Sun - General description

3/29/2015 SunWikipedia,thefreeencyclopedia

http://en.wikipedia.org/wiki/Sun 1/36

TheSun

PictureoftheSuninextremeultravioletshowingitsturbulentsurface.Observationdata

MeandistancefromEarth

1.496 108km8min19satlightspeed

Visualbrightness(V)

26.74[1]

Absolutemagnitude

4.83[1]

Spectralclassification

G2V

Metallicity Z=0.0122[2]

Angularsize 31.632.7[3]

Adjectives Solar

Orbitalcharacteristics

MeandistancefromMilkyWay

core

2.7 1017km27 200lightyears

Galacticperiod

(2.252.50) 108a

Velocity 220km/s(orbitaroundthecenteroftheMilkyWay)

SunFromWikipedia,thefreeencyclopedia

TheSun[a]isthestaratthecenteroftheSolarSystem.ItisbyfarthemostimportantsourceofenergyforlifeonEarth.TheSunisanearlyperfectsphericalballofhotplasma,[12][13]withinternalconvectivemotionthatgeneratesamagneticfieldviaadynamoprocess.[14]ThediameteroftheSunisabout109timesthatofEarth,andithasamassabout330,000timesthatofEarth,accountingforabout99.86%ofthetotalmassoftheSolarSystem.[15]Chemically,aboutthreequartersoftheSun'smassconsistsofhydrogen,whereastherestismostlyhelium,andmuchsmallerquantitiesofheavierelements,includingoxygen,carbon,neonandiron.[16]

TheSunisaGtypemainsequencestar(G2V)basedonspectralclassanditisinformallydesignatedasayellowdwarf.Itformedapproximately4.567billion[b][17]yearsagofromthegravitationalcollapseofmatterwithinaregionofalargemolecularcloud.Mostofthismattergatheredinthecenter,whereastherestflattenedintoanorbitingdiskthatbecametheSolarSystem.Thecentralmassbecameincreasinglyhotanddense,eventuallyinitiatingthermonuclearfusioninitscore.Itisthoughtthatalmostallstarsformbythisprocess.TheSunisroughlymiddleageandhasnotchangeddramaticallyforfourbillion[b]years,andwillremainfairlystableforfourbillionmore.However,afterhydrogenfusioninitscorehasstopped,theSunwillundergoseverechangesandbecomearedgiant.ItiscalculatedthattheSunwillbecomesufficientlylargetoengulfthecurrentorbitsofMercury,Venus,andpossiblyEarth.

TheenormouseffectoftheSunontheEarthhasbeenrecognizedsinceprehistorictimes,andtheSunhasbeenregardedbysomeculturesasadeity.Earth'smovementaroundtheSunisthebasisofthesolarcalendar,whichisthepredominantcalendarinusetoday.

Contents

http://en.wikipedia.org/wiki/File:The_Sun_in_extreme_ultraviolet.jpghttp://en.wikipedia.org/wiki/Angular_sizehttp://en.wikipedia.org/wiki/Venushttp://en.wikipedia.org/wiki/Solar_calendarhttp://en.wikipedia.org/wiki/Earthhttp://en.wikipedia.org/wiki/Apparent_magnitudehttp://en.wikipedia.org/wiki/Neonhttp://en.wikipedia.org/wiki/Plasma_(physics)http://en.wikipedia.org/wiki/Absolute_magnitudehttp://en.wikipedia.org/wiki/Orbithttp://en.wikipedia.org/wiki/The_Sun_in_culturehttp://en.wikipedia.org/wiki/Solar_dynamohttp://en.wikipedia.org/wiki/File:Sun_symbol.svghttp://en.wikipedia.org/wiki/Metallicityhttp://en.wikipedia.org/wiki/Thermonuclear_fusionhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Molecular_cloudhttp://en.wikipedia.org/wiki/Julian_year_(astronomy)http://en.wikipedia.org/wiki/G-type_main-sequence_starhttp://en.wikipedia.org/wiki/Galactic_yearhttp://en.wikipedia.org/wiki/Milky_Wayhttp://en.wikipedia.org/wiki/Earthhttp://en.wikipedia.org/wiki/Speed_of_lighthttp://en.wikipedia.org/wiki/Starhttp://en.wikipedia.org/wiki/Hydrogenhttp://en.wikipedia.org/wiki/Stellar_classificationhttp://en.wikipedia.org/wiki/Spectral_classificationhttp://en.wikipedia.org/wiki/Carbonhttp://en.wikipedia.org/wiki/Wikipedia:Protection_policy#semihttp://en.wikipedia.org/wiki/Velocityhttp://en.wikipedia.org/wiki/Solar_Systemhttp://en.wikipedia.org/wiki/Red_gianthttp://en.wikipedia.org/wiki/Prehistoric_timeshttp://en.wikipedia.org/wiki/Mercury_(planet)http://en.wikipedia.org/wiki/Light-yearhttp://en.wikipedia.org/wiki/Formation_and_evolution_of_the_Solar_Systemhttp://en.wikipedia.org/wiki/Wikipedia:Featured_articleshttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Star_formationhttp://en.wikipedia.org/wiki/Heliumhttp://en.wikipedia.org/wiki/Sun_deityhttp://en.wikipedia.org/wiki/Solar_mass



20km/s(relativetoaveragevelocityofotherstarsinstellarneighborhood)

370km/s[4](relativetothecosmicmicrowavebackground)

Physicalcharacteristics

Equatorialradius

696 342 65km[5]

109Earth[6]

Equatorialcircumference

4.379 106km[6]

109Earth[6]

Flattening 9 106

Surfacearea 6.09 1012km2[6]

12 000Earth[6]

Volume 1.41 1018km3[6]

1 300 000Earth

Mass (1.988 55 0.000 25) 1030kg[1]

333 000Earth[1]

Averagedensity

1.408g/cm3[1][6][7]

0.255Earth[1][6]

Centerdensity(modeled)

162.2g/cm3[1]

12.4Earth

Equatorialsurfacegravity

274.0m/s2[1]

27.94g27 542.29cgs

28Earth[6]

Escapevelocity(fromthe

surface)

617.7km/s[6]

55Earth[6]

Temperature Center(modeled):1.57 107K[1]

Photosphere(effective):5 778K[1]

Corona:5 106K

Luminosity(Lsol)

3.846 1026W[1]

3.75 1028lm

98lm/Wefficacy

Meanradiance(Isol)

2.009 107Wm2sr1

Age 4.6billionyears[8][9]

1Nameandetymology2Characteristics3Sunlight4Composition

4.1Singlyionizedirongroupelements4.2Solarandplanetarymassfractionationrelationship

5Structure5.1Core5.2Radiativezone5.3Tachocline5.4Convectivezone5.5Photosphere5.6Atmosphere5.7Photonsandneutrinos

6Magnetismandactivity6.1Magneticfield6.2Variationinactivity6.3Longtermchange

7Lifephases7.1Formation7.2Mainsequence7.3Aftercorehydrogenexhaustion7.4Earth'sfate

8Motionandlocation8.1OrbitinMilkyWay

9Theoreticalproblems9.1Coronalheatingproblem9.2FaintyoungSunproblem

10Historyofobservation10.1Earlyunderstanding10.2Developmentofscientificunderstanding10.3Solarspacemissions

11Observationandeffects12Seealso13Notes14References15Furtherreading

http://en.wikipedia.org/wiki/G-forcehttp://en.wikipedia.org/wiki/Luminosityhttp://en.wikipedia.org/wiki/Densityhttp://en.wikipedia.org/wiki/Radiushttp://en.wikipedia.org/wiki/Surface_areahttp://en.wikipedia.org/wiki/Cosmic_microwave_background_radiation#CMBR_dipole_anisotropyhttp://en.wikipedia.org/wiki/Surface_gravityhttp://en.wikipedia.org/wiki/Luminous_efficacyhttp://en.wikipedia.org/wiki/Cgshttp://en.wikipedia.org/wiki/Flatteninghttp://en.wikipedia.org/wiki/Watthttp://en.wikipedia.org/wiki/Volumehttp://en.wikipedia.org/wiki/Kelvinhttp://en.wikipedia.org/wiki/Lumen_(unit)http://en.wikipedia.org/wiki/Radiancehttp://en.wikipedia.org/wiki/Photospherehttp://en.wikipedia.org/wiki/Coronahttp://en.wikipedia.org/wiki/Escape_velocityhttp://en.wikipedia.org/wiki/Circumferencehttp://en.wikipedia.org/wiki/Densityhttp://en.wikipedia.org/wiki/Mass



Rotationcharacteristics

Obliquity 7.25[1]

(totheecliptic)

67.23(tothegalacticplane)

Rightascension

ofNorthpole[10]

286.1319h4min30s

DeclinationofNorthpole

+63.876352'North

Siderealrotationperiod(atequator)

25.05days[1]

(at16latitude) 25.38days[1]

25d9h7min12s[10]

(atpoles) 34.4days[1]

Rotationvelocity(atequator)

7.189 103km/h[6]

Photosphericcomposition(bymass)

Hydrogen 73.46%[11]

Helium 24.85%

Oxygen 0.77%

Carbon 0.29%

Iron 0.16%

Neon 0.12%

Nitrogen 0.09%

Silicon 0.07%

Magnesium 0.05%

Sulfur 0.04%

16Externallinks

Nameandetymology

TheEnglishpropernounSundevelopedfromOldEnglishsunneandmayberelatedtosouth.CognatestoEnglishsunappearinotherGermaniclanguages,includingOldFrisiansunne,sonne,OldSaxonsunna,MiddleDutchsonne,modernDutchzon,OldHighGermansunna,modernGermanSonne,OldNorsesunna,andGothicsunn.AllGermanictermsfortheSunstemfromProtoGermanic*sunnn.[18][19]

TheSunisviewedasagoddessinGermanicpaganism,Sl/Sunna.[19]ScholarstheorizethattheSun,asaGermanicgoddess,mayrepresentanextensionofanearlierProtoIndoEuropeanSundeityduetoIndoEuropeanlinguisticconnectionsbetweenOldNorseSl,SanskritSurya,GaulishSulis,LithuanianSaul,andSlavicSolntse.[19]

TheEnglishweekdaynameSundaystemsfromOldEnglish(Sunnandg"Sun'sday",frombefore700)andisultimatelyaresultofaGermanicinterpretationofLatindiessolis,itselfatranslationoftheGreek(hmrahlou).[20]TheLatinnamefortheSun,Sol,iswidelyknownbutisnotcommoningeneralEnglishlanguageusetheadjectivalformistherelatedwordsolar.[21][22]Thetermsolisalsousedbyplanetaryastronomerstorefertothedurationofasolardayonanotherplanet,suchasMars.[23]AmeanEarthsolardayisapproximately24hours,whereasameanMartian'sol'is24hours,39minutes,and35.244seconds.[24]

