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.
127
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
129
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
intothefuture
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.
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