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James B. Kaler: Heaven's Touch

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j Chapter 1

ReachingOut

earth. A beautiful word, Earth. It summons visions ofmountains,oceans,blueskiesandclouds,windandrain,prairiesandplowedfields,life.Oursmallplanetisutterly

centraltous.Whateverhappensalmostanywhereonitaffectsourlives.TheonlybodyofcomparableimportanceistheSun,whichprovidesheat,light,nearlyallofourenergy.WithouteitherSunorEarth,weobviouslycouldneverhavecometobe.Wearethesonsanddaughtersofboth.

Thenighttimeskyrevealsadifferentface.Thebluecoverisre-placedbyablackonealightwithstars,shiftingplanets,theMoon,andoccasionalcometsandmeteors.Though foundationsofart,music,philosophy,theresidentsofnightseemotherwisetohavelittleeffect.Lookingoutwardfromourhome,wefeelsuitablyiso-lated.Wemightspendalifetimewatchingtheheavens,yetnothingmuch,ifanythingatall,seemstohappentherethathasanythingtodowithourlives.

Butthatisonlybecausewedonotlookcloselyenough,orlivelong enough, to witness the whole story.While some influenceshave longbeenknown (lunar tides,for example),onlyover thepastcenturyorsohavewelearnedthatweareprofoundlyaffectednotjustbytheSunandMoon,butbypracticallyeverythingthathappens“outthere.”WearenotjustthechildrenofEarthandSun,butofthestarryUniverse.

Amongthemostamazingdiscoveriesofmodernastronomyisthatevenourday-to-dayaffairsareinfactsubjecttothevagariesofdistantplanetsandstars.Noastrologyhere:justgoodsciencethathasuncoveredthetrueplanetaryandstellarinfluences,whicharefargranderthanany“magical”onescouldeverbe.Overthe

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ninechapters that follow,theywillbe revealedaswe reachoutfromourhomeeverdeeper into theheartofouter space toex-periencenot isolation,but a grand synergy inwhich everythinginfluenceseverythingelse.Butfirstweneedasummaryofwhatisactuallyoutthere,soastoprovideacontextforwhatistocome.

Stars

Stars surround us, eight to ten thousand of them visible to theeyealone.Ahandfulbrilliantlypunctuatethedarkness,whilethefainteronesoverwhelmourvisionwiththeirsheernumbers.Thereis no recordof discovery.Stars havebeen seen,admired, loved,feared,studied,romanticized,sincehumansfirstlookedupward.Longshroudedinmystery,theywerebelievedbyourancestorstohavebeenplacedbythegodsintopatterns—constellations—totellstories,toinstruct,tocommemorate,tonotethepassageoftime.TothenorthroamthetwocelestialBears,UrsaMajorandMinor;tothesouthstalksOriontheHunter,accompaniedbyhistwoca-ninecompanions,CanisMajorandMinor.FarthersouthsailstheshipoftheArgonauts,whilegirdlingtheskyinagreatcirclearethetwelveancientanimalisticfiguresoftheZodiac,whichcradletheSun(LibraoncebeingtheScorpion’sclaws).

All cultures recognized such stellar patterns, “ours” comingdowntousfromBabyloniaandbefore.Ultimately,forty-eightofthemwerepassedtousthroughthehandsoftheancientGreeksandArabs.Thousandsofyearsaftertheirinvention,westillcele-brate them fromourownbackyards,allowingour forebears toreachouttotouchusacrosstheages.Theseancientconstellationsare supplemented by new ones culled from the interests of ourownmoremoderntimes(afurnace,sextant,microscope),givinguseighty-eightofallsizesandshapesdisplayedacrossthenorth-ernandsoutherncelestialhemispheres,towhichareaddeddozensofinformalconfigurations.

Telescopesrevealanuncountablepopulationofstars:millions,billions.Inrealitytheyareothersunsofallcolors,kinds,forms,sizes,andages.Self-luminous,theysendussignalsof light from

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Figure1.1. ThebrighthalfoftheMilkyWay,thecombinedlightofthestarsinthediskofourGalaxy,spillsacrosstheskyfromnorthatlefttosouthatright.Thecomplexdarkbandsaremadeofthickcloudsofinterstellardustthatarethebirthplacesofstars.TheSouthernCrossliesattheupperright-handcorner.Theleft-handstarofthepairjustdownandtotheleftoftheCrossisAlphaCentauri,thecloseststartoEarth,fourlight-yearsaway.AntaresinScorpiusisthebrightstarjustabovecenter.CourtesyofSergeBrunier.

distancesmeasuredinimpossiblylongunits.TheSun,150millionkilometers(nearly100millionmiles)away,providesafundamen-talmeasure.Onehundred times thediameterofEarth,our starholdsmorethan300,000timesEarth’smass.Runningonnuclearpower (the conversion of hydrogen into helium), one second’sworthofitsradiationcouldprovidealltheenergyusedonEarthforthenextmillionyears.Otherstarsrangeinsizefromthatofasmallcitytotheorbitsofthegiantplanets;inmassfromafewpercentthatoftheSuntoover100timessolar;inagefromnewlyborn tonearly theageof theUniverse itself.Oncecreated,theylivemostlyquietlivesuntiltheirinternalfuelrunsout,atwhichpointtheyenterdesperatestraits,firstswellingtogiganticpropor-tionsbeforedyingastiny,worn-outcinders.Alongthewaythey

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producesomeof themostbeautifulsightsofnature.Atinyfewevenexplode,whenceintheirnuclearfurytheymanufacturemostofthechemicalelementsofwhichwearemade.

