SCIENCE 



[N. S. Vol. XLV. No. 1169 

 Object 



e Oi'ionis 



Canis Majoris 



a Carina; 



a Orionis 



Fig. 29. Types of Stellar Spectra, Henry Draper Memorial, Harvard College Observatory. 



the evolution of planetary nebulte into stars 

 at their centers, with possible planets re- 

 volving around them, we must not conclude 

 that all stars have been formed from plane- 

 tary nebulffi. There are reasons for reject- 

 ing that view. 



1. Amongst the manj' millions of stars 

 whose images have been examined in the 

 telescopes or on photographic plates, fewer 

 than 150 planetary nebulfe have been found. 

 Unless the planetary-nebula stage of exist- 

 ence is lived very rapidly, the numbers are 

 too few to play a controlling part in the 



development of stars in general at any 

 point in stellar evolution. 



2. The average speeds of the planetary 

 nebulffi and of the different classes of stars 

 are now fairly well defined. The average 

 speed of the planetary nebulaj is about 

 seven times that of the extremely blue stars, 

 which are the only ones, we shall see later, 

 that we need consider as the immediate 

 descendants of the nebulas. There are in- 

 deed individual stars which are traveling 

 as rapidly as the individual planetary neb- 

 ulae, but on the average the discrepancy 



