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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1931 



These results may be compared with estimates arrived at in an en- 

 tirely independent way. The distance from the center seems to be 

 of the right order of magnitude. Thus Shapley from his work on 

 globular clusters located the center of the galaxy at 13,000 to 25,000 

 parsecs distance. The mass also, although higher than most current 

 estimates, is not unreasonably large. By extrapolating the results 

 of actual counts of stars. Scares and van Rhijn obtained a total of 

 30,000,000,000 stars in the galax}''. Since dark nebulae hide our 

 view, more especially in the direction of the center, it is doubtful 

 whether their survey comprehended the whole system, and the num- 

 ber may well be greater. The average mass of a star is probably 

 not more than half the mass of the sun, but there is in addition the 

 mass of the cosmic cloud and of the bright and dark nebulae to be 

 brought into account. 



How long does the galaxy take to make one complete revolution? 

 The answer is about 250 million years. We can state the figure 

 fairly definitely because it does not depend on any of the more 

 doubtful estimates; the only datum needed to determine it is the 

 magnitude of the Oort effect. It should, however, be added that 

 since the inner parts of the galaxy rotate faster than the outer 

 parts, there is no one period of revolution for the whole ; the period 

 250 million 3'ears refers to the zone in which the sun lies. It is 

 important to notice that the galaxy has made five or six rotations 

 within geological times. The sun and earth were away on the 

 far side of the center 100 million years ago — a time which geo- 

 logically does not seem to be very remote. 



We may now sum up the evidence for the hj-pothesis of a rotation 

 of the galaxy. An effect resembling differential rotation is observed 

 in all classes of distant stars and also in the cosmic cloud pervading 

 the system. These give consistent indications of the direction of the 

 center and the}' agree also as to the amount of differential rotation. 

 The evidence from proper motions has small weight, but for what 

 it is worth it supports that derived from spectroscopic radial veloc- 

 ities. The dimensions and total mass of the galactic system, inferred 

 from this effect, are reasonably consistent with current estimates 

 based on other data. Our large orbital velocity of 200-300 km per 

 sec. is confirmed to some extent by observations of globular clusters 



