presentto18.7billionyearsa.b.(afterthebang)

chapter6intothefuture

Wolfecreek,Westernaustralia

agiantmeteoriteweighingmorethan50,000tonnes

crashedtoearthheresome300,000yearsago.the

crateris200feet(60metres)deep,halfitsoriginaldepth.

ironmeteorite

broughtfromchinaforpatrick,themeteoritefellin

1516,duringtheMingDynasty.

theendisnigh

fivebillionyearsfromnow,thered-giantsunwillhave

expandedtosuchasizethattheinnerplanetsMercury

andVenuswillbesubsumed,andearthwillsuffera

fierydeath.

Whenlookingintothepastwehaveactualevidencetoexamine:intheearth'sfossilrecordwecanglimpsetheveryearlystagesofourplanet'shistory;inthecratersoftheMoonwehaveevidenceofancientcataclysmicmeteorite

impacts;inthecloudsofthecrabnebulaweseetheremnantofaviolentsupernovathatoccurredalmostonethousandyearsago.aswegazeatthefaintlightofthegalaxieswearealreadyseeingthemastheyweremillionsofyearsinthepast.ifwemeasuretherateatwhichtheyarefleeingawayfromus,wecanbuildupareliablepictureofthestateoftheUniverseasitwasbillionsofyearsago;andaswecontemplatethecosmicmicrowavebackground,weareliterallyviewingtheUniversejust300,000yearsafterthebigbang.Wecanactuallyseethepast.

the future is more problematic; we cannot see stars and galaxies as they will be in thefuture, and we have to rely on deduction, mixed with a good deal of scientific speculation.though many pages of the history of the Universe have yet to be deciphered, we knowmuch more about the Universe of six billion years ago than we do about the Universe ofsixbillionyearshence.

the earth may be insignificant in the Universe, but to us it is obviously of paramountimportance, so let us look first at what may lie in store for our own planet. on average,the earth is hit every few hundred thousand years by an asteroid large enough to causewidespread devastation. indeed, recently we have tracked several asteroids that havepassed alarmingly near the earth; a few have brushed by at a distance of only a few tensof thousands of miles, well within the orbit of the Moon. they are classed as phas orpotentially hazardousasteroids, and any one of these is capable of causing another ‘greatDying’ifitscoredadirecthit.ifaphaweretobeseenwellbeforeitwasduetocollidewiththe earth, we might be able to do something about it – perhaps by detonating a nucleardeviceclosetoit,divertingitfromitscollisioncourse.

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sumatransupervolcano?

oneofthelargestknownvolcaniceruptionsoccurred

74,000yearsagowhenthetobavolcanoeruptedin

sumatra,leavingtheapproximately1000squaremile

(3000squarekm)tobacalderabehind.thisgiant

depressionformedafterthecollapseofthevolcano's

cone,seeninthissatelliteimage(left)andatground

level(below).theislandinthecraterlakeisaresurgent

dome,wheremagmaisactiveinachamberbelowthe

surface.

intothefuture

present–18.7billionyearsa.b.128

iceonMars

threeimagesfromthehubblespacetelescopeshow

howtheicecapfluctuatesastheseasonschange

–heretheprogression(fromlefttoright)isfrom

Martianautumntospringandthensummer.

arcticseaice

theextentoftheearth’snorthpolarseaicehasrecently

beendecreasingatarateofninepercentperdecade.

thispictureshowsthesituationin2004.

but we have to admit that a collision with a body only a few miles across would bedisastrous for humanity, and we might not cope any better than the dinosaurs did.Disturbingly, in spite of efforts to detect just this sort of threat, several of the recent nearmissesweredetectedonlywhentheyhadalreadypassedbytheearth.

