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Yourwebbrowser(Safari7)isoutofdate.Formoresecurity,comfortand thebestexperienceonthissite: Updateyourbrowser Ignore sun Forthecompleteencyclopedicentrywithmediaresources,visit: http://www.nationalgeographic.org/encyclopedia/sun/ Thesunisanordinarystar,oneofabout100billioninourgalaxy,theMilkyWay. Thesunhasextremelyimportantinfluencesonourplanet:Itdrivesweather, oceancurrents,seasons,andclimate,andmakesplantlifepossiblethrough photosynthesis.Withoutthesun’sheatandlight,everythingthatmakeslife possibleonEarthwouldnotexist. About4.5billionyearsago,thesunbegantotakeshapefromamolecularcloud thatwasmainlycomposedofhydrogenandhelium.Anearbysupernovaemitteda shockwave,whichcameincontactwiththemolecularcloudandenergizedit.The molecularcloudbegantocompress,andsomeregionsofgascollapsedunder theirowngravitationalpull.Asoneoftheseregionscollapsed,italsobeganto rotateandheatupfromincreasingpressure.Muchofthehydrogenandhelium remainedinthecenterofthishot,rotatingmass.Eventually,thegasesheatedup enoughtobeginnuclearfusion,andbecamethesuninoursolarsystem. Otherpartsofthemolecularcloudcooledintoadiscaroundthebrand-newsun andbecameplanets,asteroids,comets,andotherbodiesinoursolarsystem. Thesunisabout150millionkilometers(93millionmiles)fromEarth.Thisdistance, calledanastronomicalunit(AU),isastandardmeasureofdistancefor astronomersandastrophysicists. AnAUcanbemeasuredatlightspeed,orthetimeittakesforaphotonoflightto travelfromthesuntoEarth.Ittakeslightonthesunabout8minutesand19 secondstoreachEarth. Theradiusofthesun,orthedistancefromtheverycentertotheouterlimits,is about700,000kilometers(432,000miles).Thatdistanceisabout109timesthe sizeofEarth’sradius.ThesunnotonlyhasamuchlargerradiusthantheEarth— 1of19 itisalsomuchmoremassive.Thesun’smassismorethan333,000timesthatof theEarth,andcontainsabout99.8percentofallofthemassintheentiresolar system! Composition Thesunismadeupofablazingcombinationofgases.Thesegasesareactuallyin theformofplasma.Plasmaisastateofmattersimilartogas,butwithmostofthe particlesionized.Thismeanstheparticleshaveanincreasedorreducednumber ofelectrons. Aboutthreequartersofthesunishydrogen,whichisconstantlyfusingtogether andcreatingheliumbyaprocesscallednuclearfusion.Heliummakesupalmost theentireremainingquarter.Averysmallpercentage(1.69percent)ofthesun’s massismadeupofothergasesandmetals:iron,nickel,oxygen,silicon,sulfur, magnesium,carbon,neon,calcium,andchromiumThis1.69percentmayseem insignificant,butitsmassisstill5,628timesthemassofEarth. Thesunisnotasolidmass.Itdoesnothaveeasilyidentifiableboundarieslike rockyplanetssuchasEarth.Instead,thesuniscomposedoflayersmadeup almostentirelyofhydrogenandhelium.Thesegasescarryoutdifferentfunctions ineachlayer,andthesun’slayersaremeasuredbytheirpercentageofthesun’s totalradius. Thesunispermeatedandsomewhatcontrolledbyamagneticfield.Themagnetic fieldisproducedbyacombinationofthreecomplexmechanisms:acircular electriccurrentthatrunsthroughthesun;layersofthesunthatrotateat differentspeeds;andthesun’sabilitytoconductelectricity.Nearthesun’s equator,magneticfieldlinesmakesmallloopsnearthesurface.Magneticfieldlines thatflowthroughthepolesextendmuchfarther,thousandsofkilometers,before returningtotheoppositepole. Thesunrotatesarounditsownaxis,justlikeEarth.Thesunrotates counterclockwise,andtakesbetween25and35daystocompleteasingle rotation. ThesunorbitsclockwisearoundthecenteroftheMilkyWay.Itsorbitisbetween 24,000and26,000light-yearsawayfromthegalacticcenter.Thesuntakesabout 2of19 225millionto250millionyearstoorbitonetimearoundthegalacticcenter. ElectromagneticRadiation Thesun’senergytravelstoEarthatthespeedoflightintheformof electromagneticradiation(EMR). Theelectromagneticspectrumexistsaswavesofdifferentfrequenciesand