billion kg of hydrogen{{Cite web |date=2012 |title=Ask Us: Sun |url=https://helios.gsfc.nasa.gov/qa_sun.html |url-status=dead |archive-url=https://web.archive.org/web/20180903223810/https://helios.gsfc.nasa.gov/qa_sun.html |archive-date=3 September 2018 |access-date=13 July 2017 |website=Cosmicopia |publisher=NASA}}), for 384.6 [[Yotta-|yottawatts]] ({{val|3.846|e=26|u=W}}), or 9.192{{e|10}} [[TNT equivalent|megatons of TNT]] per second. The large power output of the Sun is mainly due to the huge size and density of its core (compared to Earth and objects on Earth), with only a fairly small amount of power being generated per [[cubic metre]]. Theoretical models of the Sun's interior indicate a maximum power density, or energy production, of approximately 276.5 [[watt]]s per cubic metre at the center of the core,{{Cite web |last=Cohen |first=H. |date=9 November 1998 |title=Table of temperatures, power densities, luminosities by radius in the Sun |url=http://fusedweb.llnl.gov/CPEP/Chart_Pages/5.Plasmas/Sunlayers.html |archive-url=http://webarchive.loc.gov/all/20011129122524/http%3A//fusedweb%2Ellnl%2Egov/cpep/chart_pages/5%2Eplasmas/sunlayers%2Ehtml |archive-date=29 November 2001 |access-date=30 August 2011 |publisher=Contemporary Physics Education Project}} which, according to [[Karl Kruszelnicki]], is about the same power density inside a [[compost pile]].{{Cite web |date=17 April 2012 |title=Lazy Sun is less energetic than compost |url=http://www.abc.net.au/science/articles/2012/04/17/3478276.htm |url-status=live |archive-url=https://web.archive.org/web/20140306123113/http://www.abc.net.au/science/articles/2012/04/17/3478276.htm |archive-date=6 March 2014 |access-date=25 February 2014 |website=[[Australian Broadcasting Corporation]]}} [165] => [166] => 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.{{Cite journal |last1=Haubold |first1=H.J. |last2=Mathai |first2=A.M. |date=1994 |title=Solar Nuclear Energy Generation & The Chlorine Solar Neutrino Experiment |volume=320 |issue=1994 |pages=102–116 |journal=[[AIP Conference Proceedings]] |arxiv=astro-ph/9405040 |bibcode=1995AIPC..320..102H |doi=10.1063/1.47009|citeseerx=10.1.1.254.6033|s2cid=14622069 }}{{Cite web |last=Myers |first=S.T. |date=18 February 1999 |title=Lecture 11 – Stellar Structure I: Hydrostatic Equilibrium |url=http://www.aoc.nrao.edu/~smyers/courses/astro12/L11.html |url-status=live |archive-url=https://web.archive.org/web/20110512180052/http://www.aoc.nrao.edu/~smyers/courses/astro12/L11.html |archive-date=12 May 2011 |access-date=15 July 2009 |website=Introduction to Astrophysics II}} [167] => [168] => === Radiative zone === [169] => {{Main|Radiative zone}} [170] => [[File:Heat Transfer in Stars.svg|thumb|300x300px|Illustration of different stars' internal structure. The Sun in the middle has an inner radiating zone and an outer convective zone.]] [171] => The radiative zone is the thickest layer of the Sun, at 0.45 solar radii. From the core out to about 0.7 [[Solar radius|solar radii]], [[thermal radiation]] is the primary means of energy transfer.{{cite web |url=http://mynasa.nasa.gov/worldbook/sun_worldbook.html |publisher=NASA |title=Sun |website=World Book at NASA |access-date=10 October 2012 |url-status=dead |archive-url=https://web.archive.org/web/20130510142009/http://mynasa.nasa.gov/worldbook/sun_worldbook.html |archive-date=10 May 2013}} The temperature drops from approximately 7 million to 2 million kelvins with increasing distance from the core. 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. [[Ions]] of hydrogen and helium emit photons, which travel only a brief distance before being reabsorbed by other ions. The density drops a hundredfold (from 20 000 kg/m³ to 200 kg/m³) between 0.25 solar radii and 0.7 radii, the top of the radiative zone. [172] => [173] => === Tachocline === [174] => {{Main|Tachocline}} [175] => [176] => 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.{{Cite book |last=Tobias |first=S.M. |title=Fluid Dynamics and Dynamos in Astrophysics and Geophysics |date=2005 |publisher=[[CRC Press]] |isbn=978-0-8493-3355-2 |editor-last=A.M. Soward |pages=193–235 |chapter=The solar tachocline: Formation, stability and its role in the solar dynamo |access-date=22 August 2020 |display-editors=etal |chapter-url=https://books.google.com/books?id=PLNwoJ6qFoEC&pg=PA193 |archive-url=https://web.archive.org/web/20201029102001/https://books.google.com/books?id=PLNwoJ6qFoEC&pg=PA193 |archive-date=29 October 2020 |url-status=live}} Presently, it is hypothesized (see [[Solar dynamo]]) that a magnetic dynamo within this layer generates the Sun's [[magnetic field]]. [177] => [178] => === Convective zone === [179] => {{Main|Convection zone}} [180] => The Sun's convection zone extends from 0.7 solar radii (500,000 km) to near the surface. In this layer, the solar plasma is not dense 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 cells that carry most 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 {{convert|5,700|K|C F}} (350-fold) and the density to only 0.2 g/m³ (about 1/10,000 the density of air at sea level, and 1 millionth that of the inner layer of the convective zone). [181] => [182] => The thermal columns of the convection zone form an imprint on the surface of the Sun giving it a granular appearance called the [[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. The Sun's thermal columns are [[Bénard cells]] and take the shape of roughly hexagonal prisms.{{Cite book |last=Mullan |first=D.J |title=From the Sun to the Great Attractor |date=2000 |publisher=[[Springer Science+Business Media|Springer]] |isbn=978-3-540-41064-5 |editor-last=Page, D. |page=22 |chapter=Solar Physics: From the Deep Interior to the Hot Corona |access-date=22 August 2020 |editor-last2=Hirsch, J.G. |chapter-url=https://books.google.com/books?id=rk5fxs55_OkC&pg=PA22 |archive-url=https://web.archive.org/web/20210417080656/https://books.google.com/books?id=rk5fxs55_OkC&pg=PA22 |archive-date=17 April 2021 |url-status=live}} [183] => [184] => === Photosphere === [185] => {{Main|Photosphere}} [186] => [[File:Highest resolution photo of Sun (NSF) as of January 20, 2020.jpg|thumb|alt=A miasma of plasma|High-resolution image of the Sun's surface taken by the [[Daniel K. Inouye Solar Telescope]] (DKIST)]] [187] => [188] => The visible surface of the Sun, the photosphere, is the layer below which the Sun becomes [[opacity (optics)|opaque]] to visible light. Photons produced in this layer escape the Sun through the transparent solar atmosphere above it and become solar radiation, sunlight. The change in opacity is due to the decreasing amount of [[Hydrogen anion|H⁻ ions]], which absorb visible light easily. Conversely, the visible light we see is produced as electrons react with hydrogen atoms to produce H⁻ ions.{{Cite book |last=Gibson |first=Edward G. |date=1973 |title=The Quiet Sun (NASA SP-303) |publisher=NASA |asin=B0006C7RS0}}{{Cite book |last=Shu |first=F.H. |title=The Physics of Astrophysics |volume=1 |publisher=University Science Books |date=1991 |isbn=978-0-935702-64-4}} [189] => [190] => 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. The spectrum of sunlight has approximately the spectrum of a [[black-body]] radiating at {{convert|5,777|K|C F}}, interspersed with atomic [[absorption line]]s from the tenuous layers above the photosphere. The photosphere has a particle density of ~10²³ m⁻³ (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.{{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|doi-access=free }} [191] => [192] => 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.{{Cite web |last=Parnel |first=C. |title=Discovery of Helium |url=http://www-solar.mcs.st-andrews.ac.uk/~clare/Lockyer/helium.html |url-status=live |archive-url=https://web.archive.org/web/20151107043457/http://www-solar.mcs.st-andrews.ac.uk/~clare/Lockyer/helium.html |archive-date=7 November 2015 |access-date=22 March 2006 |publisher=University of St Andrews}} [193] => [194] => === Atmosphere === [195] => {{Main|Stellar atmosphere}} [196] => [197] => The Sun's atmosphere is composed of four parts: the photosphere (visible under normal conditions), the [[chromosphere]], the [[Solar transition region|transition region]], the [[Stellar corona|corona]] and the [[heliosphere]]. During a total solar eclipse, the photosphere is blocked, making the corona visible.{{Cite web |url= https://www.sciencedaily.com/releases/1999/08/990805111308.htm |title= "Beyond the Blue Horizon" – A Total Solar Eclipse Chase |date= 1999-08-05 |access-date= 2022-01-16 |archive-date= 2 July 2018 |archive-url= https://web.archive.org/web/20180702175721/https://www.sciencedaily.com/releases/1999/08/990805111308.htm |url-status= live }} [198] => [199] => 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}}.{{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 |url-status=live |journal=[[Bulletin of the Astronomical Society of India]] |volume=5 |pages=40–44 |bibcode=1977BASI....5...40A |archive-url=https://web.archive.org/web/20200512151641/http://prints.iiap.res.in/handle/2248/510 |archive-date=12 May 2020 |access-date=12 July 2009}} This part of the Sun is cool enough to allow for the existence of simple molecules such as [[carbon monoxide]] and water, which can be detected via their absorption spectra.{{Cite journal |last1=Solanki |first1=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 |s2cid=27696504 }} The chromosphere, transition region, and corona are much hotter than the surface of the Sun. The reason is not well understood, but evidence suggests that [[Alfvén wave]]s may have enough energy to heat the corona.{{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=1574–1577 |bibcode=2007Sci...318.1574D |doi=10.1126/science.1151747 |pmid=18063784 |s2cid=33655095|display-authors=etal}} [200] => [201] => [[File:171879main LimbFlareJan12 lg.jpg|thumb|The Sun's transition region taken by [[Hinode (satellite)|Hinode]]'s Solar Optical Telescope|left]] [202] => [203] => 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. 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 eclipses. The temperature of the chromosphere increases gradually with altitude, ranging up to around {{val|20000|u=K|fmt=commas}} near the top. In the upper part of the chromosphere helium becomes partially [[ionization|ionized]].{{Cite journal |last1=Hansteen |first1=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|doi-access=free }} [204] => [205] => Above the chromosphere, in a thin (about {{val|200|u=km}}) transition region, the temperature rises rapidly from around {{val|20000|u=K|fmt=commas}} in the upper chromosphere to coronal temperatures closer to {{val|1000000|u=K|fmt=commas}}.{{Cite journal |last1=Erdèlyi |first1=R. |last2=Ballai |first2=I. |date=2007 |title=Heating of the solar and stellar coronae: a review |journal=Astron. Nachr. |volume=328 |issue=8 |pages=726–733 |bibcode=2007AN....328..726E |doi=10.1002/asna.200710803 |doi-access=free}} The temperature increase is facilitated by the full ionization of helium in the transition region, which significantly reduces radiative cooling of the plasma. 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 [[Solar spicule|spicules]] and [[Solar filament|filaments]], and is in constant, chaotic motion. 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]].