40 The Outline of Science 



!y indefinitely. A eold luminosity due to electrification, like 

 that of the aurora horealis, would seem to fit the case better. 



\ <.v the nebular theory is that out of great "fire-mists," 

 Mich as we have described, stars are born. We do not know 

 whether gravitation is the only or even the main force at work 

 in a nebula, but it is supposed that under the action of gravity 

 the far-flung "fire-mists" would begin to condense round centres 

 of greatest density, heat being evolved in the process. Of course 

 the condensation would be enormously slow, although the sudden 

 irruption of a swarm of meteors or some solid body might hasten 

 matters greatly by providing large, ready-made centres of con- 

 densation. 



Spiral Nebulae 



It is then supposed that the contracting mass of gas would 

 begin to rotate and to throw off gigantic streamers, which would 

 in their turn form centres of condensation. The whole structure 

 would thus form a spiral, having a dense region at its centre and 

 knots or lumps of condensed matter along its spiral arms. Be- 

 sides the formless gaseous nebula? there are hundreds of thou- 

 sands of "spiral" nebula? such as we have just mentioned in the 

 heavens. They are at all stages of development, and they are 

 O>le to us at all angles that is to say, some of them face 

 directly towards us, others are edge on, and some are in inter- 

 mediate positions. It appears, therefore, that we have here a 

 striking confirmation of the nebular hypothesis. But we must 

 not go so fast. There is much controversy as to the nature of 

 these spiral nebula.-. Some eminent astronomers think they are 

 other stellar universes, comparable in size with our own. In any 

 case tl , ast structures, and if they represent stars in pro- 



I of condensation, they must be giving birth to huge agglomer- 

 ris of stars to star clusters at least. These vast and enigmatic 

 objects do not throw much light on the origin of our own 

 solar system. The nebular hypothesis, which was invented by 



