第2章

Thefirstgreatcosmologicalspeculationwhichhasbeenraisedquiteabovetheplaneofguessworkbymakingnootherassumptionthanthatoftheuniformityofnature，isthewell-knownNebularHypothesis。Everyastronomerknowsthattheearth，likeallothercosmicalbodieswhichareflattenedatthepoles，wasformerlyamassoffluid，andconsequentlyfilledamuchlargerspacethanatpresent。Itisfurtheragreed，onallhands，thatthesunisacontractingbody，sincethereisnootherpossiblewayofaccountingfortheenormousquantityofheatwhichhegenerates。 Theso-calledprimevalnebulafollowsasanecessaryinferencefromthesefacts。Therewasonceatimewhentheearthwasdistendedonallsidesawayouttothemoonandbeyondit，sothatthematternowcontainedinthemoonwasthenapartofourequatorialzone。Andatastillremoterdateinthepast，themassofthesunwasdiffusedineverydirectionbeyondtheorbitofNeptune，andnoplanethadanindividualexistence，forallwereindistinguishablepartsofthesolarmass。Whenthegreatmassofthesun，increasedbytherelativelysmallmassofalltheplanetsputtogether，wasspreadoutinthisway，itwasararevapourorgas。AttheperiodwherethequestionistakenupinLaplace’streatmentofthenebulartheory，theshapeofthismassisregardedasspheroidal；butatanearlierperioditsshapemaywellhavebeenasirregularasthatofanyofthenebulaewhichwenowseeindistantpartsoftheheavens，for，whateveritsprimitiveshape，theequalizationofitsrotationwouldintimemakeitspheroidal。ThattheQUANTITYofrotationwasthesamethenasnowisunquestionable；fornosystemofparticles，greatorsmall，canacquireorloserotationbyanyactiongoingonwithinitself，anymorethanamancouldpickhimselfupbyhiswaistbandandlifthimselfoverastonewaleSothattheprimitiverotatingspheroidalsolarnebulaisnotamatterofassumption，butisjustwhatmustoncehaveexisted，providedtherehasbeennobreachofcontinuityinnature’soperations。Nowproceedingtoreasonbackfromthepasttothepresent，ithasbeenshownthattheabandonmentofsuccessiveequatorialbeltsbythecontractingsolarmassmusthaveensuedinaccordancewithknownmechanicallaws；andinsimilarwise，underordinarycircumstances。eachbeltmusthavepartedintofragments，andthefragmentschasingeachotheraroundthesameorbit，musthaveatlastcoalescedintoaspheroidalplanet。Notonlythis，butithasalsobeenshownthatastheresultofsuchaprocesstherelativesizesoftheplanetswouldbelikelytotaketheorderwhichtheynowfollow；thattheringimmediatelysucceedingthatofJupiterwouldbelikelytoabortandproduceagreatnumberoftinyplanetsinsteadofonegood-sizedone；thattheouterplanetswouldbelikelytohavemanymoons，andthatSaturn，besideshavingthegreatestnumberofmoons，wouldbelikelytoretainsomeofhisinnerringsunbroken；thattheearthwouldbelikelytohavealongdayandJupiterashortone；thattheextremeouterplanetswouldbenotunlikelytorotateinaretrogradedirection；andsoon，throughalonglistofinterestingandstrikingdetails。Notonly，therefore，arewedriventotheinferencethatoursolarsystemwasonceavaporousnebula，butwefindthatthemerecontractionofsuchanebula，undertheinfluenceoftheenormousmutualgravitationofitsparticles，carrieswithittheexplanationofboththemoregeneralandthemoreparticularfeaturesofthepresentsystem。 Sothatwemayfairlyregardthisstupendousprocessasveritablematterofhistory，whileweproceedtostudyitundersomefurtheraspectsandtoconsiderwhatconsequencesarelikelytofollow。 Ourattentionshouldfirstbedirectedtotheenormouswasteofenergywhichhasaccompaniedthiscontractionofthesolarnebula。