{{Cite journal |last=Dwivedi |first=B.N. |date=2006 |title=Our ultraviolet Sun |url=http://www.iisc.ernet.in/currsci/sep102006/587.pdf |url-status=live |journal=[[Current Science]] |volume=91 |issue=5 |pages=587–595 |archive-url=https://web.archive.org/web/20201025001339/http://www.iisc.ernet.in/currsci/sep102006/587.pdf |archive-date=25 October 2020 |access-date=22 March 2015}}[[File:2017 Total Solar Eclipse (35909952653).jpg|thumb|During a total [[solar eclipse]], the solar corona can be seen with the naked eye, during the brief period of totality.]] [206] => [207] => The corona is the next layer of the Sun. The low corona, near the surface of the Sun, has a particle density around 10¹⁵ m⁻³ to 10¹⁶ m⁻³.{{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. 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]].{{Cite book |last=Russell |first=C.T. |title=Space Weather (Geophysical Monograph) |date=2001 |publisher=[[American Geophysical Union]] |isbn=978-0-87590-984-4 |editor-last=Song, Paul |pages=73–88 |chapter=Solar wind and interplanetary magnetic filed: A tutorial |access-date=11 July 2009 |editor-last2=Singer, Howard J. |editor-last3=Siscoe, George L. |editor-link3=George Siscoe |chapter-url=http://www-ssc.igpp.ucla.edu/personnel/russell/papers/SolWindTutorial.pdf |archive-url=https://web.archive.org/web/20181001131951/http://www-ssc.igpp.ucla.edu/personnel/russell/papers/SolWindTutorial.pdf |archive-date=1 October 2018 |url-status=live}} [208] => 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 [[Outer space|interplanetary space]] is the [[solar wind]]. [209] => [210] => The heliosphere, the tenuous outermost atmosphere of the Sun, is filled with 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 waves,{{Cite book |first1=Emslie |last1=A.G |first2=Miller |last2=J.A. |date=2003 |chapter=Particle Acceleration |chapter-url=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}} at approximately 20 solar radii ({{val|0.1|u=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,{{Cite web |date=22 April 2003 |title=A Star with two North Poles |url=https://science.nasa.gov/headlines/y2003/22apr_currentsheet.htm |url-status=dead |archive-url=https://web.archive.org/web/20090718014855/https://science.nasa.gov/headlines/y2003/22apr_currentsheet.htm |archive-date=18 July 2009 |website=Science @ NASA |publisher=NASA}}{{Cite journal |last1=Riley |first1=P. |last2=Linker |first2=J.A. |last3=Mikić |first3=Z. |date=2002 |title=Modeling the heliospheric current sheet: Solar cycle variations |journal=[[Journal of Geophysical Research]] |volume=107 |issue=A7 |pages=SSH 8–1 |bibcode=2002JGRA..107.1136R |doi=10.1029/2001JA000299 |id=CiteID 1136 |doi-access=free}} forming the solar magnetic field into a [[Parker spiral|spiral]] shape, until it impacts the [[Heliopause (astronomy)|heliopause]] more than {{val|50|u=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.{{Cite press release |title=The Distortion of the Heliosphere: Our Interstellar Magnetic Compass |date=2005 |publisher=[[European Space Agency]] |url=http://www.spaceref.com/news/viewpr.html?pid=16394 |access-date=22 March 2006 |url-status=live |archive-url=https://archive.today/20120604110953/http://www.spaceref.com/news/viewpr.html?pid=16394 |archive-date=4 June 2012}} 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]],{{Cite book |last=Anderson, Rupert W. |url=https://books.google.com/books?id=JxauCQAAQBAJ&pg=PA163 |title=The Cosmic Compendium: Interstellar Travel |publisher=Lulu.com |year=2015 |isbn=978-1-329-02202-7 |pages=163–164}} and indeed did so on August 25, 2012, at approximately 122 astronomical units (18 Tm) from the Sun.{{Cite web|url=https://voyager.jpl.nasa.gov/mission/interstellar-mission/#:~:text=On%20Aug.,billion%20kilometers)%20from%20the%20sun.|title=Voyager – the Interstellar Mission|access-date=14 May 2021|archive-date=14 September 2017|archive-url=https://web.archive.org/web/20170914060928/https://voyager.jpl.nasa.gov/mission/interstellar-mission/#:~:text=On%20Aug.,billion%20kilometers)%20from%20the%20sun.|url-status=live}} The heliosphere has a [[Heliosphere#Heliotail|heliotail]] which stretches out behind it due to the Sun's movement.{{cite web |last1=Dunbar |first1=Brian |title=Components of the Heliosphere |url=https://www.nasa.gov/mission_pages/sunearth/science/heliosphere-components.html |website=NASA |date=2 March 2015 |access-date=20 March 2021 |archive-date=8 August 2021 |archive-url=https://web.archive.org/web/20210808183941/https://www.nasa.gov/mission_pages/sunearth/science/heliosphere-components.html |url-status=live }} [211] => [212] => On April 28, 2021, during its eighth flyby of the Sun, NASA's [[Parker Solar Probe]] encountered the specific magnetic and particle conditions at 18.8 solar radii that indicated that it penetrated the [[Alfvén surface]], the boundary separating the corona from the solar wind defined as where the coronal plasma's Alfvén speed and the large-scale solar wind speed are equal.{{cite web |last1=Hatfield |first1=Miles |title=NASA Enters the Solar Atmosphere for the First Time |url=https://www.nasa.gov/feature/goddard/2021/nasa-enters-the-solar-atmosphere-for-the-first-time-bringing-new-discoveries |website=NASA |date=13 December 2021 |access-date=30 July 2022 |archive-date=27 December 2021 |archive-url=https://web.archive.org/web/20211227093247/https://www.nasa.gov/feature/goddard/2021/nasa-enters-the-solar-atmosphere-for-the-first-time-bringing-new-discoveries/ |url-status=live }}{{PD-notice}}{{cite web |title=GMS: Animation: NASA's Parker Solar Probe Enters Solar Atmosphere |url=https://svs.gsfc.nasa.gov/14036 |website=svs.gsfc.nasa.gov |access-date=30 July 2022 |language=en |date=14 December 2021 |archive-date=4 October 2022 |archive-url=https://web.archive.org/web/20221004004943/https://svs.gsfc.nasa.gov/14036 |url-status=live }} The probe measured the solar wind plasma environment with its FIELDS and SWEAP instruments.{{cite web |title=SVS: Parker Solar Probe: Crossing the Alfven Surface |url=https://svs.gsfc.nasa.gov/4958 |website=svs.gsfc.nasa.gov |access-date=30 July 2022 |language=en |date=14 December 2021 |archive-date=8 August 2022 |archive-url=https://web.archive.org/web/20220808233611/https://svs.gsfc.nasa.gov/4958 |url-status=live }}{{PD-notice}} This event was described by NASA as "touching the Sun". During the flyby, Parker Solar Probe passed into and out of the corona several times. This proved the predictions that the Alfvén critical surface is not shaped like a smooth ball, but has spikes and valleys that wrinkle its surface. [213] => [214] => === Sunlight and neutrinos === [215] => {{Main|Sunlight|Solar irradiance}} [216] => [[File:Sun in fog in Lysekil.jpg|right|thumb|The Sun seen through a light fog]] [217] => The Sun emits light across the [[visible spectrum]], so its 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. The Solar radiance per wavelength peaks in the green portion of the spectrum when viewed from space.{{cite news |title=What Color is the Sun? |work=Universe Today |url=http://www.universetoday.com/18689/color-of-the-sun/ |url-status=live |access-date=23 May 2016 |archive-url=https://web.archive.org/web/20160525215525/http://www.universetoday.com/18689/color-of-the-sun/ |archive-date=25 May 2016}}{{cite web |title=What Color is the Sun? |url=http://solar-center.stanford.edu/SID/activities/GreenSun.html |url-status=live |archive-url=https://web.archive.org/web/20171030154449/http://solar-center.stanford.edu/SID/activities/GreenSun.html |archive-date=30 October 2017 |access-date=23 May 2016 |publisher=[[Stanford University|Stanford]] Solar Center}} When the Sun is very low in the sky, [[Diffuse sky radiation|atmospheric scattering]] renders the Sun yellow, red, orange, or magenta, and in rare occasions even [[Green flash|green or blue]]. Despite its typical whiteness (white sunrays, white ambient light, white illumination of the Moon, etc.), some cultures mentally picture the Sun as yellow and some even red; the reasons for this are cultural and exact ones are the subject of debate.{{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 |url-status=dead |journal=[[Optics & Photonics News]] |pages=12–13 |archive-url=https://web.archive.org/web/20120618183229/http://www.osa-opn.org/Content/ViewFile.aspx?id=11147 |archive-date=18 June 2012}} The Sun is a [[G-type main-sequence star|G2V]] star, with ''G2'' indicating its [[effective temperature|surface temperature]] of approximately {{Convert|5,778|K|C F}}, and ''V'' that it, like most stars, is a [[main sequence|main-sequence]] star.{{cite news |author=Karl S. Kruszelnicki |date=17 April 2012 |title=Dr Karl's Great Moments In Science: Lazy Sun is less energetic than compost |newspaper=[[Australian Broadcasting Corporation]] |url=http://www.abc.net.au/science/articles/2012/04/17/3478276.htm |url-status=live |access-date=25 February 2014 |archive-url=https://web.archive.org/web/20140306123113/http://www.abc.net.au/science/articles/2012/04/17/3478276.htm |archive-date=6 March 2014 |quote=Every second, the Sun burns 620 million tonnes of hydrogen...}} [218] => [219] => 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, at or near Earth's orbit).{{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 |url-status=dead |archive-url=https://web.archive.org/web/20110801183920/http://www.pmodwrc.ch/pmod.php?topic=tsi%2Fcomposite%2FSolarConstant |archive-date=1 August 2011 |access-date=5 October 2005}} Sunlight on the surface of Earth is [[attenuation (electromagnetic radiation)|attenuated]] by [[Atmosphere of Earth|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]].{{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}} 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.{{Cite web |title=Solar radiation |url=http://curry.eas.gatech.edu/Courses/6140/ency/Chapter3/Ency_Atmos/Radiation_Solar.pdf |url-status=live |archive-url=https://web.archive.org/web/20121101070344/http://curry.eas.gatech.edu/Courses/6140/ency/Chapter3/Ency_Atmos/Radiation_Solar.pdf |archive-date=1 November 2012 |access-date=29 December 2012}} The atmosphere in particular filters out over 70% of solar ultraviolet, especially at the shorter wavelengths.{{cite web |title=Reference Solar Spectral Irradiance: Air Mass 1.5 |url=http://rredc.nrel.gov/solar/spectra/am1.5/ |url-status=live |archive-url=https://web.archive.org/web/20190512190812/https://rredc.nrel.gov/solar//spectra/am1.5/ |archive-date=12 May 2019 |access-date=12 November 2009}} Solar [[ultraviolet radiation]] ionizes Earth's dayside upper atmosphere, creating the electrically conducting [[ionosphere]].{{Cite book |last=Phillips |first=K.J.H. |title=Guide to the Sun |date=1995 |publisher=[[Cambridge University Press]] |isbn=978-0-521-39788-9 |pages=14–15, 34–38}} [220] => [221] => [[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]]. It is also the main cause of [[skin cancer]]. 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 Earth.{{Cite journal |last=Barsh |first=G.S. |date=2003 |title=What Controls Variation in Human Skin Color? |journal=[[PLOS Biology]] |volume=1 |issue=1 |page=e7 |doi=10.1371/journal.pbio.0000027 |pmc=212702 |pmid=14551921 |doi-access=free }}[[File:Earth to Sun - en.png|alt=150 million kilometers from Sun to Earth|thumb|300x300px|Once outside the Sun's surface, neutrinos and photons travel at the [[speed of light]].