^6 Nebulce and Nebular Hypotheses. [Sess. 



eye, and even to telescopic vision, the stars in this cluster 

 are perfectly normal ; but when photographed w^ith an expo- 

 sure of ten or twelve hours, they are seen to be literally bathed 

 in a mass of filmy, nebulous drapery, which winds about them, 

 stretching from star to star, and reaches out in wisps and 

 streamers far into space. To employ a chemical analogy, the 

 stars seem as if they had just been precipitated from the 

 surrounding turbid medium; but it may be supposed that 

 when they have finally absorbed their glimmering trains they 

 will shine clear-cut in keen relief upon an absolutely black 

 sky. 



You will observe that in this process of stellar evolution 

 the prime agency at work is gravity, — always drawing the 

 tenuous nebulous material inwards towards a centre, and con- 

 densing it to form stars. The process is essentially one of 

 attraction. But of late we have come to realise that there 

 are also repulsive forces in nature which, under certain 

 circumstances, may overcome gravity altogether. Matter is 

 subject to pushes as well as to pulls, and gravity has not 

 acted alone in bringing about the present state of affairs. 

 The recognition of this fact has led astronomers to believe 

 that much of the nebulous material scattered broadcast over 

 the heavens is not the aboriginal fire-mist out of which the 

 stars are made, but is composed instead of matter which has 

 been expelled from the stars. The shapes of many nebulae do 

 not suggest that they are contracting and condensing, but 

 rather that they are dominated by forces which rend and 

 perforate, — which tear matter into shreds and huddle it into 

 wisps. They are branching and straggling, and have the 

 appearance of tattered remnants rather than of matter newly 

 created. 



The idea that such nebulae are ejecta from stars is strength- 

 ened by the fact that so many of them contain helium ; for 

 helium, as I have mentioned, is not a primitive element, but 

 is one of the last products of Nature's laboratory, and results 

 from the disintegration of another element — viz., radium. 

 However that may be, it is certain that in two classes of 

 nebulae, at any rate, we find indisputable evidence of the 

 action of eruptive forces. One is the planetary class, in 

 which, as I have said, we seem to have the result of successive 