Characteristics

TheSunisaGtypemainsequencestarthatcomprisesabout99.86%ofthemassoftheSolarSystem.Onceregardedbyastronomersasasmallandrelativelyinsignificantstar,theSunhasanabsolutemagnitudeof+4.83.Thisisnowestimatedtobebrighterthanabout85%ofthestarsintheMilkyWay,mostofwhicharereddwarfs.[25][26]TheSunisaPopulationI,orheavyelementrich,[c]star.[27]TheformationoftheSunmayhavebeentriggeredbyshockwavesfromoneormorenearbysupernovae.[28]ThisissuggestedbyahighabundanceofheavyelementsintheSolarSystem,suchasgoldanduranium,relativetotheabundancesoftheseelementsinsocalledPopulationII,heavyelementpoor,stars.Theseelementscouldmostplausiblyhavebeenproducedbyendothermicnuclearreactionsduringasupernova,orbytransmutationthroughneutronabsorptionwithinamassivesecondgenerationstar.[27]

http://en.wikipedia.org/wiki/Nuclear_transmutationhttp://en.wikipedia.org/wiki/Magnesiumhttp://en.wikipedia.org/wiki/Interpretatio_germanicahttp://en.wikipedia.org/wiki/Old_Norsehttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Galactic_planehttp://en.wikipedia.org/wiki/Uraniumhttp://en.wikipedia.org/wiki/Gothic_languagehttp://en.wikipedia.org/wiki/G-type_main-sequence_starhttp://en.wikipedia.org/wiki/Nitrogenhttp://en.wikipedia.org/wiki/S%C3%B3l_(sun)http://en.wikipedia.org/wiki/Sulishttp://en.wikipedia.org/wiki/Declinationhttp://en.wikipedia.org/wiki/Sulfurhttp://en.wikipedia.org/wiki/Endothermichttp://en.wikipedia.org/wiki/Population_I_starshttp://en.wikipedia.org/wiki/Germanic_paganismhttp://en.wikipedia.org/wiki/Abundance_of_the_chemical_elementshttp://en.wikipedia.org/wiki/Earthhttp://en.wikipedia.org/wiki/Gaulish_languagehttp://en.wikipedia.org/wiki/Old_Frisianhttp://en.wikipedia.org/wiki/Axial_tilthttp://en.wikipedia.org/wiki/Neonhttp://en.wikipedia.org/wiki/Solar_rotationhttp://en.wikipedia.org/wiki/Marshttp://en.wikipedia.org/wiki/Old_Englishhttp://en.wikipedia.org/wiki/Slavic_languageshttp://en.wikipedia.org/wiki/Ecliptichttp://en.wikipedia.org/wiki/Suryahttp://en.wikipedia.org/wiki/Right_ascensionhttp://en.wikipedia.org/wiki/Saul%C4%97http://en.wikipedia.org/wiki/Hydrogenhttp://en.wikipedia.org/wiki/Heliumhttp://en.wikipedia.org/wiki/Rotationhttp://en.wikipedia.org/wiki/Old_Saxonhttp://en.wikipedia.org/wiki/Germanic_languageshttp://en.wikipedia.org/wiki/Solar_dayhttp://en.wikipedia.org/wiki/Proto-Indo-Europeanshttp://en.wikipedia.org/wiki/Indo-European_languageshttp://en.wikipedia.org/wiki/Sanskrithttp://en.wikipedia.org/wiki/Old_High_Germanhttp://en.wikipedia.org/wiki/Carbonhttp://en.wikipedia.org/wiki/Milky_Wayhttp://en.wikipedia.org/wiki/Lithuanian_languagehttp://en.wikipedia.org/wiki/Absolute_magnitudehttp://en.wikipedia.org/wiki/Red_dwarfhttp://en.wikipedia.org/wiki/Supernovahttp://en.wikipedia.org/wiki/Siliconhttp://en.wikipedia.org/wiki/Dutch_languagehttp://en.wikipedia.org/wiki/Proto-Germanichttp://en.wikipedia.org/wiki/Middle_Dutchhttp://en.wikipedia.org/wiki/Population_IIhttp://en.wikipedia.org/wiki/Photospherehttp://en.wikipedia.org/wiki/Neutron_absorption



ThisvideotakesSolarDynamicsObservatoryimagesandappliesadditionalprocessingtoenhancethestructuresvisible.Theeventsinthisvideorepresent24hoursofactivityon25September2011.

TheSunisbyfarthebrightestobjectinthesky,withanapparentmagnitudeof26.74.[29][30]Thisisabout13billiontimesbrighterthanthenextbrighteststar,Sirius,whichhasanapparentmagnitudeof1.46.ThemeandistanceoftheSuntoEarthisapproximately1astronomicalunit(about150,000,000km93,000,000mi),thoughthedistancevariesasEarthmovesfromperihelioninJanuarytoaphelioninJuly.[31]Atthisaveragedistance,lighttravelsfromtheSuntoEarthinabout8minutesand19seconds.Theenergyofthissunlightsupportsalmostalllife[d]onEarthbyphotosynthesis,[32]anddrivesEarth'sclimateandweather.

TheSun'sradiuscanbemeasuredfromitscentertotheedgeofthephotosphere,theapparentvisiblesurfaceoftheSun.[33]TheSunisanearperfectspherewithanoblatenessestimatedatabout9millionths,[34]whichmeansthatitspolardiameterdiffersfromitsequatorialdiameterbyonly10kilometres(6.2mi).[35]ThetidaleffectoftheplanetsisweakanddoesnotsignificantlyaffecttheshapeoftheSun.[36]TheSunrotatesfasteratitsequatorthanatitspoles.ThisdifferentialrotationiscausedbyconvectivemotionduetoheattransportandtheCoriolisforceduetotheSun'srotation.Inaframeofreferencedefinedbythestars,therotationalperiodisapproximately25.6daysattheequatorand33.5daysatthepoles.ViewedfromEarthasitorbitstheSun,theapparentrotationalperiodoftheSunatitsequatorisabout28days.[37]

TheSundoesnothaveadefiniteboundary,andinitsouterpartsitsdensitydecreasesexponentiallywithincreasingdistancefromitscenter.[38]Thesolarinteriorisnotdirectlyobservable,andtheSunitselfisopaquetoelectromagneticradiation.However,justasseismologyuseswavesgeneratedbyearthquakestorevealtheinteriorstructureofEarth,thedisciplineofhelioseismologymakesuseofpressurewaves(infrasound)traversingtheSun'sinteriortomeasureandvisualizeitsinnerstructure.[39]ComputermodelingoftheSunisalsousedasatheoreticaltooltoinvestigateitsdeeperlayers.

Duringatotalsolareclipse,whenthediskoftheSuniscoveredbythatoftheMoon,theSun'ssurroundingatmosphere,thecorona,canbeseen.Asthecoronaexpandsoutwardintospace,creatingthesolarwind,astreamofchargedparticles.Thespatialextentoftheinfluenceofthesolarwinddefinestheheliosphere,a"bubble"intheinterstellarmediumthatisroughly100astronomicalunitsinradius,thelargestcontinuousstructureintheSolarSystem.[40][41]Theouterboundaryoftheheliosphereistheheliopause.

Sunlight

TheSun'scoloriswhite,withaCIEcolorspaceindexnear(0.3,0.3),whenviewedfromspaceorwhenhighintheskywhenlowinthesky,atmosphericscatteringrenderstheSunyellow,red,orange,ormagenta.Despiteitstypicalwhiteness,mostpeoplementallypicturetheSunasyellowthereasonsforthisarethesubjectofdebate.[42]TheSunisaG2Vstar,withG2indicatingitssurfacetemperatureofapproximately5,778K(5,505C,9,941F),andVthatit,likemoststars,isamainsequencestar.[43][44]TheluminanceoftheSunisabout1.88gigacandelapersquaremetre,butasviewedthroughEarth'satmosphere,thisisloweredtoabout1.44Gcd/m2.[45]

http://en.wikipedia.org/wiki/Luminancehttp://en.wikipedia.org/wiki/Coriolis_effecthttp://en.wikipedia.org/wiki/Diffuse_sky_radiationhttp://en.wikipedia.org/wiki/Solar_rotationhttp://en.wikipedia.org/wiki/Helioseismologyhttp://en.wikipedia.org/wiki/Astronomical_unithttp://en.wikipedia.org/wiki/Infrasoundhttp://en.wikipedia.org/wiki/Aphelionhttp://en.wikipedia.org/wiki/Poles_of_astronomical_bodieshttp://en.wikipedia.org/wiki/Oblatenesshttp://en.wikipedia.org/wiki/Equatorhttp://en.wikipedia.org/wiki/Apparent_magnitudehttp://en.wikipedia.org/wiki/Perihelionhttp://en.wikipedia.org/wiki/Coronahttp://en.wikipedia.org/wiki/Heliopause_(astronomy)http://en.wikipedia.org/wiki/Candela_per_square_metrehttp://en.wikipedia.org/wiki/Interstellar_mediumhttp://en.wikipedia.org/wiki/Apparent_magnitudehttp://en.wikipedia.org/wiki/Solar_windhttp://en.wikipedia.org/wiki/Gigahttp://en.wikipedia.org/wiki/Seismologyhttp://en.wikipedia.org/wiki/Convectionhttp://en.wikipedia.org/wiki/Sunlighthttp://en.wikipedia.org/wiki/Photospherehttp://en.wikipedia.org/wiki/Photosynthesishttp://en.wikipedia.org/wiki/Siriushttp://en.wikipedia.org/wiki/Solar_eclipsehttp://en.wikipedia.org/wiki/CIE_1931_color_spacehttp://en.wikipedia.org/wiki/G-type_main-sequence_starhttp://en.wikipedia.org/wiki/Main_sequencehttp://en.wikipedia.org/wiki/Astronomical_unitshttp://en.wikipedia.org/wiki/Electromagnetic_radiationhttp://en.wikipedia.org/wiki/Computer_modelinghttp://en.wikipedia.org/wiki/Heliospherehttp://en.wikipedia.org/wiki/Effective_temperaturehttp://en.wikipedia.org/wiki/Solar_Dynamics_Observatory



SunlightisEarth'sprimarysourceofenergy.TheothersignificantsourceofEarth'senergyisthestoreoffissionablematerialsgeneratedbythecataclysmicdeathofotherstars.ThesefissionablematerialstrappedinEarth'scrustgiverisetogeothermalenergy,whichdrivesthevolcanismonEarthandalsomakesitpossibleforhumanstofuelnuclearreactors.ThesolarconstantistheamountofpowerthattheSundepositsperunitareathatisdirectlyexposedtosunlight.Thesolarconstantisequaltoapproximately1,368W/m2(wattspersquaremeter)atadistanceofoneastronomicalunit(AU)fromtheSun(thatis,onornearEarth).[46]SunlightonthesurfaceofEarthisattenuatedbyEarth'satmospheresothatlesspowerarrivesatthesurfacecloserto1,000W/m2inclearconditionswhentheSunisnearthezenith.[47]SunlightatthetopofEarth'satmosphereiscomposed(bytotalenergy)ofabout50%infraredlight,40%visiblelight,and10%ultravioletlight.[48]Theatmosphereinparticularfiltersoutover70%ofsolarultraviolet,especiallyattheshorterwavelengths.[49]SolarultravioletradiationionizestheEarth'sdaysideupperatmosphere,creatingtheelectricallyconductingionosphere.[50]

Solarenergycanbeharnessedbyavarietyofnaturalandsyntheticprocessesphotosynthesisbyplantscapturestheenergyofsunlightandconvertsittochemicalform(oxygenandreducedcarboncompounds),whereasdirectheatingorelectricalconversionbysolarcellsareusedbysolarpowerequipmenttogenerateelectricityortodootherusefulwork,sometimesemployingconcentratingsolarpower(thatitismeasuredinsuns).Theenergystoredinpetroleumandotherfossilfuelswasoriginallyconvertedfromsunlightbyphotosynthesisinthedistantpast.[51]

Contrarytoapopularmisconception,theSuniswhiteratherthanyellowwhenviewedfromspaceorwhensufficientlyhighinthesky.[42]

TheSunasitappearsfromthesurfaceofEarthatsunrise.

