PROBLEMS OF MODERN SCIENCE 



diameter of Betelgeuse to be 170 million miles, 

 which is nearly six times the diameter calculated 

 for our hypothetical star. We assumed, however, 

 that the absolute magnitude of our giant was i, 

 while that of Betelgeuse is 2. A recalculation 

 with this latter value makes the diameter 250 

 million miles. In any case there is now no doubt 

 that the stars may be divided into giants and 

 dwarfs, Betelgeuse being a typical giant, while 

 our sun is a typical dwarf. 



Proceeding from considerations of this kind, 

 Russell suggests a sequence of stellar evolution 

 which may be briefly sketched as follows. 



A star begins its existence, or at least begins to 

 be visible, in the condition of a red giant. Under 

 the action of gravitation it contracts, and in con- 

 sequence of this contraction its temperature rises 

 and it passes upwards through the spectral series 

 M, K, G, F, A. If massive enough, it may even 

 attain the B stage. During all this time the 

 temperature in the interior has been increasing 

 very rapidly, and Eddington has shown that the 

 pressure arising from thermal and light radiation, 

 though at first negligible, ultimately becomes 

 so great that it may overpower gravitational 

 attraction and cause the disruption of the star. 

 If the star is rotating, centrifugal force will assist 

 rupture, and in this way the production of binary 

 stars may be explained. After attaining its maxi- 

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