20 



PROFESSOR A. FOWLER, F.R.S., ON 



The second, or solar, type (G, K) stars present us with the 

 same problems as the sun, from which many of them are not 

 distinguishable. If we look, for example, at a photograph com- 

 paring the spectrum of Capella with that of the sun, even an 

 expert would find it very difficult to say which was which if 

 they had not been labelled. The Harvard classification 

 recognizes two sli^htlv different varieties. G and K. Arc lines 

 of the various elements are the chief features in both, but there 

 is evidence of a greater general absorption in the violet region 

 in the K than in the G stars. Also, the enhanced or spark 

 lines of various known elements, to which reference has already 

 been made as requiring greater energy than arc lines for their 

 production, are somewhat stronger in the G than in the K 

 stars. 



The F stars of the Harvard classification are intermediate 

 between the second and first types of Secchi. Hydrogen lines 

 are now much stronger than in the solar spectrum, while 

 metallic lines, as a whole, are somewhat enfeebled. A fact of 

 special significance, however, is that the enhanced lines of 

 various metals have gained in intensity relatively to the arc 

 lines. 



A.^ a typical specimen of type I, or A, stars we may look 

 at the spectrum of Sirins. The lines of hydrogen now 

 dominate the spectrum, but among the fainter lines we recognize 

 that the enhanced metallic lines occupy the most prominent 

 place. A most beautiful illustration of the gradual replace- 

 ment of arc lines by enhanced lines has been given by 

 Professor Hale in a series of photographs of several stars 

 showing the behaviour of two titanium lines which are almost 

 side by side ; the arc line occurs alone and with great intensity 

 in the fourth type spectrum and gradually thins out in passing 

 through the series, while the enhanced line appears about the 

 middle of the series and remains alone when the Sirian stars 

 are reached. 



In the B stars there are a few enhanced lines of magnesium 

 and calcium, but for the most part we find lines of non- 

 metallic elements. Helium, which does not show its absorption 

 lines in any of the previous types, now appears prominently 

 in association with hydrogen. Among the remaining lines, the 

 majority have been traced to oxygen, nitrogen, carbon, silicon, and 

 sulphur. The change from metallic lines of previous types to- 

 non-metallic lines in B stars is especially striking. 



In some of the B stars there are certain lines of nitrogen 

 and carbon which can only be produced in the laboratory by 