TheSunasitappearsfromthesurfaceofEarthatsunset.

http://en.wikipedia.org/wiki/File:Actual_Sunrise.jpeghttp://en.wikipedia.org/wiki/Ionospherehttp://en.wikipedia.org/wiki/File:Sun_white.jpghttp://en.wikipedia.org/wiki/Solar_cellshttp://en.wikipedia.org/wiki/File:Actual_Sunset.jpghttp://en.wikipedia.org/wiki/Nuclear_reactorshttp://en.wikipedia.org/wiki/Photosynthesishttp://en.wikipedia.org/wiki/Photosynthesishttp://en.wikipedia.org/wiki/Zenithhttp://en.wikipedia.org/wiki/Attenuation_(electromagnetic_radiation)http://en.wikipedia.org/wiki/Solar_powerhttp://en.wikipedia.org/wiki/Concentrating_solar_powerhttp://en.wikipedia.org/wiki/Astronomical_unithttp://en.wikipedia.org/wiki/Geothermal_energyhttp://en.wikipedia.org/wiki/Ultraviolet_radiationhttp://en.wikipedia.org/wiki/Solar_constant



ImagetakenbyNASASTEREOprobeslaunchedin2006utilizingtwospacecrafttoimagetheSunattheextremeUVwavelength(171).

Composition

TheSuniscomposedprimarilyofthechemicalelementshydrogenandheliumtheyaccountfor74.9%and23.8%ofthemassoftheSuninthephotosphere,respectively.[52]Allheavierelements,calledmetalsinastronomy,accountforlessthan2%ofthemass.Themostabundantmetalsareoxygen(roughly1%oftheSun'smass),carbon(0.3%),neon(0.2%),andiron(0.2%).[53]

TheSuninheriteditschemicalcompositionfromtheinterstellarmediumoutofwhichitformed.ThehydrogenandheliumintheSunwereproducedbyBigBangnucleosynthesis,andthemetalswereproducedbystellarnucleosynthesisingenerationsofstarsthatcompletedtheirstellarevolutionandreturnedtheirmaterialtotheinterstellarmediumbeforetheformationoftheSun.[54]ThechemicalcompositionofthephotosphereisnormallyconsideredrepresentativeofthecompositionoftheprimordialSolarSystem.[55]However,sincetheSunformed,someoftheheliumandheavyelementshavegravitationallysettledfromthephotosphere.Therefore,intoday'sphotospheretheheliumfractionisreducedandthemetallicityisonly84%ofthatintheprotostellarphase(beforenuclearfusioninthecorestarted).TheprotostellarSun'scompositionwasreconstructedas71.1%hydrogen,27.4%helium,and1.5%metals.[52]

IntheinnerportionsoftheSun,nuclearfusionhasmodifiedthecompositionbyconvertinghydrogenintohelium,sotheinnermostportionoftheSunisnowroughly60%helium,withthemetalabundanceunchanged.BecausetheinterioroftheSunisradiative,notconvective(seeRadiativezonebelow),noneofthefusionproductsfromthecorehaverisentothephotosphere.[56]

Thereactivecorezoneof"hydrogenburning",wherehydrogenisconvertedintohelium,isstartingtosurroundthecoreof"heliumash".ThisdevelopmentwillcontinueandwilleventuallycausetheSuntoleavethemainsequence,tobecomearedgiant[57]

ThesolarheavyelementabundancesdescribedabovearetypicallymeasuredbothusingspectroscopyoftheSun'sphotosphereandbymeasuringabundancesinmeteoritesthathaveneverbeenheatedtomeltingtemperatures.ThesemeteoritesarethoughttoretainthecompositionoftheprotostellarSunandarethusnotaffectedbysettlingofheavyelements.Thetwomethodsgenerallyagreewell.[16]

Singlyionizedirongroupelements

Inthe1970s,muchresearchfocusedontheabundancesofirongroupelementsintheSun.[58][59]Althoughsignificantresearchwasdone,theabundancedeterminationofsomeirongroupelements(e.g.cobaltandmanganese)wasstilldifficultatleastasfaras1978becauseoftheirhyperfinestructures.[58]

Thefirstlargelycompletesetofoscillatorstrengthsofsinglyionizedirongroupelementsweremadeavailablefirstinthe1960s,[60]andtheseweresubsequentlyimproved.[61]In1978theabundancesof'singlyIonized'elementsoftheirongroupwerederived.[58]

Solarandplanetarymassfractionationrelationship

http://en.wikipedia.org/wiki/Interstellar_mediumhttp://en.wikipedia.org/wiki/Big_Bang_nucleosynthesishttp://en.wikipedia.org/wiki/Astronomical_spectroscopyhttp://en.wikipedia.org/wiki/Hydrogenhttp://en.wikipedia.org/wiki/Cobalthttp://en.wikipedia.org/wiki/Meteoriteshttp://en.wikipedia.org/wiki/File:Sun_STEREO_4dec2006_lrg.jpghttp://en.wikipedia.org/wiki/Protostarhttp://en.wikipedia.org/wiki/Metallicityhttp://en.wikipedia.org/wiki/Heliumhttp://en.wikipedia.org/wiki/Hyperfine_structurehttp://en.wikipedia.org/wiki/Iron_grouphttp://en.wikipedia.org/wiki/Oscillator_strengthhttp://en.wikipedia.org/wiki/Red_gianthttp://en.wikipedia.org/wiki/Metallicityhttp://en.wikipedia.org/wiki/Manganesehttp://en.wikipedia.org/wiki/Extreme_UVhttp://en.wikipedia.org/wiki/Stellar_evolutionhttp://en.wikipedia.org/wiki/Main_sequencehttp://en.wikipedia.org/wiki/Chemical_elementhttp://en.wikipedia.org/wiki/Stellar_nucleosynthesishttp://en.wikipedia.org/wiki/%C3%85ngstrom



ThestructureoftheSun

Variousauthorshaveconsideredtheexistenceofamassfractionationrelationshipbetweentheisotopiccompositionsofsolarandplanetarynoblegases,[62]forexamplecorrelationsbetweenisotopiccompositionsofplanetaryandsolarneonandxenon.[63]Nevertheless,thebeliefthatthewholeSunhasthesamecompositionasthesolaratmospherewasstillwidespread,atleastuntil1983.[64]

In1983,itwasclaimedthatitwasthefractionationintheSunitselfthatcausedthefractionationrelationshipbetweentheisotopiccompositionsofplanetaryandsolarwindimplantednoblegases.[64]

Structure

Core

ThecoreoftheSunextendsfromthecentertoabout2025%ofthesolarradius.[65]Ithasadensityofupto150g/cm3[66][67](about150timesthedensityofwater)andatemperatureofcloseto15.7millionkelvin(K).[67]Bycontrast,theSun'ssurfacetemperatureisapproximately5,800K.RecentanalysisofSOHOmissiondatafavorsafasterrotationrateinthecorethanintherestoftheradiativezone.[65]ThroughmostoftheSun'slife,energyisproducedbynuclearfusionthroughaseriesofstepscalledthepp(protonproton)chainthisprocessconvertshydrogenintohelium.[68]Only0.8%oftheenergygeneratedintheSuncomesfromtheCNOcycle.[69]

ThecoreistheonlyregionintheSunthatproducesanappreciableamountofthermalenergythroughfusion99%ofthepowerisgeneratedwithin24%oftheSun'sradius,andby30%oftheradius,fusionhasstoppednearlyentirely.TherestoftheSunisheatedbythisenergythatistransferredoutwards,respectively,throughtheradiativeandconvectionzones.Theenergyproducedbyfusioninthecoremustthentravelthroughmanysuccessivelayerstothesolarphotospherebeforeitescapesintospaceassunlightorthekineticenergyofparticles.[43][70]

Theprotonprotonchainoccursaround9.2 1037timeseachsecondinthecore.Becausethisreactionusesfourfreeprotons(hydrogennuclei),itconvertsabout3.7 1038protonstoalphaparticles(heliumnuclei)everysecond(outofatotalof~8.9 1056freeprotonsintheSun),orabout6.2 1011kg/s.[43]

Fusinghydrogenintoheliumreleasesaround0.7%ofthefusedmassasenergy,[71]sotheSunreleasesenergyatthemassenergyconversionrateof4.26millionmetrictonspersecond,384.6yottawatts(3.846 1026W),[1]or9.192 1010megatonsofTNTpersecond.TheoreticalmodelsoftheSun'sinteriorindicateapowerdensityofapproximately276.5W/m3,[72]avaluethatmorenearly

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approximatesreptilemetabolismthanathermonuclearbomb.[e]PeakpowerproductionintheSunhasbeencomparedtothevolumetricheatgeneratedinanactivecompostheap.ThetremendouspoweroutputoftheSunisnotduetoitshighpowerpervolume,butinsteadduetoitslargesize.

Thefusionrateinthecoreisinaselfcorrectingequilibrium:aslightlyhigherrateoffusionwouldcausethecoretoheatupmoreandexpandslightlyagainsttheweightoftheouterlayers,reducingthefusionrateandcorrectingtheperturbationandaslightlylowerratewouldcausethecoretocoolandshrinkslightly,increasingthefusionrateandagainrevertingittoitspresentlevel.[73][74]

Radiativezone

Fromthecoreouttoabout0.7solarradii,thermalradiationistheprimarymeansofenergytransfer.[75]Thiszoneisnotregulatedbythermalconvectionhoweverthetemperaturedropsfromapproximately7to2millionkelvinwithincreasingdistancefromthecore.[67]Thistemperaturegradientislessthanthevalueoftheadiabaticlapserateandhencecannotdriveconvection.[67]Energyistransferredbyradiationionsofhydrogenandheliumemitphotons,whichtravelonlyabriefdistancebeforebeingreabsorbedbyotherions.[75]Thedensitydropsahundredfold(from20g/cm3toonly0.2g/cm3)from0.25solarradiitothetopoftheradiativezone.[75]

Tachocline

Theradiativezoneandtheconvectivezoneareseparatedbyatransitionlayer,thetachocline.Thisisaregionwherethesharpregimechangebetweentheuniformrotationoftheradiativezoneandthedifferentialrotationoftheconvectionzoneresultsinalargeshearaconditionwheresuccessivehorizontallayersslidepastoneanother.[76]Thefluidmotionsfoundintheconvectionzoneabove,slowlydisappearfromthetopofthislayertoitsbottom,matchingthecalmcharacteristicsoftheradiativezoneonthebottom.Presently,itishypothesized(seeSolardynamo)thatamagneticdynamowithinthislayergeneratestheSun'smagneticfield.[67]

Convectivezone

IntheSun'souterlayer,fromitssurfacetoapproximately200,000kmbelow(70%ofthesolarradiusfromthecenter),thetemperatureislowerthanintheradiativezoneandheavieratomsarenotfullyionized.Asaresult,radiativeheattransportislesseffective.Thedensityoftheplasmaislowenoughtoallowconvectivecurrentstodevelop.Materialheatedatthetachoclinepicksupheatandexpands,therebyreducingitsdensityandallowingittorise.Asaresult,thermalconvectiondevelopsasthermalcellscarrythemajorityoftheheatoutwardtotheSun'sphotosphere.Oncethematerialdiffusivelyandradiativelycoolsjustbeneaththephotosphericsurface,itsdensityincreases,anditsinkstothebaseoftheconvectionzone,whereitpicksupmoreheatfromthetopoftheradiativezoneandtheconvectivecyclecontinues.Atthephotosphere,thetemperaturehasdroppedto5,700Kandthedensitytoonly0.2g/m3(about1/6,000ththedensityofairatsealevel).[67]

ThethermalcolumnsintheconvectionzoneformanimprintonthesurfaceoftheSunasthesolargranulationandsupergranulation.Theturbulentconvectionofthisouterpartofthesolarinteriorsustains"smallscale"dynamoactionoverthenearsurfacevolumeoftheSun.[67]TheSun'sthermalcolumnsareBnardcellsandtaketheshapeofhexagonalprisms.[77]

Photosphere

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Theeffectivetemperature,orblackbodytemperature,oftheSun(5,777K)isthetemperatureablackbodyofthesamesizemusthavetoyieldthesametotalemissivepower.