Thefirstresultofsuchacontractionisthegenerationofagreatquantityofheat，andwhentheheatthusgeneratedhasbeenlostbyradiationintosurroundingspaceitbecomespossibleforthecontractiontocontinue。Thus，asconcentrationgoeson，heatisincessantlygeneratedandincessantlydissipated。Howlongthisprocessistoenduredependschieflyonthesizeofthecontractingmass，assmallbodiesradiateheatmuchfasterthanlargeones。Themoonseemstobealreadythoroughlyrefrigerated，whileJupiterandSaturnareverymuchhotterthantheearth，asisshownbythetremendousatmosphericphenomenawhichoccurontheirsurfaces。Thesun，again，generatesheatsorapidly，owingtohisgreatenergyofcontraction，andlosesitsoslowly，owingtohisgreatsize，thathissurfaceisalwayskeptinastateofincandescence。Hissurface-temperatureisestimatedatsomethreemilliondegreesofFahrenheit，andadiminutionofhisdiameterfartoosmalltobedetectedbythefinestexistinginstrumentswouldsufficetomaintainthepresentsupplyofheatformorethanfiftycenturies。Thesefactspointtoaverylongfutureduringwhichthesunwillcontinuetowarmtheearthanditscompanionplanets，butatthesametimetheycarryontheirfacethestoryofinevitableultimatedoom。Ifthingscontinuetogoonastheyhaveallalonggoneon，thesunmustbyandbygrowblackandcold，andalllifewhateverthroughoutthesolarsystemmustcometoanend。Longbeforethisconsummation，however，lifewillprobablyhavebecomeextinctthroughtherefrigerationofeachoftheplanetsintoastatelikethepresentstateofthemoon，inwhichtheatmosphereandoceanshavedisappearedfromthesurface。Nodoubtthesunwillcontinuetogiveoutheatalongtimeafterheathasceasedtobeneededforthesupportoflivingorganisms。Forthefinalrefrigerationofthesunwilllongbepostponedbythefateoftheplanetsthemselves。Theseparationoftheplanetsfromtheirparentsolarmassseemstobeafterallbutatemporaryseparation。Sonicelybalancedaretheynowintheirorbitsthattheymaywellseemcapableofrollingonintheirpresentcoursesforever。Butthisisnotthecase。Twosetsofcircumstancesareallthewhilestriving，theonetodrivetheplanetsfartherawayfromthesun，theothertodrawthemallintoit。Ontheonehand，everybodyinoursystemwhichcontainsfluidmatterhastidesraiseduponitssurfacebytheattractionofneighbouringbodies。Alltheplanetsraisetidesuponthesurfaceofthesunandtheperiodicityofsun-spots(orsolarcyclones)dependsuponthisfact。Thesetidalwavesactasadragorbrakeupontherotationofthesun，somewhatdiminishingitsrapidity。But，inconformitywithaprincipleofmechanicswellknowntoastronomers，thoughnotfamiliartothegeneralreader，allthemotionofrotationthuslostbythesunisaddedtotheplanetsintheshapeofannualmotionofrevolution，andthustheirorbitsalltendtoenlarge，——theyalltendtorecedesomewhatfromthesun。Butthisstateofthings，thoughlong-enduringenough，isafterallonlytemporary，andwillatanyratecometoanendwhenthesunandplanetshavebecomesolid。Meanwhileanothersetofcircumstancesisallthetimetendingtobringtheplanetsnearertothesun，andinthelongrunmustgainthemastery。Thespacethroughwhichtheplanetsmoveisfilledwithakindofmatterwhichservesasamediumforthetransmissionofheatandlight，andthiskindofmatter，thoughdifferentinsomerespectsfromordinaryponderablematter，isyetlikeitinexertingfriction。 Thisfrictionisalmostinfinitelylittle，yetithasawellnighinfinitelengthoftimetoworkin，andduringallthiswellnighinfinitelengthoftimeitisslowlyeatingupthemomentumoftheplanetsanddiminishingtheirabilitytomaintaintheirdistancesfromthesun。