wavelengths.Thefrequencyofawaverepresentshowmanytimesthewave repeatsitselfinacertainunitoftime.Waveswithveryshortwavelengthsrepeat themselvesseveraltimesinagivenunitoftime,sotheyarehigh-frequency.In contrast,low-frequencywaveshavemuchlongerwavelengths. Thevastmajorityofelectromagneticwavesthatcomefromthesunareinvisibleto us.Themosthigh-frequencywavesemittedbythesunaregammarays,X-rays, andultravioletradiation(UVrays).ThemostharmfulUVraysarealmost completelyabsorbedbyEarth’satmosphere.LesspotentUVraystravelthrough theatmosphere,andcancausesunburn. Thesunalsoemitsinfraredradiation,whosewavesaremuchlower-frequency. Mostheatfromthesunarrivesasinfraredenergy. SandwichedbetweeninfraredandUVisthevisiblespectrum,whichcontainsall thecolorsweseeonEarth.Thecolorredhasthelongestwavelengths(closestto infrared),andviolet(closesttoUV)theshortest. Thesunitselfiswhite,whichmeansitcontainsallofthecolorsinthevisible spectrum.Thesunappearsorange-ishyellowbecausethebluelightitemitshasa shorterwavelength,andisscatteredintheatmosphere—thesameprocessthat makestheskyappearblue. Astronomers,however,callthesuna“yellowdwarf”starbecauseitscolorsfall withintheyellow-greensectionoftheelectromagneticspectrum. EvolutionoftheSun Thesun,althoughithassustainedalllifeonourplanet,willnotshineforever.The sunhasalreadyexistedforabout4.5billionyears. 3of19 Theprocessofnuclearfusion,whichcreatestheheatandlightthatmakelifeon ourplanetpossible,isalsotheprocessthatslowlychangesthesun’scomposition. Throughnuclearfusion,thesunisconstantlyusingupthehydrogeninitscore: Everysecond,thesunfusesaround620millionmetrictonsofhydrogeninto helium. Atthisstageinthesun’slife,itscoreisabout74%hydrogen.Overthenext5 billionyears,thesunwillburnthroughmostofitshydrogen,andheliumwill becomeitsmajorsourceoffuel. Overthose5billionyears,thesunwillgofrom“yellowdwarf”to“redgiant.” Whenalmostallofthehydrogeninthesun’scorehasbeenconsumed,thecore willcontractandheatup,increasingtheamountofnuclearfusionthattakesplace. Theouterlayersofthesunwillexpandfromthisextraenergy. Thesunwillexpandtoabout200timesitscurrentradius,swallowingMercuryand Venus.AstrophysicistsdebatewhetherEarth’sorbitwouldexpandbeyondthe sun’sreach,orifourplanetwouldbeengulfedbythesunaswell. Asthesunexpands,itwillspreaditsenergyoveralargersurfacearea,whichhas anoverallcoolingeffectonthestar.Thiscoolingwillshiftthesun’svisiblelighttoa reddishcolor—aredgiant. Eventually,thesun’scorereachesatemperatureofabout100milliononthe Kelvinscale,thecommonscientificscaleformeasuringtemperature.Whenit reachesthistemperature,heliumwillbeginfusingtocreatecarbon,amuch heavierelement.Thiswillcauseintensesolarwindandothersolaractivity,which willeventuallythrowofftheentireouterlayersofthesun.Theredgiantphasewill beover.Onlythesun’scarboncorewillbeleft,andasa“whitedwarf,”itwillnot createoremitenergy. Sun’sStructure Thesunismadeupofsixlayers:core;radiativezone;convectivezone; photosphere;chromosphere;andcorona. Core 4of19 Thesun’score,morethanathousandtimesthesizeofEarthandmorethan10 timesdenserthanlead,isahugefurnace.Temperaturesinthecoreexceed15.7 millionKelvin(also15.7milliondegreesCelsius,or28milliondegreesFahrenheit). Thecoreextendstoabout25%ofthesun’sradius. Thecoreistheonlyplacewherenuclearfusionreactionscanhappen.Thesun’s otherlayersareheatedfromthenuclearenergycreatedthere.Protonsof hydrogenatomsviolentlycollideandfuse,orjointogether,tocreateahelium atom. Thisprocess,knownasaPP(proton-proton)chainreaction,emitsanenormous amountofenergy.Theenergyreleasedduringonesecondofsolarfusionisfar greaterthanthatreleasedintheexplosionofhundredsofthousandsofhydrogen bombs. Duringnuclearfusioninthecore,twotypesofenergyarereleased:photonsand neutrinos.Theseparticlescarryandemitthelight,heat,andenergyofthesun. Photonsarethesmallestparticleoflightandotherformsofelectromagnetic