Duringatotalsolareclipse,thesolarcoronacanbeseenwiththenakedeye,duringthebriefperiodoftotality.

ThevisiblesurfaceoftheSun,thephotosphere,isthelayerbelowwhichtheSunbecomesopaquetovisiblelight.[78]Abovethephotospherevisiblesunlightisfreetopropagateintospace,anditsenergyescapestheSunentirely.ThechangeinopacityisduetothedecreasingamountofHions,whichabsorbvisiblelighteasily.[78]Conversely,thevisiblelightweseeisproducedaselectronsreactwithhydrogenatomstoproduceHions.[79][80]Thephotosphereistenstohundredsofkilometersthick,beingslightlylessopaquethanaironEarth.Becausetheupperpartofthephotosphereiscoolerthanthelowerpart,animageoftheSunappearsbrighterinthecenterthanontheedgeorlimbofthesolardisk,inaphenomenonknownaslimbdarkening.[78]Thespectrumofsunlighthasapproximatelythespectrumofablackbodyradiatingatabout6,000K,interspersedwithatomicabsorptionlinesfromthetenuouslayersabovethephotosphere.Thephotospherehasaparticledensityof~1023m3(about0.37%oftheparticlenumberpervolumeofEarth'satmosphereatsealevel).Thephotosphereisnotfullyionizedtheextentofionizationisabout3%,leavingalmostallofthehydrogeninatomicform.[81]

Duringearlystudiesoftheopticalspectrumofthephotosphere,someabsorptionlineswerefoundthatdidnotcorrespondtoanychemicalelementsthenknownonEarth.In1868,NormanLockyerhypothesizedthattheseabsorptionlineswerecausedbyanewelementthathedubbedhelium,aftertheGreekSungodHelios.Twentyfiveyearslater,heliumwasisolatedonEarth.[82]

Atmosphere

ThepartsoftheSunabovethephotospherearereferredtocollectivelyasthesolaratmosphere.[78]Theycanbeviewedwithtelescopesoperatingacrosstheelectromagneticspectrum,fromradiothroughvisiblelighttogammarays,andcomprisefiveprincipalzones:thetemperatureminimum,thechromosphere,thetransitionregion,thecorona,andtheheliosphere.[78]

ThecoolestlayeroftheSunisatemperatureminimumregionabout500kmabovethephotosphere,withatemperatureofabout4,100K.[78]ThispartoftheSuniscoolenoughtoallowtheexistenceofsimplemoleculessuchascarbonmonoxideandwater,whichcanbedetectedviatheirabsorptionspectra.[83]

Thechromosphere,transitionregion,andcoronaaremuchhotterthanthesurfaceoftheSun.[78]Thereasonisnotwellunderstood,butevidencesuggeststhatAlfvnwavesmayhaveenoughenergytoheatthecorona.[84]

Abovethetemperatureminimumlayerisalayerabout2,000kmthick,dominatedbyaspectrumofemissionandabsorptionlines.[78]ItiscalledthechromospherefromtheGreekrootchroma,meaningcolor,becausethechromosphereisvisibleasacoloredflashatthebeginningandendoftotalsolar

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TakenbyHinode'sSolarOpticalTelescopeon12January2007,thisimageoftheSunrevealsthefilamentarynatureoftheplasmaconnectingregionsofdifferentmagneticpolarity.

eclipses.[75]Thetemperatureinthechromosphereincreasesgraduallywithaltitude,ranginguptoaround20,000Knearthetop.[78]Intheupperpartofthechromosphereheliumbecomespartiallyionized.[85]

Abovethechromosphere,inathin(about200km)transitionregion,thetemperaturerisesrapidlyfromaround20,000Kintheupperchromospheretocoronaltemperaturescloserto1,000,000K.[86]Thetemperatureincreaseisfacilitatedbythefullionizationofheliuminthetransitionregion,whichsignificantlyreducesradiativecoolingoftheplasma.[85]Thetransitionregiondoesnotoccuratawelldefinedaltitude.Rather,itformsakindofnimbusaroundchromosphericfeaturessuchasspiculesandfilaments,andisinconstant,chaoticmotion.[75]ThetransitionregionisnoteasilyvisiblefromEarth'ssurface,butisreadilyobservablefromspacebyinstrumentssensitivetotheextremeultravioletportionofthespectrum.[87]

ThecoronaisthenextlayeroftheSun.Thelowcorona,nearthesurfaceoftheSun,hasaparticledensityaround10151016m3.[85][f]Theaveragetemperatureofthecoronaandsolarwindisabout1,000,0002,000,000Khowever,inthehottestregionsitis8,000,00020,000,000K.[86]Althoughnocompletetheoryyetexiststoaccountforthetemperatureofthecorona,atleastsomeofitsheatisknowntobefrommagneticreconnection.[86][88]ThecoronaistheextendedatmosphereoftheSun,whichhasavolumemuchlargerthanthevolumeenclosedbytheSun'sphotosphere.WavesattheoutersurfaceofthecoronathatrandomlyblowevenfurtherfromtheSuniscalledthesolarwind,andisoneofthewaystheSuninfluencesthewholeSolarSystem.[88]

Theheliosphere,thetenuousoutermostatmosphereoftheSun,isfilledwiththesolarwindplasma.ThisoutermostlayeroftheSunisdefinedtobeginatthedistancewheretheflowofthesolarwindbecomessuperalfvnicthatis,wheretheflowbecomesfasterthanthespeedofAlfvnwaves,[89]atapproximately20solarradii(0.1AU).Turbulenceanddynamicforcesintheheliospherecannotaffecttheshapeofthesolarcoronawithin,becausetheinformationcanonlytravelatthespeedofAlfvnwaves.Thesolarwindtravelsoutwardcontinuouslythroughtheheliosphere,formingthesolarmagneticfieldintoaspiralshape,[88]untilitimpactstheheliopausemorethan50AUfromtheSun.InDecember2004,theVoyager1probepassedthroughashockfrontthatisthoughttobepartoftheheliopause.BothoftheVoyagerprobeshaverecordedhigherlevelsofenergeticparticlesastheyapproachtheboundary.[90]

TheheliosphereextendstotheouterfringeoftheSolarSystem,fartherthantheorbitofPluto,isdefinedtoendattheheliopause,whichistheendofinfluencefromtheSun,andistheboundarywiththeinterstellarmedium.

Photonsandneutrinos

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FulldiskvisiblelightimageoftheSunshowingprominentsunspotgroups.

Highenergygammarayphotonsinitiallyreleasedwithfusionreactionsinthecorearealmostimmediatelyabsorbedbythesolarplasmaoftheradiativezone,usuallyaftertravelingonlyafewmillimeters.Reemissionhappensinarandomdirectionandusuallyataslightlylowerenergy.Withthissequenceofemissionsandabsorptions,ittakesalongtimeforradiationtoreachtheSun'ssurface.Estimatesofthephotontraveltimerangebetween10,000and170,000years.[91]Incontrast,ittakesonly2.3secondsfortheneutrinos,whichaccountforabout2%ofthetotalenergyproductionoftheSun,toreachthesurface.BecauseenergytransportintheSunisaprocessthatinvolvesphotonsinthermodynamicequilibriumwithmatter,thetimescaleofenergytransportintheSunislonger,ontheorderof30,000,000years.ThisisthetimeitwouldtaketheSuntoreturntoastablestate,iftherateofenergygenerationinitscoreweresuddenlychanged.[92]

Neutrinosarealsoreleasedbythefusionreactionsinthecore,but,unlikephotons,theyrarelyinteractwithmatter,soalmostallareabletoescapetheSunimmediately.FormanyyearsmeasurementsofthenumberofneutrinosproducedintheSunwerelowerthantheoriespredictedbyafactorof3.Thisdiscrepancywasresolvedin2001throughthediscoveryoftheeffectsofneutrinooscillation:theSunemitsthenumberofneutrinospredictedbythetheory,butneutrinodetectorsweremissing23ofthem

becausetheneutrinoshadchangedflavorbythetimetheyweredetected.[93]

Magnetismandactivity

Magneticfield

TheSunhasamagneticfieldthatvariesacrossawiderangeoftimescales.Themostprominentofsuchvariationisrelatedtothequasiperiodic11yearsolarcyclewaxingandwaninginthenumberandsizeofsunspots.[14][95]SunspotsarevisibleasdarkpatchesontheSun'sphotosphereandcorrespondtoconcentrationsofmagneticfieldwheretheconvectivetransportofheatisinhibitedfromthesolarinteriortothesurface.Asaresult,sunspotsareslightlycoolerthanthesurroundingphotosphere,and,so,theyappeardark.Atatypicalsolarminimum,fewsunspotsarevisible,andoccasionallynonecanbeseenatall.Thosethatdoappearareathighsolarlatitudes.Asthesolarcycleprogressestowardsitsmaximum,sunspotstendformclosertothesolarequator,aphenomenonknownasSprer'slaw.Thelargestsunspotscanbetensofthousandsofkilometersacross.[96]

An11yearsunspotcycleishalfofa22yearBabcockLeightondynamocycle,whichcorrespondstoanoscillatoryexchangeofenergybetweentoroidalandpoloidalsolarmagneticfields.Atsolarcyclemaximum,theexternalpoloidaldipolarmagneticfieldisnearitsdynamocycleminimumstrength,butaninternaltoroidalquadrupolarfield,generatedthroughdifferentialrotationwithinthetachocline,isnearitsmaximumstrength.Atthispointinthedynamocycle,buoyantupwellingwithintheconvectivezoneforcesemergenceoftoroidalmagneticfieldthroughthephotosphere,givingrisetopairsofsunspots,roughlyalignedeastwestandhavingfootprintswithoppositemagneticpolarities.Themagneticpolarityofsunspotpairsalternateseverysolarcycle,aphenomenonknownastheHalecycle.[97][98]

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Butterflydiagramshowingpairedsunspotpattern.GraphissunspotWolfnumber.

Inthisfalsecolorultravioletimage,theSunshowsaC3classsolarflare(whiteareaonupperleft),asolartsunami(wavelikestructure,upperright)andmultiplefilamentsofplasmafollowingamagneticfield,risingfromthestellarsurface.

