66 



CHAPTER VII. A CHEMICAL CLASSIFICATION OF STARS. 



IN the attempts made to classify the stars by means of their spectra, 

 from Eutherfurd's time to quite recently, the various criteria selected 

 were necessarily for the most part of unknown origin ; with the excep- 

 tion of hydrogen, calcium, iron and carbon, in the main, chemical 

 origins could not be assigned with certainty to the spectral lines. 

 Hence the various groups defined by the behaviour of unknown lines 

 were referred to by numbers, and as the views of those employed in 

 the work of classifying differed widely as to the sequence of the 

 phenomena observed, the numerical sequences vary very considerably, so 

 that any co-ordination becomes difficult and confusing. 



The recent work referred to in the last chapter has thrown such a 

 flood of light on the chemistry of the stars that most definite chemical 

 groupings can now be established, and the object of the present 

 chapter is to give an account of the general scheme of classification in 

 which they are employed, which I have recently proposed. 



The fact that most of the important lines in the photographic region 

 of the stellar spectra have now been traced to their origins renders 

 this step desirable, although many of the chemical elements still remain 

 to be completely investigated from the stellar point of view. 



The scheme is based upon a minute inquiry into the varying inten- 

 sities, in the different stars, of the lines and flutings of the under- 

 mentioned substances : 



Certain unknown elements (probably gaseous, unless their lines 

 represent " principal series ") in the hottest stars, and the new form of 

 hydrogen discovered by Professor Pickering (which I term " proto- 

 hydrogen " for the sake of clearness), hydrogen, helium, asterium, 

 calcium, magnesium, oxygen, nitrogen, carbon, silicium, iron, titanium, 

 copper, manganese, nickel, chromium, vanadium, strontium ; the 

 spectra being observed at the highest available spark temperatures. 

 The lines thus observed I term "enhanced" lines, and I distinguished the 

 kind of vapour which produces them by the affix " proto," e.g., proto- 

 magnesium, for the sake of clearness.* 



# Boy. Soc. Proc., vol. Ixir, p. 398. 