|left]]High-energy [[gamma ray]] [[photon]]s 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 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 years.{{cite web |date=2007 |title=Ancient sunlight |url=http://sunearthday.nasa.gov/2007/locations/ttt_sunlight.php |url-status=dead |archive-url=https://web.archive.org/web/20090515085541/http://sunearthday.nasa.gov/2007/locations/ttt_sunlight.php |archive-date=15 May 2009 |access-date=24 June 2009 |website=Technology Through Time |publisher=NASA |issue=50}} In contrast, it takes only 2.3 seconds for [[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 [[Thermodynamics|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.{{Cite journal |last=Stix |first=M. |date=2003 |title=On the time scale of energy transport in the sun |journal=[[Solar Physics (journal)|Solar Physics]] |volume=212 |issue=1 |pages=3–6 |bibcode=2003SoPh..212....3S |doi=10.1023/A:1022952621810 |s2cid=118656812}} [222] => [223] => Neutrinos are also released by 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. In 2001, the discovery of the effects of [[neutrino oscillation]] resolved the discrepancy: 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.{{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 |s2cid=117848623}} [224] => [225] => == Magnetic activity == [226] => [227] => The Sun has a [[stellar magnetic field]] that varies across its surface. 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. The magnetic field 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.{{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|s2cid=17829477 |doi-access=free }}{{Cite book |last=Zirker |first=J.B. |date=2002 |title=Journey from the Center of the Sun |pages=[https://archive.org/details/journeyfromcente0000zirk/page/119 119–120] |publisher=[[Princeton University Press]] |isbn=978-0-691-05781-1 |url=https://archive.org/details/journeyfromcente0000zirk/page/119 }}{{Cite book |last=Lang |first=Kenneth R. |date=2008 |title=The Sun from Space |page=75 |publisher=[[Springer-Verlag]] |isbn=978-3-540-76952-1}} [228] => [229] => 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]]. In an approximation known as ideal [[magnetohydrodynamics]], plasma particles only move along 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. [230] => [231] => 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. 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 [[tesla (unit)|μT]] (at the photosphere) reduces with the inverse-cube of the distance, leading to a predicted magnetic field of 0.1 nT at the distance of Earth. However, according to spacecraft observations the interplanetary field at Earth's location is around 5 nT, about a hundred times greater.{{Cite journal |last1=Wang |first1=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=1248–1256 |bibcode=2003ApJ...591.1248W |doi=10.1086/375449 |s2cid=7332154}} The difference is due to magnetic fields generated by electrical currents in the plasma surrounding the Sun. [232] => [233] => === Sunspot === [234] => {{Main|Sunspot}} [235] => [[File:New-non-inv-large.gif|thumb|Sunspots time-lapse in Hydrogen-alpha captured with an amateur solar telescope]] [236] => [237] => Sunspots are visible as dark patches on the Sun's [[photosphere]] and correspond to concentrations of magnetic field where convective transport of heat is inhibited from the solar interior to the surface. As a result, sunspots are slightly cooler than the surrounding photosphere, 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 toward its [[solar maximum|maximum]], sunspots tend to form closer to the solar equator, a phenomenon known as [[Spörer's law]]. The largest sunspots can be tens of thousands of kilometers across.{{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]] |access-date=10 July 2009 |url-status=dead |archive-url=https://web.archive.org/web/20070823050403/http://www.gsfc.nasa.gov/gsfc/spacesci/solarexp/sunspot.htm |archive-date=23 August 2007}} [238] => [239] => 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-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 the 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 described by [[Hale's law]].{{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|doi-access=free }}{{cite web |date=4 January 2008 |title=NASA Satellites Capture Start of New Solar Cycle |publisher=[[PhysOrg]] |url=http://www.physorg.com/news119271347.html |access-date=10 July 2009 |archive-date=6 April 2008 |archive-url=https://web.archive.org/web/20080406132839/http://www.physorg.com/news119271347.html |url-status=live }} [240] => [241] => 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-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.{{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]] |access-date=11 July 2009 |archive-date=21 January 2015 |archive-url=https://web.archive.org/web/20150121063331/http://edition.cnn.com/2001/TECH/space/02/16/sun.flips/ |url-status=live }}{{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 |access-date=11 July 2009 |url-status=dead |archive-url=https://web.archive.org/web/20090512121817/https://science.nasa.gov/headlines/y2001/ast15feb_1.htm |archive-date=12 May 2009 }} [242] => [243] => === Solar activity === [244] => {{Main||Solar cycle}} [245] => [[File:Solar-cycle-data.png|thumb|Measurements from 2005 of solar cycle variation during the previous 30 years|left]] [246] => [247] => 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 the interplanetary magnetic field outward into the Solar System.{{Cite book |last=Zirker |first=J.B. |date=2002 |title=Journey from the Center of the Sun |pages=[https://archive.org/details/journeyfromcente0000zirk/page/120 120–127] |publisher=[[Princeton University Press]] |isbn=978-0-691-05781-1 |url=https://archive.org/details/journeyfromcente0000zirk/page/120 }} 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]]. [248] => [249] => Long-term secular change in sunspot number is thought, by some scientists, to be correlated with long-term change in solar irradiance,{{cite journal |last1=Willson |first1=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=42–44 |doi=10.1038/351042a0 |bibcode=1991Natur.351...42W|s2cid=4273483 }} which, in turn, might influence Earth's long-term climate.{{cite journal |author-link=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=1742583 |bibcode=1976Sci...192.1189E|s2cid=33896851 }} The solar cycle influences [[space weather]] conditions, including those surrounding Earth. 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.{{Cite journal |last1=Lean |first1=J. |author-link=Judith Lean |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 |bibcode=1992GeoRL..19.1591L |url=https://zenodo.org/record/1231321 |access-date=16 December 2019 |archive-date=11 May 2020 |archive-url=https://web.archive.org/web/20200511052658/https://zenodo.org/record/1231321 |url-status=live }} Earlier extended minima have been discovered through analysis of [[tree ring]]s and appear to have coincided with lower-than-average global temperatures.{{Cite book |last1=Mackay |first1=R.M. |last2=Khalil |first2=M.A.K |chapter=Greenhouse gases and global warming |chapter-url=https://books.google.com/books?id=tQBS3bAX8fUC&q=solar+minimum+dendochronology&pg=PA1 |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 |access-date=3 November 2020 |archive-date=17 April 2021 |archive-url=https://web.archive.org/web/20210417054703/https://books.google.com/books?id=tQBS3bAX8fUC&q=solar+minimum+dendochronology&pg=PA1 |url-status=live }} [250] => [251] => In December 2019, a new type of solar magnetic explosion was observed, known as forced [[magnetic reconnection]]. Previously, in a process called spontaneous magnetic reconnection, it was observed that the solar magnetic field lines diverge explosively and then converge again instantaneously. Forced Magnetic Reconnection was similar, but it was triggered by an explosion in the corona.{{cite web |last1=Johnson-Groh |first1=Mara |title=SDO sees new kind of magnetic explosion on sun |url=https://phys.org/news/2019-12-sdo-kind-magnetic-explosion-sun.html |website=phys.org |access-date=28 July 2022 |language=en |date=17 December 2019 |archive-date=27 January 2022 |archive-url=https://web.archive.org/web/20220127165038/https://phys.org/news/2019-12-sdo-kind-magnetic-explosion-sun.html |url-status=live }} [252] => [253] => == Life phases == [254] => {{Main|Formation and evolution of the Solar System|Stellar evolution}} [255] => [[File:The life cycle of a Sun-like star (annotated).jpg|thumb|upright=2|Overview of the evolution of a star like the Sun]] [256] => The Sun today is roughly halfway through the most stable part of its life. It has not changed dramatically in over four billion years and will remain fairly stable for about five billion more. However, after hydrogen fusion in its core has stopped, the Sun will undergo dramatic changes, both internally and externally. It is more massive than 71 of 75 other stars within 5 pc,{{Cite web |title=The 100 nearest star systems |url=http://www.astro.gsu.edu/RECONS/TOP100.posted.htm |access-date=2022-04-30 |website=astro.gsu.edu |archive-date=12 November 2007 |archive-url=https://web.archive.org/web/20071112173559/http://www.chara.gsu.edu/RECONS/TOP100.posted.htm |url-status=live }} or in the top ~5 percent. [257] => [258] => === Formation === [259] => {{further|Formation and evolution of the Solar System}} [260] => [261] => 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.{{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}} This age is estimated using [[computer simulation|computer models]] of [[stellar evolution]] and through [[nucleocosmochronology]]. The result is consistent with the [[radiometric dating|radiometric date]] of the oldest Solar System material, at 4.567 billion years ago.{{Cite journal |last1=Amelin |first1=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|bibcode=2002Sci...297.1678A|s2cid=24923770 }}{{Cite journal |last1=Baker |first1=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|bibcode=2005Natur.436.1127B|s2cid=4304613 }} 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 [[supernova]]e 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.{{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 |citeseerx=10.1.1.740.2876|s2cid=118354201 }} As one fragment of the cloud collapsed it also began to rotate due to [[conservation of angular momentum]] and heat up with the increasing pressure.{{Cite web |last=Glozman |first=Igor |date=2022 |title=Formation of the Solar System |url=https://people.highline.edu/iglozman/classes/astronotes/solsys_form.htm |access-date=2022-01-16 |website=[[Highline College]] |publication-place=Des Moines, WA |archive-date=26 March 2023 |archive-url=https://web.archive.org/web/20230326035535/https://people.highline.edu/iglozman/classes/astronotes/solsys_form.htm |url-status=live }} 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.{{cite journal|last=D'Angelo|first=G.|author2=Lubow, S. H. |title=Three-dimensional Disk-Planet Torques in a Locally Isothermal Disk|journal=The Astrophysical Journal|date=2010|volume=724|issue=1|pages=730–747|doi=10.1088/0004-637X/724/1/730|arxiv = 1009.4148 |bibcode = 2010ApJ...724..730D |s2cid=119204765}}{{cite book|last=Lubow|first=S. H.