TheheliosphericcurrentsheetextendstotheouterreachesoftheSolarSystem,andresultsfromtheinfluenceoftheSun'srotatingmagneticfieldontheplasmaintheinterplanetarymedium.[94]

Duringthesolarcyclesdecliningphase,energyshiftsfromtheinternaltoroidalmagneticfieldtotheexternalpoloidalfield,andsunspotsdiminishinnumber.Atsolarcycleminimum,thetoroidalfieldis,correspondingly,atminimumstrength,sunspotsarerelativelyrare,andthepoloidalfieldisatitsmaximumstrength.Withtheriseofthenext11yearsunspotcycle,differentialrotationshiftsmagneticenergybackfromthepoloidaltothetoroidalfield,butwithapolaritythatisoppositetothepreviouscycle.Theprocesscarriesoncontinuously,andinanidealized,simplifiedscenario,each11yearsunspotcyclecorrespondstoachange,then,intheoverallpolarityoftheSun'slargescalemagneticfield.[99][100]

ThesolarmagneticfieldextendswellbeyondtheSunitself.TheelectricallyconductingsolarwindplasmacarriestheSun'smagneticfieldintospace,formingwhatiscalledtheinterplanetarymagneticfield.[88]Inanapproximationknownasidealmagnetohydrodynamics,plasmaparticlesonlymovealongthemagneticfieldlines.Asaresult,theoutwardflowingsolarwindstretchestheinterplanetarymagneticfieldoutward,forcingitintoaroughlyradialstructure.Forasimpledipolarsolarmagneticfield,withoppositehemisphericalpolaritiesoneithersideofthesolarmagneticequator,athincurrentsheetisformedinthesolarwind.[88]Atgreatdistances,therotationoftheSuntwiststhedipolarmagneticfieldandcorrespondingcurrentsheetintoanArchimedeanspiralstructurecalledtheParkerspiral.[88]Theinterplanetarymagneticfieldismuchstrongerthanthedipolecomponentofthesolarmagneticfield.TheSun'sdipolemagneticfieldof50400T(atthephotosphere)reduceswiththeinversecubeofthedistancetoabout0.1nTatthedistanceofEarth.However,accordingtospacecraftobservationstheinterplanetaryfieldatEarth'slocationisaround5nT,aboutahundredtimesgreater.[101]ThedifferenceisduetomagneticfieldsgeneratedbyelectricalcurrentsintheplasmasurroundingtheSun.

Variationinactivity

TheSun'smagneticfieldleadstomanyeffectsthatarecollectivelycalledsolaractivity.Solarflaresandcoronalmassejectionstendtooccuratsunspotgroups.Slowlychanginghighspeedstreamsofsolarwindareemittedfromcoronalholesatthephotosphericsurface.BothcoronalmassejectionsandhighspeedstreamsofsolarwindcarryplasmaandinterplanetarymagneticfieldoutwardintotheSolarSystem.[102]Theeffectsof

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Measurementsofsolarcyclevariationduringthelast30years

solaractivityonEarthincludeaurorasatmoderatetohighlatitudesandthedisruptionofradiocommunicationsandelectricpower.SolaractivityisthoughttohaveplayedalargeroleintheformationandevolutionoftheSolarSystem.

Withsolarcyclemodulationofsunspotnumbercomesacorrespondingmodulationofspaceweatherconditions,includingthosesurroundingEarthwheretechnologicalsystemscanbeaffected.

Longtermchange

Longtermsecularchangeinsunspotnumberisthought,bysomescientists,tobecorrelatedwithlongtermchangeinsolarirradiance,[103]which,inturn,mightinfluenceEarth'slongtermclimate.[104]Forexample,inthe17thcentury,thesolarcycleappearedtohavestoppedentirelyforseveraldecadesfewsunspotswereobservedduringaperiodknownastheMaunderminimum.ThiscoincidedintimewiththeeraoftheLittleIceAge,whenEuropeexperiencedunusuallycoldtemperatures.[105]Earlierextendedminimahavebeendiscoveredthroughanalysisoftreeringsandappeartohavecoincidedwithlowerthanaverageglobaltemperatures.[106]

ArecenttheoryclaimsthattherearemagneticinstabilitiesinthecoreoftheSunthatcausefluctuationswithperiodsofeither41,000or100,000years.ThesecouldprovideabetterexplanationoftheiceagesthantheMilankovitchcycles.[107][108]

Lifephases

TheSuntodayisroughlyhalfwaythroughthemoststablepartofitslife.Ithasnotchangeddramaticallyforfourbillion[b]years,andwillremainfairlystableforfourbillionmore.Howeverafterhydrogenfusioninitscorehasstopped,theSunwillundergoseverechanges,bothinternallyandexternally.

Formation

TheSunwasformedabout4.57billionyearsagofromthecollapseofpartofagiantmolecularcloudthatconsistedmostlyofhydrogenandheliumandthatprobablygavebirthtomanyotherstars.[109]Thisageisestimatedusingcomputermodelsofstellarevolutionandthroughnucleocosmochronology.[8]TheresultisconsistentwiththeradiometricdateoftheoldestSolarSystemmaterial,at4.567billionyearsago.[110][111]Studiesofancientmeteoritesrevealtracesofstabledaughternucleiofshortlivedisotopes,suchasiron60,thatformonlyinexploding,shortlivedstars.ThisindicatesthatoneormoresupernovaemusthaveoccurrednearthelocationwheretheSunformed.AshockwavefromanearbysupernovawouldhavetriggeredtheformationoftheSunbycompressingthematterwithinthemolecularcloudandcausingcertainregionstocollapseundertheirowngravity.[112]Asonefragmentofthecloudcollapseditalsobegantorotateduetoconservationofangularmomentumandheatupwiththeincreasingpressure.Muchofthemassbecameconcentratedinthecenter,whereastherestflattenedoutintoadiskthatwouldbecometheplanetsandotherSolarSystembodies.Gravityandpressurewithinthecoreofthecloudgeneratedalotofheatasitaccretedmorematterfromthesurroundingdisk,eventuallytriggeringnuclearfusion.Thus,theSunwasborn.

Mainsequence

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EvolutionoftheSun'sluminosity,radiusandeffectivetemperaturecomparedtothepresentSun.AfterRibas(2010)[113]

ThesizeofthecurrentSun(nowinthemainsequence)comparedtoitsestimatedsizeduringitsredgiantphaseinthefuture

TheSunisabouthalfwaythroughitsmainsequencestage,duringwhichnuclearfusionreactionsinitscorefusehydrogenintohelium.Eachsecond,morethanfourmilliontonnesofmatterareconvertedintoenergywithintheSun'score,producingneutrinosandsolarradiation.Atthisrate,theSunhassofarconvertedaround100Mofmatterintoenergy.TheSunwillspendatotalofapproximately10billionyearsasamainsequencestar.[114]

Aftercorehydrogenexhaustion

TheSundoesnothaveenoughmasstoexplodeasasupernova.Insteaditwillexitthemainsequenceinapproximately5.4billionyearsand

starttoturnintoaredgiant.ItiscalculatedthattheSunwillbecomesufficientlylargetoengulfthecurrentorbitsoftheSolarSystem'sinnerplanets,possiblyincludingEarth.[115][116]

Evenbeforeitbecomesaredgiant,theluminosityoftheSunwillhavenearlydoubled,andEarthwillbehotterthanVenusistoday.Oncethecorehydrogenisexhaustedin5.4billionyears,theSunwillexpandintoasubgiantphaseandslowlydoubleinsizeoverabouthalfabillionyears.Itwillthenexpandmorerapidlyoverabouthalfabillionyearsuntilitisovertwohundredtimeslargerthantodayandacoupleofthousandtimesmoreluminous.Thisthenstartstheredgiantbranch(RGB)phasewheretheSunwillspendaroundabillionyearsandlosearoundathirdofitsmass.[116]

AfterRGBtheSunnowhasonlyabout120millionyearsofactivelifeleft,buttheyarehighlyeventful.Firstthecoreignitesviolentlyintheheliumflash,andtheSunshrinksbacktoaround10timesitscurrentsizewith50timestheluminosity,withatemperaturealittlelowerthantoday.Itwillthenhavereachedtheredclumporhorizontalbranch(HB),butastaroftheSun'smassdoesnotevolvebluewardalongtheHB.Insteaditjustbecomesmildlylargerandmoreluminousoverabout100millionyearsasitcontinuestoburnheliuminthecore.[116]

Whentheheliumisexhausted,theSunwillrepeattheexpansionitfollowedwhenthehydrogeninthecorewasexhausted,exceptthatthistimeitallhappensfaster,andtheSunbecomeslargerandmoreluminous.Thisistheasymptoticgiantbranch(AGB)phase,andtheSunisalternatelyburninghydrogeninashellorheliuminadeepershell.Afterabout20millionyearsontheearlyAGB,theSunbecomesincreasinglyunstable,withrapidmasslossandthermalpulsesthatincreasethesizeandluminosityforafewhundredyearsevery100,000yearsorso.Thethermalpulsesbecomelargereachtime,withthelaterpulsespushingtheluminositytoasmuchas5,000timesthecurrentlevelandtheradiustoover1AU.[117]Modelsvarydependingontherateandtimingofmassloss.ModelsthathavehighermasslossontheRGBproducesmaller,lessluminousstarsatthetipoftheAGB,perhapsonly2,000timesthe

http://en.wikipedia.org/wiki/Main_sequencehttp://en.wikipedia.org/wiki/Red_giant_branchhttp://en.wikipedia.org/wiki/Helium_flashhttp://en.wikipedia.org/wiki/Tonnehttp://en.wikipedia.org/wiki/Supernovahttp://en.wikipedia.org/wiki/Asymptotic_giant_branchhttp://en.wikipedia.org/wiki/Solar_radiushttp://en.wikipedia.org/wiki/Solar_luminosityhttp://en.wikipedia.org/wiki/1000000000_(number)http://en.wikipedia.org/wiki/Inner_planethttp://en.wikipedia.org/wiki/Subgianthttp://en.wikipedia.org/wiki/Main_sequencehttp://en.wikipedia.org/wiki/Neutrinohttp://en.wikipedia.org/wiki/Effective_temperaturehttp://en.wikipedia.org/wiki/Solar_Systemhttp://en.wikipedia.org/wiki/Thermal_pulsehttp://en.wikipedia.org/wiki/Red_gianthttp://en.wikipedia.org/wiki/Red_clumphttp://en.wikipedia.org/wiki/File:Solar_evolution_(English).svghttp://en.wikipedia.org/wiki/Main_sequencehttp://en.wikipedia.org/wiki/Horizontal_branchhttp://en.wikipedia.org/wiki/Solar_radiationhttp://en.wikipedia.org/wiki/File:Sun_red_giant.svg



EvolutionofaSunlikestar.ThetrackofaonesolarmassstarontheHertzsprungRusselldiagramisshownfromthemainsequencetothepostAGBstage.

Anartist'sdepictionoftheSunenteringitsredgiantphaseviewedfromEarth.AlllifeonEarthisextinctatthisphase.

luminosityandlessthan200timestheradius.[116]FortheSun,fourthermalpulsesarepredictedbeforeitcompletelylosesitsouterenvelopeandstartstomakeaplanetarynebula.Bytheendofthatphaselastingapproximately500,000yearstheSunwillonlyhaveabouthalfofitscurrentmass.

