darwin and modern science-第172节

按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！


nal　figure。

I　now　turn　to　the　conclusions　drawn　a　few　years　previously　by　another　observer；　where　we　shall　find　the　component　stars　not　quite　in　contact。　　This　is　the　star　Beta　Lyrae　which　was　observed　by　Goodricke；　Argelander；　Belopolsky；　Schur；　Markwick　and　by　many　others。　　The　spectroscopic　method　has　been　successfully　applied　in　this　case；　and　the　component　stars　are　proved　to　move　in　an　orbit　about　one　another。　　In　1897；　Mr。　G。W。　Myers　applied　the　theory　of　eclipses　to　the　light…curve；　on　the　hypothesis　that　the　stars　are　elongated　ellipsoids；　and　he　obtained　the　interesting　results　exhibited　in　Fig。　7。　（〃Astrophysical　Journ。〃　Vol。　VII。　（1898）；　page　1。）

The　period　of　Beta　Lyrae　is　relatively　long；　being　12d　21h　47m；　the　orbit　is　sensibly　eccentric；　and　the　two　spheroids　are　not　so　much　elongated　as　was　the　case　with　RR　Centauri。　　The　mass　of　the　system　is　enormous；　one　of　the　two　stars　being　10　times　and　the　other　21　times　as　heavy　as　our　sun。

Further　illustrations　of　this　subject　might　be　given；　but　enough　has　been　said　to　explain　the　nature　of　the　conclusions　which　have　been　drawn　from　this　class　of　observation。

In　my　account　of　these　remarkable　systems　the　consideration　of　one　very　important　conclusion　has　been　purposely　deferred。　　Since　the　light…curve　is　explicable　by　eclipses；　it　follows　that　the　sizes　of　the　two　stars　are　determinable　relatively　to　the　distance　between　them。　　The　period　of　their　orbital　motion　is　known；　being　identical　with　the　complete　period　of　the　variability　of　their　light；　and　an　easy　application　of　Kepler's　law　of　periodic　times　enables　us　to　compute　the　sum　of　the　masses　of　the　two　stars　divided　by　the　cube　of　the　distance　between　their　centres。　　Now　the　sizes　of　the　bodies　being　known；　the　mean　density　of　the　whole　system　may　be　calculated。　　In　every　case　that　density　has　been　found　to　be　much　less　than　the　sun's；　and　indeed　the　average　of　a　number　of　mean　densities　which　have　been　determined　only　amounts　to　one…eighth　of　that　of　the　sun。　　In　some　cases　the　density　is　extremely　small；　and　in　no　case　is　it　quite　so　great　as　half　the　solar　density。

It　would　be　absurd　to　suppose　that　these　stars　can　be　uniform　in　density　throughout；　and　from　all　that　is　known　of　celestial　bodies　it　is　probable　that　they　are　gaseous　in　their　external　parts　with　great　condensation　towards　their　centres。　　This　conclusion　is　confirmed　by　arguments　drawn　from　the　theory　of　rotating　masses　of　liquid。　　（See　J。H。　Jeans；　〃On　the　density　of　Algol　variables〃；　〃Astrophysical　Journ。〃　Vol。　XXII。　（1905）；　page　97。）

Although；　as　already　explained；　a　good　deal　is　known　about　the　shapes　and　the　stability　of　figures　consisting　of　homogeneous　incompressible　liquid　in　rotation；　yet　comparatively　little　has　hitherto　been　discovered　about　the　equilibrium　of　rotating　gaseous　stars。　　The　figures　calculated　for　homogeneous　liquid　can　obviously　only　be　taken　to　afford　a　general　indication　of　the　kind　of　figure　which　we　might　expect　to　find　in　the　stellar　universe。　　Thus　the　dotted　curve　in　Fig。　5；　which　exhibits　one　of　the　figures　which　I　calculated；　has　some　interest　when　placed　alongside　the　figures　of　the　stars　in　RR　Centauri；　as　computed　from　the　observations；　but　it　must　not　be　accepted　as　the　calculated　form　of　such　a　system。　　I　have　moreover　proved　more　recently　that　such　a　figure　of　homogeneous　liquid　is　unstable。　　Notwithstanding　this　instability　it　does　not　necessarily　follow　that　the　analogous　figure　for　compressible　fluid　is　also　unstable；　as　will　be　pointed　out　more　fully　hereafter。

Professor　Jeans　has　discussed　in　a　paper　of　great　ability　the　difficult　problems　offered　by　the　conditions　of　equilibrium　and　of　stability　of　a　spherical　nebula。　　（〃Phil。　Trans。　R。S。〃　Vol。　CXCIX。　A　（1902）；　page　1。　　See　also　A。　Roberts；　〃S。　African　Assoc。　Adv。　Sci。〃　Vol。　I。　（1903）；　page　6。）　　In　a　later　paper　（〃Astrophysical　Journ。〃　Vol。　XXII。　（1905）；　page　97。）；　in　contrasting　the　conditions　which　must　govern　the　fission　of　a　star　into　two　parts　when　the　star　is　gaseous　and　compressible　with　the　corresponding　conditions　in　the　case　of　incompressible　liquid；　he　points　out　that　for　a　gaseous　star　（the　agency　which　effects　the　separation　will　no　longer　be　rotation　alone；　gravitation　also　will　tend　towards　separation。。。From　numerical　results　obtained　in　the　various　papers　of　my　own；。。。I　have　been　led　to　the　conclusion　that　a　gravitational　instability　of　the　kind　described　must　be　regarded　as　the　primary　agent　at　work　in　the　actual　evolution　of　the　universe；　Laplace's　rotation　playing　only　the　secondary　part　of　separating　the　primary　and　satellite　after　the　birth　of　the　satellite。〃

