212 



KNOWLEDGE & SCIENTIFIC NEWS. 



[Sept., 1904. 



the threshold of creation. Their mutual relations in 

 time are, however, by no means obvious. 1 hey can- 

 not easily be disposed in any kind of sequence. Each 

 of the great nebula;, at any rate, exhibits features and 

 occupies a position shared by none of its fellows. The 

 most discerning cosmologist cannot pretend to say that 

 the .Argo nebula, for instance, is of greater or less 

 antiquity than the Orion or the "America" nebula. 

 They are individual growths, simultaneous, not 

 successive. The line of development indicated for 

 them is rather towards the formation of star-clusters 

 than of diverse nebular species. Thus the Pleiades 

 may illustrate the probable future condition of the 

 Orion nebula, the contained stars having gained pre- 

 dominance, though still wrapt in filmy swaddling- 

 bands, later, presumably, to be shaken off. 



I^lanetarv nebula? have much more in common than 

 irregular nebula?, and their minor varieties might, with 

 some plausibility, be associated with differences in rela- 

 tive age. They are marked chiefly by the character of 

 the nuclear star which, in nearly all such objects, ap- 

 pears to act as the pivot of the surrounding vaporous 

 structure. The supposition lies close at hand that it is 

 designed as a provision for the nourishment of the star 

 — that the star gains in mass and light at the expense 

 of the nebula, which it is eventually destined to absorb 

 wholly and supersede. On this view, planetaries like 

 the green glow-lamp at the pole of the ecliptic (N.G.C. 

 6543) should be regarded as the most advanced, while 

 Webb's planetary in Cygnus (X.G.C. 7027) would 

 exemplify an inchoate condition. In the former the 

 central star is of 9.6 magnitude, and sharply stellar ; in 

 the latter it is double and diffuse,* perhaps a wide 

 binary system in embryo. 



The question is, howev-er, still open as to the real 

 nature of the connection between planetaries and their 

 central stars. The pabulum-theory is a promising con- 

 jecture ; but no facts with which we are acquainted 

 stringently enforce it. Ideas on the subject will need 

 complete revision if the traces of spirality noted from 

 lime to time in some of these peculiar objects prove to 

 be of radical significance. The oadi, distinctive of the 

 "Owl nebula" (N.G.C. 3587) as originally shown by the 

 Parsonstown reflector, consisted of luminous traceries 

 coiled round /ico interior stars, t but the appearance was 

 either due to illusion, or became effaced by change, 

 since the camera has refused to endorse it as genuine. 

 The " helical " planetary in Draco, i however, is doubt- 

 less essentially a spiral conformation § ; and Professor 

 Schacberle, by means of exposures with a thirteen-inch 

 reflector of twenty inches focus, has compelled not only 

 the Ring nebula in Lyra, II but the Dumb-bell in Vul- 

 pecula to betray the surprising secret of their whorled 

 structuri-. Both these nebulic give a spectrum of 

 bright lines, and invention is baffled by the problem of 

 building up gaseous m.iterials into strongly charac- 

 terised edifices. The materials, however, mav not be 

 purely gaseous ;• or we possibly see (as Professor 

 Darwin long ago suggested) merely illuminated stream- 

 lines of motion furrowing .-m obscure mass. But if 

 this be indeed so, there is the further question to be 

 asked : AVhat direction does the motion take? Do the 

 tides set inward or oxitward? 



•Keeler, Lick Publications, Vol. III., p. 21.4. 



IRosse. Trans. Roy. Dublin Sorictv, Vol II,. p. 93. 



J First detected as such bv Holden .ind Schaeherle in 1SS8, 



Monthly Notices. Vol. XI.VIII. p. 38S. 

 S Deslandres. Bull. .-Islr., Feb. 1900. 

 '! .4str. Jour, Nos. 539, 547 

 "^Maunder, Knou'lcdge, Vol. XIX , p. 39. 



Our spontaneous impressions are all in favour of 

 concentrative tendencies. We cannot easily shake off 

 centripetal prejudices. Our lives are passed under a 

 regimen of central attraction, and we naturally incline 

 to universalise our experience. Hence Herschel's 

 scheme of sidereal evolution invites at first sight ready 

 acceptance. Stars seem as if they could not act other- 

 wise than as foci of condensation in nebula; ; the lucid 

 stuff involving them must, apparently, with the efflux 

 of ages, settle down towards their surfaces, and be- 

 come absorbed into their substance. Such processes 

 indeed, apart from counteracting causes, belong to 

 the inevitable order of Nature ; but these may, and 

 probably do, exist. From sundry quarters the con- 

 viction is pressed upon us that cosmic bodies can drive 

 out matter as well as draw it in. Repulsive forces 

 insist upon recognition, and their effects become more 

 palpable the more attentively they are considered. 

 Under certain conditions they get the better of gravity ; 

 and stars may possibly, like cocoon-spinning insects, 

 expend their organic energies in weaving themselves 

 unaccountably educed envelopes. The example of 

 Nova Persei is fresh in every mind, but we make no 

 pretension to decide the controversy it raised. A dog- 

 matic pronouncement is unadvisable where the un- 

 known elements of the question obscure and outweigh 

 those that are known. .A less slippery foundation for 

 reasoning is afforded by the permanently visible spiral 

 nebula;, and features charged with an emphatic mean- 

 ing have been revealed in them by photographic means. 



Looking at the entire contents of the nebular 

 heavens, we find the spiral type very largely pre- 

 dominant. It claims more specimens, and emerges 

 more distinctly with each development of delineative 

 power. Its chief prevalence, however, is among 

 " w-hite " nebulee, showing continuous spectra. 



They are vastly numerous. Gaseous nebulae are 

 reckoned by the score, white nebulae by tens of 

 thousands. Moreover, they collect near the poles of 

 the Milky Way,* while the gaseous variety crowd to- 

 wards its plane, both branches of the family thus 

 manifesting galactic relationships, though of an 

 opposite character. Now these facts of distribution 

 are not without indicative import as to relative age. 

 There is a consensus of opinion that objects showing a 

 marked preference for the Milky Way are at an earlier 

 stage of growth than those withdrawn from it, and the 

 inference derives countenance from the circumstance 

 that nebulae situated in high galactic latitudes shine 

 with continuous light, those near the galactic equator 

 with interrupted radiance. Vet it would be rash to 

 assume that any individual nebula traverses these 

 successive stages. The series could be satisfactorily 

 established only if we could point to a number of inter- 

 mediate instances, which seem to be almost wholly 

 lacking. We cannot trace in nebular as we can in 

 stellar growth the insensible gradations of progressive 

 change- They are, perhaps, complicated in nebulae by 

 influences of a different kind from those which have 

 gained the ascendencv in stars. Diffusive effects may 

 in them be more conspicuous than concentrative 

 effects ;t or a balance may be temporarily struck be- 

 tween antagonistic tendencies. 



Spiral conformation is the real crux of nebular 

 cosmogonv. The conditions from which it arises are 



* Dr. Max Wolf places the point of nebular concentration in 

 R.A. 12'" 53", D. + 61° 20', that assigned to the galactic pole 

 being in R.A. la*" 49™ , D, + 62^. Kiinigstulil Publ. Bd. I. p. 174. 



tT J. J. See, " Repulsive Forces in Nature." Pop. Asir., No. 

 100. Dec. 1902. 



