6o8 



NATURE 



[April 26, 1888 



it of passing to Class III. a or Class lll.b. Thus under 

 certain conditions its spectrum will take on the appearance 

 of Secchi's third type, Class IW.a (Vogel) ; on certain other 

 conditions it will take on the appearance of Secchi's fourth 

 type, Class III.i^ (Vogel). There is now, however, no 

 manner of doubt whatever that Secchi's Class III. a re- 

 presents stars in which the temperature is increasing, 

 and with conditions not unlike those of the nebulae — 

 that is to say, the meteorites are yet discrete, and that 

 they are on their way to form bodies of Class II. and 

 Class I. by the ultimate vaporization of all their meteoric 

 constituents. There is equally no manner of doubt that 

 the stars included in Class \l\.b have had their day ; that 

 iheir temperature has been running down, until owing to 

 reduction of temperature they are on the verge of invisi- 

 bility brought about by the enormous absorption of carbon 

 in their atmospheres. 



Pechiile was the first to object to Vogel's classification, 

 mainly on the ground that Secchi's types 3 and 4 had 

 been improperly brought together ; and my work has 

 shown how very just his objection was,^ and how clear- 

 sighted was his view as to the true position of stars of 

 Class 1 11.^. 



II. Proposed New Grouping of all Celestial 

 Bodies according to Temperature. 



Having, then, gone over the various classifications of 

 stars according to their spectra, I now proceed to consider 

 the question of the classification of celestial bodies from 

 a more advanced point of view. I pointed out in the 

 year 1886 that the time had arrived when stars with 

 increasing temperatures would require to be fundamentally 

 distinguished from those with decreasing temperatures, 

 but I did not then know that this was so easy to 

 accomplish as it now appears to be (Naiure, vol. xxxiv. 

 p. 228) ; and as I have already stated, when we consider 

 the question of classification at all, it is neither necessary 

 nor desirable that we should limit ourselves to the stars ; 

 we must include the nebulae and comets as well, and 

 the question of variability does not really concern us, 

 because it is as a rule in its extremest form the passage 

 of a body giving one spectrum to a body giving another 

 even if of a different type, owing to sudden changes of 

 temperature. 



I " M. Vogel a propose une classification suivant les diverses phases de 

 refroidissement indiquees par les spectres, dans laquelle il fait des types III. 

 et IV. de Secchi deux subdivisions d'une meme classe, Wl.a et III (J. Mais 

 je trouve certaines difficultes negatives centre cette classification relativement 

 au role qu'y joue la III.i^. En eflfet, il est admis que le IV. type de 

 Secchi se distingue nettemtnt du III. type, non seulement par la position et 

 la quantite des zones obscures, niais aussi par le fait tres-remarquable, que les 

 principales de ces zones sont bien definies et brusquement interrompues du 

 cote du violette dans le III. type du cote du rouge dans le IV. Or, si le IV. 

 type doit representer une des phases de refroidissement, par lesquelles 

 passent les etoiles, on pent faire deux hypotheses. La premiere est que le 

 spectre du IV. type soit coordonne au spectre du III. type, de maniere qu'il 

 ait des etoiles, qui passent de la phase representee par le II. type, a la 

 phase representee par le III. type, et d'autres, qui passent dTectement du 



II. type au IV. Mais cette hypothese est inadmissible. Car en connait de 

 spectres entremediaires entre le I. et le II. type, et entre le II. et III. ; mais 

 on ne connait pas, a ce que je sache, de spectres du II. type tendant au IV. 

 Reste done I'hypothese, que la phase de refroidissement, representee par le 

 spectre du IV. type, soit posterieure a la phase representee par Ic III. type, de 

 maniere que les spectres des etoiles passent du III. au IV. type. Sice passage 

 se fait peu a peu, il devrait avoir des spectres entremediaires entre le III. et 

 le IV. type ; maisquoique Secchi par exemple le 17 Jan., 1868, ait determine 

 le spectre de I'etoile 273 Schjell., comme semblant entremediaire entre le 



III. et le IV. type, il I'a plus tard reconnu du IV. type, et I'existence de 

 spectres du III. -IV. type n'est nullement prouvee. On pourrait objecter 

 que les etoiles du IV. type sont peu nombreuses et en general si petites que 

 leurs spectres sont difficiles a voir, et que par consequent il pourrait y avoir 

 parmi ces spectres quelques-uns, qui se rapprochassent du III. type. Mais 

 je reponds a cette remarque, que lesspectres du III. -IV. type, indiquant une 

 phase moins refroidie, devraient au contraire en general appartenir a des 

 Etoiles plus grandes que celles ayant des spectres du IV. type. Si on veut 

 supposer que le passage du III. au IV. typese fasse subitement, ou par une 

 catastrophe, pendant laquelle apparaissent des lignes brillantes, cette sup- 

 position meme constituerait une diffeience physique bien plus distincte entre 

 le III. et le IV. type, qu'entre le II. et le III. ; et le IV. type representerait 