Henceincourseoftimetheplanetswillallfallintothesun，oneafteranother，sothatthesolarsystemwillend，asitbegan，byconsistingofasinglemassofmatter。 Butthisisbynomeanstheendofthestory。Whentwobodiesrushtogether，eachpartswithsomeofitsenergyofmotion，andthislostenergyofmotionreappearsasheat。Intheconcussionoftwocosmicalbodies，likethesunandtheearth，anenormousquantityofmotionisthusconvertedintoheat。Nowheat，whennotallowedtoradiate，orwhengeneratedfasterthanitcanberadiated，istransformedintomotionofexpansion。Hencetheshockofsunandplanetwouldatonceresultinthevaporizationofbothbodies；andtherecanbenodoubtthatbythetimethesunhasabsorbedtheoutermostofhisattendantplanets，hewillhaveresumedsomethinglikehisoriginalnebulouscondition。Hewillhavebeendilatedintoahugemassofvapour，andwillhavebecomefitforanewprocessofcontractionandforanewproductionoflife-bearingplanets。 Wearenow，however，confrontedbyaninterestingbutdifficultquestion。Throughoutallthisgrandpastandfuturecareerofthesolarsystemwhichwehavejustbrieflytraced，wehavebeenwitnessingamostprodigaldissipationofenergyintheshapeofradiantheat。Attheoutsetwehadanenormousquantityofwhatiscalled“energyofposition，“thatis，theouterpartsofourprimitivenebulahadaverylongdistancethroughwhichtotraveltowardsoneanotherintheslowprocessofconcentration；andthisdistancewasthemeasureofthequantityofworkpossibletooursystem。Astheparticlesofournebuladrewnearerandnearertogether，theenergyofpositioncontinuallylostreappearedcontinuallyasheat，ofwhichthegreaterpartwasradiatedoff，butofwhichacertainamountwasretained。Allthegiganticamountofworkachievedinthegeologicdevelopmentofourearthanditscompanionplanets，andinthedevelopmentoflifewhereverlifemayexistinoursystem，hasbeentheproductofthisretainedheat。Atthepresentdaythesamewastefulprocessisgoingon。Eachmomentthesun’sparticlesarelosingenergyofpositionastheydrawcloserandclosertogether，andtheheatintowhichthislostenergyismetamorphosedispouredoutmostprodigallyineverydirection。Letusconsiderforamomenthowlittleofitgetsusedinoursystem。Theearth’sorbitisanearlycircularfiguremorethanfivehundredmillionmilesincircumference，whileonlyeightthousandmilesofthispathareatanyonetimeoccupiedbytheearth’smass。Throughtheseeightthousandmilesthesun’sradiatedenergyisdoingwork，butthroughtheremainderofthefivehundredmillionitisidleandwasted。Butthecaseisfarmorestrikingwhenwereflectthatitisnotintheplaneoftheearth’sorbitonlythatthesun’sradianceisbeingpouredout。Itisnotanaffairofacircle，butofasphere。Inordertoutilizeallthesolarrays，weshouldneedtohaveanimmensenumberofearthsarrangedsoastotoucheachother，formingahollowspherearoundthesun，withthepresentradiusoftheearth’sorbit。WemaywellbelieveProfessorTyndall，therefore，whenhetellsusthatallthesolarradiancewereceiveislessthanatwo-billionthpartofwhatissentflyingthroughthedesertregionsofspace。Someoftheimmenseresidueofcoursehitsotherplanetsstationedinthewayofit，andisutilizedupontheirsurfaces；buttheplanets，allputtogether，stopsolittleofthetotalquantitythatourstartlingillustrationisnotmateriallyalteredbytakingthemintotheaccount。Nowthistwo-billionthpartofthesolarradiancepouredoutfrommomenttomomentsufficestobloweverywind，toraiseeverycloud，todriveeveryengine，tobuildupthetissueofeveryplant，tosustaintheactivityofeveryanimal，includingman，uponthesurfaceofourvastandstatelyglobe。