thereareotherwhollypossiblenaturalscenariosinwhichlifeonearthcouldcometoaprematureend.geologistshaverecentlybecomeawareofthepotentialeruptionofsupervolcanoes,whichcouldresultfromvastreservoirsofmagmaunderextremepressure,oneofwhichhasbeendiscoveredundertheyellowstonenationalparkinWyoming.theeruptionofanyofthesecouldresultinaplanet-widecloudofdebrisintheatmosphere,sodenseandpersistentthatmostplantandanimallifewoulddiefromlackofsunlight.itisnowthoughtthatsomepastextinctionsmayhavebeenduetosupervolcanoes.aman-madedisasterisalsopossible.Wenowhavethecapabilitytodestroyourselves,andwemaylackthecivilizationnottodoso.Whateverhappensthough,theultimate

Wearetalkinghereofchangesonverylong

timescales.however,whataboutminorfluctuations

inthesun’soutput?recentlyagreatdealhasbeen

heardaboutglobalwarming,believedtobedueto

ourownactions;therecanbenodoubtthatthe

releaseofcarbondioxide,methaneandothergases

thatareknownasgreenhousegasesmustinfluence

temperaturesatgroundlevel,andthesegreenhouse

gasesarenowpresentinouratmosphereata

greaterconcentrationthaneverbefore.

concernedenvironmentalistsfrommany

countrieshaveraisedthealarminrecentyears.

thereareattemptstoreducetheemissionsby

switchingtoothersourcesofenergy,butasyet

thelargestproducerofgreenhousegases,the

Unitedstates,hassofarrefusedtoagreetolimit

itsoutput.

yetitisfairtosaytherearesceptics.nobody

doubtsthattheworldisatpresentwarmingup,

butcanwereallyblameourselves?forexample,

between1645and1715thesunwas‘quiet’;there

werefew,ifany,sunspotsandnoaurorae.edmond

halleynotedin1715thathehadseenhisfirst

largesunspot.thecoronawaslackingduringtotal

solareclipses,anditseemsthattheusualsolar

cyclewassuspendedforreasonsthatare

globalwarming

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tunguska

inJune1908somethingexplodedabovethestony

tunguskariverinsiberia.Witnessesreportedseeinga

brightfireball.theexplosionwasheard600miles(960

km)away;380squaremiles(970squarekm)offorest

wereflattened;treesupto30miles(48km)awaywere

felledbytheshockwave.itisprobablethattheexplosion

wascausedbyameteorite160feet(49metres)in

diameterenteringtheatmosphereandvaporizingfive

miles(8km)abovetheground.

stillunclear.thiswasthefamousMaunder

Minimum,socalledbecauseitwasdiscoveredin

thelate19thcenturybye.W.Maunder,whomade

astudyofsunspotrecordscoveringatimespanof

300years.

curiously,itseemstohavecorrespondedwitha

changeinclimate,atleastineuropewherereliable

recordswerekept.theperiodwasknownasthe

‘littleiceage’;theriverthames,forexample,

frozeeverywinterduringthe1680s,andfrostfairs

wereheldonit.WhentheMaunderMinimum

ended,theworldwarmedup–andtherewereno

significantemissionsofgreenhousegasesthen.

Moreover,thereareearlierrecordsofvariationsin

thesun,whichhadsimilareffectsontheearth.

cosmologicallyspeaking,inthelong-term

developmentofthesunandearth,these

fluctuationsareveryinsignificant–itisonlyof

crucialimportancetoushumansforwhomavery

smallriseintemperaturecanbecatastrophic.

itseemsclear,therefore,thatchangesinthe

sun’sactivitycanhaveamarkedeffectonthe

earth’sclimate.however,therehasbeenno

equivalentchangeobservedinthesun'sactivity

duringthelate20thandearly21stcenturiesto

accountfortherecentriseintemperature.

fateoftheearthlieswiththesun.itistothesunthatweoweourexistence,anditisthissamesunthatwilldestroyourplanet.

theendoflifeonearththesunisusingupitsnuclearfuelbut,surprisingly,becomingmoreluminous.thishappensveryslowly–imperceptibly,asfarasweareconcerned.asthehydrogeninthecentreofthestarisusedup,sothesuncontractsalittle,puttingmorepressureonthecoreandraisingitstemperature.therateatwhichthereactionsproceeddependsstronglyonthecoretemperature,andsofuelisusedupfaster.abillionyearsfromnow,thesunwillbepowerfulenoughtogiveearthanuncomfortablytorridclimate;itsinhabitantsmaywellhavetoabandontheequatorialregionsaltogether,andhuddlenearthepoles.

