Jan. 5, 1888] 



NATURE 



^35 



In the columns headed *"' Calculated " are the numbers obtained 

 by multiplying by 4/3 the numbers of the stars in the 

 classes of different magnitude given in the " Wunder des 

 Ilimmels," by Littrow, 5th ed. p. 577, deduced from the Durcli- 

 inusterung of Argelander, and then dividing the result by 15 

 and 750 respectively. For the stars I II. a the agreement is 

 almost perfect up to the magnitude of 5 "9 inclusive, and for 

 III.^ up to 6 '9; up to 7 "9 the agreement is pretty good, whilst 

 after that the numbers observed are more and more in arrear of 

 the numbers obtained by cxlculation. We may conclude there- 

 fore that our knowledge of the spectra W.\.a is almo.t complete 

 up to the magnitude of 5*9 inclusive, and of spectra \W.b up to 

 7 '5 inclusive. The researches of M. Vogel have not added any 

 new star \\\.a with a magnitude higher than 5*0, and only very 

 few between 5*0 and 60, 'and, as I have already said, no new 

 star III./' above the magnitude 7'5, although he has examined all 

 the stars up to this limit of magnitude between - 7." and + 20' 

 of declination. As to the difference existing between " observa- 

 tion" and "calculation " in the caie of the III. a feebler than 

 6*o, we must remember that as yet no systematic spectroscopic 

 research has been made of the stars between - 2° and — 23", 

 nor between + 20'' and the North Pole. Consequently the num- 

 ber of stars III. a between 6"o and 7"5 will probably be much 

 increased before very long, and will approach the theoretic num- 

 ber. On this account I imagine the stars of III.^ are hfty times 

 rarer than those of I II. a. 



The list of these rare stars is probably already very complete 

 for that part of the sky visible in Europe, for the nearer the 

 researches of an astronomer are to the present time the feebler 

 are the stars with spectra of this class which he discovers 

 (Secchi 67, D'Arrest 7'o, Vogel 7'l, Daner 8'3, Pickering 

 9*l). The conclusions, therefore, which we can draw as to the 

 manner in which these stars are distributed over the heavens 

 deserve some c ')nftdence. Such a research is very interesting. 

 We have already seen that the principal bands in these spectra 

 owe their origin to the presence of a carbon compound in the 

 atmosphere of the stars. It is important to know whether there 

 is a certain direction in the heavens in which these stars are 

 more numerous than in others, especially when we consider that 

 the sam2 substance is present in comets, which come from inter- 

 stellar space. I have made such a research, and have come to 

 the conclusion that the objects in question are grouped similarly 

 to stars in general, being closer together in the neighbourhood 

 of the Milky Way. Setting out from the position of the Pole 

 of the Milky Way given by Heis, R.A. = I2h. 42m., Decl. 

 — -f 26" 8', for the equinox igDO'o, I have calculated the 

 quantities P, or the distances of the stars from this Pole, given 

 in my catalogue. But to have my list a little more complete for 

 the part of the heavens invisible in Europe, I have calculated 

 the same quantity also for the following stars, whose spectra 

 have been examined by M. Pechiile (" Expedition Danoise pour 

 rObservation du Passage de Venus," 1882, pp. 40-43). 



By the help of the P's found, I have obtained the following 

 table, wliich indicates the numbers of those stars which are 

 between the different limits of distance of the Poles, boreal or 

 austral, of the Milky Way. 

 Limits of Polar Distance. No. of -Stars. Mean Ma^initude. 



0-35 .. 3 ••• <^'(> 



35-60 ... 8 ... 6-6 



63-70 ... 8 ... 7-2 



70-80 ... 13 ... 7-4 



80-90 ... 29 ... 8-3 



It is at once seen that there is an immense accumulation 

 between 80^ and 90° of polar distance, and that the polar regions 

 are totally empty up to 19° distance from the Pole; and this 

 relation would doubtless become still more striking if our know- 

 ledge of these stars which are invisible in Europe was more 

 complete ; for, whilst the two Polar regions are for the most 

 part visible, a great part of the Milky Way is always below our 

 horizon. Besides the number of stars in the different zones, I 

 have also calculated their m;an mignitules, and it will be seen 



that for them, as well as for other stars, there is this rule — that 

 in the Milky Way the faint stars are much closer together than 

 in the neighbourhood of its Poles. 