radiation.Neutrinosaremoredifficulttodetect,andonlyaccountforabout2%of thesun’stotalenergy.Thesunemitsbothphotonsandneutrinosinalldirections, allthetime. RadiativeZone Theradiativezoneofthesunstartsatabout25%oftheradius,andextendsto about70%oftheradius.Inthisbroadzone,heatfromthecorecools dramatically,frombetween7millionto2millionK. Intheradiativezone,energyistransferredbyaprocesscalledthermalradiation. Duringthisprocess,photonsthatwerereleasedinthecoretravelashort distance,areabsorbedbyaclose-byion,releasedbythation,andabsorbedagain byanother.Onephotoncancontinuethisprocessforalmost200,000years! Aneutrino’stripthroughtheradiativezoneandtherestofthesun’slayersis muchquicker:Inabout2.3seconds,neutrinospassthroughmorethan643,738 kilometers(400,000miles)ofthesun’slayers.NeutrinosaimedatEartheasily passthroughEarth’satmosphere,water,land,ice,animals,plants,andhumans. 5of19 Thisispossiblebecauseneutrinosdonotcarryanelectriccharge.Theydonot interactwithmostparticlesthattheycomeincontactwith. TransitionZone:Tachocline Betweentheradiativezoneandthenextlayer,theconvectivezone,thereisa transitionzonecalledthetachocline.Thisregioniscreatedasaresultofthesun’s differentialrotation. Differentialrotationhappenswhendifferentpartsofanobjectrotateatdifferent velocities.Thesunismadeupofgasesundergoingdifferentprocessesat differentlayersanddifferentlatitudes.Thesun’sequatorrotatesmuchfasterthan itspoles,forinstance. Therotationrateofthesunchangesrapidlyinthetachocline. ConvectiveZone Ataround70%ofthesun’sradius,theconvectivezonebegins.Inthiszone,the sun’stemperatureisnothotenoughtotransferenergybythermalradiation. Instead,ittransfersheatbythermalconvectionthroughthermalcolumns. Similartowaterboilinginapot,orhotwaxinalavalamp,gasesdeepinthesun’s convectivezoneareheatedand“boil”outward,awayfromthesun’score, throughthermalcolumns.Whenthegasesreachtheouterlimitsoftheconvective zone,theycooldown,andplungebacktothebaseoftheconvectivezone,tobe heatedagain. Photosphere Thephotosphereisthebrightyellow,visible"surface"ofthesun.Thephotosphere isabout400kilometers(250miles)thick,andtemperaturestherereachabout 6,000K(5,700°C,10,300°F). Thethermalcolumnsoftheconvectionzonearevisibleinthephotosphere, bubblinglikeboilingoatmeal.Throughpowerfultelescopes,thetopsofthe columnsappearasgranulescrowdedacrossthesun.Eachgranulehasabright center,whichisthehotgasrisingthroughathermalcolumn.Thegranules’dark edgesarethecoolgasdescendingbackdownthecolumntothebottomofthe convectivezone. 6of19 Althoughthetopsofthethermalcolumnslooklikesmallgranules,theyareusually morethan1,000kilometers(621miles)across.Mostthermalcolumnsexistfor about8to20minutesbeforetheydissolveandformnewcolumns.Therearealso “supergranules”thatcanbeupto30,000kilometers(18,641miles)across,and lastforupto24hours. Thesunisconstantlycoveredbyabout4,000granules. Sunspots,solarflares,andsolarprominencestakeforminthephotosphere, althoughtheyaretheresultofprocessesanddisruptionsinotherlayersofthe sun. Photosphere:Sunspots Asunspotisjustwhatitsoundslike—adarkspotonthesun.Asunspotforms whenintensemagneticactivtiyintheconvectivezonerupturesathermalcolumn. Atthetopoftherupturedcolumn(visibleinthephotosphere),temperatureis temporarilydecreasedbecausehotgasesarenotreachingit.Thiscooler temperature(still6,000K,or2,727°C,or4,940°F)causesthetopofthecolumn toappearmuchdarkerthanthesurroundingarea. Photosphere:SolarFlares Theprocessofcreatingsunspotsopensaconnectionbetweenthecorona(the veryouterlayerofthesun)andthesun’sinterior.Solarmattersurgesoutofthis openinginformationscalledsolarflares.Theseexplosionsaremassive:Inthe periodofafewminutes,solarflaresreleasetheequivalentofabout160billion megatonsofTNT,oraboutasixthofthetotalenergythesunreleasesinone