November, 1903.1 



KNOWLEDGE. 



253 



A variable of the same tyjie, but much faiuter, was made 

 tlie subject of a similar enquiry by Mr. Myers in 1898.* 

 U Pegasi never attains ninth magnitude ; hence spectro- 

 scopic complications equally with spectroscopic verification 

 remain at present out of sight. The star, nevertheless, 

 excites keen interest, and claims sustained attention. Its 

 light-i'urve has been laiil down with exquisite accuracy- at 

 Harvard College, and shows two slightly unequal minima 

 to be comprised within a period of nine hours, signifying, 

 on the adopted theory, the occurrence of alternating 

 eclipses at intervals of 4^ hours. The distance from 

 centre to centre of the occulting stars, the smaller of which 

 is of about eight-tenths the brightness of the larger, 

 " does not materially differ," Mr. Myers tells us, "from 

 the sum of their radii, suggesting the probable existence 

 of the ' apioidal ' form of Poiucarc." If they do not 

 actually coalesce, the component bodies revolve in contact, 

 and rotate synchronously. Thus it is hard to say whether 

 U Pegasi should be accounted as a single pear-shaped mass 

 spinning in the time of light-change, or as a close couple 

 circulating freely in an identical period. The mean density 

 of the system appears to lie between one-third and one- 

 fourth that of the sun. 



Another specimen of the " dumb-bell " system is 

 perhaps met with in R^ Centauri. The narrow range of 

 its variation makes it a delicate object to observe ; but 

 Mr. A. W. Roberts, who first noticed its peculiarity in 

 1896, has since accumulated an extensive series of wonder- 

 fully accurate visual determinations of its fluctuating 

 brightness, and has besides rendered them the basis of an 

 able and exhaustive theoretical discussion. f The double 

 pci-idcl of R- Centauri is restricted to 1-lh. 32m. Within 

 this !)rief span quadruple phases are included — that is to 

 say, two evenly-balanced maxima, and two inconsiderably 

 disparate minima. These result, Mr. Roberts concludes, 

 from the mutual eclipses of interpenetrating ellipsoids, one 

 somewhat more luminous than the other, revolving — if 

 they can properly be said to revolve — in an orbit inclined 

 'S2'^ to the visual plane. They are of just one-third the 

 solar density, and the forms satisfying photometric 

 requirements by the varying areas of luminous surface 

 presented to sight in different sections of their path show 

 a surprising agreement with the bi-prolate figure given by 

 Prof. Darwin's analysis as the shape of a body on the 

 verge of disruption through accelerated rotatory move- 

 ment. The inference is, then, almost irresistible that R° 

 Centauri really exemplities the nascent stage of binai-y 

 stars. To establish this completely, however, spectroscopic 

 data are needed ; and they are difficult to procure for a 

 star below the seventh magnitude. 



No such obstacle impedes the investigation of the analo- 

 gous, but much brighter star V Puppis. Detected as a 

 spectroscopic binary by Prof. Pickering in 1895, this star 

 traverses so wide an orbit in the short period of thirty-five 

 hours as to imply — if the published details are correct — 

 that the pair possess no less than 348 times the gravita- 

 tional power of the sun. They are, nevertheless, 

 according to Mr. Roberts, fifty times more tenuous, and 

 each globe should have a diameter of about 16i million 

 miles ; all which, though startling, is not incredible. The 

 light-curve of V Puppis, as traced by Mr. Roberts, is closely 

 modelled upon that of U I'egasi. And he postulates 

 similar conditions of eclipse. It rests, however, with the 

 spectroscope to determine whether those conditions are 

 realised or not. 