Planets

AgainstthedistantstellarbackgroundlietheplanetsofourSolarSystem.Allbutthefarthestofthemappeartooutshinethestars,whicharehundredsofthousands,millions,oftimesfartheraway.AstheplanetsorbittheSun,eachonitsownpath,eachtakingitsowntime,theirmovementsagainsttheZodiaclongagocapturedtheimagination,somuchsothatinancienttimestheywererelatedtogods.TherefliesfleetMercurytheMessenger.Nowyouseehimineveningtwilight,butlookquicklybeforeheflitsintomorning.VisitnextwithpeacefulVenus,theclassicbrillianteveningormorningstar,Aphrodite,goddessofloveandbeauty,whoseglowcanbeseeninfulldaylightandcancasteerieshadowsatnight.JumpingoverEarth,findreddishMars,theleeringgodofwar,halfagainfartherfromtheSunthanwe.ThenwalkwithstatelyJupiter,kingZeushimself.FivetimesEarth’sdistancefromtheSun,hespendsayearvisitingeachofthedozenzodiacalfigures,every20yearspassinghisdoublydistant,slower,fainter,defeatedfather,Saturn.Perhapsfromtheirzodiacalhomesthegodscantellusthemeaningsofourlives,canrevealourfates,thefortune-tellingartofastrologynotsplittingofffromastronomyuntilwebegantoseetheplanetsforwhattheyactuallyare:other“earths”castintoavarietyofforms.Tothese,addthetwodiscoveredinmoderntimes.Anothernear-doublingofdistancegetsustoUranus,namedaftertheembodi-mentoftheheavensthemselves.HalfagainfartheroutgetsustoNeptune,namedforthegodoftheSea.Discoveredonlyin1846,ittakesacenturyandahalftomakeafullloopoftheSun.

Nearby,orbitingourEarthandbrighterstill,isoursmallsisterplanet,theMoon,whichconstantlychangesshapeasshemakeshermonthlyrounds.NotonlycanweseetheMoonresolvedasadiskwiththenakedeye,it’ssoclosethatwecanevenseedarkfea-turesonitssurface,ofwhichtheold-timersmadefancifulfigures,

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butwhichwenowknowareancientlavaflowscloselyrelatedtoabattered,beaten,punctured,crateredsurface.Inbetweenistheleftoverdebrisoftheformationofourplanetarysystem,madeofasteroidsthatflockbetweenMarsandJupiterandoftheicycom-etsthatthrivemostlybeyondNeptuneandthatarerelatedtothe“lastplanet,”tinyPluto.WhilerareamongthesmallplanetsoftheinnerSolarSystem,dozensofmoonsflockaroundthegiantplan-etsfromJupiteronout.

Whiletheinteractionsbetweenthestars,Sun,Moon,andplanetswith our Earth are deep and complex,our first appreciation ofthemallisstillthroughthelighttheysendus.StarsandtheSunmaketheirownradiantenergy,whiletheplanetsandtheMoonshinebyreflection—asdoestheEarth.Howeveritisproduced,ourfirstthoughtisalwaystolooktothe

Light.

Light: our human window to our surroundings and to the sky.Nothing is so fast! Indeed,nothing can be so fast.Turn on animaginaryflashlightandthree-billionthsofasecondlatertheradi-antbeamisameteraway;inasecondandahalfithaspassedtheMoon(384,000kilometers,239,000miles),ineightminutestheSun(150or93million).Inthreehoursitbeginstoexittheplan-etarysystem,zippingbyNeptuneat300,000kilometers(186,000miles) per second, then an hour later past Pluto’s path. Such abeamwouldnextgoonalonglonelyrideforatleastfour years beforeitwouldencounteranotherstar,onethensaidtobefour“light-years”away.Anditwouldflyforthousandsofyearsbeforeitpassedthelaststarvisiblewithoutatelescope.

Light:ourhumanwindowtothepast.Reversetheflowandletnaturalstarlightfalltowardyou.Becauseofthetimeneededforlighttotravel,weseethemostdistantstarsastheywerethousandsofyearsago,Plutoasitwasfourhoursago.Sunlightiseightmin-utesold.Evenyourfriendsappearastheywereafewbillionthsofasecondago.The“present”isasweseeitwithourowneyes.Everyonethushasadifferentviewofreality.