but this will only provide a temporary escape. the deserts will expand as the lowerlatitudes become uninhabitable, and the land available for cultivation of crops will become

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present–18.7billionyearsa.b.130

furthestoutpost

titanseemstopossessconditionssimilartothose

onearthbeforelifebegan,whichiswhyitisofgreat

interesttoscientists.thankstothesuccessofthe

huygensprobeinearly2005,itisthemostdistant

worldonwhichaspacecrafthaslanded.huygens

droppedthroughthethickatmosphereandsent

backavideotoitscreators,showingaviewoftitan’s

mountainouslandscapeasitdescended.thisimageis

fromhuygens’mothership,cassini,employingafilter

toseethroughtheclouds.

scarce indeed. the shifting of the continental plates will have long since destroyed thefamiliar shapes of the continents. any remaining ice caps will melt, causing an enormousriseinsealevel;muchoftheremaininglandwillbeflooded.

the relentless heat will increase; by three billion years in the future a critical point willhave been reached. the sun will be 40 per cent brighter than it is now, so that all surfacewater on the earth will have evaporated; the oceans will be gone, and our world will havebecomeaveryhostileplace.

if humanity still exists on earth when the changes in the environment become obvious,how will our remote descendants react? the onset of these changes will be detectable, andalarm bells will ring – but it seems unlikely that even a highly advanced civilization couldcontrol the sun. no doubt a committee meeting would be called, but what would be on theagenda?tomove theearthout toasafedistancemightbepossible,buteven thiswouldnotprovideapermanentsolution,asweshallsee. itmightbepossible to remove theearth fromthe solar system altogether and somehow make it self-sufficient, so that it could survivewithout a sun. if this proved too difficult the human race might consider mass migrationto another world – in another solar system – or else the construction of an enormous, self-supportingspacestationtoaccommodatethesurvivors.

if nothing can be done, as time passes it seems likely that the entire earth will becomea molten, seething mass of magma. there can then be no reprieve; ultimately all life will

131

betelgeux

thisimageofbetelgeuxwasthefirstdirectpictureof

thesurfaceofastarotherthanoursun.therearesome

unexpectedfeatures,forexamplethehotspotbelow

thecentre.

swallowedbythesun

thesunwillswelltosuchasizethatMercuryand

Venusareconsumed.Whilethecurrentorbitofearth

willbewithintheredgiant,thestar'slossofmasswill

causeearthtoswingoutwardandsoescape.bythen,

lifeonearthwilllonghaveceasedtoexist.thediagram

belowshowsthesizeoftheswollenred-giantsun,

comparedtothesizeoftheinnersolarsystemtoday.

be wiped out. so much for earth. in the rest of the solar system, things may becometemporarily more promising for life. Mars will be much warmer than it is now, and itsmassive ice caps (composed of both carbon dioxide and water) will begin to melt. anatmosphere will develop, and for a short while – a few tens of millions of years or so– Mars will briefly be a hospitable place. however, this situation cannot last for long.Mars is simply too small, and has too weak a gravitational pull to retain its newly foundatmosphereforlong.

ithasbeensuggestedthathumanitymightfindarefugeontitan,thelargestsatelliteofsaturn,whichhasathick,nitrogen-richatmosphere.alas,thisisnotso.titanhasalowescapevelocity,andretainsitsatmosphereonlybecauseitissocold;atalowtemperature,gasmoleculesaresluggish.raisethetemperaturebyonlyafewdegrees,andthewholeoftitan’satmospherewillescape.

