HABITABLE WORLDS

Evening Post, Volume CIX, Issue 150, 29 June 1925, Page 3

HABITABLE WORLDS

ARE THERE OTHERS ?

LIMITING TEMPERATURES

ORBITS OP THE STARS.

j When we calculate the central temperatures of stars of various kinds, using Eddington's latest formulas, it appears that at the higher densities which are actually found to occur among the.stars of various masses, the central temperatures ar c nearly the same, equalling I about 30,000,000 degrees Centigrade, declares a writer in the "Scientific American." It looks, then, as if there were some restrictive limitation, such that even the cores of the stars could not get hotter than a certain limit. '. What looks' like a good explanation of this comes as a by-product of the modern theory of the origin of solar and stellar heat, lhe sun certainly has. been shining with pretty nearly its present intensity during geological time—that is, according to. the best present estimates, for a billion years or so. No known source of energy would suffice to keep it going so long. It gradually has become clear that there must be some enormous.; internal source' of-energy in the sun and in other: stars.The limiting temperature is now easy to understand. ■ At temperatures above jO 000,000; degrees the supply of heat inside the star. becomes rmore rapid than the escape of heat. A star which/, got hotter than this inside' would be forced to expand, and so , to; .diminish its . Internal temperature.; But wo may still" ask, "What is this strange process' by* which this vast store of heat is created; m the_ stars?" Several answers have been given,, of which the most radical of. al] appears now to be thebest. 7 .;■;.: ■ ■.■ : " RADIO 'ACTIVITY; : : Ever since the' discovery.of radio activ-. ityifc has b'eeri realised that roatter-^the' heavy atoms at least—^possess" enormous' internal energy by virtue of its own: constitution. This relation, at first" vague and speculative; was.reduced 'to: quantitative form by -Einstein, who. con-; eluded that all energy possesses -mass, 1 and that- mass r.nd energy; should be" interconvertible." ...;:*■..; ■■•;■-.■."■.''■■■

1 If; then, in some Httle'-uhderstood way, this' transformation, of; matter into heat .becomes actual withiri; the stars, -a tre-. mendous amount of .heat may; be created at. the expense of a slow decrease in mass. When a star has reached-the cri-' tical-;-, central: -temperature:- mentioned above it ma,y'L be . Expected gradually to' "burn itself, away/—as Eddingtoiv puts itr-;diminishing in. rnass•-.■'■ arid ' passing. down the. series: from the" great white stars like Sirius to those resembling the sun and^the redder, dwarfs. .-:'-■ :. : '-The .flood of 'outgoing-, heat from :thel sun is,so-huge:;that,-: on .Einstein's prin-. ciples^ the mass of our luminary must be; diminishing: at the rate of .four million' tons per second.- :: But the sun is somas--sive that, even at this almost incredible rate of heat: lossj :it, will require 150 billion years to" use up one per cent, of: the sun's "mass. The whole past history; of the':, sun'- is estimated :by -Jeans - as: ;about 7000 billion years. - Its'futuro span: of life should be', fully twenty times as long.;; .--..-.;-.' ,:;': ;: X■: .■■•.:;-'^ -■]:'■: :'::: -:\

I Gbbd^^e^^sVf6^Vfega^dmg's^qh■'■a\be-': wildering stretch :oi ; time ; as' real- are found in a recent paper by Jeans, which discusses various; results', of, such' a- slow 'decrease in the• mass of■ ■ a star....',. '.'■■'•. _'-'; So long as the mass of. any- star— say!'" oi the sun—remains' constant,: the size; of the orbit of.a planet'revolving 'around, it (or of.that of one: ofapair of double stars';•'. revolving.' abput .the' other) -will: remain unaltered unless other forces than gravitation are : :atv work. :\ Such another: force is found in the friction' of the tides which each Component of a' double^ star pair raises on' the" other.: It has long been known;"th'at; this; tidal 'action tends' to' separate the componentsand enlarge the orbit.; During Uiis' process,: an 'orbit.which.was originally 1 nearly circular, (as'it must;have-been when : the: components were nearly.' in contact) could increase.somewhat in/eccentricity,! but no one'^ has 1 ever, been able: toshow* mathematically: how. tidal .'action could; produce the■■i-,veryv,- high •> eccentricities''--which are. common in double star orbits; ;* '•■ A *e^' years ' ago' Jeans .showed' that these high eccentricities'could be explained by the effect 'of-chance.-'.encounters of other stars. with : such double _• star. pairs. If, while, the -components^ are pursuing nearly circular orbits, another star passes close to them,ats attraction will'disturb; their relative motion..: " This will change ' an originally circular "orbit into an ellipse, ;and Jeans has proved that if, a large number of stars were subjected to numerous encounters the : final ;distribu-! tion of high and low eccentricities would be hot far from that: which is observed among' the known' double stars.- • "v '.!.- - .■-•■. .;,;' STARS' IN SPACE. '-■ :■< V; : ' : ' Now\an, elementary calculation, based :oh the known masses of : the . stars^ahd' on the thickness with which ,:they.. are sown'...in:space, shows that auy .given double star would;, encounter another star closely enough to produce'much diaiige ■in .thejjcbit only once in some 4000 billion rears. - .-When the life of a star .was j believed to cover: only a .few billious i of years at .the most, these encounters an'-.', peared.tb. be of negligible imporlauce. Even with the enormously increased time .scale . allowed by our- new:. outlook, it "does not seem al, first sight that there 'would be time.-enough for the necessary encounters; but Jeans points out that if,_as is very probable, the stars of the Milky Way are kept from wandering into .outer regions , of; space by the attraction of the whole mass, and: if they are slowly losing: mass, the. whole galactic system must be .slowly expanding. ■In :remote. times, then, the stars .must have .been nearer '; together',.! and-" encounter must have been more frequent.: ■ This /correction is found to give us jnst about enough encounters to"produce- the observed distribution o£ double star orbits. -; . - -. ■■-■■■■ . :- :: - -■;.-.-.

11'rom -two independent'-'.-'lines of evidence, therefore, there is« good reason to believe tliab the past-history-of-;our starry universe, runs back for billions of years. This has a.most important .bear-' ing on another question which: lias;long excited 1 great interest—that of ;the plural'cy .of -worlds. Are there among the stars, many planetary system like our own solar system? If so, there may well be _ habitable worlds among them, .with living creatures on them-. If not^ our peopled world.may be unique.. ' Everybody knows, of course, that, we' could not', see planets revolving about even the; nearest stars—they' would be far too- faint to be detected." We mustdepend upon inference. iSTow there is 'good reason.to., believe that the planets, of our system were produced by eruptions from the ;sun, under the influence, of an encounter .-with a star which passed unusually close , to.it. , So close an, encounter, according ■to ■ Jeans's calculations, should happen not oftoner than once in ten trillion years, perhaps even more rarely. With the shorter time scale for the universa it appeared that planetary systems should be exceedingly rare —indeed, it was possible that our solar system was. the only one. With the longer time scale the genesis of suchsystems becomes in. Jeans's words, "a somewhat, although not excessively, rare event." We may now believe that there may be millions of planetary systems nmong the stars and perhaps thousands ol world? which aro tie abode a! Jifo,