622 SCIENCE PROGRESS 



This minute percentage would seem to stamp the planetary 

 at once as an exceptional case, a sporadic manifestation of a 

 path which has been but rarely followed in stellar evolution." 



Dr. Wright, too, in the section devoted to the spectra of 

 the gaseous nebulae, finds that about half the planetary nebulae 

 have bright nuclei of the Wolf-Rayet type of spectrum, and 

 has " no hesitancy in advancing the opinion that the nebular 

 nuclei belong to the same general division as the Class O stars, 

 irrespective of whether they exhibit bright bands or not."^ 

 These nuclei of the planetary nebulae have been found to give 

 a continuous spectrum strong in ultra-violet light. This is 

 generally supposed to be an indication of very high temperature. 

 But there can be no possible doubt as to the close connection 

 between the planetary nebulae and the Class O stars, and, as 

 we have seen, new stars pass in their spectral changes into 

 planetary nebulae and finally into Class O stars. 



Another link between new stars and nebulae is furnished by 

 Hubble's discovery of a variable nebula, the light radiations 

 of which are exactly matched by the bright bands in the early 

 spectrum of Nova Aurigae. Nor must we forget in this con- 

 nection the apparently expanding nebula about Nova Persei, 

 photographed by Ritchey at Yerkes with the two-foot reflector 

 on September 20, 1901, and again on November 18. Four 

 well-marked condensations in these nebulous clouds had moved 

 one minute of arc in a month. If the distance of the nebula 

 was that of the nearer fixed stars, this angular displacement 

 would have indicated a velocity of about 2,800 miles per second. 

 The more probable view is that the nebula was already there 

 before the outburst in Nova Persei, and that the light waves 

 were reflected from the successive nebulous clouds, as sunlight 

 is reflected from successive ranges of mountains. There is no 

 doubt at all as to the connection of new stars with nebulae. 

 The question at issue is whether the nebula represents a real 

 expansion of the materials of the star, or whether, being already 

 present as a dark invisible mass, it is thrown into sympathetic 

 luminous vibrations by the tremendous outburst that takes 

 place in its midst. Nor, again, can there be any reasonable 

 doubt that the preliminary explosion, however caused, and 

 whatever may be the agency that carries the tenuous gases 

 into space, perhaps light-pressure, was a solar eruption on a 

 greatly magnified scale. 



Reverting again to the successive phases in the progressive 

 spectra of new stars, Lieut. -Col. F. J. M. Stratton, of the Solar 

 Physics Observatory, Cambridge, in a study of the spectra of 

 Nova Geminorum, has shown that the appearance of these 

 spectra was accompanied by a succession of jets or impulses, 



^ Loc. cit., p. 252, 