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Light:strangestuff.Asaflowofalternatingelectricandmagneticfields(thetwoforeverlinkedtogether,eachcausingtheother),noonehaseveractually“seen”lightitself;weinsteadsensetheeffectithasonoureyes.AmajormeansofmovingenergyfromoneplaceintheUniversetoanother,lightbehavesinpartlikeacontinuouswave,much like thewavepatterns thatglideacross the sea.Theshorter thedistancebetween wave crests (the“wavelength”),themorewaveshityoupersecond,andthemoreenergyawavetraincancarry.Butlightalsoactslikeapackofspeedingbullets,aspar-ticles,“photons”thathurdlealongfromsourceofthelighttotheeye.Light’sstrangenessisthatitbehavesaswavesandparticlesat the same time,aconceptthatrendersourbestintuitiveimaginationspowerless.

Anotherstrangeness(tous,nottoNature)isthatphotonshavenomass,no“weight”;theyaretheonlyparticlesknownthatdonot.Inhisrelativitytheory,Einsteinshowedusthatmass(M)andenergy(E)arerelatedthroughperhapsthemostfamousequationeverwritten,E =Mc2,wherec isthevelocityoflight.Whilethespeedof lightalone ishuge,squaring it (multiplying itby itself)makesthenumbervastlylarger,suchthatatinyamountofmasscanbeturnedintoastartlingamountofenergy—whichisthekeytonuclearandstellarpower.

Energy,whichcomesinmanyforms,caninitscrudestsensebethoughtofastheabilityofabodytoaccelerateortogiveheattoanother.Withmoremassandhighervelocity,aspeedingcarobvi-ouslyhasmoreenergythanarunninghuman.Einsteinalsothenrevealedthatasthevelocity(henceenergy)ofaparticleincreases(“relative”tous),sodoesitsperceivedmass.Atlight-speedapar-ticle’smasswouldbecomeinfinite,whichisimpossible.However,sincephotonshavenomasstobeginwith,onlytheyareallowedto runat the limit.Anythingwithmass is confined to less thanlight-speed.

For all its richness,our personal window on the Universe isterriblysmallwithinastunningrangeofwavelengths,withinthe“electromagnetic spectrum.”Withour eyes,we see thosewavesthat fallbetween0.00004and0.00008ofacentimeter (where,

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notsooddly,theSunandstarsgenerallyemitthemostoftheirenergy).We sense the different visible wavelengths as differentcolors.Atthelongend,weseered,attheshortendviolet,withorange, yellow, green, blue, and their hundreds of overlappingshadesinbetween.

Outside of this visual band, our eyes cannot register wave-photons,nomatterhowpowerfulorhowmanytheremaybe.Lon-gerthanthevisualwavelengthlimit—uptoaboutamillimeter—liesthe“infrared.”Longerwaves,intokilometer-wavelengthstowardanunknownend,welooselycall“radio.”Conceptually,however,allarethesame.Allarestill“light”thatcarriesenergy,allrunning(inthevacuum)atthe“speedoflight.”Longwavesmeanlowen-ergy,suchthat(unlessathighintensityoratspecificwavelengthssuchasthosefoundbouncingwithinamicrowaveoven)theyposelittledangertousorotherlivingthings.

Shorterthanthevisuallimit,morevioletthanviolet,istheultra­violet.Iflessthanapercentorsoofthewavelengthofvisuallight,the waves are called X-rays.Another factor of 100 smaller,weenterthedomainof“gammarays.”Shortwavescarryhigherener-gies,resultinginincreasinglyhigherdanger.UltravioletlightfromtheSunwillburnyourskin,whileX-rays(unlessusedproperlyfortheirmedicalbenefit)aredownrightdangerous;gammarayscanbe lethal.Fortunately,the latter twoandmostof theultravioletrealmareblockedbytheEarth’sprotectingatmosphere.

Together,stars,thedustygasesinthespacebetweenthestars,andrelatedcosmicobjects,emitacrossthisentirewavelengthar-ray.Among the great triumphs of twentieth-century astronomywastheopeningoftheelectromagneticspectrumtoourviewviaastartlingarrayofnewtechnologies.Theexpansionofourvisionbeganinthe1930swithradioastronomy,andendedwithgammaraysandX-raysobservedfromsatellitesorbitinginthedepthsofspaceabovethesurroundingatmosphere.

Thevisualspectrumfromviolet tored isbutoneoctaveonanimaginaryelectromagneticpianowithakeyboardhundredsofkilo-meterslong.Allofitisavailableforinspection,allowingustoexplorethedepthsofthesystemofstarsinwhichwelive,ofourGalaxy.

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Figure1.2. TheHubbleSpaceTelescopeorbitsagainstthebackgroundofEarthanditsclouds.Launchedin1990,theHSTepitomizesthedozensofspaceobservatoriesthatcan“see”withexquisitedetailacrosstheelectromagneticspectrum.NASA.