It　is　desirable　to　add　a　word　in　explanation　of　the　expression　〃gravitational　instability〃　in　this　passage。　　It　means　that　when　the　concentration　of　a　gaseous　nebula　（without　rotation）　has　proceeded　to　a　certain　stage；　the　arrangement　in　spherical　layers　of　equal　density　becomes　unstable；　and　a　form　of　bifurcation　has　been　reached。　　For　further　concentration　concentric　spherical　layers　become　unstable；　and　the　new　stable　form　involves　a　concentration　about　two　centres。　　The　first　sign　of　this　change　is　that　the　spherical　layers　cease　to　be　quite　concentric　and　then　the　layers　of　equal　density　begin　to　assume　a　somewhat　pear…shaped　form　analogous　to　that　which　we　found　to　occur　under　rotation　for　an　incompressible　liquid。　　Accordingly　it　appears　that　while　a　sphere　of　liquid　is　stable　a　sphere　of　gas　may　become　unstable。　　Thus　the　conditions　of　stability　are　different　in　these　two　simple　cases；　and　it　is　likely　that　while　certain　forms　of　rotating　liquid　are　unstable　the　analogous　forms　for　gas　may　be　stable。　　This　furnishes　a　reason　why　it　is　worth　while　to　consider　the　unstable　forms　of　rotating　liquid。

There　can　I　think　be　little　doubt　but　that　Jeans　is　right　in　looking　to　gravitational　instability　as　the　primary　cause　of　fission；　but　when　we　consider　that　a　binary　system；　with　a　mass　larger　than　the　sun's；　is　found　to　rotate　in　a　few　hours；　there　seems　reason　to　look　to　rotation　as　a　contributory　cause　scarcely　less　important　than　the　primary　one。

With　the　present　extent　of　our　knowledge　it　is　only　possible　to　reconstruct　the　processes　of　the　evolution　of　stars　by　means　of　inferences　drawn　from　several　sources。　　We　have　first　to　rely　on　the　general　principles　of　stability；　according　to　which　we　are　to　look　for　a　series　of　families　of　forms；　each　terminating　in　an　unstable　form；　which　itself　becomes　the　starting…point　of　the　next　family　of　stable　forms。　　Secondly　we　have　as　a　guide　the　analogy　of　the　successive　changes　in　the　evolution　of　ideal　liquid　stars；　and　thirdly　we　already　possess　some　slender　knowledge　as　to　the　equilibrium　of　gaseous　stars。

From　these　data　it　is　possible　to　build　up　in　outline　the　probable　history　of　binary　stars。　　Originally　the　star　must　have　been　single；　it　must　have　been　widely　diffused；　and　must　have　been　endowed　with　a　slow　rotation。　　In　this　condition　the　strata　of　equal　density　must　have　been　of　the　planetary　form。　　As　it　cooled　and　contracted　the　symmetry　round　the　axis　of　rotation　must　have　become　unstable；　through　the　effects　of　gravitation；　assisted　perhaps　by　the　increasing　speed　of　rotation。　　（I　learn　from　Professor　Jeans　that　he　now　（December　1908）　believes　that　he　can　prove　that　some　small　amount　of　rotation　is　necessary　to　induce　instability　in　the　symmetrical　arrangement。）　　The　strata　of　equal　density　must　then　become　somewhat　pear…　shaped；　and　afterwards　like　an　hour…glass；　with　the　constriction　more　pronounced　in　the　internal　than　in　the　external　strata。　　The　constrictions　of　the　successive　strata　then　begin　to　rupture　from　the　inside　progressively　outwards；　and　when　at　length　all　are　ruptured　we　have　the　twin　stars　portrayed　by　Roberts　and　by　others。

As　we　have　seen；　the　study　of　the　forms　of　equilibrium　of　rotating　liquid　is　almost　complete；　and　Jeans　has　made　a　good　beginning　in　the　investigation　of　the　equilibrium　of　gaseous　stars；　but　much　more　remains　to　be　discovered。　　The　field　for　the　mathematician　is　a　wide　one；　and　in　proportion　as　the　very　arduous　exploration　of　that　field　is　attained　so　will　our　knowledge　of　the　processes　of　cosmical　evolution　increase。

From　the　point　of　view　of　observation；　improved　methods　in　the　use　of　the　spectroscope　and　increase　of　accuracy　in　photometry　will　certainly　lead　to　a　great　increase　in　our　knowledge　within　the　next　few　years。　　Probably　the　observational　advance　will　be　more　rapid　than　that　of　theory；　for　we　know　how　extraordinary　has　been　the　success　attained　within　the　last　few　years；　and　the　theory　is　one　of　extreme　difficulty；　but　the　two　ought　to　proceed　together　hand　in　hand。　　Human　life　is　too　short　to　permit　us　to　watch　the　leisurely　procedure　of　cosmical　evolution；　but　the　celestial　museum　contains　so　many　exhibits　that　it　may　become　possible；　by　the　aid　of　theory；　to　piece　together　bit　by　bit　the　processes　through　which　stars　pass　in　the　course　of　their　evolution。

In　the　sketch　which　I　have　endeavoured　to　give　of　this　fascinating　subject；　I　have　led　my　reader　to　the　very　confines　of　our　present　knowledge。　　It　is　not　much　more　than　a　quarter　of　a　century　since　this　class　of　observation　has　claimed　the　close　attention　of　astronomers；　something　c