|author2=Ida, S. |chapter=Planet Migration |bibcode=2010exop.book..347L| title=Exoplanets |publisher=University of Arizona Press, Tucson, AZ| editor=S. Seager. |pages=347–371|year=2011|arxiv=1004.4137 }} Gravity and pressure within the core of the cloud generated a lot of heat as it accumulated more matter from the surrounding disk, eventually triggering [[stellar nucleosynthesis|nuclear fusion]].{{Cite web |last=Jones |first=Andrew Zimmerman |date=May 30, 2019 |title=How Stars Make All of the Elements |url=https://www.thoughtco.com/stellar-nucleosynthesis-2699311 |access-date=2023-01-16 |website=[[ThoughtCo]] |language=en |archive-date=11 July 2023 |archive-url=https://web.archive.org/web/20230711191648/https://www.thoughtco.com/stellar-nucleosynthesis-2699311 |url-status=live }} [262] => [263] => The stars [[HD 162826]] and [[HD 186302]] share similarities with the Sun and are thus hypothesized to be its stellar siblings, formed in the same molecular cloud.{{cite web|url=http://www.natureworldnews.com/articles/6974/20140509/astronomers-find-suns-sibling-called-hd-162826.htm|title=Astronomers Find Sun's Sibling 'HD 162826'|date=May 9, 2014|publisher=Nature World News|access-date=2022-01-16|archive-date=3 March 2016|archive-url=https://web.archive.org/web/20160303235530/http://www.natureworldnews.com/articles/6974/20140509/astronomers-find-suns-sibling-called-hd-162826.htm|url-status=live}}{{cite web |url=https://www.universetoday.com/140598/astronomers-find-one-of-the-suns-sibling-stars-born-from-the-same-solar-nebula-billion-of-years-ago/ |title=Astronomers Find One of the Sun's Sibling Stars. Born From the Same Solar Nebula Billions of Years Ago |author=Matt Williams |date=2018-11-21 |website=[[Universe Today]] |access-date=2022-10-07 |archive-date=26 March 2023 |archive-url=https://web.archive.org/web/20230326035623/https://www.universetoday.com/140598/astronomers-find-one-of-the-suns-sibling-stars-born-from-the-same-solar-nebula-billion-of-years-ago/ |url-status=live }} [264] => [265] => === Main sequence === [266] => [[File:Evolution of a Sun-like star.svg|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.|300x300px]] [267] => The Sun is about halfway through its main-sequence stage, during which nuclear fusion reactions in its core fuse hydrogen into helium. Each second, more than four billion kilograms of matter are converted into energy within the Sun's core, producing neutrinos 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 to 11 billion years as a main-sequence star before the [[red giant]] phase of the Sun.{{Cite book |last1=Goldsmith |first1=D. |last2=Owen |first2=T. |title=The search for life in the universe |url=https://books.google.com/books?id=Q17NmHY6wloC&pg=PA96 |page=96 |publisher=University Science Books |date=2001 |isbn=978-1-891389-16-0 |access-date=22 August 2020 |archive-date=30 October 2020 |archive-url=https://web.archive.org/web/20201030203521/https://books.google.com/books?id=Q17NmHY6wloC&pg=PA96 |url-status=live }} At the 8 billion year mark, the Sun will be at its hottest point according to the [[Gaia (spacecraft)|ESA's Gaia space observatory mission]] in 2022.{{Cite web |last=Source |first=News Staff / |date=2022-08-12 |title=ESA's Gaia Mission Sheds New Light on Past and Future of Our Sun {{!}} Sci.News |url=https://www.sci.news/astronomy/sun-future-11093.html |access-date=2022-08-15 |website=Sci.News: Breaking Science News |language=en-US |archive-date=4 April 2023 |archive-url=https://web.archive.org/web/20230404001136/https://www.sci.news/astronomy/sun-future-11093.html |url-status=live }} [268] => [269] => The Sun is gradually becoming hotter in its core, hotter at the surface, larger in radius, and more luminous during its time on the main sequence: since the beginning of its main sequence life, it has expanded in radius by 15% and the surface has increased in temperature from {{Convert|5,620|K|C F}} to {{Convert|5,772|K|C F}}, resulting in a 48% increase in luminosity from 0.677 [[solar luminosity|solar luminosities]] to its present-day 1.0 solar luminosity. This occurs because the helium atoms in the core have a higher mean [[molecular weight]] than the [[hydrogen atom]]s that were fused, resulting in less thermal pressure. The core is therefore shrinking, allowing the outer layers of the Sun to move closer to the center, releasing [[gravitational potential energy]]. According to the [[virial theorem]], half of this released gravitational energy goes into heating, which leads to a gradual increase in the rate at which fusion occurs and thus an increase in the luminosity. This process speeds up as the core gradually becomes denser.{{cite book |last1=Carroll |first1=Bradley W. |title=An introduction to modern astrophysics |last2=Ostlie |first2=Dal A |date=2017 |publisher=Cambridge University Press |isbn=978-1-108-42216-1 |edition=Second |location=Cambridge, United Kingdom |pages=350, 447, 448, 457}} At present, it is increasing in brightness by about 1% every 100 million years. It will take at least 1 billion years from now to deplete liquid water from the Earth from such increase.{{cite web |url=https://www.science.org/content/article/earth-wont-die-soon-thought |title=Earth Won't Die as Soon as Thought |date=22 January 2014 |access-date=24 May 2015 |archive-date=12 November 2020 |archive-url=https://web.archive.org/web/20201112023013/https://www.sciencemag.org/news/2014/01/earth-wont-die-soon-thought |url-status=live }} After that, the Earth will cease to be able to support complex, multicellular life and the last remaining multicellular organisms on the planet will suffer a final, complete [[mass extinction]].{{cite journal |last1=Snyder-Beattie |first1=Andrew E. |last2=Bonsall |first2=Michael B. |date=30 March 2022 |title=Catastrophe risk can accelerate unlikely evolutionary transitions |journal=Proceedings of the Royal Society B |volume=289 |issue=1971 |doi=10.1098/rspb.2021.2711 |pmid=35350860 |pmc=8965398 }} [270] => [271] => === After core hydrogen exhaustion === [272] => [273] => [[File:Sun red giant.svg|thumb|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]] [274] => The Sun does not have enough mass to explode as a [[supernova]]. Instead, when it runs out of hydrogen in the core in approximately 5 billion years, core hydrogen fusion will stop, and there will be nothing to prevent the core from contracting. The release of gravitational potential energy will cause the luminosity of the Sun to increase, ending the main sequence phase and leading the Sun to expand over the next billion years: first into a [[subgiant]], and then into a [[red giant]].{{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 |access-date=20 February 2016 |archive-date=9 February 2016 |archive-url=https://web.archive.org/web/20160209042249/http://www.space.com/22471-red-giant-stars.html |url-status=live }}{{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 |issue=1 |pages=155–163 |year=2008 |arxiv=0801.4031 |bibcode=2008MNRAS.386..155S|s2cid=10073988 }} The heating due to gravitational contraction will also lead to expansion of the Sun and hydrogen fusion in a shell just outside the core, where unfused hydrogen remains, contributing to the increased luminosity, which will eventually reach more than 1,000 times its present luminosity. When the Sun enters its [[red-giant branch]] (RGB) phase, it will engulf (and very likely destroying) [[Mercury (planet)|Mercury]] and [[Venus]]. According to a 2008 paper, Earth's orbit will have initially expanded to at most {{Convert|1.5|AU|e6km e6mi|abbr=unit|sigfig=2}} due to the Sun's loss of mass. However, Earth's orbit will then start shrinking due to [[tidal forces]] (and, eventually, drag from the lower chromosphere) so that it is engulfed by the Sun during the [[tip of the red-giant branch]] phase 7.59 billion years from now, 3.8 and 1 million years after Mercury and Venus have respectively suffered the same fate. [275] => [276] => By the time the Sun reaches the tip of the red-giant branch, it will be about 256 times larger than it is today, with a radius of {{Convert|1.19|AU|e6km e6mi|abbr=unit}}.{{Cite journal |last1=Boothroyd |first1=Arnold I. |last2=Sackmann |first2=I.-Juliana |date=January 1, 1999 |orig-date=19 December 1995 |title=The CNO Isotopes: Deep Circulation in Red Giants and First and Second Dredge-up |url=https://iopscience.iop.org/article/10.1086/306546 |journal=The Astrophysical Journal |publisher=The American Astronomical Society (AAS), The Institute of Physics (IOP) |volume=510 |issue=1 |pages=232–250 |arxiv=astro-ph/9512121 |bibcode=1999ApJ...510..232B |doi=10.1086/306546 |s2cid=561413}} The Sun will spend around a billion years in the RGB and lose around a third of its mass. [277] => [278] => 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]]; 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]].{{Cite web|url=http://faculty.wcas.northwestern.edu/~infocom/The%20Website/end.html|title=The End Of The Sun|access-date=24 May 2015|archive-date=22 May 2019|archive-url=https://web.archive.org/web/20190522175414/http://faculty.wcas.northwestern.edu/~infocom/The%20Website/end.html|url-status=live}} 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 metallicity 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 react helium in the core. [279] => [280] => When the helium is exhausted, the Sun will repeat the expansion it followed when the hydrogen in the core was exhausted. This time, however, 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 reacting 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. Despite this, the Sun's maximum AGB radius will not be as large as its tip-RGB maximum: 179 {{Solar radius|link=yes}}, or about {{Convert|0.832|AU|e6km e6mi|abbr=unit}}.{{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 |bibcode=1993ApJ...413..641V|doi-access=free }} [281] => [282] => 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. 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. [283] => [284] => 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 ionized into a [[planetary nebula]] as the exposed core reaches {{Convert|30,000|K|C F|sigfig=}}, as if it is in a sort of [[blue loop]]. The final naked core, a [[white dwarf]], will have a temperature of over {{Convert|100,000|K|C F|sigfig=}} and contain an estimated 54.05% of the Sun's present-day mass. 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 super-dense [[black dwarf]].{{Cite journal |bibcode = 1995A&A...297..727B|title = Stellar evolution of low and intermediate-mass stars. I. Mass loss on the AGB and its consequences for stellar evolution|last1 = Bloecker|first1 = T.|journal = Astronomy and Astrophysics|year = 1995|volume = 297|page = 727}}{{Cite journal |bibcode = 1995A&A...299..755B|title = Stellar evolution of low- and intermediate-mass stars. II. Post-AGB evolution|last1 = Bloecker|first1 = T.|journal = Astronomy and Astrophysics|year = 1995|volume = 299|page = 755}} As such, it would give off no more energy for an even longer time than it was a white dwarf.{{cite web | last=Johnson-Groh | first=Mara | title=The end of the universe may be marked by 'black dwarf supernova' explosions | website=Live Science | date=August 25, 2020 | url=https://www.livescience.com/black-dwarf-supernovae-end-universe.html | access-date=November 24, 2023 | archive-date=2 June 2023 | archive-url=https://web.archive.org/web/20230602022731/https://www.livescience.com/black-dwarf-supernovae-end-universe.html | url-status=live }} [285] => [286] => == Location == [287] => === Solar System === [288] => {{Main|Solar System}} [289] => [[File:Solar System true color.jpg|alt=see caption|thumb|upright=1.3|The Solar System, with sizes of the Sun and planets to scale. The terrestrial planets are on the right, the gas and ice giants are on the left.]] [290] => The Sun has eight known planets orbiting it. 