During the followinghalf abillionyears thesunwill swell toover twice its present size,and although the surface temperature will fall, its luminosity will double. there will alsobe effects on the earth’s orbit. the sun’s stellar wind will increase in power and our starwillbeginto losemassas itevolves intoaredgiant.this lossofmassmeansthat thesun’sgravitationalpullwillbeweakenedand, in response,planetswill start tomoveoutward; theearthwillswingouttoadistanceofaround120millionmiles(200millionkm)–notnearlyfarenoughforittoescapefromtheintenseheatofthenowmassively-swollensun.

intothefuture

thered-giantsunMoving further into the future, at about five billion years from the present day, hydrogen‘burning’ in thesun’scorewillcease; therewillbenohydrogen left– itwillallhavebeenconverted to helium in the process of nuclear fusion. the core suddenly will no longerbe supported by the pressure of radiation emitted from nuclear reactions. gravitationalcollapse cannot be prevented; the outer material will rush in, compressing the core andheating the material. Until this point, helium nuclei had been unable to participate innuclear reactions. in a matter of seconds, however, the temperature will become highenough for another level of fusion to occur.the helium nuclei combine to form berylliumandlithium.this isamuchmoreefficientreaction; thesunwill radiateover2000timesasfiercelyasitdoesnowanditwillballoonouttosuchanextentthatMercuryandVenuswillbeswallowedup.thesunwillhave,eventually,becomearedgiant.

redspidernebula(ngc6532)

thetangledwebofthisplanetarynebulaistheresultof

asun-likestarejectingitsgasesandbecomingawhite

dwarf.inthiscasethewhitedwarfisoneofthehottest

everobserved.

ringnebula(M57)

thiscelebratedplanetarynebulahastheappearance

ofasmoke-ringcentredaroundtheremainsofthe

supernovaexplosionthatproducedit.ifwecouldsee

itinthreedimensions,ourviewwouldbestraightdown

theaxisofatube.

thefutureoftheMoon

theMoonwillremainlinkedwiththeearth–there

isnoreasontosupposeotherwise–butitsorbit

willchange.atthemomentitismovingaway

fromtheearthattherateofabout4centimetres

(11/2inches)peryear,becauseoftidaleffects.

thecruxofthematteriswhatistermedangular

momentum.theangularmomentumofamoving

bodyisobtainedbymultiplyingtogetheritsmass,

thesquareofitsdistancefromthecentreofmotion,

andthespeedarounditsorbit–thatistosay,the

rateofaxialrotation.aswehaveseen,theMoon’s

axialrotationisthesameasitsorbitalperiod(27.3

days),whichiswhyitalwayskeepsthesameface

turnedtowardus(allthelargesatellitesinthesolar

systembehaveinthesamewaywithrespectto

theirprimaryplanets).angularmomentumcan

neverbedestroyed;itcanonlybetransferred.ifthe

rateofaxialrotationissloweddown,ashappened

earlyintheearth-Moonsystem,somethingelsehas

toincrease,andthis‘something’isthedistance

betweenthetwobodies.thesituationisratherlike

thatofaniceskater,mid-spin.Whenshebringsher

armsintohersides,angularmomentummustbe

conservedandsoshespeedsup.

theprocessisnotcompleteevennow,because

theearth’srotationisstillbeingbrakedbythepull

oftheMoon,andeachdayis0.00000002seconds

longerthanitspredecessor,thoughtherearealso

irregularfluctuationsnotconnectedwiththeMoon.

itisthesethatareresponsiblefortheoccasional

leapsecondsthatareaddedandsubtractedfrom

theofficialtime.however,theMooncouldnotgo

onrecedingindefinitely.ifitmovedoutto350,000

milesitwouldstarttodrawinwardagain,because

oftidaleffectsduetothesun:itsorbitalperiod

andtheearth’saxialrotationperiodwouldthenbe

equal,47timesaslongasourpresentday.ifour

worldsurvivesthesun’sredgiantstagethismay

actuallyhappen,butofcoursenotuntillongafterall

lifehasvanishedfromtheearth.

butterflynebula(M2–9)

ifthisnebulaisseensliceddownthemiddle,itis

knownasthe‘twinJetnebula’,whichwouldbehighly

appropriatebecausethevelocityofthegashasbeen

measuredat200miles(320km)persecond!

