5 8o 



NA TURE 



[October 28, 1922 



faint globular clusters, N.G.C. 7006, lor which a value 

 of about 65.000 parsecs (more than 200,000 light-years) 

 has been obtained, and checked by three or four different 

 photometric methods. The most recent determination 

 of its distance involved the discovery and study with 

 the 100-inch reflector at Mount Wilson of Cepheid 

 variables of the 19th apparent magnitude — the faintest 

 periodic variable stars on record. It is likely that more 

 remote objects, with distances as yet unmeasured, have 

 been seen or photographed — possibly among the faintest 

 spiral nebulae or among the faint stars in the Milky 

 Way. 



VIII. A recent investigation of Cepheid variable 

 stars in the Small Magellanic Cloud has shown that the 

 very faintest variables have periods of less than one 

 day. 28 This result, which permits the direct ex- 

 tension of the period-luminosity law to the short- 

 period type of Cepheid, is further evidence of the high 

 absolute luminosity of the kind of Cepheid variable 

 which is most frequent in globular clusters. 



IX. The proposal of a larger scale for the galactic 

 system brings us face to face with the " island uni- 

 verse " theory of spiral nebulae, which, with varying 

 success, has for many years maintained that the spirals 

 are other " universes " of stars — that they are galaxies 

 comparable with our own, and that our Galaxy, seen 

 from a sufficient distance, would appear as a spiral 

 nebula. A theory of " comparable galaxies " immedi- 

 ately becomes very difficult to maintain along with the 

 larger dimensions of the galactic system. In a paper 

 published three years ago I discussed at some length 

 this problem of external galaxies. 29 The conclusion 

 reached at that time, that the nebulas of the spiral 

 family are probably neither galactic in size nor stellar 

 in composition, has been strengthened rather than 

 weakened by subsequent investigations, particularly by 

 van Maanen's remarkable work on the motions in the 

 brighter spirals. 30 



X. Since the brighter spiral nebula;, according to 

 the present view, are probably within the boundaries 

 of our galactic system, it may be that the novae 

 occurring so frequently in the Andromeda nebula re- 

 present the encounter of this enormous, rapidly moving 

 object with galactic stars. The suggestion is in har- 

 mony with the Seeliger-Monck hypothesis of the cause 

 of ordinarv nov ; andas, moreover, it is in line with 

 the only hypothesis that has yet been advanced to 

 account for the peculiar irregular variable stars in the 

 diffuse nebulae, such as those in Orion. 31 This in- 

 terpretation of the variables of the relatively near 

 Orion nebula would certainly be of significance for 

 historical geology, since disturbances of our sun, much 

 less serious in character than those observed for novae 

 and for the Orion variables, would be of paramount 

 importance in matters pertaining to terrestrial climates 

 and organisms. 32 



XI. Conversely we can use the geological records to 

 show that the radiating equilibrium of the sun probably 

 has been uncommonly stable compared with that of 

 many stars. The investigation at Harvard under 

 Prof. Bailey's direction of the frequency of galactic 

 novae brought out the remarkable result that at least 

 fifteen novas, brighter than the tenth magnitude at maxi- 

 mum, have appeared every vear during the last three 

 decades. 33 If a frequency of even one-fifth that 



NO. 3765, VOL. I io] 



amount has been maintained throughout the hundreds 

 of millions of years of approximately constant solar 

 radiation (shown by the geological records), more novae 

 have occurred than there are known stars. Our sun, 

 however, which has certainly escaped not only disasters 

 of this kind but even much less serious disturbances, 

 apparently moves in an uneventful region of space. 



XII. The attractive and somewhat futile specula- 

 tions on the probability of the occurrence of proto- 

 plasmic life and its slow evolution elsewhere in the 

 sidereal system must, of course, take account of the 

 frequency of these calamitous stellar outbursts that we 

 call novas. 



In connexion with this attempt at a partial inter- 

 pretation of galactic structure it might be well to 

 emphasise the following points. 



(j) Many of the fundamental laws and assumptions 

 of physics are involved in this sidereal superstructure, 

 so that developments in thought or observation, which 

 hereafter greatly affect these laws and customary 

 assumptions, may at the same time seriously disturb 

 existing conceptions of the sidereal system. 



(2) The complete elucidation of the source of stellar 

 energy may bring with it modifications both in our 

 views of the evolution of stars and in our assumption 

 of the importance of gravitational organisation of stellar 

 bodies. 



(3) The question of the obstruction of light in space 

 is not in a satisfactory condition, and the nature of 

 the radiation of the diffuse nebulae is little understood ; 

 we have essentially no information concerning the pre- 

 giant stage of stars and its relation to the diffuse 

 nebulse, and the dust and gases in space. 



(4) Cepheid variables, though comparable with each 

 other, may possibly be sufficiently different from other 

 stars that we cannot use their speed of evolution as a 

 quantitative measure of the speed of evolution for all 

 stars. 



For the present I take little heed of these warnings, 

 and merely record them as examples of underlying 

 uncertainties. They serve to remind us that the con- 

 clusions are based not only on favourable observations 

 and theory, but also on the absence (for the time 

 being) of seriously unfavourable data. 



REFERENCES. 



8. Lund Meddelanden, Series 2, No. 19. 



9. Bui. Nat. Research Council, No. u, p. 174. 



10. Proc. Acad. Sci. Amsterdam, 20, p. 1108 ; 21, p. 36. 



11. Mt. W. Contr. 15s. and 175, p. 11. 



12. I first tried out the method six years ago (Mt. W. Contr. 116, p. 81), 

 but abandoned it as wholly unsuited to the brighter stars in globular clusters. 

 I in' Kapteyn luminosity curves for separate spectral types, however, may 

 be of high value. 



13. Bui. Ast. Inst. Neth., No. 8. 



14. Kapteyn and van Rhijn state that eight Cepheids of long period are 

 known in two globular clusters. In my paper from which they get their 

 data (Mt. W. Contr. 151) I show that twelve long-period Cepheids occur in 

 the five globular clusters Messier 3, 5, 13, 15, and Omega Centauri. Four 

 of these clusters contain also large numbers of short-period Cepheids only 

 one or two magnitudes fainter than the long-period Cepheids. Unpublished 

 results obtained at Harvard show that long-period Cepheids occur in other 

 globular clusters. 



15. .1/;. W. Contr. 153, Mt. W. An. Rep. for 1918, and elsewhere. 

 11.. Russell, Astropk. Jour. 54, p. 140. 



17. Obsen atory, May 1922. 



IS. Bui. Nat. Research Council, No. II. p. 1S4, 190. 



19. The photovisual magnitudes for Messier n, however, are probably 

 affected by a serious scale error ; the colour indices do not agree with the 

 -p.w tr.t -uiiseipicntlv determined (.1//. Jt\ Contr. 120 and 228). 



20. Mt. II. Commun. 02, p. 6 ; cf. also Hubble, Mt. W. An. Rep. for 1921, 



D. 2SI. 



, p. 10, and 161, p. 13. 