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 nine bodies generally considered as [[dwarf planet]]s and some more [[list of possible dwarf planets|candidates]], an [[asteroid belt]], numerous [[comets]], and a large number of icy bodies which lie beyond the orbit of Neptune. Six of the planets and many smaller bodies also have their own [[natural satellite]]s: in particular, the satellite systems of Jupiter, Saturn, and Uranus are in some ways like miniature versions of the Sun's system.{{cite book |title=Physics and Chemistry of the Solar System |date=2004 |publisher=Elsevier |editor-first=John |editor-last=Lewis |edition=2 |page=147 }} [291] => [292] => The Sun is moved by the gravitational pull of the planets. The center of the Sun is always within 2.2 solar radii of the barycenter. This motion of the Sun is mainly due to the four large planets. Each planet in the series Jupiter, Saturn, Neptune, Uranus has about twice as much effect (moment of inertia) as the next. For some periods of several decades (when Neptune and Uranus are in [[Opposition (astronomy)|opposition]]) the motion is rather regular, forming a [[trefoil]] pattern, whereas between these periods it appears more chaotic.{{bettersourceneeded|reason=Once a paper has been retracted, the entire paper can no longer be considered an RS even if the particular details being retracted was not part of the reason for the retraction. If the paper cited by the retract paper is sufficient to support this it would be a suitable replacement.|date=September 2023}}See Figure 5 and reference in {{cite journal |last1=Valentina Zharkova |title=Oscillations of the baseline of solar magnetic field and solar irradiance on a millennial timescale |journal=Scientific Reports |date=Jun 24, 2019 |doi=10.1038/s41598-019-45584-3 |display-authors=etal|arxiv=2002.06550 |volume=9 |issue=1 |page=9197 |pmid=31235834 |pmc=6591297 }} Although this paper was retracted by the journal because of an error about the distance between the Sun and Earth, Figure 5 is based on another paper and is unaffected by the problem. After 179 years (nine times the [[synodic period]] of Jupiter and Saturn), the pattern more or less repeats, but rotates by about 24°.{{cite journal |last1=Paul Jose |title=Sun's Motion and Sunspots |journal=[[The Astronomical Journal]] |date=Apr 1965 |volume=70 |pages=193–200 |doi=10.1086/109714 |bibcode=1965AJ.....70..193J |url=http://www.landscheidt.info/pdf/jose1965.pdf |access-date=22 March 2020 |archive-date=22 March 2020 |archive-url=https://web.archive.org/web/20200322184010/http://www.landscheidt.info/pdf/jose1965.pdf |url-status=live }} The value of 24° comes from (360)(15 J − 6 S)/(S − J), where S and J are the periods of Saturn and Jupiter respectively. The orbits of the inner planets, including those of the Earth, are similarly displaced by the same gravitational forces, so the movement of the Sun has little effect on the relative positions of the Earth and the Sun or on solar irradiance on the Earth as a function of time.{{cite journal |title=Retraction Note: Oscillations of the baseline of solar magnetic field and solar irradiance on a millennial timescale |journal=Scientific Reports |date=Mar 4, 2020 |doi=10.1038/s41598-020-61020-3 |volume=10 | last1 = Zharkova | first1 = V. V. | last2 = Shepherd | first2 = S. J. | last3 = Zharkov | first3 = S. I. | last4 = Popova | first4 = E.|issue=1 |page=4336 |pmid=32132618 |pmc=7055216 |bibcode=2020NatSR..10.4336Z | doi-access = free }} [293] => [294] => === Celestial neighborhood === [295] => {{Excerpt|Solar System|Celestial neighborhood}} [296] => [297] => == Motion == [298] => {{Further|Stellar kinematics}} [299] => [[File:Motion of Sun, Earth and Moon around the Milky Way.jpg|thumb|The general motion and orientation of the Sun, and Earth and the Moon as its Solar System satellites.]] [300] => [301] => Being part of the Milky Way galaxy the Sun, taking along the whole Solar System, moves in an orbital fashion around [[Galactic Center|the galaxy's center of mass]] at an average speed of 230 km/s (828,000 km/h) or 143 mi/s (514,000 mph),{{cite web |url=http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question18.html |website=NASA |title=StarChild Question of the Month – Does the Sun move around the Milky Way? |date=February 2000 |url-status=live |archive-url=https://web.archive.org/web/20231030090914/https://starchild.gsfc.nasa.gov/docs/StarChild/questions/question18.html |archive-date= Oct 30, 2023 }} taking about 220–250 million [[Earth year]]s to complete a revolution (a [[Galactic year]]),{{cite web | last=Siegel | first=Ethan | title=Our Motion Through Space Isn't A Vortex, But Something Far More Interesting | website=Forbes | date=August 30, 2018 | url=https://www.forbes.com/sites/startswithabang/2018/08/30/our-motion-through-space-isnt-a-vortex-but-something-far-more-interesting/ | access-date=November 25, 2023 |url-status=live |archive-url=https://web.archive.org/web/20231125013457/https://www.forbes.com/sites/startswithabang/2018/08/30/our-motion-through-space-isnt-a-vortex-but-something-far-more-interesting/?sh=1cfdeca37ec2 |archive-date=November 25, 2023 }} having done so about 20 times since the Sun's formation.{{cite web | last=Currin | first=Grant | title=How long is a galactic year? | website=Live Science | date=August 30, 2020 | url=https://www.livescience.com/how-long-galactic-year.html | access-date=November 25, 2023 | archive-date=25 November 2023 | archive-url=https://web.archive.org/web/20231125013457/https://www.livescience.com/how-long-galactic-year.html | url-status=live }} The direction of the Sun's motion, the [[Solar apex]] is roughly in the direction of the star [[Vega]]. [302] => [303] => [[File:Milky Way Arms ssc2008-10.svg|thumb|The Sun's idealized orbit around the Galactic Center in an artist's top-down depiction of the current layout of the Milky Way.]] [304] => [305] => == Observational history == [306] => [307] => === Early understanding === [308] => {{See also|The Sun in culture}} [309] => [[File:Solvognen DO-6865 2000.jpg|thumb|The [[Trundholm sun chariot]] pulled by a horse is a sculpture believed to be illustrating an important part of [[Nordic Bronze Age]] mythology.]] [310] => 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]] causes 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. The Sun has played an important part in many world religions, as described in a later section. [311] => [312] => 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 perihelion and moving slower when it is farther away at aphelion.{{Cite book |title=Babylon to Voyager and beyond: a history of planetary astronomy |first=David |last=Leverington |publisher=[[Cambridge University Press]] |date=2003 |isbn=978-0-521-80840-8 |pages=6–7}} [313] => [314] => 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.{{Cite journal |last=Sider |first=D. |title=Anaxagoras on the Size of the Sun |jstor=269068 |journal=[[Classical Philology (journal)|Classical Philology]] |volume=68 |issue=2 |pages=128–129 |date=1973 |doi=10.1086/365951|s2cid=161940013 }} 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 third 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 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 first 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}}.{{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|jstor=1006040}} [315] => [316] => 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 third 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]]. [317] => [318] => === Development of scientific understanding === [319] => [[File:Sun-bonatti.png|thumb|Sol, the Sun, from a 1550 edition of [[Guido Bonatti]]'s ''Liber astronomiae'']] [320] => 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.{{cite book |last=Ead |first=Hamed A. |title=Averroes As A Physician |publisher=[[University of Cairo]]}} The invention of the telescope in the early 17th century permitted detailed observations of sunspots 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.{{cite web |title=Galileo Galilei (1564–1642) |url=https://www.bbc.co.uk/history/historic_figures/galilei_galileo.shtml |publisher=BBC |access-date=22 March 2006 |archive-date=29 September 2018 |archive-url=https://web.archive.org/web/20180929134432/http://www.bbc.co.uk/history/historic_figures/galilei_galileo.shtml |url-status=live }} [321] => [322] => [[Astronomy in medieval Islam|Arabic astronomical contributions]] include [[Al-Battani]]'s 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.''A short History of scientific ideas to 1900'', C. Singer, Oxford University Press, 1959, p. 151. (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]].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. [323] => [324] => From an observation of a [[transit of Venus]] in 1032, the Persian astronomer and polymath [[Avicenna|Ibn Sina]] concluded that Venus was closer to Earth than the Sun.{{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 |pages=39–47 [44] |doi=10.1086/350839|bibcode=1972Isis...63...39G |s2cid=120700705 }} In 1672 [[Giovanni Cassini]] and [[Jean Richer]] determined the distance to Mars and were thereby able to calculate the distance to the Sun. [325] => [[File:BBSO full-disk H-alpha 2002-07-26 153931 color.png|thumb|Sun as seen in Hydrogen-alpha light]] [326] => [[File:The Sun by the Atmospheric Imaging Assembly of NASA's Solar Dynamics Observatory - 20100819.jpg|thumb|The Sun as seen in ultraviolet light]] [327] => 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.{{cite news |title=Sir Isaac Newton (1643–1727) |newspaper=BBC Teach |url=https://www.bbc.co.uk/history/historic_figures/newton_isaac.shtml |publisher=BBC |access-date=22 March 2006 |archive-date=10 March 2015 |archive-url=https://web.archive.org/web/20150310093436/http://www.bbc.co.uk/history/historic_figures/newton_isaac.shtml |url-status=live }} In 1800, [[William Herschel]] discovered [[infrared]] radiation beyond the red part of the solar spectrum.{{cite web |title=Herschel Discovers Infrared Light |url=http://coolcosmos.ipac.caltech.edu/cosmic_classroom/classroom_activities/herschel_bio.html |publisher=Cool Cosmos |access-date=22 March 2006 |url-status=dead |archive-url=https://web.archive.org/web/20120225094516/http://coolcosmos.ipac.caltech.edu/cosmic_classroom/classroom_activities/herschel_bio.html |archive-date=25 February 2012 }} 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]]. The 20th century brought about several specialized systems for observing the Sun, especially at different narrowband wavelengths, such as those using Calcium H (396.9 nm), K (393.37 nm) and [[H-alpha|Hydrogen-alpha]] (656.46 nm) filtering. [328] => [329] => 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.{{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 |access-date=25 August 2006 |archive-date=25 September 2006 |archive-url=https://web.archive.org/web/20060925190954/http://zapatopi.net/kelvin/papers/on_the_age_of_the_suns_heat.html |url-status=live }} 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.