Birth or Stars 7 



why the original form of the Nebular Hypothesis has 

 been abandoned. The four inner planets should have 

 possessed greater masses than the four outer ones, 

 which is very far from being the case. The residual 

 sun, the heart of the nebula, should be revolving on 

 its axis much more quickly than it actually does — 

 once in twenty-five days. And, again, the equatorial 

 plane of the sun ought, on Laplace's hypothesis, to 

 lie precisely in the mean plane of the planets' orbits, 

 whereas it is inclined seven degrees to such a mean 

 plane. Let these difficulties suffice. 



§3. Spiral Nebulce. 



The characteristic nebula observable in the heavens 

 to-day seems to be a whirling mass of gas with a cen- 

 tral nucleus and spirally-twisted ejected arms emerg- 

 ing symmetrically from opposite ends. Condensa- 

 tions in the arms of these whirling nebulae are stars, 

 as Dr. Jeans says, "in the process of birth." When 

 these condensations began to move as detached bodies, 

 vast clusters of stars may have arisen, great systems 

 like the Milky Way, each system born out of a gigan- 

 tic rotating nebula. "In the spiral nebula," Dr. Jeans 

 writes, "we are watching, not the birth of planets, 

 which Laplace attempted to explain by his nebular 

 hypothesis, but the birth of the stars themselves. The 

 process is, in its main outlines, identical with that 

 imagined by Laplace, but on a more stupendous 

 scale." 



It might be supposed that the separated stars, set 

 free from a giant nebula, like sparks from a Cath- 