rotteneggnebula(oh231)

thisimagegivesusaninsightintothefateofoursun,

sinceweseetheplanetarynebulabeingformed.gas

movingatmillionsofmilesperhourramsintothe

surroundinggas–asupersonicshockfrontwherethe

gasglowsblue.itshoulddevelopintoafullbipolar

planetarynebulaliketheoneaboveoverthenext

1000years.Whyrottenegg?Muchsulphurhasbeen

detected,androtteneggssmellofsulphur.

redrectangle(afgl915)

theladder-likestructuresinthisplanetarynebulacould

benestedexpandingmaterialejectedbythestarin

conicalshapesthatweareseeingexactlyedge-on.

eskimonebula(ngc2392)

thissmallplanetarynebulahaspatternsofdustand

gasthatarecomplexandnotfullyexplained.itis

supposedtolooklikeafacesurroundedbyafur-

trimmedparkahood(althoughthisismoreobviousin

medium-sizedtelescopesthaninthisbeautifulpicture).

present–18.7billionyearsa.b.136

ananomaloussupernova?

Whensupernova1987ablewup,accordingto

thetheoryexplainedhere,aneutronstarorblack

holeoughttohavebeenleftatthecentreofthe

expandingring.asyet,noevidencehasbeen

foundofeither.

brightwhitedwarf

alsoknownastheDogstar,siriusisthebrighteststar

innorthernskies.telescopesrevealthatitisinfacttwo

stars.inopticaltelescopesthebrighteststarissiriusa,

whilesiriusb,awhitedwarfis10,000timesdimmer.

however,whenseeninX-rays,asbelow,thesituationis

reversed,andthewhitedwarfisthestrongemitter.

at some stage in its evolution, the ageing red-giant sun will become increasinglyunstable. its outer envelope will be blown to a distance from the main star by a series ofviolentpulsations,formingwhatisknownasaplanetarynebula.

it isworthnoting that aplanetarynebulahasnothing todowithplanets,but is simply thediscarded outer envelope of a highly-evolved star. these are the butterflies of the Universe,with many beautiful and varied forms but with lives of only a few tens of thousands ofyears.themostfamousof theseobjects, theringnebulainlyra, iseasytolocateeveninasmall telescope, because it is midway between two naked-eye stars, β (beta) and γ (gamma)lyrae, close to the brilliant Vega; even moderately powerful binoculars will show it. in atelescope it looks rather like a dimly luminous cycle tyre.M57 looks symmetrical, but otherplanetaries show an amazing range of shapes, which must depend on the exact processesby which material is ejected from the central star; it seems that the most common is anhourglass shape, with most material being directed along the axes of the star’s magneticfield. according to this model, the planetary nebula appears either as an hourglass or as aring, depending on whether we are looking at it edge- or face-on. this picture seems tobe accurate on the broadest scales, but much of the detail is more difficult to account for.planetary nebulae are among the most chemically interesting regions of our Universe, withmany molecules being produced by reactions driven by the light of the central star in theearlystagesofthenebula’sformation.

137

Whitedwarf–thebankruptsunatthesametime,backatthecentralstar,nowthatthefuelavailableisexhausted,therewillnolongerbeanythingtopreventourstarcollapsingunderitsowngravity,andthiscollapsewillproceedrapidly.eventually,thedensitywillbecomesogreatthatanewresistingforce,degeneracypressure,willbegintoworkagainstgravity.Degeneracypressureisaconsequenceoftheexclusionprinciple,afundamentalaxiomofthetheoryofquantummechanics,whichholdsthatnotwoparticlescaneverbeinthesamestate–thatistosay,iftwoparticleswithidenticalcharge,massandenergycometooclosetogetherthentheywillstarttorepeleachother.thestarwillshrinkuntildegeneracypressureexactlybalancesthecrushingforceofgravity,andatthispointthecollapseceases.thenewstablestateisanincrediblydensecorenolargerthantheearth,knownasawhitedwarf.asingleteaspoonfulofwhitedwarfmaterialwouldweighseveraltonnes.bynowtheearthwillhavewithdrawntoadistanceof170millionmiles(270millionkm)fromtheexhaustedfeebleremnantofthesun.

