522 WRIGHT— RECENT SPECTROSCOPIC OBSERVATIONS. 



are intended to represent only in the most general way the two 

 classes or hypotheses which form part of the very speculative sub- 

 ject of stellar evolution, and are introduced to show at a glance the 

 especial interest which attaches to the relationship of the nobulae to 

 one or the other of the stellar groups. It may be said with regard 

 to the respective merits of the two hypotheses that the first has in 

 its favor a pretty straightforward sequence of spectral similarities, 

 with spectral evidence in the earlier groups of high temperature and 

 intense electrical excitement, which fades through the spectra in the 

 order indicated, while the second appears to receive support from 



Th:'v.i-9 0/ S;ili-i^ Enf/u/ion. 



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XX 



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Lockyer. 



Fig. 2. 



considerations of a general nature which indicate an exceptonally 

 low density for some of the red stars. 



It will be observed that both of these hypotheses assume the 

 nebulas as antecedent to the stars. 



In searching for a nebular origin there are four general classes 

 of objects among which it is necessary to distinguish: the spiral 

 nebulae, the extended amorphous nebulae with continuous spectra, 

 the extended gaseous nebulae, and the small gaseous, or planetary 

 nebulae. With respect to the spirals, it is doubtful whether they 

 belong to our stellar system, and in any event they are bodies of 

 such stupendous size and mass that they can not be regarded as 

 single stars in the process of formation. We know little of the 