Consideringthewondrousrichnessandvarietyoftheterrestriallifewroughtoutbythefewsunbeamswhichwecatchinourcareerthroughspace，wemaywellpauseoverwhelmedandstupefiedatthethoughtoftheincalculablepossibilitiesofexistencewhicharethrownawaywiththepotentactinismthatdartsunceasinglyintotheunfathomedabysmsofimmensity。Whereitgoestoorwhatbecomesofit，nooneofuscansurmise。 Nowwhen，intheremotefuture，oursunisreducedtovapourbytheimpactoftheseveralplanetsuponhissurface，theresultingnebulousmassmustbeaveryinsignificantaffaircomparedwiththenebulousmasswithwhichwestarted。Inordertomakeasecondnebulaequalinsizeandpotentialenergytothefirstone，alltheenergyofpositionatfirstexistingshouldhavebeenretainedinsomeformorother。Butnearlyallofithasbeenlost，andonlyaninsignificantfractionremainswithwhichtoendowanewsystem。Inordertoreproduce，infutureages，anythinglikethatcosmicaldevelopmentwhichisnowgoingoninthesolarsystem，aidmustbesoughtfromwithout。Wemustendeavourtoframesomevalidhypothesisastotherelationofoursolarsystemtoothersystems。 Thusfarourviewhasbeenconfinedtothecareerofasinglestar，——oursun，——withthetiny，easily-coolingballswhichithascastoffinthecourseofitsdevelopment。Thusfar，too，ourinferenceshavebeenverysecure，forwehavebeendealingwithacircumscribedgroupofphenomena，thebeginningandendofwhichhavebeenbroughtprettywellwithinthecompassofourimagination。Itisquiteanotherthingtodealwiththeactualorprobablecareerofthestarsingeneral，inasmuchaswedonotevenknowhowmanystarsthereare，whichformpartsofacommonsystem，orwhat。aretheirprecisedynamicrelationstooneanother。Neverthelesswehaveknowledgeofafewfactswhichmaysupportsomecautiousinferences。Allthestarswhichwecanseeareundoubtedlyboundtogetherbyrelationsofgravitation。Nodoubtoursunattractsalltheotherstarswithinourken，andisreciprocallyattractedbythem。Thestars，too，liemostlyinoraroundonegreatplane，asisthecasewiththemembersofthesolarsystem。Moreover，thestarsareshownbythespectroscopetoconsistofchemicalelementsidenticalwiththosewhicharefoundinthesolarsystem。Suchfactsasthesemakeitprobablethatthecareerofotherstars，whenadequatelyinquiredinto，wouldbefoundtobelikethatofourownsun。Observationdailyenhancesthisprobability，forourstudyofthesiderealuniverseiscontinuallyshowingusstarsinallstagesofdevelopment。Wefindirregularnebulae，forexample；wefindspiralandspheroidalnebulae；wefindstarswhichhavegotbeyondthenebulousstage，butarestillatawhiterheatthanoursun；andwealsofindmanystarswhichyieldthesamesortofspectrumasoursun。Theinferenceseemsforceduponusthatthesameprocessofconcentrationwhichhasgoneoninthecaseofoursolarnebulahasbeengoingoninthecaseofothernebulae。Thehistoryofthesunisbutatypeofthehistoryofstarsingeneral。Andwhenweconsiderthatallothervisiblestarsandnebulaearecoolingandcontractingbodies，likeoursun，towhatotherconclusioncouldweverywellcome？WhenwelookatSirius，forinstance，wedonotseehimsurroundedbyplanets，foratsuchadistancenoplanetcouldbevisible，evenSiriushimself，thoughfourteentimeslargerthanoursun，appearingonlyasa“twinklinglittlestar。“ButacomparativesurveyoftheheavensassuresusthatSiriuscanhardlyhavearrivedathispresentstageofconcentrationwithoutdetaching，planet-formingrings，forthereisnoreasonforsupposingthatmechanicallawsoutthereareatalldifferentfromwhattheyareinourownsystem。