second. Cloudsofions,atoms,andelectronseruptfromsolarflares,andreachEarthin abouttwodays.Solarflaresandsolarprominencescontributetospaceweather, whichcancausedisturbancestoEarth’satmosphereandmagneticfield,aswell asdisruptsatelliteandtelecommunicationssystems. Photosphere:CoronalMassEjections Coronalmassejections(CMEs)areanothertypeofsolaractivitycausedbythe 7of19 constantmovementanddisturbanceswithinthesun’smagneticfield.Eruptions fromCMEstravelfrom20to3,200kilometers(12to2,000miles)persecond. AlthoughCMEstypicallyformneartheactiveregionsofsunspots,thecorrelation betweenthetwohasnotbeenproven.ThecauseofCMEsisstillbeingstudied, anditishypothesizedthatdisruptionsineitherthephotosphereorcoronaleadto theseviolentsolarexplosions. Photosphere:SolarProminence Solarprominencesarebrightloopsofsolarmatter.Theycanburstfarintothe coronallayerofthesun,expandinghundredsofkilometerspersecond.These curvedandtwistedfeaturescanreachhundredsofthousandsofkilometersin heightandwidth,andlastanywherefromafewdaystoafewmonths. Solarprominencesarecoolerthanthecorona,andtheyappearasdarkerstrands againstthesun.Forthisreason,theyarealsoknownasfilaments. Photosphere:SolarCycle Thesundoesnotconstantlyemitsunspotsandsolarejecta;itgoesthrougha cycleofabout11years.Duringthissolarcycle,thefrequencyofsolarflares changes.Duringsolarmaximums,therecanbeseveralflaresperday.During solarminimums,theremaybefewerthanoneaweek. Thesolarcycleisdefinedbythesun’smagneticfields,whichlooparoundthesun andconnectatthetwopoles.Every11years,themagneticfieldsreverse,causing adisruptionthatleadstosolaractivityandsunspots. ThesolarcyclecanhaveeffectsonEarth’sclimate.Forexample,thesun’s ultravioletlightsplitsoxygeninthestratosphereandstrengthensEarth’s protectiveozonelayer.Duringthesolarminimum,therearelowamountsofUV rays,whichmeansthatEarth’sozonelayeristemporarilythinned.Thisallows moreUVraystoenterandheattheEarth’satmosphere. SolarAtmosphere Thesolaratmosphereisthehottestregionofthesun.Itismadeupofthe chromosphere,thecorona,andatransitionzonecalledthesolartransitionregion thatconnectsthetwo. 8of19 Thesolaratmosphereisobscuredbythebrightlightemittedbythephotosphere, anditcanrarelybeseenwithoutspecialinstruments.Onlyduringsolareclipses, whenthemoonmovesbetweentheEarthandthesunandhidesthephotosphere, cantheselayersbeseenwiththeunaidedeye. Chromosphere Thepinkish-redchromosphereisabout2,000kilometers(1,250miles)thickand riddledwithjetsofhotgas. Atthebottomofthechromosphere,whereitmeetsthephotosphere,thesunis atitscoolest,atabout4,400K(4,100°C,7,500°F).Thislowtemperaturegives thechromosphereitspinkcolor.Thetemperatureinthechromosphereincreases withaltitude,andreaches25,000K(25,000°C,45,000°F)attheouteredgeof theregion. Thechromospheregivesoffjetsofburninggasescalledspicules,similartosolar flares.Thesefierywispsofgasreachoutfromthechromospherelikelong, flamingfingers;theyareusuallyabout500kilometers(310miles)indiameter,and moveoutwardfromthesunatabout20kilometers(12miles)persecond. Spiculesonlylastforabout15minutes,butcanreachthousandsofkilometersin heightbeforecollapsinganddissolving. SolarTransitionRegion Thesolartransitionregion(STR)separatesthechromospherefromthecorona. BelowtheSTR,thelayersofthesunarecontrolledandstayseparatebecauseof gravity,gaspressure,andthedifferentprocessesofexchangingenergy.Above theSTR,themotionandshapeofthelayersaremuchmoredynamic.Theyare dominatedbymagneticforces.Thesemagneticforcescanputintoactionsolar eventssuchascoronalloopsandthesolarwind. Thestateofheliuminthesetworegionshasdifferencesaswell.BelowtheSTR, heliumispartiallyionized.Thismeansitstillhasoneelectron.AroundtheSTR, heliumabsorbsabitmoreheatandlosesitslastelectron.Itstemperaturesoars toalmost1millionK(1million°C,1.8million°F). 