PROBLEMS OF ASTROPHYSICS 457 



Cassiopeiae, Pleione, and Mu Centauri are examples. Closely related 

 to the foregoing are the helium stars. 1 Their absorption lines include 

 the Huggins hydrogen series complete, a score or more of the con- 

 spicuous helium lines, frequently a few of the Pickering hydrogen 

 series, and usually some inconspicuous metallic lines. Calcium ab- 

 sorption 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 2 are not thoroughly 

 understood; but atmospheres of higher temperatures than their 

 underlying strata, or very extensive simple atmospheres, seem to 

 be demanded. The former condition, on the large scale required, 

 involves some difficulties, 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- 

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

 of Orion, 8 or the remnants of nebulosity in which the individual stars 

 of the Pleiades 4 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. Vega and Sirius 5 are con- 

 spicuous 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. 6 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 



1 Clerke's Problems in Astrophysics, p. 189. 



2 Frost's Scheiner's Astronomical Spectroscope, p. 250. 



3 Harvard Annals, xxxn, 66, and plate in, fig. 4. 



* Clerke's System of the Stars, p. 224, and frontispiece. 



5 Huggins, An Atlas of Representative Stellar Spectra, plates v and vi. 



8 Ibid, plate vii. 



