darwin and modern science-第193章

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ngated　that　they　overlap；　as　exhibited　in　his　figure　titled　〃The　shape　of　the　star　RR　Centauri〃　（Fig。　6。）。　　The　dotted　curve　shows　a　form　of　equilibrium　of　rotating　liquid　as　computed　by　me　some　years　before；　and　it　was　added　for　the　sake　of　comparison。

On　turning　back　to　Fig。　5　the　reader　will　see　in　the　smooth　dotted　curve　the　light　variation　which　would　be　exhibited　by　such　a　binary　system　as　this。　　The　curve　is　the　result　of　computation　and　it　is　impossible　not　to　be　struck　by　the　closeness　of　the　coincidence　with　the　series　of　black　dots　which　denote　the　observations。

It　is　virtually　certain　that　RR　Centauri　is　a　case　of　an　eclipsing　binary　system；　and　that　the　two　stars　are　close　together。　　It　is　not　of　course　proved　that　the　figures　of　the　stars　are　ellipsoids；　but　gravitation　must　deform　them　into　a　pair　of　elongated　bodies；　and；　on　the　assumptions　that　they　are　not　enveloped　in　an　absorptive　atmosphere　and　that　they　are　ellipsoidal；　their　shapes　must　be　as　shown　in　the　figure。

This　light…curve　gives　an　excellent　illustration　of　what　we　have　reason　to　believe　to　be　a　stage　in　the　evolution　of　stars；　when　a　single　star　is　proceeding　to　separate　into　a　binary　one。

As　the　star　is　faint；　there　is　as　yet　no　direct　spectroscopic　evidence　of　orbital　motion。　　Let　us　turn　therefore　to　the　case　of　another　star；　namely　V　Puppis；　in　which　such　evidence　does　already　exist。　　I　give　an　account　of　it；　because　it　presents　a　peculiarly　interesting　confirmation　of　the　correctness　of　the　theory。

In　1895　Pickering　announced　in　the　〃Harvard　Circular〃　No。　14　that　the　spectroscopic　observations　at　Arequipa　proved　V　Puppis　to　be　a　double　star　with　a　period　of　3d　2h　46m。　　Now　when　Roberts　discussed　its　light…curve　he　found　that　the　period　was　1d　10h　54m　27s；　and　on　account　of　this　serious　discrepancy　he　effected　the　reduction　only　on　the　simple　assumption　that　the　two　stars　were　spherical；　and　thus　obtained　a　fairly　good　representation　of　the　light…curve。　　It　appeared　that　the　orbit　was　circular　and　that　the　two　spheres　were　not　quite　in　contact。　　Obviously　if　the　stars　had　been　assumed　to　be　ellipsoids　they　would　have　been　found　to　overlap；　as　was　the　case　for　RR　Centauri。　　（〃Astrophysical　Journ。〃　Vol。　XIII。　（1901）；　page　177。）　　The　matter　rested　thus　for　some　months　until　the　spectroscopic　evidence　was　re…examined　by　Miss　Cannon　on　behalf　of　Professor　Pickering；　and　we　find　in　the　notes　on　page　177　of　Vol。　XXVIII。　of　the　〃Annals　of　the　Harvard　Observatory〃　the　following：　　〃A。G。C。　10534。　　This　star；　which　is　the　Algol　variable　V　Puppis；　has　been　found　to　be　a　spectroscopic　binary。　　The　period　1d。454　（i。e。　1d　10h　54m）　satisfies　the　observations　of　the　changes　in　light；　and　of　the　varying　separation　of　the　lines　of　the　spectrum。　　The　spectrum　has　been　examined　on　61　plates；　on　23　of　which　the　lines　are　double。〃　　Thus　we　have　valuable　evidence　in　confirmation　of　the　correctness　of　the　conclusions　drawn　from　the　light…curve。　　In　the　circumstances；　however；　I　have　not　thought　it　worth　while　to　reproduce　Dr　Roberts's　provisional　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　co