9of19 Corona Thecoronaisthewispyoutermostlayerofthesolaratmosphere,andcanextend millionsofkilometersintospace.Gasesinthecoronaburnatabout1millionK(1 million°C,1.8million°F),andmoveabout145kilometers(90miles)persecond. Someoftheparticlesreachanescapevelocityof400kilometerspersecond(249 milespersecond).Theyescapethesun’sgravitationalpullandbecomethesolar wind.Thesolarwindblastsfromthesuntotheedgeofthesolarsystem. Otherparticlesformcoronalloops.Coronalloopsareburstsofparticlesthat curvebackaroundtoanearbysunspot. Nearthesun’spolesarecoronalholes.Theseareasarecolderanddarkerthan otherregionsofthesun,andallowsomeofthefastest-movingpartsofthesolar windtopassthrough. Electromagneticradiationthatescapesfromthecoronaisclassifiedintothree types:K-corona,F-corona,andE-corona. ThelightfromtheK-coronacomesfromsunlightscatteringfreeelectrons.Light fromtheF-coronaiscausedbysunlightbouncingoffofcosmicdustparticles. LightfromtheE-coronaresultsfromchangesinthequantityofphotonsinan otherwiseuniformarea.TheE-coronasuppliesinformationaboutthecorona’s composition. SolarWind Thesolarwindisastreamofextremelyhot,chargedparticlesthatarethrownout fromtheupperatmosphereofthesun.Enormousamountsofmaterialare releasedfromthesun:about4-6billionmetrictonseveryhour.Thismeansthat every150millionyears,thesunlosesamassequaltothatoftheEarth.However, evenatthisrateofloss,thesunhasonlylostabout0.01%ofitstotalmassfrom solarwind. Thesolarwindblowsinalldirections,atabout1millionkph(620,000mph).It continuesmovingatthatspeedforabout10billionkilometers(6billionmiles). 10of19 SomeoftheparticlesinthesolarwindslipthroughEarth’smagneticfieldandinto itsupperatmospherenearthepoles.Astheycollidewithatmosphere,these chargedparticlessettheatmosphereaglowwithcolor,creatingauroras,colorful lightdisplaysknownastheNorthernandSouthernLights.Solarwindscanalso causesolarstorms.Thesestormscaninterferewithsatellitesandknockout powergridsonEarth. Thesolarwindfillstheheliosphere,themassivebubbleofchargedparticlesthat encompassesthesolarsystem. Thesolarwindeventuallyslowsdownneartheborderoftheheliosphere,ata theoreticalboundarycalledtheheliopause.Thisboundaryseparatesthematter andenergyofoursolarsystemfromthematterinneighboringstarsystemsand theinterstellarmedium. Theinterstellarmediumisthespacebetweenstarsystems.Thesolarwind,having traveledbillionsofkilometers,cannotextendbeyondtheinterstellarmedium. StudyingtheSun Thesunhasnotalwaysbeenasubjectofscientificdiscoveryandinquiry.For thousandsofyears,thesunwasknowninculturesallovertheworldasagod,a goddess,andasymboloflife. TotheancientAztecs,thesunwasapowerfuldeityknownasTonatiuh,who requiredhumansacrificetotravelacrossthesky.InBalticmythology,thesunwas agoddessnamedSaule,whobroughtfertilityandhealth.Chinesemythologyheld thatthesunistheonlyremainingof10sungods. In150CE,GreekscholarClaudiusPtolemycreatedageocentricmodelofthesolar systeminwhichthemoon,planets,andsunrevolvedaroundtheEarth.Thiswas themodelforalmost1,200years,reachingpeopleinWesternEurope,North Africa,andtheMiddleEast. Itwasnotuntilthe16thcenturythatPolishastronomerNicolausCopernicusused mathematicalandscientificreasoningtoprovethatplanetsrotatedaroundthe sun.Thisheliocentricmodelistheoneweusetoday. 11of19 Inthe17thcentury,thetelescopeallowedpeopletoexaminethesunindetail.The sunismuchtoobrighttoallowustostudyitwithoureyesunprotected.Witha telescope,itwaspossibleforthefirsttimetoprojectaclearimageofthesunonto ascreenforexamination. EnglishscientistSirIsaacNewtonusedatelescopeandprismtoscatterthelight ofthesun,andprovedthatsunlightwasactuallymadeofaspectrumofcolors. In1800,infraredandultravioletlightwerediscoveredtoexistjustoutsideofthe visiblespectrum.Anopticalinstrumentcalledaspectroscopemadeitpossibleto separatevisiblelightandotherelectromagneticradiationintoitsvarious wavelengths.Spectroscopyalsohelpedscientistsidentifygasesinthesun’s