6l2 



NATURE 



[October 20, 1904 



1S79, Sir Norman Lockyer ' made the important discovery 

 that several of these " unknovi'n " stellar lines w;ere 

 coincident with lines of iron which were enhanced in bright- 

 ness in passing from the arc to the spark spectrum. What 

 is meant precisely by " enhanced lines " may be gathered 

 from Fig. i, and the first idea of their relation to stellar 

 spectra is well brought nut in Kitj. 2. 





2. 



The laboratory experiments suggested that in a space 

 heated to the temperature of the hottest spark, and shielded 

 from the effects of a lower temperature, the spectrum of 

 iron would consist of these enhanced lines alone, and the 

 outcome of the investigation was, in fact, to show that this 

 condition is realised in the stars which are judged to be 

 verv hot by the extension of their spectra into the violet. 

 Thiis in a Orionis the continuous spectrum in the violet is 

 feeble, and the arc lines and flutings appear 

 without the enhanced lines ; in a. Cygni the 

 violet radiation is more intense, and the 

 enhanced lines are relatively much stronger 

 than the arc lines ; while in Rigel, with still 

 stronger violet radiation, the enhanced lines 

 appear in the absence of the arc lines. .\t 

 a still higher stage, represented in the 

 diagram by 7 Orionis, the metallic lines 

 have disappeared altogether, and are re- 

 placed by lines chiefly due to gases. 



In a subsequent paper - it was shown that 

 similar results were obtained in the case of 

 other elements, and the presence or absence 

 of enhanced lines, or their intensities a-- 

 compared with those of the arc line^, 

 appeared to afford a ready means nl 

 arranging stars at certain stages in order 

 of temperature independently of a special 

 study of the violet radiation. Adopting this 

 mode of bringing together stars of approxi- 

 mately the same mean temperature, it was 

 found, as before, that at each stage th( 

 stars were divisible into two groups, .iini F ar",~n"s " 



that these groups naturally fell into Iw ' ' 



series, in each of which there was an alnm-i i. Mi 



unbroken sequence of changes in the line %^- 



spectra. As determined in this way, stars 



of one series differ from those of the other Fu,. ..— Illu>iia 



at the same stage of heat : — " (i) in the 



greater continuous absorption in the violet 



or ultra-violet, (2) in the generally greater intensity and 



breadth of the metallic lines, (3) in the smaller thickness of 



the hydrogen lines, (4) in the greater thickness of the helium 



lines at those stages in which they are visible." The 



differences indicated in (2) and (3) are well illustrated by the 



comparison of the spectra of .Sirius and a Cygni given in 



Kitr. 3. 



It is to be noted that while the relative intensities of the 

 arc and enhanced lines of the same metal are the same in 

 both stars, thus indicating probable near equality of 

 temperature, the metallic lines generally are weaker in 

 .Sirius than in a Cygni, while the lines of hydrogen behave 

 in an e.xactly opposite manner. 



The differences between the two series were explained by 

 supposing, as before, that one of them comprises stars of 

 increasing temperature and the other those which are be- 

 coming cooler. On the meteoritic hypothesis, stars of the 

 first series would still be in the state of unconden^ed swarms, 

 and the greater thickness of effective absorbing vapours 

 would account for the increased continuous absorption at 

 the violet end of the spectrum, as well as for the greater 

 thickness of the metallic lines, as compared with those stars 

 in which a photosphere has been formed. 



In 1899, in view of the fruitful results of the continued 

 investigation of enhanced lines in relation to the stars. Sir 

 Norman Lockyer ' concluded that the time had arrived for 

 a complete revision of the nomenclature of the stellar 

 groups, and a more extensive definition of their chemical 

 peculiarities. This new classification, in a slightlv revised 

 form, is fully stated and applied to the spectra of 470 of 

 the brighter stars in a recent publication of the Solar 

 Physics Committee.- On account of divergences of opinion 

 among those engaged in these investigations, the same 

 type of spectrum was referred to differently numbered 

 groups in the various classifications which had been 

 previously proposed, and to avoid the confusion to which 

 this gave rise the use of numbers was entirely dispensed 

 with. The idea underlying the new nomenclature cannot 

 be better stated than in the words of the author, namely : — 

 " As we know beyond all question that a series of geological 

 strata from the most ancient to the most recent brings us 

 in presence of different organic forms, of which the most 

 recent are the most complex, it is natural to suppose that 

 the many sharp changes of spectra observed in a series of 

 stars from the highest temperature to the lowest, brings us 

 in presence of a series of chemical forms which become more 

 complex as the temperature is reduced. Hence we can in 

 the stars study the actual facts relating to the workings of 

 inorganic evolution on lines parallel to those which have 



^ 



I Roy. Soc. rroc, vol. Ix. p. 475 (1896). = /i:,i, vol. Ix 



NO. 1825, VOL. 70] 



,pp. 148-209(1897). 



g the gradual replacement of arc lines of iron by_enhanced line.^ in stars or 

 increasing temperature. 



already been made available in the case of organic evolu- 

 tion. If then we regard the typical stars as the equivalents 

 of the typical strata, such as the Cambrian, Silurian, &c., 

 it is convenient that the form of the words used to define 

 them should be common to both." .An adjectival form end- 

 ing in ian was therefore suggested. 



1 Roy. Soc. Proc.f vol. Ixv. p. 186. 



- "Catalogue of 470 of the Brighter Stars, classified according to their 

 Chemistry." (London: H.M. Stationery Office, 1902.) 



