PROBLEMS OF MODERN SCIENCE 



relative to our system with very great velocities : 

 whereas the average speed of a star is of the order 

 of 20 miles per second, that of the nebulas is about 

 500 miles per second. In the second place, with 

 very few exceptions, they appear to be receding 

 from our system. 



Thirdly, in some of them temporary stars 

 have appeared, and the average maximum magni- 

 tude attained by these stars, when compared with 

 the average maximum attained by new stars known 

 to belong to our own system, indicates that the 

 spirals are a hundred times as distant as the Milky 

 Way, which is the region in which new stars almost 

 invariably appear. This estimate of distance agrees 

 with deductions made by Curtis from consideration 

 of proper motion and radial velocity. For the 

 spirals examined he obtains an average distance 

 of 10,000 light-years. 



An explanation of the spiral form has recently 

 been given by Jeans. He has studied the dynamics 

 of a rotating mass of gas, and has shown that under 

 certain conditions, depending upon density and 

 speed of rotation, it will disintegrate. A sharp 

 edge will be formed along the equator, and from 

 two opposite points on the edge matter will spill 

 out. Filaments will thus be ejected in a continuous 

 manner, forming spiral arms in the equatorial 

 plane of the nebula. 



This explanation makes the spiral formation 



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