October, 1902.] 



KNOWLEDGE 



225 



PFow tame and artificial seemed the pleasures and 

 luxuries of civilization soon after our return home. How 

 we siffhed for the wild freedom of camp life. Little 

 liardsliips, aud even the tormeutiuL; mosquitoes of Lapland, 

 were forjjiotten iu a feverish longiu<,' to roam once more over 

 some country untouched bv man — the desire to be alone with 

 Nature in her wildest aspects was our strongest passion. 



NEBULOUS STARS AND THEIR SPECTRA. 



By Miss Agnes M. Clerke. 

 With his great photographic binociilar. Dr. Max Wolf 

 seciired, iu July, lilOl, two simultaneous impressions of 

 tiie extraordinary oliject named by him the "America" 

 nebula. One, lately reproduced iu these pages, has 

 excited the wonder and admiration of all competent 

 judges ; the second has not yet been published. The 

 comparison of the twin pictures would be of extreme 

 interest. To take just one example. Even a cursory 

 examination of the July plate shows tbe nebulous 

 lijtht to be interrupted, often where it is brightest, by 

 isolated spots of absolute blackness, round or oval, 

 and sharply terminated. Their identification in the 

 companion-photograph would leave no doubt of their 

 objective reality. The peculiarity is the more note- 

 worthy from its extensive sidereal prevalence. Stellar and 

 nebular formations are alike liable to perforation ; whether 

 as an incident in their development, or through tbe action 

 of external forces, we cannot pretend to determine. The 

 pits and chasms in the Trifid Nebula strike the eye at 

 once in the beautiful pictures obtained by the late Prof. 

 Keeler with the Crossley reflector. Dark areas aud " black, 

 tortuous rifts," assert their presence emphatically on 

 Dr. Roberts's plates of the nebulous cluster in Monoceros,* 

 (N.G.C. 2237-9). The " Key-hole " Nebula manifests the 

 same characteristics on an enlarged scale ; while the Milky 

 Way itself bears witness to its dominance over the distri- 

 bution of cosmical masses to the utmost bounds of the 

 visible universe. Moreover, globular clusters are apt to 

 be riddled with " holes.'' Prof. Barnard's recent study of 

 M. 5 Serpeutis, with the forty-inch Yerkes refractor, 

 brought to his surprised cognisance a number of " inky 

 black spots," close south-preceding and south-following, 

 the densest part of that singular aggregation. f He had, 

 indeed, been previously aware of the presence iu M. 13 — 

 the great Hercules cluster — of similar disaggregative 

 symj>toms (if as such we are justified iu interpreting 

 them). Nor are they absent from nebulous stars. The 

 luminous halos surrounding these rare objects are not, as 

 a rule, symmetrically disposed. They are frequently 

 irregular aud heterogeneous, and show vacancies and 

 mottlings altogether inconsistent with the supposition of 

 their condition being one of statical equilibrium. 



Nebulous stars probably represent the earliest phase of 

 stellar evolution. They irresistibly suggest incij)ience ; 

 they have seemingly not yet fully appropriated the 

 material allotted for their construction. In the course of 

 a few or of many millenniums, they will, it is reasonable to 

 suppose, have absorbed the outstanding supply and will 

 shine as finished suns. Hence, particular importance 

 attaches to the investigation of spectra emanating, 

 presumably, from inchoate photospheres. For the ascer- 

 tainment of their nature serves to determine, with 

 approximate security, the starting-point of the grand 

 process by which suns grow and decay. And, judging 

 from the scanty information so far available, this starting- 

 point is from stars marked by helium-absorption. By 



* " Celestial Photographs, 

 t Aatr. yac/i., No. 3519. 



Vol. II., p. 171. 



helium-absorption, associated — as Mr. McClean and Miss 

 Maury independently concluded — with light-stoppage by 

 all the three series of hydrogen, by oxygen, perhaps 

 invariably, aud very frequently by nitrogen and silicon. 

 The action of metals is, in these early spectra, quite 

 inconspicuous, and metallic rays form no part of the 

 emissions of gaseous nebuhe, so that the prominence of 

 metalloids in stars of itself suggests their proximity to a 

 nebulous condition. 



Now the spectra of stars with nebulous appurtenances 

 are mainly impressed with dark lines of helium, hydrogen, 

 and oxygen. It is too soon to pronounce the rule an 

 invariable one ; but until some one exception to it has 

 been estal>lished, it may be admitted as a provisional 

 generalisation. At the first sight of Dr. Max Wolf's 

 photograph (already adverted to) of the "America" nebula, 

 such an exception seemed to present itself ; for the 

 strongly nebulous star close to its western border was 

 named in the ^ext $ Cygni, and J Cygni shows a spectrum 

 of the solar type. Further enquiry, however, proved that 

 thei-e had been an accidental error of identification, and 

 that the star in question is no other than -57 Cygni, the 

 spectrum of which has the letter " B " affixed to it in the 

 Draper Catalogue.* This means that it shines with the 

 " Onon " quality of light ; aud on turning to the supple- 

 mentary remarks, we note the record of a distinctive line 

 of cosmic hydrogen, iu addition to those of helium and of 

 vacuum-tube hydrogen. The real ^ Cygni lies on the 

 Pacific side of the nebulous continent — looking at it as if 

 it were depicted on a terrestrial map — and in about the 

 latitude of San Francisco. But in the ocean of space it is 

 probably situated indefinitely nearer to us than the nebula 

 projected beside it upon the sensitive plate. It may, in 

 actual fact, be just as completely disconnected from it as a 

 reticle from the celestial bodies it serves tp divide and define. 



This cannot be said of the diffuse object off the Atlantic 

 shore of the nebula. Prof. Wolf states that, iu the 

 original negative, " many interesting arms and wings of 

 nebulosity " can be perceived to radiate from it. And, 

 although he does not assert categorically their organic 

 relationship with it, the chance of the fortuitous production 

 of an arrangement so express aud particular must be 

 regarded as extremely remote. It is, indeed, admittedly 

 difficult to discriminate, in long-exposure photographs, 

 between stars genuinely and stars only optically nebulous, 

 yet the visible origination of nebulous appendages can 

 scarcely be fictitious. Thus, the situation of 57 Cygni, 

 with regard to the filmy protrusions apparently issuing 

 from it, is too critical to be casual ; aud, as we have seen, 

 the spectrum of the star corresponds perfectly "with a 

 nebvdous coudition. 



Its brightness — 4-6 magnitude— is almost that of 

 w Orionis, detected as nebulous, both photographically 

 and visually, by Barnard in September, 1893 (Knowledge, 

 Vol. XVII., p. ] 7). The two stars are also spectroscopically 

 alike; they constitute similarfociof nebulous concentration; 

 each would seem to be a diffuse globe but recently equipped 

 with the machinery requisite for intense radiation. With 

 them may be classed a- Scorpii, one of the finest nebulous 

 stars in the heavens, described by Barnard as structurally 

 involved iu the great Rho Ophiuchi formation ; e and 

 I Orionis, the "atmospheres" of which were noted ijy 

 Sir John Herschel ; and X Orionis, disclosed as haze- 

 encompassed by a long fixed stare with Barnard's Willard 

 lens. This object is one of Struve's pairs (2 738) ; the 



* Dr. Max Wolf desires to acknowledge Misa Gierke's correction, 

 and would ask readers of KNOWLEDaE, in his article on page 156 of 

 Knowledge for July, 1902, at the 20th line, for "| Cygni," to read 

 " 57 Cygni." 