ThepostAGBevolutionisevenfaster.Theluminositystaysapproximatelyconstantasthetemperatureincreases,withtheejectedhalfoftheSun'smassbecomingionisedintoaplanetarynebulaastheexposedcorereaches30,000K.Thefinalnakedcoretemperaturewillbeover100,000K,afterwhichtheremnantwillcooltowardsawhitedwarf.Theplanetarynebulawilldisperseinabout10,000years,butthewhitedwarfwillsurvivefortrillionsofyearsbeforefadingtoblack.[118][119]

Earth'sfate

DuringtheSun'slifeinthemainsequence,theSunisbecomingmoreluminous(about10%every1billionyears,atthepresenttime).ThesurfacetemperatureoftheSunisalmostconstant.Theincreaseofluminosityisessentiallyduetoaslowincreaseinthesolarradius.TheincreaseinsolarluminosityissuchthatinaboutanotherbillionyearsEarth'swaterwillevaporateandescapeintospace,renderingitinhospitabletoallknownterrestriallife.[120][121]EarthisnotexpectedtosurvivetheSun'stransitionintoaredgiant.Atitslargest,theSunwillhaveamaximumradiusbeyondEarth'scurrentorbit,1AU(1.5 1011m),250timesthepresentradiusoftheSun.[120]BythetimetheSunhasenteredtheasymptoticredgiantbranch,theorbitsoftheplanetswillhavedriftedoutwardsduetoalossofroughly30%oftheSun'spresentmass.Mostofthismasswillbelostasthesolarwindincreases.Also,tidalaccelerationwillhelpboostEarthtoahigherorbit(similartowhatEarthdoestotheMoon).Ifitwereonlyforthis,EarthwouldprobablyremainoutsidetheSun.However,currentresearchsuggeststhataftertheSunbecomesaredgiant,Earthwillbepulledinowingtotidaldeceleration.[120]

Motionandlocation

OrbitinMilkyWay

TheSunliesclosetotheinnerrimoftheMilkyWay'sOrionArm,intheLocalInterstellarCloudortheGouldBelt,atahypothesizeddistanceof7.58.5kpc(25,00028,000lightyears)fromtheGalacticCenter,[122][123][124][125][126][127]containedwithintheLocalBubble,aspaceofrarefiedhotgas,possiblyproducedbythesupernovaremnantGeminga.[128]Thedistancebetweenthelocalarmandthenextarmout,thePerseusArm,isabout6,500lightyears.[129]TheSun,andthustheSolarSystem,isfoundinwhatscientistscallthegalactichabitablezone.TheApexoftheSun'sWay,orthesolarapex,isthedirectionthattheSuntravelsthroughspaceintheMilkyWay,relativetoothernearbystars.Thegeneral

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IllustrationoftheMilkyWay,showingthelocationoftheSun(clicktoenlarge)

MotionofthebarycenteroftheSolarSystemrelativetotheSun

MapofthefullSunbySTEREOandSDOspacecraft

directionoftheSun'sgalacticmotionistowardsthestarVegaintheconstellationofLyraatanangleofroughly60skydegreestothedirectionoftheGalacticCenter.Ofthe50neareststellarsystemswithin17lightyearsfromEarth(theclosestbeingthereddwarfProximaCentauriatapproximately4.2lightyears),theSunranksfourthinmass.[130]

TheSun'sorbitaroundtheMilkyWayisexpectedtoberoughlyellipticalwiththeadditionofperturbationsduetothegalacticspiralarmsandnonuniformmassdistributions.InadditiontheSunoscillatesupanddownrelativetothegalacticplaneapproximately2.7timesperorbit.IthasbeenarguedthattheSun'spassagethroughthehigherdensityspiralarmsoftencoincideswithmassextinctionsonEarth,perhapsduetoincreasedimpactevents.[131]IttakestheSolarSystemabout225250millionyearstocompleteoneorbitthroughtheMilkyWay(agalacticyear),[132]soitisthoughttohavecompleted2025orbitsduringthelifetimeoftheSun.TheorbitalspeedoftheSolarSystemaboutthecenteroftheMilkyWayisapproximately251km/s(156mi/s).[133]Atthisspeed,ittakesaround1,190yearsfortheSolarSystemtotraveladistanceof1lightyear,or7daystotravel1AU.[134]

TheSun'smotionaboutthecenterofmassoftheSolarSystemiscomplicatedbyperturbationsfromtheplanets.ThebarycenterisjustoutsidethevolumeoftheSunwhenJupiterandSaturn(thetwoplanetswiththegreatestmasses)areroughlyinthesamedirection,asseenfromtheSun.Whentheyareinoppositedirections,andtheotherplanetsarealignedappropriately,thebarycentercanbeveryclosetothecenteroftheSun.Everyfewhundredyearsthismotionswitchesbetweenprogradeandretrograde.[135]

Theoreticalproblems

Coronalheatingproblem

Thetemperatureofthephotosphereisapproximately6,000K,whereasthetemperatureofthecoronareaches1,000,0002,000,000K.[86]Thehightemperatureofthecoronashowsthatitisheatedbysomethingotherthandirectheatconductionfromthephotosphere.[88]

Itisthoughtthattheenergynecessarytoheatthecoronaisprovidedbyturbulentmotionintheconvectionzonebelowthephotosphere,andtwomainmechanismshavebeenproposedtoexplaincoronalheating.[86]Thefirstiswaveheating,inwhichsound,gravitationalormagnetohydrodynamicwavesareproducedbyturbulenceintheconvectionzone.[86]Thesewavestravelupwardanddissipateinthecorona,depositingtheirenergy

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TheTrundholmsunchariotpulledbyahorseisasculpturebelievedtobeillustratinganimportantpartofNordicBronzeAgemythology.Thesculptureisprobablyfromaround1350BC.ItisdisplayedattheNationalMuseumofDenmark.

intheambientmatterintheformofheat.[136]Theotherismagneticheating,inwhichmagneticenergyiscontinuouslybuiltupbyphotosphericmotionandreleasedthroughmagneticreconnectionintheformoflargesolarflaresandmyriadsimilarbutsmallereventsnanoflares.[137]

Currently,itisunclearwhetherwavesareanefficientheatingmechanism.AllwavesexceptAlfvnwaveshavebeenfoundtodissipateorrefractbeforereachingthecorona.[138]Inaddition,Alfvnwavesdonoteasilydissipateinthecorona.Currentresearchfocushasthereforeshiftedtowardsflareheatingmechanisms.[86]

FaintyoungSunproblem

TheoreticalmodelsoftheSun'sdevelopmentsuggestthat3.8to2.5billionyearsago,duringtheArcheanperiod,theSunwasonlyabout75%asbrightasitistoday.SuchaweakstarwouldnothavebeenabletosustainliquidwateronEarth'ssurface,andthuslifeshouldnothavebeenabletodevelop.However,thegeologicalrecorddemonstratesthatEarthhasremainedatafairlyconstanttemperaturethroughoutitshistory,andthattheyoungEarthwassomewhatwarmerthanitistoday.TheconsensusamongscientistsisthattheatmosphereoftheyoungEarthcontainedmuchlargerquantitiesofgreenhousegases(suchascarbondioxide,methaneand/orammonia)thanarepresenttoday,whichtrappedenoughheattocompensateforthesmalleramountofsolarenergyreachingit.[139]

Historyofobservation

TheenormouseffectoftheSunontheEarthhasbeenrecognizedsinceprehistorictimes,andtheSunhasbeenregardedbysomeculturesasadeity.

Earlyunderstanding

Likeothernaturalphenomena,theSunhasbeenanobjectofvenerationinmanyculturesthroughouthumanhistory.Humanity'smostfundamentalunderstandingoftheSunisastheluminousdiskinthesky,whosepresenceabovethehorizoncreatesdayandwhoseabsencecausesnight.Inmanyprehistoricandancientcultures,theSunwasthoughttobeasolardeityorothersupernaturalphenomenon.WorshipoftheSunwascentraltocivilizationssuchastheancientEgyptians,theIncaofSouthAmericaandtheAztecsofwhatisnowMexico.InreligionssuchasHinduism,theSunisstillconsideredaGod.Manyancientmonumentswereconstructedwithsolarphenomenainmindforexample,stonemegalithsaccuratelymarkthesummerorwintersolstice(someofthemostprominentmegalithsarelocatedinNabtaPlaya,EgyptMnajdra,MaltaandatStonehenge,England)Newgrange,aprehistorichumanbuiltmountinIreland,wasdesignedtodetectthewintersolsticethepyramidofElCastilloatChichnItzinMexicoisdesignedtocastshadowsintheshapeofserpentsclimbingthepyramidatthevernalandautumnequinoxes.

TheEgyptiansportrayedthegodRaasbeingcarriedacrosstheskyinasolarbarque,accompaniedbylessergods,andtotheGreeks,hewasHelios,carriedbyachariotdrawnbyfieryhorses.FromthereignofElagabalusinthelateRomanEmpiretheSun'sbirthdaywasaholidaycelebratedasSolInvictus(literally"UnconqueredSun")soonafterthewintersolstice,whichmayhavebeenanantecedentto

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SincetheobservationofsunspotsbyGalileoin1609,scientistshavecontinuedtostudytheSun.

Christmas.Regardingthefixedstars,theSunappearsfromEarthtorevolveonceayearalongtheeclipticthroughthezodiac,andsoGreekastronomersconsideredittobeoneofthesevenplanets(Greekplanetes,wanderer),afterwhichthesevendaysoftheweekarenamedinsomelanguages.[140][141][142]

Developmentofscientificunderstanding

IntheearlyfirstmillenniumBC,BabylonianastronomersobservedthattheSun'smotionalongtheeclipticwasnotuniform,thoughtheywereunawareofwhythiswasitistodayknownthatthisisduetothemovementofEarthinanellipticorbitaroundtheSun,withEarthmovingfasterwhenitisnearertotheSunatperihelionandmovingslowerwhenitisfartherawayataphelion.[143]

OneofthefirstpeopletoofferascientificorphilosophicalexplanationfortheSunwastheGreekphilosopherAnaxagoras,whoreasonedthatitwasagiantflamingballofmetalevenlargerthanthePeloponnesusratherthanthechariotofHelios,andthattheMoonreflectedthelightoftheSun.[144]Forteachingthisheresy,hewasimprisonedbytheauthoritiesandsentencedtodeath,thoughhewaslaterreleasedthroughtheinterventionofPericles.EratosthenesestimatedthedistancebetweenEarthandtheSuninthe3rdcenturyBCas"ofstadiamyriads400and80000",thetranslationofwhichisambiguous,implyingeither4,080,000stadia(755,000km)or804,000,000stadia(148to153millionkilometersor0.99to1.02AU)thelattervalueiscorrecttowithinafewpercent.Inthe1stcenturyAD,Ptolemyestimatedthedistanceas1,210timestheradiusofEarth,approximately7.71millionkilometers(0.0515AU).[145]

ThetheorythattheSunisthecenteraroundwhichtheplanetsmovewasfirstproposedbytheancientGreekAristarchusofSamosinthe3rdcenturyBC,andlateradoptedbySeleucusofSeleucia(seeHeliocentrism).Thislargelyphilosophicalviewwasdevelopedintofullypredictivemathematicalmodelofaheliocentricsysteminthe16thcenturybyNicolausCopernicus.Intheearly17thcentury,theinventionofthetelescopepermitteddetailedobservationsofsunspotsbyThomasHarriot,GalileoGalileiandotherastronomers.GalileomadesomeofthefirstknowntelescopicobservationsofsunspotsandpositedthattheywereonthesurfaceoftheSunratherthansmallobjectspassingbetweenEarthandtheSun.[146]SunspotswerealsoobservedduringtheHanDynasty(206BCAD220)byChineseastronomerswhomaintainedrecordsoftheseobservationsforcenturies.Averroesalsoprovidedadescriptionofsunspotsinthe12thcentury.[147]

ArabicastronomicalcontributionsincludeAlbatenius'discoverythatthedirectionoftheSun'sapogee(theplaceintheSun'sorbitagainstthefixedstarswhereitseemstobemovingslowest)ischanging.[148](Inmodernheliocentricterms,thisiscausedbyagradualmotionoftheaphelionoftheEarth'sorbit).IbnYunusobservedmorethan10,000entriesfortheSun'spositionformanyyearsusingalargeastrolabe.[149]

ThetransitofVenuswasfirstobservedin1032byPersianastronomerandpolymathAvicenna,whoconcludedthatVenusisclosertoEarththantheSun.[150]In1672GiovanniCassiniandJeanRicherdeterminedthedistancetoMarsandweretherebyabletocalculatethedistancetotheSun.

