Mat 25, 1917J 



SCIENCE 



543 



fluence our present question one way or the 

 other. Aside from nebulium, the hydrogen 

 and helium lines are the most prominent 

 ones in the bright-line nebula3. They are 

 the most prominent ones in the nuclei of the 

 planetary nebulse. Thej' are prominent in 

 the isolated Wolf-Rayet stars, in the Class 

 B stars containing bright lines, and in the 



Fig. 33. The Ring Nebula in Lyra, photographed 

 by Curtis with the Crossley Reflector of the Lick 

 Observatory. 



Class B stars containing only dark lines. 

 Helium has never been found in any yel- 

 low or red stars, except feebly in a certain 

 stratum of our sun's atmosphere, and that 

 is due to our abilitj^ to observe our sun 

 somewhat in detail ; and the hydrogen is 

 relatively feeble in the later classes of the 

 Harvard sequence. 



The continuous spectra of stars as ar- 

 ranged in the Harvard sequence decrease 

 continuously in the relative strength of the 

 blue, violet and ultra-violet regions as we 

 pass from the central stars of the planetary 

 nebulae, which are wonderfully strong in 

 ultra-violet light, down through the blue 

 stars with both bright and dark lines. 



through the blue stars" containing dark 

 lines onh^, and on through the yellow stars 

 to the red stars. The central star in the 

 ring nebula in Lyra (Fig. 33) is invisible 

 to many inexperienced observers, even in 

 the great Lick telescope. Yet it is so rich 

 in violet light, and especially in ultra-violet 

 light, that it can be photographed in the 

 great reflecting telescopes with an exposure 

 of only two or three seconds. Wright has 

 recently made the interesting discovery that 

 the continuous spectra of many planetai'y 

 nebulffi are remarkably strong in a long 

 stretch of the ultra-violet region. The se- 

 quence of decreasing richness in blue and 

 violet light extends unbroken from the 

 nebula and nebular nuclei down to the red 

 stars. 



The si^ectra of the stars are indicative of 

 conditions existing in their surface strata. 

 We do not know definitely just what con- 

 ditions produce or accompany certaiii types 

 of spectra, but surface temperature seems 

 to be a prime influence, and there are some 

 reasons for thinking that electrical condi- 

 tions may also be extremely important.. 

 Now all determinations of the surface tem- 

 peratures of the stars make the extreme 

 blue-violet stars the hottest of all, with the 

 effective temperatures decreasing continu- 

 ouslj^ as we pass from the nuclear stars of 

 the planetary nebula down through the 

 Harvard sequence to the red stars. Fabry 

 and his associates in Marseilles have re- 

 cently arrived at the result, by physical 

 methods, that the temperature of the Orion 

 nebula is very high, vastly higher than the 

 temperatures of the red and yellow stars, 



s Huggins has called attention to a reduction in 

 the brightness of the ultra-violet spectrum of the 

 first-magnitude star Vega (Class A) by virtue of 

 ■(That seems to be an absorption band many hundred 

 Angstrom units in width. It is not yet known to 

 what extent the ultra-violet spectra of blue stars 

 in general may be aifected by such an absorption 

 band. 