What lies ahead?the answer must be ‘very little’.thewhite dwarf is bankrupt; it has noenergy reserve, and all it can do is to shine very dimly as it cools, eventually reaching theambient temperature.the transition to a cold, inert, deadblackdwarf takes anunimaginablylong time – in fact, it may be that the Universe is not yet old enough for any black dwarfsto have been formed – it seems likely that our sun will end its life as a tiny, dead star stillorbitedbytheghostsofitsremainingplanets.

neutronstarsandblackholeslarger stars meet a different fate. in particular, when the star is so large that the coreforming a white dwarf has a mass greater than the so-called chandrasekhar mass, 1.4 timesthe mass of the sun, even the quantum effect of degeneracy pressure is not sufficient tohalt the collapse. instead, the pressure is so great that individual protons and electrons donot survive. forced to combine, they form neutrons, and we are left with what is known asa neutron star, the density of which far exceeds even that of a white dwarf – a single sugarcube of neutron star material would weigh the same as all of humanity! neutron stars areextremely small, no more than 15 miles across, but on average are one and a half times asmassive as the sun. if you could stand on the surface of a neutron star your weight wouldbe of the order of 10 billion tonnes. the neutron star is actually the most common form ofsupernovaremnant.Weobservethemintheguiseofmysteriousobjectscalledpulsars.

insupernovaeventsinverylargestars,evenaneutronstarisnottheendofthelineintherapidshrinkingofthecore.onceallitsnuclearreserveshavebeenusedup,thecollapsestarts,butthistimeitissocatastrophicthatnothingcanstopit.themonsterstargoesonshrinkingandshrinkingandbecomingdenseranddenser,passingthroughtheneutron-starstage.asthishappens,theescapevelocitygoesup.anystarwithlessthanabouteighttimesthemassofthesunwillenditslifeaseitherawhitedwarforaneutronstar.ifthestarismoremassivethanthis,thecollapseisliterallyunstoppableand,aswehavealreadyseen,ablackholewillform.

pulsarspulsarsarerapidlyspinningneutronstars,whichweseeaspulsatingsourcesofradiowaves,withseveralpulsesarrivingeachsecond.Wehavealreadydiscussedtheroleofangularmomentuminplanetformation,anditisimportanthere,too.asthematerialof

intothefuture

139

barrel-shapednebula

thisbeautifulsupernovaremnantintheMilkyWayhas

asecretthatisrevealedwhenviewedintheX-raypart

ofthespectrum,ashere,infalsecolour.thebrilliant

bluebandmaybetheremainsofagamma-rayburst,

oneofnature’smostpowerfulexplosionsofenergy.

guitarnebula(WnJ2225)

thewakeleftbyaneutronstartravellingthroughspace

atabout1000miles(1600km)persecondcreatedthis

extraordinarycosmicguitarintheinterstellarmedium.

thestarcollapsestoformtheneutronstar,itcarriesitsangularmomentumwithit,andjustastheiceskaterbringinghisarmsintohissidespeedsup,sotheformingneutronstarspinsfasterandfaster.oncethecollapseiscomplete,thepulsarwillspinataroughlyconstantrate.Manypulsarsthatspinthousandsoftimesasecondarenowknown.thesemustbeyoung;theneutronstarswillgraduallyslowdownovertime.