The Milky Way

ImagineascenenotfromEarth’ssurface,whereourplanetcreatesahorizonthatcutsouthalfthesky,butfromdeepspaceoutsidethe Solar System,where you can get a full view of the celestialsphere that seems to surroundus,and thusof all the stars thatmakethemselvesknowntotheunaidedeye.NolongertwinklingfromthedisturbingeffectsofEarth’satmosphere,theyarepoint-likejewelssetonablackcosmiccloth.Splittingtheheavensintwois a luminous,highly irregular bandof light—theMilkyWay—madeofthecombinedlightofbillionsofstarsthatareindividu-allyinvisible.TowardthezodiacalconstellationofSagittarius(theArcher),theMilkyWayisbrightandbroad,whileintheoppositedirection,towardTaurus(theBull)andAuriga(theCharioteer),itappearsfaintandindistinct.

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TheMilkyWay,thesubjectofcountlesstalesandpoems,isthevisualmanifestationofourGalaxy,ourownrealhome,acollec-tionofover200billionstarsofwhichourSunisjustone.MostoftheGalaxy’sstarsarearrangedinathindiskthatisfilledinthemiddlewithaneventhinnerlayeroffreegasanddust.Thesystemissobigthatitwouldtakealightraymorethan100,000yearstomakethejourneyfromonesidetotheother.Therereallyisnodefinablevisualedge:ourstellarsystemjustgraduallyfadesaway.We,setwithinthedisk,seeitaroundourheadsasthesto-riedbandoflight,whilethelayerofdustygasclouds(fromwhichnewstars continuouslycondense)divides theband in two,likethefillingofacake.Thoughlocatedwellwithinthedisk,wearefarfromthecenter,hencethereisadramaticvariationinbright-nessaswegazearoundtheMilkycircle.Surroundingthewholeaffair is a vast and encompassing, though sparsely populated,sphericalhalo.

Withinthedisksomestarsgangtogetherintoclustersthattellofcommonbirth.Amongthemostbelovedofcelestialsightsarethe Pleiades—the Seven Sisters—and their mythological half-sisters,theHyades,bothinresidenceinTaurus.Underthecurveof the Larger Bear’s Big Dipper flows another,Coma Berenices(Berenices’s Hair), while off in Cancer’s direction the Beehivebuzzeswithitsdelightfulswarm.Thetelescopeshowssuchclus-ters to containhundreds,even thousands,ofmembers,while intheGalactichaloareahandfulofgiganticclustersthatcancon-tainmillionsofstars,someoftheirfuzzyglowsalsovisibletothenakedeye.Evenif—perhapsliketheSun—starshavedriftedawayfromtheirbirthmates,theyarestillcompanionable,agoodlyfrac-tionofthemdouble,triple,quadruple,evenmore,inahierarchyofgravity-boundneighbors.

TokeeptheGalaxyfromcollapsingonitselfthroughthecom-binedgravityofitsstars,thestarsmustbecirculating—orbiting—causingourGalaxytorotate,thoughnotasasolidbody.Instead,theinnerportionrotatesinmuchlesstimethantheouterregions.OurownSuntakes250millionyearstomakeafullcircuit.Allthestarsaroundushavetheirownuniquepathsthatdependon

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Figure1.3. No,nottheDipper,butthePleiades(SevenSisters)starclusterthatgracestheconstellationTaurus.Whileonlysixstarsarevisibletotheordinaryeye,thecluster—430light-yearsaway—containshundredsoffainterones.Boundtogetherbygravity,thePleiadesisbutoneofthousandsofclusters,someofwhichcontainmillionsofstars.Thecluster’shottestandbrighteststarsilluminateathindustygascloudthroughwhichtheclusterisnowpassing.CourtesyofMarkKillion.

thegravitiestheyfeel,suchthatallseemtodriftpastusorwepastthem.Overthenextthousandsofmillennia,theconstellationswillslowly change their shapesuntil theybecomeunrecognizable toourcurrenteyes,astheSunanditsSolarSystemtravelonaslowbut steady journey to theGalaxy’s other side.By then,noneofthestarswenowseewillbevisible,butwillinsteadbereplacedbyothers thatwe inourpresent era cannot see at all.Gravita-tionaldisturbancesandrotationalsohelpspreadoutthestarsintoahugepinwheelofgraceful“spiralarms,”renderingoursaclassic“spiralgalaxy.”

And if this system, our Galaxy, seems almost overwhelming,thereismore:wearenotalone.

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Figure1.4. WanttoseewhatourGalaxymightlooklike?TheAndromedagalaxyis(likeourown)aflatdiskover100,000light-yearsacrossthatliessome2.5millionlight-yearsaway.Newhotbluestarscrowdouterspiralarms,whileancientreddishstarsflocktowardthecenter.Belowcenterandtotheupperrightareapairofsmallcompanionellipticalgalaxies.NOAO/AURA/NSF/WIYN.

galaxies

Offinvasterdistanceslieothergalaxies.Fourarevisibletothena-kedeye.FromtheEarth’ssouthernhemisphereyoucaneasilyspotapairofsmallonessome180,000light-yearsaway,thetwoirreg-ularlyshapedMagellanicClouds,whichorbitourlargerGalaxyassatellites.InnorthernautumneveningslooktotheconstellationAndromedatofindasmallcottonypatchnicelyvisiblewithoutthe

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telescope.Themostdistantthingtheunaidedhumaneyecansee,theAndromedaGalaxyliestwoandahalfmillionlight-yearsoff.Anotherspiralgalaxyevengranderthanourown,itmaycontaintwiceasmanystars.Nearby,in theneighboringconstellationofTriangulum(theeponymousTriangle),andatasimilardistance,liesamuchsmallerspiralavailableonlytoyoungereyesviewingfromextremelydarksites.

