NEBULiE 125 



ship between the gases and the stars has yet to be 

 decided. 



In shape, nebulae vary : some are more or less 

 spherical ; some annular ; but the greater majority 

 are spiral, often with two spiral arms, and usually 

 with stars following the curves of the spiral. The 

 cause of the shape of nebulae, and especially the 

 cause of the spiral formation, is a question that 

 has been much debated. Arrhenius holds that 

 the nebulae are the result of collisions, and that 

 the shape depends on the character of the collision. 

 When direct face to face, an elliptic or more or 

 less spherical nebula would be produced ; while 

 what may be called a half-hall-cannon collision 

 would result in a rotating spiral nebula with two 

 arms — a shape frequently seen. " At the collision, 

 matter will have been ejected from both these 

 celestial bodies, at right angles to the relative direc- 

 tions of their motions, in two powerful torrents, 

 which would be situated in the plane in which the 

 two bodies were approaching each other." 



The double arm seen in most well-developed 

 spiral nebulae is explained in a somewhat similar 

 way by Chamberlin. He holds that such nebulae 

 are formed, not by actual colHsion, but by tidal 

 disruption. When two bodies in space, of stellar 

 size, pass within a certain distance of each other, 

 the smaller is apt to be not only tidally deformed 



