544 



SCIENCE 



[N. S. Vol. XL\\ No. 1169 



and comparable with the temperatures of 

 the blue stars. There again is a sequence 

 with the nebulffi and the blue stars at one 

 end and the red stars at the other. 



Fowler has called attention to the re- 

 markable facts that certain lines of helium, 

 etc., requiring the most powerful electric 

 discharges at command to produce them in 

 the laboratory — super-spark lines, he calls 

 them — are found in the greatest relative 

 strength in the gaseous nebulas ; next in 

 order of strength in the bright-line or' Wolf- 

 Rayet stars, which class includes the central 

 stars of the planetary nebulee ; and in lesser 

 strength in the first subdivision of the Class 

 B stars, that is, in the bluest of the blue 

 stars containing dark lines only; secondly, 

 that the lines in the spectra of the later 

 Class B stars, of the Class A and Class F 

 stars are prevailingly those produced under 

 the less intense conditions of the ordinary 

 electric spark; and, thirdly, that the lines 

 in the yellow-red stars are prevailingly the 

 arc lines which indicate relatively weak 

 electrical conditions. That is another se- 

 quence of conditions running from the 

 nebulse down to the reel stars. 



There are still other sequences running 

 harmoniously through the Harvard classifi- 

 cation. For example, the velocities of the 

 stars in their travels within the stellar sj-s- 

 tem increase as we pass by spectral classes 

 from the large bright-line nebulas and the 

 very blue stars down through the yellow 

 stars to the red stars. The distances apart 

 of the two components of double stars, and 

 consequently their periods of revolution 



Fig. 34. The Irregular Nebulse near Gamma 

 Cassiopeia?, photographed by Curtis with the 

 Crossley Eefleetor of the Lick Observatory. 



[The immensely over-exposed image of the bril- 

 liant star Gamma Cassiopeise is at the lower left- 

 hand corner. The lines radiating from the center 

 of the star image are diffraction effects produced 

 within the telescope. The upper right half of the 

 figure contains much nebulosity, the brighter angles 

 of two nebular structures pointing approximately 

 toward the star Gamma.] 



around each other, increase consecutively 

 as we pass from the blue stars to the red. 

 It might be said that, so far as all these 

 stellar sequences are concerned, the course 

 of evolution could begin with either the red 

 stars or the blue stars and proceed to the 

 other end of the sequence. Not to mention 

 several very weighty objections to the as- 

 sumption that the red stars are effectively 

 young and the blue stars effectively old, we 

 submit the case on the evidence of the neb- 

 ula. The planetarj^ nebulffi, the irregular 

 nebuhT?, the Wolf-Rayet stars, the Class B 

 .stMrs with biialit lines, and the Class B 



Fig. 35. The Spectrum of Gamma Cassiopeise, Henry D 



Memorial, Harvard College Observatory. 



