CHAPTER XIX. 



THE PROBABLE ORIGIN OF NEBUL.E. 



Motion in space as a source of heat. — Reason why nebula? occupy so 

 much space. — Reason why nebulas are of such various shapes. — 

 Reason why nebula? emit such feeble light. — Heat and light of 

 nebulas cannot result from condensation. — The gaseous state the 

 first condition of a nebula. — Star-clusters. — Objections 

 considered. 



The object of the present chapter is to examine the 

 bearings of the modern science of energy on the ques- 

 tion of the origin of nebulae, and in particular to 

 consider the physical cause of the dispersion of matter 

 into stellar space in the nebulous form. In doing so, 

 I have studiously avoided the introduction of mere 

 hypotheses and principles not generally admitted by 

 physicists. These remarks may be necessary, as the 

 heading to the present chapter might otherwise lead 

 to the belief that it is on a speculative subject lying 

 outside the province of the physicist. 



The question of the origin of nebulae is simplified by 

 the theory, now generally received, that stars are suns 

 like our own, and that nebulae are in all probability 

 stars in process of formation. The problem will there- 

 fore be most readily attacked by considering, first, the 

 origin of our sun, as this orb, being the one most 

 accessible to us, is that with which we are best 

 acquainted. 



By the origin of the sun I do not, of course, mean 

 the origin of the matter constituting the sun — this 