One might perhaps suppose that there is a certain portion of 

 the Milky Way where the stars III.i^ are more frequent than 

 elsewhere. In order to decide this it is necessary first to calcu- 

 late for each star the quantity which has the same relation to the 

 Milky Way as the right ascensions have to the equator ; and 

 then make a table, on the distribution, having this quantity as 

 its foundation. Such a research cannot, however, lead to good 

 results as long'as our acquaintance with the stars between 25" of 

 south declination and the South Pole is almost nil. I will only 

 say, then, that there is a great number of these stars around 

 R.A. 305'', Decl. + 40°, but almost an equal number around 

 R.A. 85°, Decl. + 25°. Now both these points are precisely 

 those in which, in the northern hemisphere, the stars are closest 

 together. It seems that they are grouped almost according to 

 the sane laws as all other stars, and that, properly speaking, 

 there is no region where stars of the Class \\\.b abound. 



A similar research of the stars \\\.a could not give exact 

 results, as our acquaintance with these stars below magnitude 

 6 'O is still too imperfect. However, the researches which M. 

 Pechiile undertook, with the aid of the UranoinctHa Argentina, 

 on the distribution of the coloured stars, render it probable that 

 these also are closest together in the neighbourhool of the Milky 

 Way. 



I have already said that in all probability the spectra of fixed 

 stars must be subject to variations on account of the diminution 

 in the temperature of stars which must take place sooner or later, 

 and I observed that it is precisely on tha supposition of such a 

 diminution that the classes of M. Vogel are based. There are, 

 however, eminent savants who have combated the correctness of 

 this opinion, and who have formed ingenious hypotheses to prove 

 the possibility that the sun, and consequently the stars also, may 

 regain the heat which emanates from them. But it would be too 

 much to say that these theories have victoriously withstood 

 caticism, and the spectroscopic examination of the stars has 

 given results fatal to them. Although the spectra of stars 

 may be divided into very distinct classes, according to their 

 characteristics, there are, on the other hand, numerous spectra 

 of all possible grades between any two classes, so that it may be 

 difficult, if not impossible, to decide to what class a star belongs, 

 and that even when it is sufficiently brilliant for all the details of 

 its spectra to be distinctly recognized. Besides, we see that the 

 more the star resembles the first class, the brighter is its violet 

 part, whereas the violet part becomes fainter and fainter or even 

 invisible when the spectrum resembles that of a Orionis (III. a). 

 On that account it seems certain that the spectra owe their 

 characteristics to the greater or less degree of incandes:ence of 

 the stars, so that the temperature of stars of Class III. must be 

 relatively low. 



Doubtless these changes do take place in the stellar spectra, 

 although we must suppose that, as regards the spectra of the 

 first two classes, they are almost exclusively secular, and operate 

 so slowly that millions of years may pass before they become 

 apparent. 



It is different with stars of Class III. These being probably 

 already much cooler than the others, we may reasonably expect 

 that the changes will take place more rapidly, and perhaps also 

 that from time to time temporary augmentations in activity will 

 take place on their surface, followed by periodic changes in their 

 spectra. 



In the course of his observations Secchi arrived at the con- 

 clusion that the colours and spectra of these stars were subject 

 to remarkable changes in a very short period. My observations 

 led to the same conclusion, if observations from the years i865 

 to 1874 may be trusted without reserve. For, without counting 

 the few and unimportant discrepancies which I disco vered 

 between the aspect of several spectra and the descriptions given 

 by earlier observers, I found that there are forty stars whi ch have 

 been comprised in Class III., among which there is scarcely one 

 which now belongs to it, and there are some which ought to have 

 been transported from one sub -class into the other. But, for 

 reasons which I will here explain, such a conclusion would 

 certainly be too hasty. 



On the one hand, Secchi's observations date from a time which 

 we may call the infancy of spectrum analysis, and the instru- 

 ments employed were very imperfect ; on the other hand, he was 

 the first to introduce a classification of the stars according to 