Whatcausesthepulses?theemissionfrommaterialaroundtheneutronstarischannelledintonarrowbeamsnearthepolesoftheobject.asthestarrotates,sothesebeamsflashacrosstheearthlikealighthousebeamcrossesmomentarilyoverashipfaroutatseaorawatcherontheshore.Whenthebeamispointingtowardus,ourtelescopesdetectapulse.

pulsarsarethemostaccurateclocksintheUniverse;thereareoccasionallyglitchesduetosomepoorly-understoodprocessesdeepinthestar,butapartfromtheserareeventsandtheslowing-downoverlongtimescales,theykeepperfecttime.theythusprovideuniquelaboratoriesforastronomers.inparticularthereisararesystemknownasthedoublepulsar,aboutwhichwewillhavemoretosaylater.therehavebeenreportsofthepresenceofplanetsorbitingpulsars,thesuggestionbeingthattheseplanetsareresponsiblefortinychangesinthetimingofthepulses.however,itisdifficulttoseehowplanetscouldhavesurvivedtheexplosionthataccompaniedthepulsar’sbirth.

intothefuture

present–18.7billionyearsa.b.140

arp299

thispairofcollidinggalaxiesmaybethebestplaceto

lookforanewsupernovaexplosion.asuperstarcluster

inarp299sawitspeakofstarformation6to8million

yearsago,andmanystarsarenowendingtheirlivesin

supernovaexplosions.foursupernovaehavebeenseen

since1990!

theantennae(ngc4038&4039)

thecoresofthesetwocollidinggalaxiesaretheorange

blobs,andawidebandofchaoticduststretches

betweenthem.namedfortheirresemblancetothe

antennaeofaninsect,thesimilarityisfarlessmarkedin

thiswonderfulimagefromthehubblespacetelescope

thanitiswithalesssophisticatedground-based

instrument.eventuallythetwogalaxieswillendtheir

cosmicdanceandmerge,butfornowtheybothshine

brightly.thisismostlyduetoaburstofstarformation

triggeredbythecollision.

remember,wehavebeendiscussingtheevolutionofthecoreofthestar,butsomethingmoredramaticishappeningoutside.asthecollapseissuddenlyhalted,theouterenvelopereboundsinastupendousreleaseofenergy.ithasbecomeasupernova.

starworldsincollisionasoursungrowsold,sotoothroughouttheUniverseoldstarswilldieandnewstarswillbeformed.galaxiesalsoareevolvingandmoving.ourlocalgroupofgalaxiescontainsonlythreereallymajorstarsystems;theandromedaspiral,thetriangulumspiralandourownMilkyWaygalaxy.ofthese,andromedaisthelargestandtriangulumthesmallest.andromeda,atadistanceofbetweentwoandthreemillionlight-years,isalsothenearestand,caughtbythemutualgravitationalattractionbetweenitandourgalaxy,isapproachingusatarateof190miles(300km)persecond.inthreebillionyearstime,therefore,inourpartoftheUniversesomethingreallydramaticwilloccur:acollisionbetweentwolargegalaxies.

ifasmallgalaxycollideswithamuchlargerone,itissimplyabsorbedandwillusuallyloseitsseparateidentitycompletely;inanycase,itisboundtobeseverelydisruptedbytidalforces;itsstarswillbeliterallystrippedfromiteverytimeitgoesnearthelargergalaxy.thingsareverydifferentwhentwomajorgalaxiescollide.

perhapsitisbesttosayatthispointthatalthoughwetalkaboutcollisionsbetweengalaxies,wedonotmeantoimplythatindividualstarsmightcollide.thespacebetweenthem–rememberthesunismorethanfourlight-yearsawayfromitsnearestneighbour,proximacenturi–issimplytoovastandstellarcollisionswillremainextremelyrare,eveninthechaoticenvironmentofamergerbetweentwogalaxies.

thecollisionwilltakeseveralbillionsofyears.ifcomputersimulationsaretobetrusted,andromedawillfirstswingpastourgalaxy,andtoanywatcherspresentthetinypatchoflightwouldbecomelargerandlargeruntilitcametodominatethenightskyasthemaininteractionsbegin.asthereservoirsofgasineachgalaxycollide,theresultingshockwavestriggertheformationofmanythousandsofnewstars,andmanyofthesewillbeinbrilliantclustersdominatedbythehot,bluestars.thecreationofmanymassive

141intothefuture

theMice(ngc4676)

threehundredmillionlight-yearsaway,inthe

constellationcomaberenices,thecosmiccapersof

thepairofcollidinggalaxiesknownastheMicewill

ultimatelyendwiththepairmergingintoasinglegiant

galaxy.theyarenearertocompletingthismergerthan

aretheantennae,andareclassifiedasasinglesystem

inthenewgeneralcatalogue(ngc).