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Sol,theSun,froma1550editionofGuidoBonatti'sLiberastronomiae.

In1666,IsaacNewtonobservedtheSun'slightusingaprism,andshowedthatitwasmadeupoflightofmanycolors.[151]In1800,WilliamHerscheldiscoveredinfraredradiationbeyondtheredpartofthesolarspectrum.[152]The19thcenturysawadvancementinspectroscopicstudiesoftheSunJosephvonFraunhoferrecordedmorethan600absorptionlinesinthespectrum,thestrongestofwhicharestilloftenreferredtoasFraunhoferlines.Intheearlyyearsofthemodernscientificera,thesourceoftheSun'senergywasasignificantpuzzle.LordKelvinsuggestedthattheSunwasagraduallycoolingliquidbodythatwasradiatinganinternalstoreofheat.[153]KelvinandHermannvonHelmholtzthenproposedagravitationalcontractionmechanismtoexplaintheenergyoutput,buttheresultingageestimatewasonly20millionyears,wellshortofthetimespanofatleast300millionyearssuggestedbysomegeologicaldiscoveriesofthattime.[153]In1890JosephLockyer,whodiscoveredheliuminthesolarspectrum,proposedameteoritichypothesisfortheformationandevolutionoftheSun.[154]

Notuntil1904wasadocumentedsolutionoffered.ErnestRutherfordsuggestedthattheSun'soutputcouldbemaintainedbyaninternalsourceofheat,andsuggestedradioactivedecayasthesource.[155]However,itwouldbeAlbertEinsteinwhowouldprovidetheessentialcluetothesourceoftheSun'senergyoutputwithhismassenergyequivalencerelationE=mc2.[156]In1920,SirArthurEddingtonproposedthatthepressuresandtemperaturesatthecoreoftheSuncouldproduceanuclearfusionreactionthatmergedhydrogen(protons)intoheliumnuclei,resultinginaproductionofenergyfromthenetchangeinmass.[157]ThepreponderanceofhydrogenintheSunwasconfirmedin1925byCeciliaPayneusingtheionizationtheorydevelopedbyMeghnadSaha,anIndianphysicist.Thetheoreticalconceptoffusionwasdevelopedinthe1930sbytheastrophysicistsSubrahmanyanChandrasekharandHansBethe.HansBethecalculatedthedetailsofthetwomainenergyproducingnuclearreactionsthatpowertheSun.[158][159]In1957,MargaretBurbidge,GeoffreyBurbidge,WilliamFowlerandFredHoyleshowedthatmostoftheelementsintheuniversehavebeensynthesizedbynuclearreactionsinsidestars,someliketheSun.[160]

Solarspacemissions

ThefirstsatellitesdesignedtoobservetheSunwereNASA'sPioneers5,6,7,8and9,whichwerelaunchedbetween1959and1968.TheseprobesorbitedtheSunatadistancesimilartothatofEarth,andmadethefirstdetailedmeasurementsofthesolarwindandthesolarmagneticfield.Pioneer9operatedforaparticularlylongtime,transmittingdatauntilMay1983.[162][163]

Inthe1970s,twoHeliosspacecraftandtheSkylabApolloTelescopeMountprovidedscientistswithsignificantnewdataonsolarwindandthesolarcorona.TheHelios1and2probeswereU.S.GermancollaborationsthatstudiedthesolarwindfromanorbitcarryingthespacecraftinsideMercury'sorbitatperihelion.[164]TheSkylabspacestation,launchedbyNASAin1973,includedasolarobservatorymodulecalledtheApolloTelescopeMountthatwasoperatedbyastronautsresidentonthestation.[87]Skylabmadethefirsttimeresolvedobservationsofthesolartransitionregionandofultravioletemissionsfromthesolarcorona.[87]Discoveriesincludedthefirstobservationsofcoronalmassejections,thencalled"coronaltransients",andofcoronalholes,nowknowntobeintimatelyassociatedwiththesolarwind.[164]

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TheSungivingoutalargegeomagneticstormon1:29pm,EST,13March2012

AlunartransitoftheSuncapturedduringcalibrationofSTEREOB'sultravioletimagingcameras[161]

AsolarprominenceeruptsinAugust2012,ascapturedbySDO

In1980,theSolarMaximumMissionwaslaunchedbyNASA.Thisspacecraftwasdesignedtoobservegammarays,XraysandUVradiationfromsolarflaresduringatimeofhighsolaractivityandsolarluminosity.Justafewmonthsafterlaunch,however,anelectronicsfailurecausedtheprobetogointostandbymode,andit

spentthenextthreeyearsinthisinactivestate.In1984SpaceShuttleChallengermissionSTS41Cretrievedthesatelliteandrepaireditselectronicsbeforerereleasingitintoorbit.TheSolarMaximumMissionsubsequentlyacquiredthousandsofimagesofthesolarcoronabeforereenteringEarth'satmosphereinJune1989.[165]

Launchedin1991,Japan'sYohkoh(Sunbeam)satelliteobservedsolarflaresatXraywavelengths.Missiondataallowedscientiststoidentifyseveraldifferenttypesofflares,anddemonstratedthatthecoronaawayfromregionsofpeakactivitywasmuchmoredynamicandactivethanhadpreviouslybeensupposed.Yohkohobservedanentiresolarcyclebutwentintostandbymodewhenanannulareclipsein2001causedittoloseitslockontheSun.Itwasdestroyedbyatmosphericreentryin2005.[166]

OneofthemostimportantsolarmissionstodatehasbeentheSolarandHeliosphericObservatory,jointlybuiltbytheEuropeanSpaceAgencyandNASAandlaunchedon2December1995.[87]Originallyintendedtoserveatwoyearmission,amissionextensionthrough2012wasapprovedinOctober2009.[167]Ithasprovensousefulthatafollowonmission,theSolarDynamicsObservatory(SDO),waslaunchedinFebruary2010.[168]SituatedattheLagrangianpointbetweenEarthandtheSun(atwhichthegravitationalpullfrombothisequal),SOHOhasprovidedaconstantviewoftheSunatmanywavelengthssinceitslaunch.[87]Besidesitsdirectsolarobservation,SOHOhasenabledthediscoveryofalargenumberofcomets,mostlytinysungrazingcometsthatincinerateastheypasstheSun.[169]

AllthesesatelliteshaveobservedtheSunfromtheplaneoftheecliptic,andsohaveonlyobserveditsequatorialregionsindetail.TheUlyssesprobewaslaunchedin1990tostudytheSun'spolarregions.ItfirsttravelledtoJupiter,to"slingshot"intoanorbitthatwouldtakeitfarabovetheplaneoftheecliptic.Serendipitously,itwaswellplacedtoobservethecollisionofCometShoemakerLevy9withJupiterin1994.OnceUlysseswasinitsscheduledorbit,itbeganobservingthesolarwindandmagneticfieldstrengthathighsolarlatitudes,findingthatthesolarwindfromhighlatitudeswasmovingatabout750km/s,whichwasslowerthanexpected,andthattherewerelargemagneticwavesemergingfromhighlatitudesthatscatteredgalacticcosmicrays.[170]

http://en.wikipedia.org/wiki/Sungrazing_comethttp://en.wikipedia.org/wiki/Solar_eclipsehttp://en.wikipedia.org/wiki/Atmospheric_reentryhttp://en.wikipedia.org/wiki/Comet_Shoemaker%E2%80%93Levy_9http://en.wikipedia.org/wiki/Jupiterhttp://en.wikipedia.org/wiki/Ulysses_probehttp://en.wikipedia.org/wiki/Cosmic_rayhttp://en.wikipedia.org/wiki/Yohkohhttp://en.wikipedia.org/wiki/Solar_Maximum_Missionhttp://en.wikipedia.org/wiki/File:Giant_prominence_on_the_sun_erupted.jpghttp://en.wikipedia.org/wiki/Ultraviolethttp://en.wikipedia.org/wiki/Solar_Dynamics_Observatoryhttp://en.wikipedia.org/wiki/Solar_flarehttp://en.wikipedia.org/wiki/Lagrangian_pointhttp://en.wikipedia.org/wiki/Gamma_rayhttp://en.wikipedia.org/wiki/Space_Shuttle_Challengerhttp://en.wikipedia.org/wiki/European_Space_Agencyhttp://en.wikipedia.org/wiki/X-rayhttp://en.wikipedia.org/wiki/Comethttp://en.wikipedia.org/wiki/NASAhttp://en.wikipedia.org/wiki/Solar_and_Heliospheric_Observatoryhttp://en.wikipedia.org/wiki/File:Sunspots_and_Solar_Flares.jpghttp://en.wikipedia.org/wiki/NASA



TheSun,asseenfromlowEarthorbitoverlookingtheInternationalSpaceStation.Thissunlightisnotfilteredbytheloweratmosphere,whichblocksmuchofthesolarspectrum

Elementalabundancesinthephotospherearewellknownfromspectroscopicstudies,butthecompositionoftheinterioroftheSunismorepoorlyunderstood.Asolarwindsamplereturnmission,Genesis,wasdesignedtoallowastronomerstodirectlymeasurethecompositionofsolarmaterial.GenesisreturnedtoEarthin2004butwasdamagedbyacrashlandingafteritsparachutefailedtodeployonreentryintoEarth'satmosphere.Despiteseveredamage,someusablesampleshavebeenrecoveredfromthespacecraft'ssamplereturnmoduleandareundergoinganalysis.[171]

TheSolarTerrestrialRelationsObservatory(STEREO)missionwaslaunchedinOctober2006.Twoidenticalspacecraftwerelaunchedintoorbitsthatcausethemto(respectively)pullfurtheraheadofandfallgraduallybehindEarth.ThisenablesstereoscopicimagingoftheSunandsolarphenomena,suchascoronalmassejections.[172][173]

TheIndianSpaceResearchOrganisationhasscheduledthelaunchofa100kgsatellitenamedAdityafor201516.ItsmaininstrumentwillbeacoronagraphforstudyingthedynamicsoftheSolarcorona.[174]

Observationandeffects

ThebrightnessoftheSuncancausepainfromlookingatitwiththenakedeyehowever,doingsoforbriefperiodsisnothazardousfornormalnondilatedeyes.[175][176]LookingdirectlyattheSuncausesphosphenevisualartifactsandtemporarypartialblindness.Italsodeliversabout4milliwattsofsunlighttotheretina,slightlyheatingitandpotentiallycausingdamageineyesthatcannotrespondproperlytothebrightness.[177][178]UVexposuregraduallyyellowsthelensoftheeyeoveraperiodofyears,andisthoughttocontributetotheformationofcataracts,butthisdependsongeneralexposuretosolarUV,andnotwhetheronelooksdirectlyattheSun.[179]LongdurationviewingofthedirectSunwiththenakedeyecanbegintocauseUVinduced,sunburnlikelesionsontheretinaafterabout100seconds,particularlyunderconditionswheretheUVlightfromtheSunisintenseandwellfocused[180][181]conditionsareworsenedbyyoungeyesornewlensimplants(whichadmitmoreUVthanagingnaturaleyes),Sunanglesnearthezenith,andobservinglocationsathighaltitude.

