ASTRONOMY 



of light emitted per square mile of surface must 

 also be equal. Since the difference in magnitude 

 between the two groups is on the average 9 units, 

 it can be shown that the brighter stars must have 

 surface areas 4,000 times greater than the fainter 

 ones, or that their diameters are in the ratio of 

 64 to i. 



It has been found by examination of binary 

 stars that the masses of the stars do not differ very 

 greatly from that of the sun, the ratio in the case 

 of the majority ranging from three to one-third. 

 Suppose then that a faint M star has a mass equal 

 to that of the sun. Being cooler, it will be some- 

 what denser, and we may therefore take its diameter 

 to be about 700,000 miles. The diameter of a 

 bright M star of equal mass would then be over 

 30 million miles, so that it is a veritable giant. 



Russell therefore propounded the theory that 

 the members of the upper group of bright stars 

 are giants, while those in the lower one are dwarfs 

 by comparison. Within the last two months a 

 brilliant verification of this theory has been ob- 

 tained. On December 13, 1920, the diameter 

 of a Orionis (Betelgeuse) was measured at the 

 Mount Wilson observatory by means of an inter- 

 ferometer attached to the loo-inch reflector, and 

 was found to be one twenty-second part of a 

 second of arc. The distance is uncertain, but 

 assuming it to be 130 light-years, we find the 



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