{{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.|doi-access=free }} 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.{{Cite journal |last=Lockyer |first=J.N. |title=The meteoritic hypothesis; a statement of the results of a spectroscopic inquiry into the origin of cosmical systems |journal=London and New York |date=1890 |bibcode=1890mhsr.book.....L}} [330] => [331] => 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.{{cite web |last=Darden |first=L. |title=The Nature of Scientific Inquiry |url=http://www.philosophy.umd.edu/Faculty/LDarden/sciinq/ |date=1998 |access-date=25 August 2006 |archive-date=17 August 2012 |archive-url=https://web.archive.org/web/20120817040843/http://www.philosophy.umd.edu/Faculty/LDarden/sciinq/ |url-status=live }} 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''²}}.{{Cite book |last=Hawking |first=S.W. |author-link = Stephen Hawking |date=2001 |title=The Universe in a Nutshell |publisher=Bantam Books |isbn=978-0-553-80202-3}} 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.{{cite web |title=Studying the stars, testing relativity: Sir Arthur Eddington |url=http://www.esa.int/esaSC/SEMDYPXO4HD_index_0.html |website=Space Science |publisher=[[European Space Agency]] |date=2005 |access-date=1 August 2007 |archive-date=20 October 2012 |archive-url=https://web.archive.org/web/20121020174459/http://www.esa.int/esaSC/SEMDYPXO4HD_index_0.html |url-status=live }} 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]]. 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.{{Cite journal |last1=Bethe |first1=H. |title=On the Formation of Deuterons by Proton Combination |journal=[[Physical Review]] |volume=54 |issue=10 |page=862 |date=1938 |doi=10.1103/PhysRev.54.862.2 |last2=Critchfield |first2=C.|bibcode=1938PhRv...54Q.862B}}{{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 |pmid=17835673|bibcode=1939PhRv...55..434B|s2cid=36146598 |doi-access=free }} 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.{{Cite journal |first1=E.M. |last1=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 |url=https://authors.library.caltech.edu/45747/1/BURrmp57.pdf |doi-access=free |access-date=12 April 2020 |archive-date=23 July 2018 |archive-url=https://web.archive.org/web/20180723054833/https://authors.library.caltech.edu/45747/1/BURrmp57.pdf |url-status=live }} [332] => [333] => === Solar space missions === [334] => {{See also|Solar observatory}} [335] => [[File:Pioneer-6-9.jpg|thumb|Illustration of [[Pioneer 6, 7, 8, and 9]]]] [336] => The first satellites designed for long term observation of the Sun from interplanetary space were NASA's [[Pioneer program|Pioneers]] 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.{{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]] |access-date=22 March 2006 |url-status=dead |archive-url=https://web.archive.org/web/20060422075141/http://www.astronautix.com/craft/pio6789e.htm |archive-date=22 April 2006 |df=dmy-all}}{{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]] |access-date=30 October 2010 |quote=NASA maintained contact with Pioneer 9 until May 1983 |url-status=dead |archive-url=https://web.archive.org/web/20120402205810/http://solarsystem.nasa.gov/missions/profile.cfm?MCode=Pioneer_09 |archive-date=2 April 2012 |df=dmy-all}} [337] => [338] => In the 1970s, two [[Helios (spacecraft)|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's orbit at perihelion. 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. Skylab made the first time-resolved observations of the solar transition region and of ultraviolet emissions from the solar corona. Discoveries included the first observations of coronal mass ejections, then called "coronal transients", and of [[coronal hole]]s, now known to be intimately associated with the solar wind.{{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=1619–1627 |doi=10.1016/S0032-0633(01)00098-8 |bibcode=2001P&SS...49.1619B |url=https://zenodo.org/record/1259695 |access-date=25 August 2019 |archive-date=13 July 2020 |archive-url=https://web.archive.org/web/20200713051926/https://zenodo.org/record/1259695 |url-status=live }} [339] => [340] => In the 1970s, much research focused on the abundances of [[iron group|iron-group]] elements in the Sun.{{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 |issue=4 |pages=683–694 |bibcode=1978MNRAS.184..683B |doi=10.1093/mnras/184.4.683 |doi-access=free}}Ross and Aller 1976, Withbroe 1976, Hauge and Engvold 1977, cited in Biemont 1978. 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. The first largely complete set of [[oscillator strength]]s of singly ionized iron-group elements were made available in the 1960s,Corliss and Bozman (1962 cited in Biemont 1978) and Warner (1967 cited in Biemont 1978) and these were subsequently improved.Smith (1976 cited in Biemont 1978) In 1978, the abundances of singly ionized elements of the iron group were derived. [341] => Various authors have considered the existence of a gradient in the [[isotope|isotopic]] compositions of solar and planetary [[noble gas]]es,Signer and Suess 1963; Manuel 1967; Marti 1969; Kuroda and Manuel 1970; Srinivasan and Manuel 1971, all cited in Manuel and Hwaung 1983 e.g. correlations between isotopic compositions of [[neon]] and [[xenon]] in the Sun and on the planets.Kuroda and Manuel 1970 cited in Manuel and Hwaung 1983:7 Prior to 1983, it was thought that the whole Sun has the same composition as the solar atmosphere.{{Cite journal |last1=Manuel |first1=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}} 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. [342] => [[File:Smm.jpg|left|thumb|Drawing of a [[Solar Maximum Mission]] probe]] [343] => In 1980, the [[Solar Maximum Mission]] probes were launched by NASA. This spacecraft was designed to observe gamma rays, [[X-ray]]s and [[Ultraviolet|UV]] radiation from solar flares during a time of high solar activity and 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.{{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 |access-date=22 March 2006 |archive-url=https://web.archive.org/web/20060405183758/http://web.hao.ucar.edu/public/research/svosa/smm/smm_mission.html |archive-date=5 April 2006}} [344] => [345] => 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 annular eclipse in 2001 caused it to lose its lock on the Sun. It was destroyed by atmospheric re-entry in 2005.{{cite press release |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 |access-date=22 March 2006 |archive-date=10 August 2013 |archive-url=https://web.archive.org/web/20130810150641/http://www.jaxa.jp/press/2005/09/20050913_yohkoh_e.html |url-status=dead }} [346] => [347] => 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. Originally intended to serve a two-year mission, a mission extension through 2012 was approved in October 2009.{{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 |website=ESA Science and Technology |access-date=16 February 2010 |archive-date=2 May 2013 |archive-url=https://web.archive.org/web/20130502023453/http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=45685 |url-status=live }} It has proven so useful that a follow-on mission, the [[Solar Dynamics Observatory]], was launched in February 2010.{{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 |website=NASA Press Release Archives |access-date=16 February 2010 |archive-date=10 August 2013 |archive-url=https://web.archive.org/web/20130810082825/http://www.nasa.gov/home/hqnews/2010/feb/HQ_10-040_SDO_launch.html |url-status=live }} 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. 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.{{cite web |title=Sungrazing Comets |url=http://sungrazer.nrl.navy.mil/ |publisher=[[Large Angle and Spectrometric Coronagraph|LASCO]] ([[US Naval Research Laboratory]]) |access-date=19 March 2009 |archive-date=25 May 2015 |archive-url=https://web.archive.org/web/20150525060147/http://sungrazer.nrl.navy.mil/ |url-status=live }} [348] => [[File:The Ulysses spacecraft undergoes testing at the vacuum spin-balancing facility in ESTEC.jpg|thumb|[[Ulysses (spacecraft)|''Ulysses'' spacecraft]] testing at the vacuum spin-balancing facility]] [349] => [[File:Parker Solar Probe spacecraft model.png|thumb|Artist rendition of the [[Parker Solar Probe]]]] [350] => 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 (spacecraft)|''Ulysses'' probe]] was launched in 1990 to study the Sun's polar regions. It first traveled 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 km/s, which was slower than expected, and that there were large magnetic waves emerging from high latitudes that scattered galactic cosmic rays.{{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 |access-date=22 March 2006 |url-status=dead |archive-url=https://web.archive.org/web/20060106150819/http://ulysses.jpl.nasa.gov/science/mission_primary.html |archive-date=6 January 2006 }} [351] => [352] => 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.{{Cite journal |last1=Calaway |first1=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. |bibcode=2009NIMPB.267.1101C |url=https://zenodo.org/record/1259269 |access-date=13 July 2019 |archive-date=11 May 2020 |archive-url=https://web.archive.org/web/20200511052700/https://zenodo.org/record/1259269 |url-status=live }} [353] => * [[STEREO|Solar Terrestrial Relations Observatory]] (STEREO) mission was launched in October 2006. Two identical spacecraft were launched into orbits that caused 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.{{cite web |date=8 March 2006 |url=http://www.nasa.gov/mission_pages/stereo/spacecraft/index.html |title=STEREO Spacecraft & Instruments |website=NASA Missions |access-date=30 May 2006 |archive-date=23 May 2013 |archive-url=https://web.archive.org/web/20130523040216/http://www.nasa.gov/mission_pages/stereo/spacecraft/index.html |url-status=live }}{{Cite journal |title=Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) |last1=Howard |first1=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 |bibcode=2008SSRv..136...67H |doi=10.1007/s11214-008-9341-4 |s2cid=122255862 |url=https://orbi.uliege.be/bitstream/2268/21196/1/secchi%20space%20sci%20rev.pdf |doi-access=free |access-date=25 August 2019 |archive-date=14 December 2019 |archive-url=https://web.archive.org/web/20191214040251/https://orbi.uliege.be/bitstream/2268/21196/1/secchi%20space%20sci%20rev.pdf |url-status=live }} [354] => * [[Parker Solar Probe]] was launched in 2018 aboard a [[Delta IV Heavy]] rocket and will reach a perihelion of {{val|0.046|u=AU}} in 2025, making it the closest-orbiting manmade satellite as the first spacecraft to fly low into the solar corona.{{cite news|url=https://www.space.com/three-big-missions-spotlight-the-sun.html|title=Our sun will never look the same again thanks to two solar probes and one giant telescope|author=Meghan Bartels|publisher=Space.com|access-date=March 9, 2020|archive-date=2 March 2020|archive-url=https://web.archive.org/web/20200302130801/https://www.space.com/three-big-missions-spotlight-the-sun.html|url-status=live}} [355] => * [[Solar Orbiter]] mission (SolO) was launched in 2020 and will reach a minimum perihelion of {{val|0.28|u=AU}}, making it the closest satellite with sun-facing cameras.