AN ADDRESS ON ASTROPHYSICS. 3° 7 



to the foregoing are the helium stars. Their absorption lines include the 

 Huggins hydrogen series complete, a score or more of the conspicuous 

 helium lines, frequently a few of the Pickering hydrogen series, and 

 usually some inconspicuous metallic lines. Calcium absorption is 

 absent, or scarcely noticeable. The white stars in Orion and the 

 Pleiades are typical of this age. 



The causes which produce bright lines in stars are not thoroughly 

 understood; but atmospheres of higher temperatures than their under- 

 lying strata, or very extensive simple atmospheres, seem to be demanded. 

 The former condition, on the large scale required, involves some diffi- 

 culties, and mildly suggests the possibility that external influences may 

 be acting upon the radiating strata of bright-line stars. 



The assignment of the foregoing types to an early place in stellar 

 life was first made upon the evidence of the spectroscope. The photo- 

 graphic discovery of nebulous masses in the regions of a large propor- 

 tion of the bright-line and helium stars affords extremely strong con- 

 r firmation of their youth. Who that has seen the nebulous background 

 of Orion, or the remnants of nebulosity in which the individual stars 

 of the Pleiades are immersed, can doubt that the stars in these groups 

 are of recent formation ? 



With the lapse of time, stellar heat radiates into space ; and, so far 

 as the individual star is concerned, is lost. On the other hand, the 

 force of gravity in the surface strata increases. The inevitable con- 

 traction in volume is accompanied by increasing average temperature. 

 Changes in the spectrum are the necessary consequence. The second 

 hydrogen series vanishes, the ordinary hydrogen absorption is in- 

 tensified, the helium lines become indistinct, and calcium and iron 

 absorptions begin to assert themselves. Yega and Sirius are conspic- 

 uous examples of this period. Increasing age gradually robs the 

 hydrogen lines of their importance, the H and K lines broaden, the 

 metallic lines develop, the bluish- white color fades in the direction 

 of the yellow, and, after passing through types exemplified by many 

 well-known stars, the solar stage is reached. The reversing layer in 

 solar stars represents but four or five hydrogen absorption lines of 

 moderate intensity; the calcium lines are commandingly prominent; 

 and some 20,000 metallic lines are observable. The solar type seems 

 to lie near the summit of stellar life. The average temperature of 

 the mass must be nearly a maximum, for the low density indicates a 

 constitution that is still gaseous. 



Passing time brings a lowering of average temperature. The color 

 passes from yellow to the red, in consequence of lower radiating tem- 

 peratures and increasing general absorption by the atmosphere. The 

 hydrogen lines become indistinct, metallic absorption remains prom- 

 inent, and broad absorption bands are introduced. In one type, of 

 which Alpha Herculis is an example, these bands are of unknown 



