452 



Mi*. J. Norman Lockyer. 



metals has enabled me to apply new criteria in the classification of 

 stellar spectra. 



A study of these lines shows at once that B Cephei is hotter than 

 either a. Tauri or the snn, and that the difference between its spectrnm 

 and that of a, Tanri or the sun ife certainly in part due to this differ- 

 ence of temperature of the absorbing vapours. The lines which 

 are stronger in B Cephei than in the sun include many of those which 

 have been found to be enhanced in the spark spectra of metals, so 

 that they are no longer to be regarded as unknown lines. Similarly, 

 many of the lines of a. Cygni for which no origins could previously 

 be assigned have been shown to be lines of common metals under con- 

 ditions of high temperature. Still, the mere presence of the enhanced 

 lines in a star spectrum affords us no criterion as to whether the 

 temperature of a star is increasing or decreasing. But I have also 

 shown that if we take the relative intensities of the enhanced lines 

 and the arc lines as an indication of stellar temperatures, and in this 

 way bring together a sufficient number of stars of about the same 

 temperature as 7 Cygni or B Cephei, such spectra may be divided 

 into two well-marked groups, of w r hich 7 Cygni and Castor may be 

 taken as types. The chief generic differences between the two 

 groups of stars at the temperature of d Cephei were thus summarised 

 in the paper to which reference has been made : — 



Cephei. 



(1) Considerable absorption in 



ultra-violet. 



(2) Hydrogen lines relatively 



thin. 



(3) Metallic lines of moderate 



intensity. 



Castor. 



(1) Very little continuous ab- 



sorption in ultra-violet. 



(2) Hydrogen lines relatively 



very thick. 



(3) Metallic lines relatively 



feeble. 



I further showed that these differences are simply and sufficiently 

 explained on the supposition that stars like 7 Cygni and B Cephei 

 are uncondensed swarms of meteorites, while those like Castor, which 

 have about the same mean temperature, are stars approaching the 

 condition of the sun in which photospheres and relatively quiescent 

 atmospheres have formed. The foregoing considerations indicate 

 that there are three chief periods in the history of a star during its 

 stages of luminosity : — 



(1) A period during which it exists as an uncondensed swarm, when 

 the " atmosphere " is disturbed by meteoritic bombardment from 

 without. 



At this stage the atmosphere is a mass of heterogeneous vapour 

 at various temperatures and moving with different velocities in 

 different regions. 