{{Cite web |title=Solar Orbiter |url=https://www.esa.int/Science_Exploration/Space_Science/Solar_Orbiter |access-date=2022-03-29 |website=esa.int |language=en |archive-date=29 March 2022 |archive-url=https://web.archive.org/web/20220329191606/https://www.esa.int/Science_Exploration/Space_Science/Solar_Orbiter |url-status=live }} [356] => * [[CubeSat for Solar Particles]] (CuSP) was launched as a rideshare on [[Artemis 1]] on 16 November 2022 to study [[Solar energetic particles|particles]] and [[#Magnetic field|magnetic fields]]. [357] => * [[Indian Space Research Organisation]] has launched a {{val|100|u=kg}} satellite named ''[[Aditya-L1]]'' on 2 September 2023.{{Cite web|date=February 2, 2022|first=Chethan|last=Kumar|title=2 key Gaganyaan crew abort tests, Aditya top priority|url=https://timesofindia.indiatimes.com/india/2-key-gaganyaan-crew-abort-tests-aditya-top-priority/articleshow/89305880.cms|access-date=2022-02-02|website=The Times of India|language=en|archive-date=18 February 2022|archive-url=https://web.archive.org/web/20220218203122/https://timesofindia.indiatimes.com/india/2-key-gaganyaan-crew-abort-tests-aditya-top-priority/articleshow/89305880.cms|url-status=live}} Its main instrument will be a [[coronagraph]] for studying the dynamics of the solar corona.{{Cite web|url=https://www.firstpost.com/tech/science/aditya-l-1-after-chandrayaan-2-isro-to-pursue-indias-first-mission-to-the-sun-in-2020-7053851.html|title=Aditya L-1: After Chandrayaan 2, ISRO to pursue India's first mission to the Sun in 2020|date=25 July 2019|website=Tech2|access-date=2 August 2019|archive-date=2 August 2019|archive-url=https://web.archive.org/web/20190802145416/https://www.firstpost.com/tech/science/aditya-l-1-after-chandrayaan-2-isro-to-pursue-indias-first-mission-to-the-sun-in-2020-7053851.html|url-status=live}} [358] => [359] => === Unsolved problems === [360] => [361] => ==== Coronal heating ==== [362] => {{Main|Stellar corona}} [363] => {{unsolved|astronomy|Why is the Sun's corona so much hotter than the Sun's surface?}} [364] => [365] => The temperature of the photosphere is approximately 6,000 K, whereas the temperature of the corona reaches {{val|1000000|-|2000000|u=K|fmt=commas}}. The high temperature of the corona shows that it is heated by something other than direct [[heat conduction]] from the photosphere. [366] => [367] => 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. The first is wave heating, in which sound, gravitational or magnetohydrodynamic waves are produced by turbulence in the convection zone. These waves travel upward and dissipate in the corona, depositing their energy in the ambient matter in the form of heat.{{Cite journal |last=Alfvén |first=H. |date=1947 |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 |bibcode=1947MNRAS.107..211A |doi=10.1093/mnras/107.2.211 |doi-access=free}} The other is magnetic heating, in which magnetic energy is continuously built up by photospheric motion and released through [[magnetic reconnection]] in the form of large solar flares and myriad similar but smaller events—[[nanoflares]].{{Cite journal |last=Parker |first=E.N. |date=1988 |title=Nanoflares and the solar X-ray corona |journal=[[Astrophysical Journal]] |volume=330 |issue=1 |page=474 |bibcode=1988ApJ...330..474P |doi=10.1086/166485}} [368] => [369] => Currently, it is unclear whether waves are an efficient heating mechanism. All waves except Alfvén waves have been found to dissipate or refract before reaching the corona.{{Cite journal |last1=Sturrock |first1=P.A. |last2=Uchida |first2=Y. |date=1981 |title=Coronal heating by stochastic magnetic pumping |journal=[[Astrophysical Journal]] |volume=246 |issue=1 |page=331 |bibcode=1981ApJ...246..331S |doi=10.1086/158926 |hdl-access=free |hdl=2060/19800019786}} In addition, Alfvén waves do not easily dissipate in the corona. Current research focus has therefore shifted towards flare heating mechanisms. [370] => [371] => ==== Faint young Sun ==== [372] => {{Main|Faint young Sun paradox}} [373] => {{unsolved|astronomy|How could the early Earth have had liquid water if the Sun's output is predicted to have only been 70% as intense as it is today?}} [374] => [375] => Theoretical models of the Sun's development suggest that 3.8 to 2.5 billion years ago, during the [[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.{{Cite journal |last1=Kasting |first1=J.F. |last2=Ackerman |first2=T.P. |date=1986 |title=Climatic Consequences of Very High Carbon Dioxide Levels in the Earth's Early Atmosphere |url=https://zenodo.org/record/1230890 |url-status=live |journal=[[Science (journal)|Science]] |volume=234 |issue=4782 |pages=1383–1385 |bibcode=1986Sci...234.1383K |doi=10.1126/science.11539665 |pmid=11539665 |archive-url=https://web.archive.org/web/20190926171220/https://zenodo.org/record/1230890 |archive-date=26 September 2019 |access-date=13 July 2019}} [376] => [377] => However, examination of [[Archean|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.{{cite journal |author1=Rosing, Minik T. |author2=Bird, Dennis K. |author3=Sleep, Norman H. |author4=Bjerrum, Christian J. |date=1 April 2010 |title=No climate paradox under the faint early Sun |journal=Nature |volume=464 |issue=7289 |pages=744–747 |bibcode=2010Natur.464..744R |doi=10.1038/nature08955 |pmid=20360739 |s2cid=205220182}} [378] => [379] => == Observation by eyes == [380] => [[File:- panoramio (785).jpg|thumb|The Sun seen from Earth, with [[Glare (vision)|glare]] from the lenses. The eye also sees glare when looked towards the Sun directly.|left]] [381] => 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-[[Mydriasis|dilated]] eyes.{{Cite journal |first1=T.J. |last1=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 |pmid=5551296}}{{Cite journal |first1=M.O.M. |last1=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}} Looking directly at the Sun ([[sungazing]]) causes [[phosphene]] visual artifacts and temporary partial blindness. It also delivers about 4 milliwatts of sunlight to the retina, slightly heating it and potentially causing damage in eyes that cannot respond properly to the brightness.{{Cite journal |last1=Hope-Ross |first1=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|pages=29–33|doi-access=free }}{{Cite journal |title=Solar Retinopathy from Sun-Gazing Under Influence of LSD |last1=Schatz |first1=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|pmc=1214879 |pages=270–273}} Viewing of the direct Sun with the naked eye can cause UV-induced, sunburn-like lesions on the retina beginning after about 100 seconds, particularly under conditions where the UV light from the Sun is intense and well focused.{{Cite journal |first1=W.T. Jr. |last1=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 |pmid=815821|bibcode=1976Natur.260..153H|s2cid=4283242 }}{{Cite book |first1=W.T. Jr. |last1=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=978-0-306-40328-6}} [382] => [383] => 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.{{Cite book |first=T. |last=Kardos |title=Earth science |url=https://books.google.com/books?id=xI6EDV_PRr4C&pg=PT102 |page=87 |publisher=J.W. Walch |date=2003 |isbn=978-0-8251-4500-1 |access-date=22 August 2020 |archive-date=3 November 2020 |archive-url=https://web.archive.org/web/20201103234006/https://books.google.com/books?id=xI6EDV_PRr4C&pg=PT102 |url-status=live }} Brief glances at the midday Sun through an unfiltered telescope can cause permanent damage.{{cite book |last=Macdonald |first=Lee |chapter=Equipment for Observing the Sun |date=2012 |title=How to Observe the Sun Safely |publisher=Springer Science + Business Media |place=New York |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. |series=Patrick Moore's Practical Astronomy Series |isbn=978-1-4614-3824-3}} [384] => [385] => 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,{{Cite journal |last1=Haber |first1=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 |s2cid=2349082}} 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.{{Cite journal |title=Diurnal asymmetries in global radiation |first=I.G. |last=Piggin |journal= Archiv für Meteorologie, Geophysik und Bioklimatologie, Serie B|date=1972 |volume=20 |issue=1 |doi=10.1007/BF02243313 |pages=41–48|bibcode=1972AMGBB..20...41P|s2cid=118819800 }} [386] => [387] => 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.{{cite web |title=The Green Flash |url=https://www.bbc.co.uk/weather/features/understanding/greenflash.shtml |publisher=BBC |access-date=10 August 2008 |archive-url=https://web.archive.org/web/20081216135504/http://www.bbc.co.uk/weather/features/understanding/greenflash.shtml |archive-date=16 December 2008}} [388] => [389] => == Religious aspects == [390] => {{Main|Solar deity}} [391] => [[File:太阳神鸟金饰 Golden Sun Bird.png|thumb|[[Golden Sun Bird|Sun and Immortal Birds Gold Ornament]] by ancient Shu people. The center is a sun pattern with twelve points around which four [[Three-legged crow|birds]] fly in the same counterclockwise direction. [[Shu (kingdom)|Ancient Kingdom of Shu]], coinciding with the [[Shang dynasty]].]] [392] => Solar deities play a major role in many world religions and mythologies.{{cite book |last1=Coleman |first1=J.A. |last2=Davidson |first2=George |title=The Dictionary of Mythology: An A–Z of Themes, Legends, and Heroes |date=2015 |publisher=Arcturus Publishing Limited |location=London |isbn=978-1-78404-478-7 |page=316}} [[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, known as [[Surya]]. Many ancient monuments were constructed with solar phenomena in mind; for example, stone [[megalith]]s accurately mark the summer or winter [[solstice]] (for example in [[Nabta Playa]], Egypt; [[Mnajdra]], Malta; and [[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. [393] => [394] => The ancient [[Sumer]]ians believed that the Sun was [[Utu]],{{cite book |last1=Black |first1=Jeremy |first2=Anthony |last2=Green |title=Gods, Demons and Symbols of Ancient Mesopotamia: An Illustrated Dictionary |url=https://books.google.com/books?id=05LXAAAAMAAJ&q=Inana |publisher=The British Museum Press |year=1992 |isbn=978-0-7141-1705-8 |pages=182–184 |access-date=22 August 2020 |archive-date=20 November 2020 |archive-url=https://web.archive.org/web/20201120094829/https://books.google.com/books?id=05LXAAAAMAAJ&q=Inana |url-status=live }}{{citation |last=Nemet-Nejat |first=Karen Rhea |author-link=Karen Rhea Nemet-Nejat |date=1998 |title=Daily Life in Ancient Mesopotamia |publisher=Greenwood |isbn=978-0-313-29497-6 |page=[https://archive.org/details/dailylifeinancie00neme/page/203 203] |url=https://archive.org/details/dailylifeinancie00neme/page/203}} the god of justice and twin brother of [[Inanna]], the [[Queen of Heaven (antiquity)|Queen of Heaven]], who was identified as the planet Venus. Later, Utu was identified with the [[East Semitic]] god [[Shamash]]. Utu was regarded as a helper-deity, who aided those in distress. [395] => [[File:Maler der Grabkammer der Nefertari 001.jpg|thumb|Ra from the [[tomb of Nefertari]], 13th century BC]] [396] => [397] => From at least the [[Fourth 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]]. 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]].{{cite book |last1=Teeter |first1=Emily |title=Religion and Ritual in Ancient Egypt |date=2011 |publisher=Cambridge University Press |location=New York |isbn=978-0-521-84855-8}}{{cite book |last1=Frankfort |first1=Henri |title=Ancient Egyptian Religion: an Interpretation |date=2011 |publisher=Dover Publications |isbn=978-0-486-41138-5}}[[File:Ra Barque.jpg|thumb|Ra on the solar barque, adorned with the sun-disk]]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]].