atmosphere—eachelementhasitsownwavelengthpattern. However,themethodbywhichthesungenerateditsenergyremainedamystery. Manyscientistshypothesizedthatthesunwascontracting,andemittingheat fromthatprocess. In1868,EnglishastronomerJosephNormanLockyerwasstudyingthesun’s electromagneticspectrum.Heobservedbrightlinesinthephotospherethatdid nothaveawavelengthofanyknownelementonEarth.Heguessedthattherewas anelementisolatedonthesun,andnameditheliumaftertheGreeksungod, Helios. Lockyer’sdiscoveryrevolutionizedsolarscience.Overthenext30years, astronomersconcludedthatthesunhadahot,pressurizedcorethatwascapable ofproducingmassiveamountsofenergythroughnuclearfusion. Technologycontinuedtoimproveandallowedscientiststouncovernewfeatures ofthesun.Infraredtelescopeswereinventedinthe1960s,andscientists observedenergyoutsideofthevisiblespectrum.Twentieth-centuryastronomers usedballoonsandrocketstosendspecializedtelescopeshighabovetheEarth, andexaminedthesunwithoutanyinterferencefromtheEarth'satmosphere. Solrad1wasthefirstspacecraftdesignedtostudythesun,andwaslaunchedby theUnitedStatesin1960.Thatdecade,NASAsentfivePioneersatellitestoorbit 12of19 thesunandcollectinformationaboutthestar. In1980,NASAlaunchedamissionduringthesolarmaximumtogather informationaboutthehigh-frequencygammarays,UVrays,andX-raysthatare emittedduringsolarflares. TheSolarandHeliosphericObservatory(SOHO)wasdevelopedinEuropeandput intoorbitin1996tocollectinformation.SOHOhasbeensuccessfullycollecting dataandforecastingspaceweatherfor12years. Voyager1and2arespacecrafttravelingtotheedgeoftheheliosphereto discoverwhattheatmosphereismadeofwheresolarwindmeetstheinterstellar medium.Bothspacecraftarescheduledtoreachthisboundaryin2014! Anotherdevelopmentinthestudyofthesunishelioseismology,thestudyofsolar waves.Theturbulenceoftheconvectivezoneishypothesizedtocontributeto solarwavesthatcontinuouslytransmitsolarmaterialtotheouterlayersofthe sun.Bystudyingthesewaves,scientistsunderstandmoreaboutthesun’sinterior andthecauseofsolaractivity. EnergyfromtheSun Photosynthesis Sunlightprovidesnecessarylightandenergytoplantsandotherproducersinthe foodweb.Theseproducersabsorbthesun’sradiationandconvertitintoenergy throughaprocesscalledphotosynthesis. Producersaremostlyplants(onland)andalgae(inaquaticregions).Theyarethe foundationofthefoodweb,andtheirenergyandnutrientsarepassedontoevery otherlivingorganism. FossilFuels PhotosynthesisisalsoresponsibleforallofthefossilfuelsonEarth.Scientists estimatethatabout3,000millionyearsago,thefirstproducersevolvedinaquatic settings.Sunlightallowedplantlifetothriveandadapt.Aftertheplantsdied,they decomposedandshifteddeeperintotheEarth,sometimesthousandsofmeters. Thisprocesscontinuedformillionsofyears. 13of19 Underintensepressureandhightemperatures,theseremainsbecamewhatwe knowasfossilfuels.Thesemicroorganismsbecamepetroleum,naturalgas,and coal. Peoplehavedevelopedprocessesforextractingthesefossilfuelsandusingthem forenergy.However,fossilfuelsareanonrenewableresource.Theytakemillions ofyearstoform. SolarEnergyTechnology Solarenergytechnologyharnessesthesun’sradiationandconvertsitintoheat, light,orelectricity. Solarenergyisarenewableresource,andmanytechnologiescanharvestit directlyforuseinhomes,businesses,schools,andhospitals.Somesolarenergy technologiesincludesolarvoltaiccellsandpanels,solarthermalcollectors,solar thermalelectricity,andsolararchitecture. Photovoltaicsusethesun’senergytospeedupelectronsinsolarcellsand generateelectricity.Thisformoftechnologyhasbeenusedwidely,andcan provideelectricityforruralareas,largepowerstations,buildings,andsmaller devicessuchasparkingmetersandtrashcompactors. Thesun’senergycanalsobeharnessedbyamethodcalled“concentratedsolar power,”inwhichthesun’sraysarereflectedandmagnifiedbymirrorsandlenses. Theintensifiedrayofsunlightheatsafluid,whichcreatessteamandpowersan