1 8 THE EVOLUTION OF THE UNIVERSE 



that the spiral nebula are masses of glowing metal 

 being formed into worlds. They are nebulae in an 

 advanced stage of evolution. Other nebulae are 

 plainly proved by the spectroscope to be clouds of 

 billions of miles of glowing gas. At all events, our 

 "cloud" is not an imaginary thing at all. Turn it 

 into the Latin *' nebula," and there are about 200,000 

 of them known to us. 



Apart from a few recent speculations, which do not 

 seem likely to be accepted, the general belief is that 

 the stars were made by the gathering together into 

 solid globes of vast loose clouds of material in this 

 way. Whether the material was gas, or solid particles, 

 or both mixed together, is disputed ; and we need not 

 go into the point. ^ How far metals like radium add 

 to their heat is also disputed. These things must be 

 studied in larger books. The general truth is enough 

 here. The stars are evolved from great clouds of 

 loose matter by condensation. 



Sometimes you read in the papers that a "new 

 star" has appeared in the heavens. It is not thought 

 that this is the ordinary birth of stars. They grow, 

 and die down, too rapidly. The life of a star, 



^ Some may care to have a word on these disputes. There are 

 three chief theories. The Nebular Hypothesis conceives the cloud 

 as mainly of gas. The Msteoritic Hypothesis thinks the starting 

 point was an immense swarm of meteors. The Plantesimal Hypo- 

 thesis takes as its starting point a vast cloud of solid particles, and 

 these are supposed to have come from the ripping open (by another 

 star) of a dead sun. 