{{cite web |url=http://www.oxforddictionaries.com/definition/american_english/planet |publisher=Oxford Dictionaries |title=Planet |access-date=22 March 2015 |date=December 2007 |archive-date=2 April 2015 |archive-url=https://web.archive.org/web/20150402154243/http://www.oxforddictionaries.com/definition/american_english/planet |url-status=dead }}{{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 |bibcode=1997JHA....28....1G|doi=10.1177/002182869702800101|s2cid=118875902 }}{{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}} [398] => [399] => In [[Proto-Indo-European religion]], the Sun was personified as the goddess [[Sun deity|''*Seh₂ul'']].{{cite encyclopedia |date=1997 |title=Encyclopedia of Indo-European Culture |editor1-last=Mallory |editor1-first=James P. |editor1-link=J. P. Mallory |editor2-last=Adams |editor2-first=Douglas Q. |editor2-link=Douglas Q. Adams |place=London |publisher=Routledge |id=(EIEC) |url=https://books.google.com/books?id=tzU3RIV2BWIC&q=Sun+goddess |isbn=978-1-884964-98-5 |access-date=20 October 2017 |archive-date=31 March 2017 |archive-url=https://web.archive.org/web/20170331204930/https://books.google.com/books?id=tzU3RIV2BWIC&q=Sun+goddess |url-status=live }}{{cite book |last=Mallory |first=J.P. |date=1989 |title=In Search of the Indo-Europeans: Language, Archaeology and Myth |url=https://archive.org/details/insearchofindoeu00jpma |url-access=registration |page=[https://archive.org/details/insearchofindoeu00jpma/page/129 129] |publisher=[[Thames & Hudson]] |isbn=978-0-500-27616-7}} Derivatives of this goddess in [[Indo-European languages]] include the [[Old Norse]] ''[[Sól (sun)|Sól]]'', [[Sanskrit]] ''[[Surya]]'', [[Gaulish language|Gaulish]] ''[[Sulis]]'', [[Lithuanian language|Lithuanian]] ''[[Saulė]]'', and [[Slavic languages|Slavic]] ''Solntse''. In [[ancient Greek religion]], the sun deity was the male god Helios,[[Hesiod]], ''[[Theogony]]'' [https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.01.0130%3Acard%3D371 371] {{Webarchive|url=https://web.archive.org/web/20210915222218/https://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.01.0130:card%3D371 |date=15 September 2021 }} who in later times was [[syncretism|syncretized]] with [[Apollo]].{{cite book |last=Burkert |first=Walter |author-link=Walter Burkert |date=1985 |title=Greek Religion |publisher=Harvard University Press |location=Cambridge |isbn=978-0-674-36281-9 |page=120}} [400] => [401] => In the [[Bible]], Malachi 4:2 mentions the "Sun of Righteousness" (sometimes translated as the "Sun of Justice"),{{Bibleverse||Malachi|4:2|9}}{{citation |title=Bible, Book of Malachi |publisher=King James Version |url=https://www.biblegateway.com/passage/?search=Malachi+4&version=KJV |access-date=20 October 2017 |archive-date=20 October 2017 |archive-url=https://web.archive.org/web/20171020140215/https://www.biblegateway.com/passage/?search=Malachi+4&version=KJV |url-status=live }} which some [[Christianity|Christians]] have interpreted as a reference to the [[Messiah]] ([[Christ]]).{{cite book |last=Spargo |first=Emma Jane Marie |title=The Category of the Aesthetic in the Philosophy of Saint Bonaventure |url=https://books.google.com/books?id=SUkWAAAAMAAJ&q=sol+iustitiae+malachiae+IV+2&pg=PA86 |page=86 |year=1953 |publisher=The Franciscan Institute |location=St. Bonaventure, New York; E. Nauwelaerts, Louvain, Belgium; F. Schöningh, Paderborn, Germany |access-date=3 November 2020 |archive-date=17 April 2021 |archive-url=https://web.archive.org/web/20210417054707/https://books.google.com/books?id=SUkWAAAAMAAJ&q=sol+iustitiae+malachiae+IV+2&pg=PA86 |url-status=live }} In ancient Roman culture, [[Sunday]] was the day of the sun god. In paganism, the Sun was a source of life, giving warmth and illumination. 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]]). It was adopted as the [[Sabbath]] day by Christians. The symbol of light was a pagan device adopted by Christians, and perhaps the most important one that did not come from Jewish traditions. Christian churches were built so that the congregation faced toward the sunrise.{{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 |isbn=978-0-312-18723-1 |access-date=15 November 2015 |archive-date=18 May 2016 |archive-url=https://web.archive.org/web/20160518085001/https://books.google.com/books?id=qugouOh3KjMC&pg=PA22 |url-status=live }} [402] => [403] => [[Tonatiuh]], the Aztec god of the sun,{{cite book |title=State and Cosmos in the Art of Tenochtitlan |url=https://archive.org/details/statecosmosinart00town |url-access=registration |last=Townsend |first=Richard |publisher=Dumbarton Oaks |year=1979 |location=Washington, DC |page=[https://archive.org/details/statecosmosinart00town/page/66 66]}} was closely associated with the practice of [[human sacrifice]]. The sun goddess [[Amaterasu]] is the most important deity in the [[Shinto]] religion,{{cite book |last=Roberts |first=Jeremy |title=Japanese Mythology A To Z |location=New York |publisher=[[Chelsea House Publishers]] |year=2010 |edition=2nd |isbn=978-1-60413-435-3 |pages=4–5}}{{cite book |last=Wheeler |first=Post |title=The Sacred Scriptures of the Japanese |location=New York |publisher=Henry Schuman |pages=393–395 |year=1952 |isbn=978-1-4254-8787-4}} and she is believed to be the direct ancestor of all [[List of Emperors of Japan|Japanese emperors]]. [404] => [405] => == See also == [406] => {{Portal|Astronomy|Stars|Solar System|Weather|Physics}} [407] => {{div col|colwidth=30em}} [408] => * {{Annotated link |Advanced Composition Explorer}} [409] => * {{Annotated link |Analemma}} [410] => * {{Annotated link |Antisolar point}} [411] => * [[Circled dot (disambiguation)|Circled dot]]{{snd}} other uses of the Sun symbol and similar symbols [412] => * {{Annotated link |List of brightest stars}} [413] => * {{Annotated link |List of nearest stars and brown dwarfs}} [414] => * {{Annotated link |Midnight sun}} [415] => * {{slink|Planets in astrology|Sun}} [416] => * {{Annotated link |Solar telescope}} [417] => * {{Annotated link |Sun path}} [418] => * {{Annotated link |Sun-Earth Day}} [419] => * {{Annotated link |Timeline of the far future}} [420] => {{div col end}} [421] => [422] => == Notes == [423] => {{reflist|group=note}} [424] => {{notelist [425] => | notes = [426] => {{efn [427] => | name = heavy elements [428] => | In [[astronomy|astronomical sciences]], the term ''heavy elements'' (or ''metals'') refers to all chemical elements except hydrogen and helium. [429] => }} [430] => {{efn [431] => | name = particle density [432] => | Earth's atmosphere near sea level has a particle density of about 2{{e|25}} m⁻³. [433] => }} [434] => {{efn [435] => | name=rotation [436] => | Counterclockwise is also the direction of revolution around the Sun for objects in the Solar System and is the direction of axial spin for most objects. [437] => }} [438] => }} [439] => [440] => == References == [441] => {{reflist}} [442] => [443] => == Further reading == [444] => * {{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=978-1-4000-6875-3}} [445] => * {{Cite journal |last=Hudson |first=Hugh |date=2008 |title=Solar Activity |journal=[[Scholarpedia]] |volume=3 |issue=3 |page=3967 |doi=10.4249/scholarpedia.3967|bibcode=2008SchpJ...3.3967H |doi-access=free }} [446] => * {{Cite journal |last=Thompson |first=M.J. |date=August 2004 |title=Solar interior: Helioseismology and the Sun's interior |journal=[[Astronomy & Geophysics]] |volume=45 |issue=4 |doi=10.1046/j.1468-4004.2003.45421.x |pages=21–25|bibcode=2004A&G....45d..21T |doi-access=free }} [447] => [448] => == External links == [449] => {{Spoken Wikipedia|date=2021-06-07|En-Sun.ogg}} [450] => {{Sister project links|Sun|v=no|n=no|b=no|s=no}} [451] => * [http://www.astronomycast.com/astronomy/episode-30-the-sun-spots-and-all/ Astronomy Cast: The Sun] [452] => * [http://www.acrim.com/ Satellite observations of solar luminosity] {{Webarchive|url=https://web.archive.org/web/20170611210135/http://acrim.com/ |date=11 June 2017 }} [453] => * [https://www.youtube.com/watch?v=qpMRtvFD8ek&hl=fr Animation – The Future of the Sun] [454] => * [https://www.youtube.com/watch?v=6tmbeLTHC_0 "Thermonuclear Art – The Sun In Ultra-HD"] | [[Goddard Space Flight Center]] [455] => * [https://www.youtube.com/watch?v=l3QQQu7QLoM "A Decade of Sun"] | Goddard Space Flight Center [456] => [457] => {{The Sun|state=uncollapsed}} [458] => {{Sun spacecraft}} [459] => {{Solar System}} [460] => {{Star}} [461] => {{Nearest star systems|1}} [462] => {{Astronomy navbar}} [463] => {{Authority control}} [464] => [465] => [[Category:Sun| ]] [466] => [[Category:Articles containing video clips]] [467] => [[Category:Astronomical objects known since antiquity]] [468] => [[Category:G-type main-sequence stars]] [469] => [[Category:Light sources]] [470] => [[Category:Space plasmas]] [471] => [[Category:Stars with proper names]] [] => )

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Sun

The Sun is the star at the center of the Solar System, and it is by far the most important source of energy for life on Earth. It is composed of hot plasma and has an approximate diameter of about 1.

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It is composed of hot plasma and has an approximate diameter of about 1. 39 million kilometers, which is about 109 times that of Earth. The Sun accounts for about 99. 86% of the total mass of the Solar System and its gravitational pull governs the motion of all other celestial bodies. The Sun is classified as a G-type main-sequence star, commonly referred to as a yellow dwarf. Its surface temperature is around 5,500 degrees Celsius (9,932 degrees Fahrenheit), and it radiates energy in the form of light, heat, and other types of electromagnetic radiation. This energy is produced through the process of nuclear fusion, where hydrogen atoms combine to form helium, releasing huge amounts of energy in the process. The Sun plays a crucial role in sustaining life on Earth, as it provides the heat and light necessary for photosynthesis, the process by which plants convert sunlight into food. Additionally, the Sun's gravitational pull influences the orbits of planets and other objects in the Solar System. Scientists have conducted extensive research on the Sun, and various space missions and telescopes have been deployed to study its structure, behavior, and effects on Earth. These studies have led to a deeper understanding of solar physics, the formation of the Solar System, and the potential of harnessing solar energy for various practical applications. In popular culture, the Sun has been a subject of fascination and worship for centuries. It has been depicted in ancient mythologies, artworks, and literature, and it continues to inspire awe and curiosity among people worldwide.

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A supernova is a powerful and luminous event that occurs when a star explodes, releasing an incredible amount of energy.

Thermodynamics

Thermodynamics is a branch of physics that deals with the relationship between heat, work, and energy.

Watt

Watt is the SI unit of power, named after the Scottish engineer James Watt.

X-ray

X-ray, a type of electromagnetic radiation, is the topic of the Wikipedia page.