 une phase bien distincte, laderniere peut-etre avant I'extinction totale. Le 

 r6Ie physique du IV. type est done encore si mysterieux, que j'ai cru pouvoir 

 encore me conformer a I'exemple de d'Arrest, en suivant la classification 

 formelle de Secchi." — C. F. Pechiile," Expedition Danoise pour I'Observation 

 du Passage de Venus, 1882," p. 25 (Copenhagen, J. H. Schultz, 1883). 



In the first classification on these lines, which is 

 certain to be modified as our knowledge gets more 

 exact, it is desirable to keep the groups as small in 

 number as possible ; the groups being subsequently 

 broken up into sub-groups, or, as I prefer to call them, 

 species, as the various minute changes in spectra brought 

 about by variations of temperature are better made out. 



In my paper of November 17 (Nature, vol. xxxvii. 

 p. 84), I gave a diagram of the " temperature curve," on 

 which is shown the distribution of nebulae and of stars as 

 divided into classes by Vogel, on the two arms of the 

 curve. 



On one arm of this we have those stages in the various 

 heavenly bodies in which in each case the temperature is 

 increasing, while on the other arm we have that other 

 condition in which we get first vaporous combination, 

 and then ultimately the formation of a crust due to the 

 gradual cooling of the mass in dark bodies like, say, the 

 companion to Sirius. At the top we of course laave that 

 con' tion in which the highest temperature must be 

 assumed to exist. 



To begin, then, a more general classification with the 

 lowest temperatures, it is known that the nebulae and 

 comets are distinguished from most stars by the fact that 

 we get evidence of radiation. Absorption has been sus- 

 pected in the spectra of some nebulae,^ and has been 

 observed beyond all doubt in some comets.^ But there are 

 some stars in which we also get radiation, accompanied 

 by certain absorption phenomena ; but there is no 

 difficulty in showing that these bodies are more special 

 on account of their bright lines than on account of their 

 absorption bands. We may therefore form the first group 

 of bodies which are distinguished by the presence of 

 bright hnes or flutings in the spectrum. 



The presence or absence of carbon will divide this 

 group into two main divisions, which, however, we may 

 neglect in the following very brief sketch which I give in 

 advance of a more detailed treatment. 



The first species in this group would contain the nebulae, 

 in which only the spectrum of the meteoric constituents 

 is observed. In the second species we find the spectrum 

 of hydrogen added. 



Another early species would contain those bodies in 

 which the nebula spectrum gets almost masked by a con- 

 tinuous one, such as Comets 1866 and 1867, and the great 

 nebula in Andromeda. 



In the second division will be more condensed 

 swarms still, in which, one by one, new lines are added 

 to the spectra, and carbon makes its appearance ; while 

 probably the last species in this group would be bodies 

 represented by 7 Cassiopeiae. 



The great distinction between the first group and the 

 second would be that evidences of absorption now become 

 prominent, and side by side with the bright flutings of 

 carbon and occasionally the lines of hydrogen we have 

 well-developed fluting absorption. 



The second group, therefore, is distinguished from the 

 first by mixed flutings — that is to say, the presence of 

 bright and dark flutings as well as lines in the spectrum. 



I give a detailed examination into the species of this 

 group in the next part of this memoir. 



' "Nebula [No. 117, sih. 32 M. R.A. oh. 35m. 5-33. ; N.P.D. 49° 54 

 I2"'7. Very, very bright ; large, round ; pretty suddenly much brighter in 

 the middle]. — This small but bright companion of the great nebula in 

 Andromeda presents a spectrum exactly similar to that of 31 M [the great 

 nebula in Andromeda]. The spectrum appears to end abruptly in the 

 orange ; and throughout its length is not uniform, but is evidently crossed 

 either by lines of absorption or by bright lines" (Huggins, Pkii. Trans, vol. 

 cliv. p. 441). 



^ "A dark band was noticed at wave-length s^j'g" (Copeland, Comet 

 III., 1881, Copernicus, vol. ii. p. 226). 



" May 20. — With none of these dispersions could any bright bands, 

 properly so called, be distinguished ; but two faint broad dark bands, or 

 what gave that impression, crossed the spectrum. ... A third dark band 

 was suspected near D on the blue side of that line " (M.iuiider, Comet a 1882 

 (Wells), "Greenwich Spectroscopic Observations, 1882," p 34). 



The dark bands were observed again, and their wave-lengths measured 

 on May 31 " {ibid. p. 35). 