andthereforeshort-livedstarsmeansthatsupernovaewillbecommon,andtheshockwavesfromtheirexplosionswilltriggerfurthermassiveboutsofstarformation.theskywillbelitteredwithcloudsofglowinggasanddust.afterswingingby,whatremainsofandromedawilltakeperhaps100millionyearstodescribeastatelyU-turn,beforeplungingheadlongbackintotheheartofwhatwasoncetheMilkyWay.Muchofthematerialwillbeleftbehindinlongstreamers,butovertimethesetoowillfallintothecentreandalargeellipticalgalaxyseemsthelikelyresult.eventuallytheblackholeatthecentreofourgalaxymaywellmergewiththeblackholewhichalmostcertainlyliesattheheartofandromeda.

itisgenerallybelievedthattwoblackholescollidingwillcombinetoformasingle,moremassiveblackhole.intenseradiationisboundtobereleased,alongwithwhatarecalledgravitationalwaves.

gravitationalwavesgravitationalwavesarepredictedbyeinstein’stheoryofrelativity,andcanbethoughtofasripplesinspaceitself.theyareproducedinsignificantquantitiesonlybythemostenergeticofevents.however,eventhentheeffectmustbesmall,andgravitationalwaveshavenotyetbeendetected.Manyattemptshavebeenmade,buttodetecttheeffectasspaceripplesaroundus,formidableprecisionisrequired–equivalenttomeasuringthelengthofamile-longrodtoanaccuracyoflessthanthesizeofasingleatomicnucleus.perhapsthebesthopeliesinusingsatellites,andvariousprojectsarebeingplanned.Detectinggravitationalwaveswouldenableustoprobeawholenewsetofsituationsandobjects,includingsomeoftherarestphenomenaintheUniverse.

althoughwehavenotyetdetectedgravitationalwaves,thereiscompellingevidencefortheirexistenceintheformofasystem,uniqueinourexperienceandknownasthedouble

pulsar,inwhichtwocompactneutronstarsorbiteachother.sincetheseremarkableobjectsemitextremelyregularpulsesofenergythatcanbeseenacrosshugedistances,weareabletotimetheirorbitswithgreataccuracy.astronomershavediscoveredthatthesetwopulsarsarespirallingintowardseachother,whichmeansthatenergymustbebeinglostfromthesystem;theamountbeinglostcorrespondsquitewelltothepredictedenergythatwouldgointogravitationalwaves–butuntilwedetectthewavesthemselves,wecannotbesurethatwehavetheanswer.

theend?Whateverhappenstothecentralblackholes,bythistimetheearthwillbelonggoneasahabitableworld,andthesunwillbenearingthelastpartofitscareerasaluminousstar;itmayevenhavealreadybecomeawhitedwarf.Wewillnotbetheretoseeit–butwillanybody?

Muchofthereleasedenergywillbedangerous,forexampleintheformofX-rays,andanylife-bearingplanetswillbedelugedwithhigh-energyradiationthatwilldisruptmetabolicprocessesanddamagelivingtissue.theradiationmaywellbesufficienttowipeouteventhemosttechnicallyadvancedcivilizations.atleastwemaybeconfidentthatfinallytheactivitywillsubside,andthenewlyformedgalaxywillsettledown.Mostofthegaswillhavebeenusedupinthefireworksthatfollowedthecollision,andsotherateofstarformationtoowillhavepeaked.perhapstheeventualoutcomewillbeasystemthatiscalmandstable,butalsolifeless.

throughoutthenextfivebillionyearsfromnowthesevariousprocesseswillcontinue–stardeaths,starbirths,supernovaeandcollisionsbetweengalaxies.themostsignificantlong-termchangewillbetheincreasingdistancesbetweentheclustersofgalaxies.WearedriftingslowlybutinexorablyintothelongtwilightoftheUniverse.