On the Chemistry of the Hottest Stars. 



L81 



The Absorptions not Identical. 



Early in my researches I pointed out* that the absorption pheno- 

 mena in stellar spectra need not be identical at the same mean 

 temperature on the ascending and descending sides of the curve, 

 since, on the meteoritic hypothesis, there mast be a tremendous 

 difference in the physical conditions. 



In a condensing swarm, the centre of which is undergoing 

 meteoritic bombardment from all sides, there cannot be the equi- 

 valent of the solar chromosphere ; the whole mass is made up of 

 heterogeneous vapour at different temperatures, and moving with 

 different velocities in different regions. 



In a condensed swarm, of which we can take the sun as a type, all 

 action produced from without has practically ceased ; we get rela- 

 tively a quiet atmosphere and an orderly assortment of the vapours 

 from top to bottom. But still, on the view that the differences in 

 the spectra of the heavenly bodies chiefly represent differences in 

 degree of condensation and temperature, there can be, au fond, no 

 great chemical difference between bodies of increasing and bodies of 

 decreasing temperature. Hence, it is exceedingly probable that at 

 equal mean temperatures on opposite sides of the temperature curve, 

 this chemical similarity of the absorbing vapours will result in many 

 points of resemblance in the spectra. 



The identical behaviour of the enhanced metallic and cleveite 

 lines on both sides of the curve shows us that we have here a pre- 

 viously undreamt-of test to apply to this question, and it is impor- 

 tant to make sure of this ground before I proceed to apply it to others 

 opened up and rendered clearer by the new investigations. 



The New Criteria. 



With the photographs at present available, the test can be applied 

 at three stages of temperature, the stars of equal mean temperature 

 being determined by the relative intensities of the enhanced and arc 

 metallic lines. When several stars of the same mean temperature 

 are thus brought together, it soon becomes evident that they are 

 divisible into two groups, which differ considerably in other respects. 

 To take one instance, the average temperature of the absorbing iron 

 vapour is about the same in e Pegasi as in Arcturus, since the spectro- 

 scopic difference between these stars, so far as the line spectra are 

 concerned, is very slight. But the continuous absorption in the 

 violet is much greater in e Pegasi than in Arcturus, while the metallic 

 lines are also somewhat broader. The difference between the stars, 

 therefore, does not appear to be due alone to a difference of temperature. 



* 'Hoy. Soc. Proc.,' vol. 44, p. 26. 



o 2 