Hugeasthesedistancesseem,theyarejustasmallstepintothecosmos.Ours,thesefourgalaxies,plusanotherfewdozensmallscrappyonesaretiedtogetherthroughtheirownmutualgravitiesintoapoorishclustercalledsimply“TheLocalGroup.”Fartherout,150millionlight-yearsaway,isamuchgrandercluster.NearlyfillingthelargezodiacalconstellationVirgo,theVirgoClustercon-tains thousandsof galaxies,thebiggest an ellipsoidal,nonspiralstructuremorethantentimesasmassiveasourownMilkyWaysystem.

Thefartherawaywelook,intodistancesmeasuredinbillionsoflight-years,themorewesee—individualgalaxiesplushundreds,thousandsoftheirclusters,eachcontainingthousandsofindivid-ualmembers.Thenumberofgalaxiesnowseemsalmostinfinite,onepilingontopoftheother,moregalaxiesthantherearestarswithintheMilkyWay.Ifwecouldaddupallwecouldpotentiallyseeinourbesttelescopes,thenumberwouldapproachatrilliongalaxies, each one different, the larger ones containing billions,hundredsofbillions,ofstars,themostdistantseenasitappearedbillionsofyearsinthepast.

All—fromourveryselves,toourplanets,tothenightlystars,totheGalaxy,toothergalaxies—arelockedtogetheraspartofthegreatestofallstructures,onethatincludeseverything.

The Universe

Inakindofoverwhelmingsimplicity,theUniverseis“allthereis.”Wehavenoideahow“big”itis,ifthatconcepthasanymeaningatall.Itmightbeinfinite.Howeverlarge,ithasonegreatandsingularcharacteristic:expansion.Fornearlyacentury,we’veknownthat

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thefartheragalaxyisfromus,theredderitslight.Thedegreeofthis“redshift”isinterpretedintermsofspeedofrecession:doublethedistance,doublethevelocity,andsoon.Itwouldseemthatweareatsomekindofcenter,withalltheothergalaxiesmadlyfleeingfromus.Thatviewisanillusion.Sincethevelocitiesareindirectproportiontodistance,everygalaxyismovingawayfromeveryothergalaxy.Nomatterinwhichoneyoumightlive,youwouldseethesamething.

At the same time, we cannot ignore the powerful effects ofgravity.The expansion involves only large scales.The Earth isnotexpanding,andneither is theGalaxy; themutualgravityofitscomponentsoverwhelmsanyexpansioneffect.Ifgalaxiesarenear eachother, likeours andAndromeda,theirmutual gravityisstillstrongenoughtoovercometheexpansion,andkeepsthemtogether.Likewise, gravity maintains the integrity of even largeclustersofgalaxies.Itismoreaccuratetosaythatisolatedgalaxiesandclustersofgalaxiesareseparatingatvelocitiesproportionaltothedistancesbetweenthem.

TheUniverse then looks as if it has undergone somekindofgiganticexplosion,withgalaxiesandtheirclustershurlingthem-selvesthroughspace.Anotherillusion.Einsteinrevealedanextra-ordinarydepthtoNature,inwhichtimeandspacearecombinedintoasingle four-dimensionalentitycalled“spacetime.”Theap-parent recessionof galaxies is the result not of theirmovementthroughspace,butoftheexpansionofspacetimeitself.Galaxies,caughtinitsweb,aremerelygoingalongfortheride.

Theredshift iscommonlymisinterpretedasa“Dopplershift,”whichcausesthelightwavesofarecedingbodytoseemlongerandthusredder,thoseofanapproachingbodytoappearshorterandbluer.Wehearthesamethinginaudio:anapproachingcarhasahigher-pitchedsoundthanarecedingvehicle.Dopplershiftsinthespectraofstarsandotherobjectsarecriticaltoourunderstandingof themandof theGalaxy.What isreallyhappening is that thephotonsfromdistantgalaxiesarestretchedalongwithspacetime’sexpansionand thereforenaturally lengthen,or“redden,”duringtheirlongjourneystooureyes.Theexpansionrategivestheagesincethebeginningofthestill-mysteriousexpansion.We—inthe

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broadest sense—were born 13.7 billion years ago at a momentwhenallmatterandenergywerecrushedintoa“hot,densestate”whoseactualoriginiscompletelyunknown.OurSunanditsplan-ets,bornfromcondensingdustygasesofinterstellarspace,camealong9billionyearslater.

