NATURE 



[November 2, 1899 



ON THE DISTRIBUTION OF THE VARIOUS 

 CHEMICAL GROUPS OF STARS} 



II. 



THE results so far referred to have regard to the stars 

 with dark lines in their spectra, but besides these 

 there are many so-called bright-line stars. 



I should state that there has necessarily been a change 

 of front in our views with regard to these bright-line stars 

 since they were first classified with nebuke by Pickering 

 and myself. 



The nebulas are separated from stars by the fact that 

 in their case we have to deal with bright lines, that is 

 to say, we are dealing with radiation phenomena, and not 

 with absorption phenomena, as in the case of the stars so 

 far considered ; and in the first instance it was imagined 

 that the bright-line stars were, from the chemical point of 

 view, practically nebulae, although they appeared as 

 stars, because the brightest condensations of them were 

 go limited or so far away that they gave a star-like 

 appearance in the telescope. 



Since that first grouping of bright-line stars, by the 

 Avork chiefly of the American astronomers it has been 

 found that in a large number of cases they have also 

 dark lines in their spectra, and that being so we must 

 classify them by their dark lines instead of by their bright 

 ones ; and the bright-line stars thus considered chiefly 



generality toonly two degrees, and the greatest departure, 

 the greatest galactic latitude, was something within 

 nine degrees. That was the story in 1891. Two years 

 afterwards Campbell, another distinguished American 

 astronomer, also interested himself in this question of the 

 bright-line stars, and he discussed them, his catalogue 

 containing fifty-five as opposed to Pickering's thirty-three. 

 He found also that they were collected almost exclusively 

 in the Milky Way, and that outside the Milky Way 

 practically none had ever been observed. The importance 

 of this result I will indicate by and by, but in the mean- 

 time I can throw on the screen a very useful map which 

 Campbell prepared. The central line of that map repre- 

 sents the galactic zone, the plane of the Milky Way, and 

 he marks along it the different galactic longitudes, show- 

 ing above and below the plane just a few degrees of 

 i galactic latitude north and south, sufficient to enable him 

 j to plot upon it all the bright-line stars which he dis- 

 j cussed. The diagram shows that all these bright-line 

 stars really are close to the central plane of the Milky 

 ; Way. Only one out of the fifty-five is more than nine 

 degrees from it, and this lies in a projecting spur, so that 

 j we cannot really say that that is out of the Milky Way. It is 

 I remarkable that these bright-line stars are not equally 

 I distributed along the Milky Way. They are chiefly con- 

 \ densed in two opposite regions, and there is one region 

 I in which they are markedly absent. The glass globe will 



Fig. 3.— Distribution of the Wolf-Rayet stars in the Milky Way. 



turn out to be gaseous stars, 7m'th a difference. What i s 

 that difference ? It is this, I think : in the case of the 

 bright-line stars we are dealing with the condensations 

 ttf themost disturbed nebulae in the heavens; together with 

 the light which we get from the nucleus of that nebula 

 which appears as a star and can be spectroscopically 

 classified with the other dark-line stars, inasmuch as the 

 surrounding vapours close to the star produce absorption, 

 and therefore give us dark lines ; other parts of the 

 nebuUe, probably those further afield, give us bright lines 

 which mix with the dark ones. Therefore we get both 

 bright lines and dark lines under these conditions. So 

 far as the result goes up to the present moment, it looks 

 as we have now to consider that these bright-line stars, 

 instead of being nebulae merely, are gaseous stars at a 

 very high temperature, m consequence of the fact that 

 the nebula which is surrounding them, which is falling 

 upon them, is increasing the temperature of the central 

 mass by the change of vis viva into heat. Pickering,^ in 

 his discussion of these stars, had thirty-three to deal with, 

 and he found that there was a wonderful tendency among 

 these to group themselves along the Milky Way ; that 

 very few of them, in fact, lay outside its central plane ; 

 that is to say, the galactic latitude, as it is called, the 

 distance in degrees from the plane was limited in the 



1 A Lecture to Working Men, delivered at the Museum of Practical 

 Geology, on April lo, by Prof. Sir Norman Lockyer, K.C.B., F.R.S. 

 (Continued from vol. Ix., p. 620.) 



2 Astr. Nach., No. 2025. 



NO. 1566, VOL. 61] 



show how the matter stands, I think, rather conveniently. 

 We have the Milky Way represented by red tape. The 

 secondary Milky Way, which starts from it at one point 

 of the heavens and meets it again, is also indicated. Dark 

 wafers mark the galactic longitudes and latitudes of the 

 bright-line stars. We find that these stars begin just 

 before the doubling commences. They go on, and are 

 sometimes very numerous, and they end just after the 

 doubling ends ; and we notice there is a long range of 

 the Milky Way where it is single in which there is abso- 

 lutely no bright-line star at all. It looks, therefore, very 

 much as if there is a something connected with this 

 doubling of the Milky Way which produces the conditions 

 which generate these bright-line stars. 



By the labours of Dundr, Pickering, McClean and 

 Campbell, we are beginning to get very definite notions 

 as to the distribution of the various chemically different 

 stars in relation to the Milky Way. How about the 

 nebula; from the point of view of chemical distribution ? 

 Here we are in difficulties. 



I have already stated that with regard to the general 

 question of the nebula; it is impossible to speak with 

 certainty, because at present there has not been suffi- 

 cient time and there has not been a sufficient number of 

 observers at work to classify the thousands of " nebulcs" 

 which we now know of into those which give us the 

 gaseous spectrum, and those which are entirely different, 

 apparently, in their constitution, and only give us what 

 is called a continuous spectrum ; but still we can go a 



