ASTRONOMY 



a consequence of rotation, and rotation has 

 actually been observed in a few nebulae. For 

 example, by comparison of photographs taken at 

 different periods, Von Maanen has found that the 

 matter at a distance of five minutes of arc from the 

 centre of a nebula in Ursa Major would make a 

 revolution round the nucleus in 85,000 years, 

 and that there is also an outward motion of the 

 matter along the spirals. 



Photographs of other nebulae which are seen 

 sideways show nuclei with sharp equatorial edges, 

 such as Jeans' theory demands. 



If we attempt to make an estimate of the 

 amount of matter in any one of these nebulae, 

 basing our calculations upon the observed angular 

 dimensions, and making reasonable assumptions 

 regarding their distances from us, we find that 

 there is sufficient matter to construct many thou- 

 sands, even millions of stars, equal in mass to 

 our sun. 



It seems therefore not at all improbable that 

 these nebulae are the stuff out of which universes 

 of stars are made. The actual steps in the process 

 can only be dimly guessed. All the appearances 

 suggest that gravitation breaks up the spiral arms 

 ejected from the rotating central nucleus into 

 comparatively short portions which ultimately con- 

 dense into stars. Stars so formed would continue 

 to rotate about the centre of the nebula in a plane 



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