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Groupsofcelestialobjects-setsofstars,cosmicdustorinterstellar
medium,darkmatterandotherelementsorbitingaroundacommoncentre
ofmassarereferredtoasagalaxy.TheEarthisoneofthebodiesbuildingthe
MilkyWay8.
Galaxiesarebuiltbyplanets:astronomicalobjectsinwhosecorenuclear
fusionreactionprocessesoccur9(Weintraub2014,pp.3,223),andother
celestialbodiesorbitingtheSunoranyothercentralstar,thusforming
aplanetarysystem.TheplanetarysysteminwhichtheEarthislocatedinthe
SolarSystem.TheSolarSystemconsistsofgravitationallyboundcelestial
bodies:physicalobjectsandtheirstructurescontainedinouterspace
(outsidetheEarth)satmosphere),i.e.theSun,planetsanddwarfplanets,
planetarymoonsandotherbodiessuchas:asteroids,comets,meteoroidsand
interplanetarydust.
TheSunisthecloseststarandthecentralbodyofthesolarsystem,
containing99.87%ofitsmass.Itsmassisequalto333thousandEarth
masses,itsmeanradiusis696thousandkmandthemeandensityoftheSunis
1.41g/cm3.TheSun)sgravitationalaccelerationonthesurfaceisapproximately
274m/s2.TheboundarybetweentheinterioroftheSunanditsatmosphereis
thephotosphere,itemitsthevisiblelightandprecedesthechromosphere-
thetransitionallayer,andthesolarcorona-theoutermostsolarsphere.The
solartemperaturedistributionshowsanincreasewithheight,whichbegins
fromthelowerboundaryofthechromosphereto1millionK,reachedinthe
lowerboundaryofthesolarcorona.Theuppercorona,ontheotherhand,is
responsiblefortheoutfowofplasmaintheformofthesolarwind:astream
ofelectricallychargedparticlesmovingintotheinterstellarspaceataspeed
of300to800km/s.Theoutfowofthematterandotherrelatedphenomena
(e.g.solarfares,changesinthesizeofthecorona)areknownassolaractivity.
TheSunisagaseousbodycomposedof,bymass,72%hydrogen,approx.
26%heliumand2%heavierelementssuchas:carbon,nitrogen,oxygen,neon,
magnesium,silicon,argon,calcium,nickelandiron.Thesolarenergyisderived
fromnuclearfusionreactionsduringwhichhydrogenfusestobecomehelium,
andconsequentlyemitting4.27*10-12Jofenergy.Theprerequisitecondition
fortheoccurrenceofthereactioninquestionisasufficientlyhightemperature
explanationfortheaccelerationofspaceexpansionandthemissingmassthattheuniverse
shouldcontain(Nally2011)-71.5%;darkmatter-24%,cosmicgas,interplanetarymedium
-4%)(Howdidouruniversebegin?2019).
8
Etymologically,theMilkyWayisderivedfromtheGreekmythologyanditsnamehas
remainedunchangedtothisday.
9
Nuclearfusion(nucleosynthesisorthermonuclearreaction)isanexothermicreaction
duringwhichtwosmallernucleimergeintoonelargernucleuswithalargeratomicnumber.
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