The evidence that the creation event, the“Big Bang,”actuallytookplaceisdeepandrich.Amongthebestisthatthe“fireball”from the Big Bang produced not only the mass of the Universebut—fromtheintenseenergyofitssuperheatedstate—afloodofshort-wave,high-energygammarays.Justlikethelightfromdis-tantgalaxies,overthepastbillionsofyears,theywerestretched,becominglonger,thatis,“redder.”AnotherwayoflookingatitisthattheheatoftheoriginalearlyUniversecooledsuchthatitnowcontainsonlylow-energyradiowaves.Weliveinaconstantbathof themcalled theCosmicMicrowaveBackground,whoseradiophotonssurroundusasifradiatedfromabodyatamerethreeCel-siusdegreesabovethelowestpossibletemperature(absolutezero,–273C),whichisthevaluepredictedbyBigBangtheory.Ripplesinthisbackgroundrepresentthebeginningsofassembliesofgalaxiesandalsogiveusthesameageasfoundfromthegalaxies’velocities.Andnearly14billionyears later,hereweare,herearethestars,herearetheplanets,herearetheheavensthemselves.

darkness

Yet for all our seeming knowledge, as did the stars of old, theUniverseand its treasuresremainshrouded inmystery.Wehavedoneasortofinventoryofthestoresofenergyandmassthatitcontains.ThekeytounderstandingliesintheUniverse’s“shape.”Spacetimeisratherlikeafour-dimensionalsheetofrubber(how-ever unimaginable that might be) that might be bent,distorted,evenfoldedbackuponitselfinavarietyofwaysthatdependcriti-callyupontheaverageamountofmattercontainedwithinaspeci-fiedhugevolume.Shapeiscrucialtoknow,asthecalculatedageoftheUniversedependsonit.Thepractitionersoftheartof“cosmol-ogy,”thestudyofthecosmosatlarge,cancalculatetheaverage,

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Figure1.5. TheHubble“Ultra-DeepField”revealsthousandsofgalaxieswithinapinheadoftheskyameretwentiethofadegreeacross,atenththeangularsizeofthefullMoon.Afewbigbrightonesarerelativelynearby,whilethefainteststretchouttobillionsoflight-yearsaway.Thefarthertheyare,thefastertheymoveawayfromus,thefirstcluethattheUniversewascreatedina“BigBang.”Thoughtheirnumberseemsoverwhelming,galaxiessuchastheserepresentlessthan1percentofthemass-energyoftheUniverse.NASA,ESA,R.Windhorst(ArizonaStateUniversity),H.Yan(SpitzerScienceCenter,Caltech).

smeared out,density of mass (including the mass equivalent ofenergy)thatisrequiredtorolloutspacetimewithnobendswhat-ever,tomakeit,inaweirdmultidimensionalway,“flat”(suchthatEuclid’sfamedplanegeometryactuallyworks).

Addupthestarsandthestuffinthespacesbetweenthem,andittotalstojustunderapercentofthatneeded.MoremassisindicatedbyBigBangtheory,whichpredictshowprimitivehydrogenatoms

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combinedtocreateavarietyoflightchemicalelements.Much,ifnotmost,of thismatterseemstobe in the formofhotgas thatliesbetweenthegalaxiesoflargeclusters,andevenbetweenclus-ters,whereitisobservedwithX-raytelescopes.Someismatterleftoverfromgalaxyformation,whilemuchwasalsoejectedfromthecluster’smembersby stellar explosions.Or so it’s thought.Sum-marizingthevariouspredictionsandobservationsgetsuscloseto5percentoftheflatnessrequirement.

Thendescendstheshroud.OurGalaxy,itsstarsrevolvingaroundthecenterundertheinfluenceoftheircombinedgravity,isspinningtoofastforwhatwesee.Galaxiesinclustersorbitaroundtheclus-ter’scentersundertheinfluenceoftheir mutualgravities,butagain,theymovefasterthanexpected.Theremustbesomethingouttherewithenoughofagravitationalholdtodothejob,tospeedthingsup,butitiscompletelyunseen.Dark matter.Itsurroundsgalaxies,pervadestheirclusters.Wehavenoideawhatconstitutesit.Rather,therearemany ideas,butnonethatcanbeproven.Additallupbasedontheamountofmassneededtoyielddarkmatter’sgravity,andlo,onefindsanother20percent,gettingusup(oncethenum-bersareroundedoff)toaquarterofthatrequiredtounfoldtheUniverse,butstillnotenough.