electricgenerator. Solarpowercanalsobecollectedanddistributedwithoutmachineryor electronics.Forexample,roofscanbecoveredwithvegetationorpaintedwhiteto decreasetheamountofheatabsorbedintothebuilding,therebydecreasingthe amountofelectricityneededforairconditioning.Thisissolararchitecture. Sunlightisabundant:Inonehour,theEarth’satmospherereceivesenough sunlighttopowertheelectricityneedsofallpeopleforayear.However,solar technologyisexpensive,anddependsonsunnyandcloudlesslocalweathertobe effective.Methodsofharnessingthesun’senergyarestillbeingdevelopedand 14of19 improved. Vocabulary Term Partof Speech astronomer noun astronomical unit noun Definition personwhostudiesspaceandtheuniversebeyond Earth'satmosphere. (AU)(150millionkilometers/93millionmiles)unitof distanceequaltotheaveragedistancebetweenthe Earthandthesun. brightlycoloredbandsoflight,visiblearoundEarth's geomagneticpoles,causedbysolarwindinteractingwith particlesinEarth'smagneticfield. transparentlayerofgassurroundingthephotosphere (visibledisc)ofastar. allweatherconditionsforagivenlocationoveraperiod oftime. topresstogetherinasmallerspace. aurora noun chromosphere noun climate noun compress verb convection noun core noun corona coronalmass ejection noun transferofheatbythemovementoftheheatedpartsof aliquidorgas. theextremelyhotcenterofEarth,anotherplanet,ora star. outermostpartofthesunoranotherstar'satmosphere. noun hugeburstofsolarwindandotherchargedparticles. dense differential rotation dust electricity havingpartsormoleculesthatarepackedclosely adjective together. patternwheredifferentpartsofanobjectrotateat noun differentspeedsorvelocities. microscopicparticlesofrocksormineralsdriftingin noun space.Alsocalledcosmicdustorspacedust. setofphysicalphenomenaassociatedwiththepresence noun andflowofelectriccharge. electromagnetic noun spectrum continousbandofallkindsofradiation(heatandlight). 15of19 Term Equator Partof Speech noun escapevelocity noun foodweb noun fossilfuel noun frequency noun galaxy noun generator noun granule noun gravitational pull noun heliopause noun helioseismology noun infrared radiation interstellar medium noun noun ionize verb IsaacNewton noun Kelvinscale noun magneticfield noun Definition imaginarylinearoundtheEarth,anotherplanet,orstar runningeast-west,0degreeslatitude. speedandforcethatanobjectmusthavetoescapethe gravityofalargerbody,insteadoforbitingaroundit. allrelatedfoodchainsinanecosystem.Alsocalleda foodcycle. coal,oil,ornaturalgas.Fossilfuelsformedfromthe remainsofancientplantsandanimals. rateofoccurrence,orthenumberofthingshappening inaspecificareaoverspecifictimeperiod. collectionofstars,planets,gases,andothercelestial bodiesboundtogetherbygravity. machinethatconvertsonetypeofenergytoanother, suchasmechanicalenergytoelectricity. topofaconvectivecellonthesun'sphotosphere,where hotgasesriseandquicklydissipate. physicalattractionbetweentwomassiveobjects. outerboundaryofthesolarsystem,wherethesolar windisstoppedbytheinterstellarmedium. studyofthesun'sinteriorbyanalysisofthe(mostly acoustic)wavesitemits. partoftheelectromagneticspectrumwithwavelengths longerthanvisiblelightbutshorterthanmicrowaves. materialinthespacebetweenstarsystems,mostly hydrogenplasmaandcosmicdust. toaddorremoveanelectrontoanatomorgroupof atoms,givingthemanelectriccharge. (1642-1727)Englishphysicist,mathematician,and philosopher. scaleformeasuringtemperaturewherezeroKelvinis absolutezero,theabsenceofallenergy. areaaroundandaffectedbyamagnetorcharged particle. 