Probably all short-period variables are binaries, with 

 coincident orbital- aud light-cycles. But all are not occul- 



* Astrophi/sical Journal, Vol. VIII., p. 163. 

 t MontJilii Notices, Vol. LXIII., p. 627. 



ting binaries. There are some — we are still ignorant of 

 their pr<i]M)rtionate numbers — which undergo a course of 

 light-ebange, apparently compatible with an occulting 

 hypothesis, yet certainly escape eclipse. Prof. Campbell 

 has made it unmistakably cleir that ? Geininorum is thus 

 constituted.* Two stars are present, but their plane of 

 motion is inclined at an unknown angle to the line of 

 sight ; it does not ajsproximate to coincidence with it. 

 Now the possibility is not excluded that V Puppis belongs 

 to the same class. Mr. Roberts's assumptions are indeed 

 in themselves extremely plausible, and they may at an}' 

 moment be pri)ved, by a few well-timed spectrographs to 

 be undeniably true. The one conclusive test is the cessa- 

 tion of radial movement at epochs of minimum. Evidently, 

 if the diminution in lustre be in fact due to an eclipse, the 

 eclipsing and eclipsed bodies must be crossing the line of 

 sight just when the obscuration is deejjest. There is no 

 evading this geometrical requirement ; and it must be 

 rigorously complied with in the circular orbits traversed 

 by bodies revolving in contact. Before, then, Mr. Roberts's 

 theory of V Puppis can be accepted with implicit con- 

 fidence, it has to be ascertained whether a zero of radial 

 speed is reached concurrently with the jshotometric minima. 

 If so, they may be unhesitatingly accounted eclipse- 

 phenomena ; if, on the contrary, the decline in brightness 

 prove to 1)8 unrelated to a slackening of speed, then the 

 supposition that it accompanies and indicates a transit must 

 be peremptorily discarded. Moreover, the spectroscopic 

 verdict as regards V Puppis can safely be applied to stars 

 with similar light-curves, especially to R- Centauri and 

 U Pegasi, and may serve to clear away some of the intri- 

 cacies connected with the exceptional system of [i Lyra;. 

 The measurement of a single speetrographic plate might 

 thus be made essentially to supply the lack of many 

 desirable, but at present unattainable, determinations. 



The existence of stellar systems of the " dumb-bell " 

 type would violate no mechanical law. " Roche's limit " 

 does not apply to globes comparable in size. The range 

 of disjjarity within which it holds good has not, it is true, 

 been theoretically established ; but it may be said, in 

 general terms, to concern the relations of planets and 

 satellites (to use a purposely vague phrase), not those of 

 double stars. What the law asserts is that a subordinate 

 small body cannot, if their mean density be the same, re- 

 volve intact at a less distance than 244 radii of its primary 

 from that primary's centre. For satellites of slighter 

 consistence the limit should be extended. Ourownmoou, 

 for instance, i^ould never have circulated, without being 

 rent in pieces by tidal strains, in an orbit less than 22,00t) 

 miles in diameter. f Bodies of co-ordinate mass are, 

 however, exempted from the prohibitive rule against 

 mutual approach. No analytical veto is imposed upon 

 the origin by fission of double stars, or upon the subsis- 

 tence of stellar Siamese twins. The inequalities in attrac- 

 tive power of co-ordinate masses avail for distortion, not 

 for disruption. Their individuality, therefore, once created, 

 is in a manner indestructible. It tends, in fact, to 'oecome 

 more pronour.ced as the orbital span gradually widens 

 through tidal friction. The " dumb-bell " condition may, 

 accordingly, be, comparatively speaking, transitory. Nor 

 can we be assured of its subsistence otherwise than by the 

 peculiar nature of the eclipses attending upon it, taken in 

 connection with spectroscopic observations of decisive 

 import. The disclosure, by such means, of systems so 

 strangely conditioned, affords a deejier insight than would 

 else have been possible into the cosmical order, and fills a 

 blank page in the genetic history of the sidereal world. 



• Aslroph. Jour., Vol. XIII., p. 90; Science, July 27tli, 1900. 

 t Ot. H. Darwin, " Tho Tides," p. 325. 



