1 907-1908.] Nebulce and Nebular Hypotheses. 77 



outflows of matter from a centre ; and the other is the spiral 

 class — the most numerous of all. A close examination of 

 their photographs suggests very strongly that such nebulse are 

 a species of sidereal wreckage which is now developing into a 

 new system. For they are never simple watch-spring spirals, 

 but are invariably two-branched. They consist of a nucleus, 

 generally elongated, from opposite ends of which issue a pair 

 of nebulous arms which enfold the nucleus in double con- 

 volutions. This arrangement is quite irreconcilable with the 

 hypothesis that the spiral can have been produced by the 

 action of gravity ; but it affords powerful presumptive evidence 

 that this type of nebula exhibits the effects of an explosive 

 projection of matter from the nucleus. 



The cause of such a disruption may conceivably be in- 

 ternal, — analogous to the forces which compel eruptive 

 outbursts on the sun to take place at opposite ends of a 

 solar diameter. But it is more likely to be external, and 

 due to the tidal strains produced in a star by the near 

 approach of another star. For all the stars are in rapid 

 motion through space, and it seems certain that sooner or 

 later every star must pass dangerously near to another star. 

 Actual collisions may sometimes occur, but a near approach 

 will be much more frequent. Our own sun-star, for example, 

 is travelling towards a certain bright star in the sky at a 

 speed of 15 miles per second. That star is many hundreds 

 of thousands of millions of miles away — but at 15 miles per 

 second even hundreds of thousands of millions of miles will 

 be covered sometime. Moreover, our sun is a sluggard among 

 the stars, — his 1 5 miles per second is almost a " dead march," 

 and certainly no more than a leisurely saunter, in comparison 

 with the terrific velocities of the stars in general. Hence the 

 probability of close approach is greater in their case than in 

 his. Now consider what the result of such an approach would 

 be, and remember that the stars are huge globes composed 

 solely of gases and incandescent vapours, and having nothing 

 solid about them at all. Each star would raise great tides in 

 the other, — one tide on the side next the approaching star, 

 and one on the opposite side, in exactly the same manner as 

 the moon raises tides on opposite sides of our earth. Each 

 star would thus become elongated, and as they approached 