Toresolvetheissue(andtoaddtothemysteryatthesametime),look deeper at the expansion.Modern telescopes, imagers, andaboveall,knowledge,haveallowedcosmologiststomeasuretheexpansionratetodistancesofbillionsoflight-years,allowingusto lookfarback into time.Wemightexpectaslowdownof theexpansionasthecombinedgravityofeverythingintheUniverseactstoholditback.Velocityandredshiftareindeedseennot tobequiteindirectproportiontoeachother.Insteadofshowingaslowdown,theobservedvariancerevealstheopposite,asurpris-ingspeedup!Theexpansionisgettingeverfaster.Anyaccelerationrequiresenergy,whichfromitsmassequivalentyieldsthemissing75percent.Within theuncertaintiesof thedata,theUniverse isflat!Butwehaveevenlessofanideawherethisenergycomesfromthanwedoaboutthenatureofdarkmatter.Sotodarkmatter,adddark energy.Either thator something is terriblyamisswithourconceptofgravity.

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We seem almost to be back to primitive times when we firstlookeduptothestarstowonderwhattheyare.Wearestillwon-deringjustashard,perhapsnotaboutstarsassuch,butaboutthestuffthatsurroundsthem,whichsurelyhasaroleinmakingthem,andthat thenplaysarole inmakingus.Sowekeepwonderingandexploring.Andaswereachoutinourattemptstounderstandthemall,fromplanetstogalaxies,theyinturnreachbacktoustoaidinourquest.

Heaven’s Touch

ReturntotheoutpostindeepspacefromwhichweobservedtheMilkyWay.Thesceneryisquiet,serene,strikinglybeautiful.Noth-ingseemstohappen.Floatinginthevoidwefeelisolated,justaswewouldbackonEarthwhereourownplanetdominatesusandour thoughts.Whatdo these starshavedowithus?We cannotreachthem.Theirgreatdistancesmakeitlikelythatnospaceshipeverwill.Andthereisnoevidencethatanyoneoutthere—ifthereisanyone—hasever,orwillever,comehere.Alltheconnectionswehavewiththedistantcosmosseemtobefrombeautifulandbenignstarlight,whichallowsustoadmireandstudytheUniverse,butnevertoreachoutandtouchit.

Similarconceptsofalienationextendtonearspace,toourSolarSystem.TheSunwarmsuswithitslightandheat,butotherwisewepayitlittleheed.Onehundredmillionmilesaway—ahundredtimesthesolardiameter,tenthousandtimesEarth’sdiameter—itseemsotherwisenottoaffectorbotherus.TheMoonandplanets,going through theironce-mystifyingmovements (whichwenowfullyunderstandfromgravitationalandorbitaltheory),arepleas-ing and fun to watch,but again they appear to have no directbearinguponourownsmallworld.Thoughwehavegonetothem,imagedthemupclose,haveevenlandeduponthemoutofcurios-ity and a yearning for exploration,we could as easily have leftthemalone,astheydous.

Ordothey?Thestoryofdiscoveryoverthepastcenturyormorehasstarklyrevealedthatsuchisolationistviewsarethereverseof

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reality.Our sensesare limited,andour lifeonEarth short com-paredwiththeflowofcelestialtime,bothofwhichveilmanyoftheremarkablyvariedwaysinwhichtheheavensdirectlyinteractwithus.Indeed,lifeitselfwouldbeimpossiblewithoutallthesedirectinteractions.

BeginbystandingatthebeachtowatchthewatergoupanddowninsynchronywiththepositionandphaseoftheMoon,allpoweredbygravityfromasatelliteaquartermillionmilesaway,thecyclemodifiedbythe farmoredistantSun.Not just for thewatching,tidesareapartofthefabricoflife,perhapseveninpartresponsibleforitscreation.They—thetides—areonEarth,butnotofEarth,astheirproductionliesintheheavens,whichreachesouttotouchourworld.

Throughthetides,theSunbarelyrevealsitspower.Northernerswill tell you more,about the shining lights in thenight sky,thenorthernlightsthathang,thatflow,thatshootluminouscannon-ballsacross thesky,allcausedby thepiecesofatomsshotatusbytheSun,acceleratedtowardusinassociationwithragingsolarmagnetism.Farmorethancausingprettylights,solarstormsdis-ruptourplanet’smagneticfield,makecompassesgoawry,breakthe electronics that ride aboard billion-dollar satellites, and canevenmakeourlightsgoout.Whenthestormssubsideoverlongperiods—decades—ourEarthchills.WithouttheSun’smagneticef-fects,it’sevenconceivablethatterrestriallifecouldneverhavebe-gun.Moreinvisibly,billionsofsubatomicparticlespassharmlesslythroughuseachsecondfromthedeeplyburiednuclearfurnacethatpowerssunlight.TheSun,Helios,indeedreachesouttotouchusinwaysthatweareonlybeginningtounderstand.

Moveontotheplanets,theEarth’sbrethrenthatorbittheSun,fromMercuryclosein,toUranusandNeptunefaraway,theEarthnumberthree.AllaresatellitesoftheSunthatwerecreatedalongwithitsomefourandahalfbillionyearsago.AsdoestheMoon,theyaffectusthroughtheirgravity.Toofarawaytocausetidesofanymeasurablesort,theyinsteadacttopullonthewholeEarthandalteritsorbit.Overtheaeons,thedistancebetweentheEarthandSunchangesfirstinonedirection,thenintheother,asdoestheorbitaloblateness.Couplethatwitha26,000-yearwobbleinour

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rotationaxiscausedbytheMoonandSunactingonanequatorialbulge that comes from terrestrial rotation,andour seasonsandclimategraduallychangefromvariancesinsolarheating.Iceagesmaybeaconsequence.