ViewingtheSunthroughlightconcentratingopticssuchasbinocularsmayresultinpermanentdamagetotheretinawithoutanappropriatefilterthatblocksUVandsubstantiallydimsthesunlight.WhenusinganattenuatingfiltertoviewtheSun,thevieweriscautionedtouseafilterspecificallydesignedforthatuse.SomeimprovisedfiltersthatpassUVorIRrays,canactuallyharmtheeyeathighbrightnesslevels.[182]Herschelwedges,alsocalledSolarDiagonals,areeffectiveandinexpensiveforsmalltelescopes.Thesunlightthatisdestinedfortheeyepieceisreflectedfromanunsilveredsurfaceofapieceofglass.Onlyaverysmallfractionoftheincidentlightisreflected.Therestpassesthroughtheglassandleavestheinstrument.Iftheglassbreaksbecauseoftheheat,nolightatallisreflected,makingthedevicefailsafe.Simplefiltersmadeofdarkenedglassallowthefullintensityofsunlighttopassthroughiftheybreak,endangeringtheobserver'seyesight.Unfilteredbinocularscandeliverhundredsoftimesasmuchenergyasusingthenakedeye,possiblycausingimmediatedamage.ItisclaimedthatevenbriefglancesatthemiddaySunthroughanunfilteredtelescopecancausepermanentdamage.[183]

http://en.wikipedia.org/wiki/Herschel_wedgehttp://en.wikipedia.org/wiki/File:STS-134_EVA4_view_to_the_Russian_Orbital_Segment.jpghttp://en.wikipedia.org/wiki/Cataractshttp://en.wikipedia.org/wiki/Solar_windhttp://en.wikipedia.org/wiki/Opticshttp://en.wikipedia.org/wiki/Binocularshttp://en.wikipedia.org/wiki/Stereoscopichttp://en.wikipedia.org/wiki/Phosphenehttp://en.wikipedia.org/wiki/Naked_eyehttp://en.wikipedia.org/wiki/Astronomical_spectroscopyhttp://en.wikipedia.org/wiki/Aditya_(spacecraft)http://en.wikipedia.org/wiki/Genesis_(spacecraft)http://en.wikipedia.org/wiki/Ultraviolethttp://en.wikipedia.org/wiki/Indian_Space_Research_Organisationhttp://en.wikipedia.org/wiki/Coronal_mass_ejectionshttp://en.wikipedia.org/wiki/Coronagraphhttp://en.wikipedia.org/wiki/Parachutehttp://en.wikipedia.org/wiki/Infraredhttp://en.wikipedia.org/wiki/International_Space_Station



Halowithsundogs

Partialsolareclipsesarehazardoustoviewbecausetheeye'spupilisnotadaptedtotheunusuallyhighvisualcontrast:thepupildilatesaccordingtothetotalamountoflightinthefieldofview,notbythebrightestobjectinthefield.DuringpartialeclipsesmostsunlightisblockedbytheMoonpassinginfrontoftheSun,buttheuncoveredpartsofthephotospherehavethesamesurfacebrightnessasduringanormalday.Intheoverallgloom,thepupilexpandsfrom~2mmto~6mm,andeachretinalcellexposedtothesolarimagereceivesuptotentimesmorelightthanitwouldlookingatthenoneclipsedSun.Thiscandamageorkillthosecells,resultinginsmallpermanentblindspotsfortheviewer.[184]Thehazardisinsidiousforinexperiencedobserversandforchildren,becausethereisnoperceptionofpain:itisnotimmediatelyobviousthat

one'svisionisbeingdestroyed.

Duringsunriseandsunset,sunlightisattenuatedduetoRayleighscatteringandMiescatteringfromaparticularlylongpassagethroughEarth'satmosphere,[185]andtheSunissometimesfaintenoughtobeviewedcomfortablywiththenakedeyeorsafelywithoptics(providedthereisnoriskofbrightsunlightsuddenlyappearingthroughabreakbetweenclouds).Hazyconditions,atmosphericdust,andhighhumiditycontributetothisatmosphericattenuation.[186]

Arareopticalphenomenonmayoccurshortlyaftersunsetorbeforesunrise,knownasagreenflash.TheflashiscausedbylightfromtheSunjustbelowthehorizonbeingbent(usuallythroughatemperatureinversion)towardstheobserver.Lightofshorterwavelengths(violet,blue,green)isbentmorethanthatoflongerwavelengths(yellow,orange,red)butthevioletandbluelightisscatteredmore,leavinglightthatisperceivedasgreen.[187]

UltravioletlightfromtheSunhasantisepticpropertiesandcanbeusedtosanitizetoolsandwater.Italsocausessunburn,andhasothermedicaleffectssuchastheproductionofvitaminD.UltravioletlightisstronglyattenuatedbyEarth'sozonelayer,sothattheamountofUVvariesgreatlywithlatitudeandhasbeenpartiallyresponsibleformanybiologicaladaptations,includingvariationsinhumanskincolorindifferentregionsoftheglobe.[188]

Seealso

Notes

AdvancedCompositionExplorerAntisolarpointListofbrighteststarsSolarastronomySolarenergySundogSunpathSunEarthDaySungazing

http://en.wikipedia.org/wiki/Halo_(optical_phenomenon)http://en.wikipedia.org/wiki/Refractionhttp://en.wikipedia.org/wiki/Ozone_layerhttp://en.wikipedia.org/wiki/Vitamin_Dhttp://en.wikipedia.org/wiki/Temperature_inversionhttp://en.wikipedia.org/wiki/Sun_doghttp://en.wikipedia.org/wiki/Optical_phenomenonhttp://en.wikipedia.org/wiki/Sun_doghttp://en.wikipedia.org/wiki/File:Doppelsonne_Halo_Echzell_Hessen_12-08-2012.jpghttp://en.wikipedia.org/wiki/Surface_brightnesshttp://en.wikipedia.org/wiki/Latitudehttp://en.wikipedia.org/wiki/Rayleigh_scatteringhttp://en.wikipedia.org/wiki/Sun-Earth_Dayhttp://en.wikipedia.org/wiki/Sun_pathhttp://en.wikipedia.org/wiki/Moonhttp://en.wikipedia.org/wiki/Sungazinghttp://en.wikipedia.org/wiki/Antisolar_pointhttp://en.wikipedia.org/wiki/Ultraviolethttp://en.wikipedia.org/wiki/Solar_eclipsehttp://en.wikipedia.org/wiki/Advanced_Composition_Explorerhttp://en.wikipedia.org/wiki/Green_flashhttp://en.wikipedia.org/wiki/Mie_theoryhttp://en.wikipedia.org/wiki/Solar_energyhttp://en.wikipedia.org/wiki/Antiseptichttp://en.wikipedia.org/wiki/Solar_astronomyhttp://en.wikipedia.org/wiki/Human_skin_colorhttp://en.wikipedia.org/wiki/Rayleigh_scatteringhttp://en.wikipedia.org/wiki/Sunburnhttp://en.wikipedia.org/wiki/List_of_brightest_stars



a. Itissometimessaidthat"Sol"isanalternativenamefortheSun.Itisthewordfor"Sun"inLatinandsomemodernEuropeanlanguagesandistherootofsomeEnglishwords,suchas"solar".However,itisnotincluded,onitsownandinthissense,inEnglishdictionaries,soitisnotconsideredcorrectEnglish.Nevertheless,itissometimesfoundinsciencefiction.

b. Allnumbersinthisarticleareshortscale.Onebillionis109,or1,000,000,000.c. Inastronomicalsciences,thetermheavyelements(ormetals)referstoallelementsexcepthydrogenand

helium.d. Hydrothermalventcommunitieslivesodeepundertheseathattheyhavenoaccesstosunlight.Bacteria

insteadusesulfurcompoundsasanenergysource,viachemosynthesis.

e. A50kgadulthumanhasavolumeofabout0.05m3,whichcorrespondsto13.8watts,atthevolumetricpowerofthesolarcenter.Thisis285kcal/day,about10%oftheactualaveragecaloricintakeandoutputforhumansinnonstressfulconditions.

f. Earth'satmospherenearsealevelhasaparticledensityofabout2 1025m3.

References

1. Williams,D.R.(1July2013)."SunFactSheet"(http://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html).NASA.Retrieved12August2013.

2. Asplund,M.Grevesse,N.Sauval,A.J.(2006)."ThenewsolarabundancesPartI:theobservations".CommunicationsinAsteroseismology147:7679.Bibcode:2006CoAst.147...76A(http://adsabs.harvard.edu/abs/2006CoAst.147...76A).doi:10.1553/cia147s76(https://dx.doi.org/10.1553%2Fcia147s76).

3. "Eclipse99:FrequentlyAskedQuestions"(http://education.gsfc.nasa.gov/eclipse/pages/faq.html).NASA.Retrieved24October2010.

4. Hinshaw,G.etal.(2009)."FiveyearWilkinsonMicrowaveAnisotropyProbeobservations:dataprocessing,skymaps,andbasicresults".TheAstrophysicalJournalSupplementSeries180(2):225245.arXiv:0803.0732(https://arxiv.org/abs/0803.0732).Bibcode:2009ApJS..180..225H(http://adsabs.harvard.edu/abs/2009ApJS..180..225H).doi:10.1088/00670049/180/2/225(https://dx.doi.org/10.1088%2F00670049%2F180%2F2%2F225).

5. Emilio,M.Kuhn,J.R.Bush,R.I.Scholl,I.F.(2012)."MeasuringtheSolarRadiusfromSpaceduringthe2003and2006MercuryTransits".TheAstrophysicalJournal750(2):135.arXiv:1203.4898(https://arxiv.org/abs/1203.4898).Bibcode:2012ApJ...750..135E(http://adsabs.harvard.edu/abs/2012ApJ...750..135E).doi:10.1088/0004637X/750/2/135(https://dx.doi.org/10.1088%2F0004637X%2F750%2F2%2F135).

6. "SolarSystemExploration:Planets:Sun:Facts&Figures"(http://web.archive.org/web/20080102034758/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sun&Display=Facts&System=Metric).NASA.Archivedfromtheoriginal(http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sun&Display=Facts&System=Metric)on2January2008.

7. Ko,M.(1999).Elert,G.,ed."DensityoftheSun"(http://hypertextbook.com/facts/1999/MayKo.shtml).ThePhysicsFactbook.

8. Bonanno,A.Schlattl,H.Patern,L.(2008)."TheageoftheSunandtherelativisticcorrectionsintheEOS".AstronomyandAstrophysics390(3):11151118.arXiv:astroph/0204331(https://arxiv.org/abs/astroph/0204331).Bibcode:2002A&A...390.1115B(http://adsabs.harvard.edu/abs/2002A&A...390.1115B).doi:10.1051/00046361:20020749(https://dx.doi.org/10.1051%2F00046361%3A20020749).

http://dx.doi.org/10.1088%2F0004-637X%2F750%2F2%2F135http://en.wikipedia.org/wiki/Digital_object_identifierhttp://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.htmlhttp://en.wikipedia.org/wiki/Chemical_elementhttp://dx.doi.org/10.1553%2Fcia147s76http://education.gsfc.nasa.gov/eclipse/pages/faq.htmlhttp://en.wikipedia.org/wiki/ArXivhttp://en.wikipedia.org/wiki/Digital_object_identifierhttp://adsabs.harvard.edu/abs/2012ApJ...750..135Ehttp://en.wikipedia.org/wiki/Bib

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Sun - General description