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ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 3 



with the stars thinning out gradually with increasing distance from 

 the sun, more rapidly of course in the direction perpendicular to the 

 plane of the Milky Way, On Kapteyn's system, the diameter along 

 the direction of the Milky Way, where the stars are only one-tenth as 

 thickly spaced as in the neighborhood of the sun, is about 18,000 

 light-years corresponding to Walkey's and Eddington's dimensions, 

 with a thickness of about 3,500. If, however, one goes farther out- 

 ward until the stars are only one-hundredth as thickly spaced as near 

 the sun, and hence very sparsely scattered indeed, the diameter would 

 be 55,000 and the thickness 11,000 light-years. 



FiGUEB 3. — The nearest stars. 



But as Kapteyn himself fully realized, all the earlier estimates 

 as well as his own referred to a simplified ideal watch-shaped system 

 of stars, thinning out gradually as you go outward from the sun, 

 which is taken as the center, while the real system is much more 

 complex. Even the most cursory glance at the Milky Way on a 

 clear, moonless night suffices to show that the stellar system has no 

 such regular distribution as that pictured above. It seems rather to 

 be formed of great aggregations of stars, star clouds as they are 

 usually called, separated by sparser regions arranged in the most 

 irregular way, and one is forced to realize, especially when one sees 