Benignthoughtheymayseem, theplanetsevencometovisit, theirattentionssometimesquiteunwelcome.Theywerebornthroughsuccessivecollisionsofsmallerbodies(builtfromdustgrains)thatorbitedtheprimitiveSun.Theprocess,however,wasfarfrom100percentefficient,resultinginalargeamountofleftoverdebristhatconsistsoftherocky-metallicsmallasteroidsmostlybetweenMarsandJupiterandtheicycometsthatmostlyresidebeyondtheplan-etarysystem.Actingas“dirtysnowballs,”cometarybodiesmea-sured in tens or hundreds of kilometers across lie in two greatreservoirs.One,aflatbutthickplatebeyondNeptune,ismadeofcometsthatweretoothinlyspreadtoassembleintoaplanet.Theother,holdingcometsthatwerekickedoutoftheplanetarysystembythegiantouterplanets,mayextendhalfwaytotheneareststar.Ifcaughtinalong,loopingorbit,acometcangetcloseenoughtotheSunsothatitsicesturntogas,which,withthereleaseofdust,producesthegracefultailsiconictoastronomy.

Collisions among asteroids coupled with the gravitational ef-fectsofplanetscantosstheseshreddedbodiestotheEarth.Hittingouratmosphereandheating,theyfirstappearasstreaksoflight—meteors—crossingthesky.Iflargeenough,theysmackintousasme-teoritestobevisitedinmuseums.Thelargestcandiggiantcratersintheground,andinthemostdevastatingofsucheventsevenwipeoutwholespeciesoflife.Smallrocksanddustgrainsflakedoffcometsproduceasteadyrainofsuchmeteors,sometimesshowersofthem,sometimeswholestormsof themtobeadmired.Andcometscanhitustoo,withawesomeforce.GiantimpactsonotherplanetscanbesoenergeticastolaunchrocksintoorbitaroundtheSun,someofwhicheventuallyalsohitus,givingusacheap“spaceprogram,”wherebywecantakepiecesoftheMoonandMarsintoourlabora-tories.Evenstarscanhavesucheffectsbygravitationallydisturbingtheoutericycometarybodies,tossingthembacktowardus.

TheSunshinesthesamedayafterday.Insideisahighlycon-trolledfusionmachinethatconvertsthesolarhydrogenfuelinto

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heliumwiththecreationofenergy.Asthefuelrunsoutoverlongperiods—billionsofyears—theSunwillgraduallychange.Foratime,itwillswelltoswallowMercury,perhapsVenus,perhapsintheultimatesolareffectevenconsumingEarth.Itwillthenquietlyshrinktoacoolingdenseballcomparabletoourownplanet’ssize.Otherstarsarenotsolucky.Raremassiveonesexplode,where-upontheyacceleratetheirsurroundingsoutwardatspeedscloseto thatof light.The stuff rainsdownuponusas“cosmic rays”thatmanufacturetheradioactivecarbonthatarchaeologistsusetodateancientruins.Cosmicraysmayevenbetheseedsthattriggerlightningbolts,perhapsevensomecloudformation,allfromdyingstarsthatarethousandsoflight-yearsaway.

Ifnearenough,theradiationfromsuchexplosionsmaybehaz-ardoustolife,indeedmayevenhavecausedoneormoreextinc-tioneventsonEarth.Ordinarystellarexplosionsneedstayatleast30light-yearsaway,whileultra-raremaximumdetonationscoulddamageusfromthousandsoflight-yearsaway.Nowonder,sincetheireffectscanevenbeseenwiththenakedeyefrombillions oflight-years’distance.Theremainsofthesedestroyedstarscanbejust as dangerous.Stellar blowups leave behind dense remnantsthatpackthemassoftheSunintoballsnomorethanafewtensof kilometers across, into neutron stars that can have magneticfieldsamillionbilliontimesstrongerthanEarth.Adjustmentsinthese extreme stars and in their magnetic fields send out burstsof radiation sopowerful that even though tensof thousandsoflight-yearsaway,theycanturnofforbitingsatellitesanddisruptcommunication.

Intheultimateconnection,wecamefromoutthere.Allthesub-atomic particles that make atoms and therefore ourselves werecreated in the Big Bang, the event that began our Universe.Alltheheavierchemicalelementsweremadeinagingandexplodingstars,whichalsoproducedmuchoftheenergyneededtodrivestarandultimatelyplanetformation.Wearenotjustin thecosmos,weareof thecosmos,we,alongwithalltheotherpartsofitare thecosmos,allofitone,allofitallowingustobebornandtoliveourlivessoastounderstandandappreciateitsgrandbeauty.