The Spectrum of u Aquilm, 



235 



According to the earlier work at Kensington, stars like P Arietis 

 were classed in Group Va,"^ that is, between stars like Sirius and those 

 like Procyon. The work on enhanced lines which has been done since 

 then enables us to carry on the work of classification with much 

 greater precision, since we have now a means of estimating relative 

 temperatures with considerable accuracy. In this way we learn that 

 stars at each stage of temperature fall into two groups, one of which 

 represents stars of increasing temperature, and the other including 

 stars of decreasing temperature, a Aquilae and ^ Arietis fall in the 

 latter group, and are to be regarded therefore as stars in which photo- 

 spheres have formed. The later work on the classification of spectra 

 has shown that it is sufficient for all practical purposes to include both 

 in the Sirian group of stars. 



This question of classification is further elucidated by a more 

 detailed examination of the spectrun of a Aquilse in relation to a Cygni 

 and the Sirian stars. The foregoing demonstration of the likeness 

 between a Aquilae and P Arietis leads us to expect that the origins of 

 the lines in the spectrum of a Aquilas will be the same in the main as 

 those of P Arietis and Sirius. In these stars the temperature of the 

 absorbing vapours is intermediate between that of the arc and that at 

 which enhanced lines appear alone, so that the spectra show both arc 

 and enhanced lines. The origins of the chief enhanced lines in the 

 spectrum of Sirius have already been investigated,! and practically 

 the same lines occur in P Arietis. Besides these enhanced lines, 

 there are several well-known arc lines, such as the iron triplets and 

 the blue line of calcium, which can be readily identified. The origins, 

 of some of the lines of both classes are shown in the plate which 

 accompanies this paper, enhanced lines being shown at the bottom and 

 arc lines at the top. 



It will be seen that enhanced lines of iron appear in a Aquilae, but 

 have not the same relative intensity as in a Cygni ; the most enhanced 

 line of iron (X 4233*3), for example, which in a Cygni is represented 

 by a very strong and well-defined line, is in a Aquilse very weak and 

 hazy. On the other hand, some of the enhanced lines of iron less- 

 refrangible than Hy are fairly prominent. The principal enhanced 

 lines of magnesium, strontium, and titanium are also certainly present, as. 

 shown in the plate. The enhanced double line of silicium at AA. 4128*1, 

 4131*1, if present, is very weak, a moderately strong hazy line, rather 

 less refrangible than the silicium double, making it rather difficult to- 

 determine whether the latter is certainly present. 



Among the arc lines present are those of the iron triplet in the 

 violet {XX 4045-90, 4063*76, 4071*79), which are clearly seen, but the 

 iron triplet in the blue {XX 4383*70—4415*27) cannot be identified 



* * Phil. Trans.,' A, vol. 184 (1893), p. 726. 

 t * Koy. Soc. Pron.,* vol. 65 (1899). Plate 7. 

 VOL. LXVI. U 



