THE STAKS AND NEBULAE 273 



material require to be mastered and disciplined. How- 

 ever, fortunately for science, we do not need to wait to 

 see one particular case through before arriving at con- 

 clusions, since there are many thousands of these curious 

 objects in the sky in different stages of deployment and 

 evolution, so that by putting two and two together we 

 can write the individual histories of all. Hitherto the 

 teaching has been, that the moment of momentum of any 

 segregated system cannot change, and that a nebula is 

 to all intents and purposes a system and amenable to the 

 rule. But that here, again, fact and theory do not gibe, 

 note these words of Kapetyn : 



The phenomenon of the increase of velocity with the evolu- 

 tional stage of the stars must give rise to speculation as to its 

 cause. The observational results contained in our table naturally 

 lead us to conclude that the matter from which the stars originate 

 must have little or no velocity. How is this possible under the 

 influence of the combined attraction of the rest of the system? Is 

 it not as if gravitation had no effect on the cosmical matter in its 

 primordial state? If this be so, as soon as matter changes from 

 this state to another in which gravity begins to act, or to act freely, 

 motion will arise, and it is evident that, as a rule, the motion must 

 be accelerated, at least during immense periods, so that the longer 

 the period lapsed since the birth of the stars the greater must be 

 their average velocity. 



It would take us too far afield to enter into a detailed 

 discussion of the numerous capricious forms of nebulae, 

 caprices that the very nature of their origin as hereinbe- 

 fore set forth sufficiently accounts for; and we shall 

 therefore confine this inquiry to the far more important 

 consideration of the origin and significance of spiral 

 nebulae, which obviously involve structural dynamics of 

 basic character. These peculiar nebulae so far outnumber 

 all others put together that the inference is unescapable 

 that they are the advanced stages of a cosmic process by 

 which all nebulae are being molded. They are simply sys- 

 tems of planetary bodies enshrouded in star-dust ex- 

 pelled by the explosion of their central sun or suns, which 

 they are gradually redding up by their gravitational suc- 

 tion while regularly rounding their orbits. To visualize 

 such a spiral nebula you need only imagine our sun to ex- 