16of19 Term Partof Speech Definition molecularcloud noun regionofgas,plasma,andcosmicdustwheremolecules (usuallymolecularhydrogen)form. neutrino noun subatomicparticlethatcarriesnoelectricalchargeand interactsweaklywithallothermatter. nonrenewable resource noun naturalresourcethatexistsinalimitedsupply. nuclearfusion noun processwherethenucleiofoneelement,usually hydrogen,fusewitheachothertoformthenucleiof anotherelement,usuallyhelium. nutrient noun orbit noun ozonelayer noun photon noun photosphere noun photosynthesis noun photovoltaic noun powergrid noun radiativezone noun radius noun redgiant noun resource rotate whichthestar'sdiameterismeasured. processbywhichplantsturnwater,sunlight,and carbondioxideintowater,oxygen,andsimplesugars. adjectiveabletoconvertsolarradiationtoelectricalenergy. plasma renewable substanceanorganismneedsforenergy,growth,and life. pathofoneobjectaroundamoremassiveobject. layerintheatmospherecontainingthegasozone,which absorbsmostofthesun'sultravioletradiation. smallestunitoflight,orelectromagneticradiation. lowestvisiblelayerofastarandtheboundaryfrom noun verb stateofmatterwithnofixedshapeandmolecules separatedintoionsandelectrons. networkofcablesorotherdevicesthroughwhich electricityisdeliveredtoconsumers.Alsocalledan electricalgrid. areaofastar'sinteriorwhereenergyistransferred throughthermalradiation. rayextendingfromthecenterofacircleorspheretoits surfaceorcircumference. "mainsequence"starwithhugesurfacearea,low surfacetemperature,andreddishcolor. resourcethatcanreplenishitselfatasimilarratetoits usebypeople. toturnaroundacenterpointoraxis. 17of19 Term rupture satellite solar architecture solarcycle Partof Definition Speech verb tobreakortear. objectthatorbitsaroundsomethingelse.Satellitescan noun benatural,likemoons,ormadebypeople. theplanninganddesignofbuildingstomakethemost noun useofthesun'sheatandlight. fairlypredictablechangesinsolaractivity(suchas noun sunspotsandsolarflares)overan11-yearperiod. solareclipse noun solarenergy noun solarflare noun solar prominence noun solarstorm noun solarsystem noun solarwind noun spaceweather noun spectroscopy noun spicule noun star noun sunspot noun supernova noun eventwhenthesunisblockedbythemoonpassing betweenitandtheEarth. radiationfromthesun. explosioninthesun'satmosphere,whichreleasesa burstofenergyandchargedparticlesintothesolar system. hugeeruptionofcoolgasesfromthesurfaceofthe sun,oftenshapedlikeagiantloop. suddenchangeintheEarth'smagnetosphere,caused bythesolarwindinteractingwiththeEarth'smagnetic field.Alsocalledageomagneticstorm. thesunandtheplanets,asteroids,comets,andother bodiesthatorbitaroundit. flowofchargedparticles,mainlyprotonsandelectrons, fromthesuntotheedgeofthesolarsystem. changesintheenvironmentoutsidetheEarth's atmosphere,usuallyinfluencedbythesun. scienceofthemeasurementoflightthatisreflected, absorbed,oremittedbydifferentmaterials. thin,dynamicjetofsolarmaterialejectedfromthe chromosphere(upperatmosphere)ofthesun. largeballofgasandplasmathatradiatesenergy throughnuclearfusion,suchasthesun. dark,coolerareaonthesurfaceofthesunthatcan move,change,anddisappearovertime. sudden,violentexplosionofamassivestar. 18of19 Term tachocline Partof Speech noun transitionzone noun ultraviolet radiation noun wavelength noun whitedwarf noun Definition transitionzoneinthesun,betweeninner,radiativezone andtheouter,convectivezone. areabetweentwonaturalorartificialregions. powerfullightwavesthataretooshortforhumansto see,butcanpenetrateEarth'satmosphere.Ultravioletis oftenshortenedtoUV. thedistancebetweenthecrestsoftwowaves. small,dense"mainsequence"starthathasusedup mostofitsnuclearfuel. Articles&Profiles NationalGeographicNews:SunisRoundestNaturalObjectKnown Images NationalGeographicScience:SunPhotos Video NationalGeographicVideo:Sun101 Websites NationalGeographicScience:TheSun—LivingWithaStormyStar NASA:SolarSystemExploration—Sun NASA:SolarandHeliosphericObservatory(SOHO) NASA:Heliophysics ©1996–2017NationalGeographicSociety.Allrightsreserved. 19of19