DIAMETERS OF THE STARS — DANJON. 



173 



The following data shows the magnitudes, corresponding to the 

 maxima of the curves. 



The mean frequency of the giant stars of class G and that of the 

 dwarfs of the same type are separated by an interval of five magni- 

 tudes. Calculation easily shows that the former are therefore 1,000 

 times less dense. In class M the ratio of volumes amounts to 

 1,000,000. Greater values naturally are obtained by comparing the 

 greatest giant stars with the smallest dwarfs. Our conclusions of 

 the preceding section are strikingly confirmed. 



Let us consider a moment the cosmogony developed by Eussell. 

 He was the first to call attention to this separation of the stars into 

 dwarfs and giants. According to him, giant stars are young stars. 

 As they age they contract and grow warmer. The dwarfs are stars 

 already condensed and in the process of cooling off and becoming 

 dark. 



A star thus commences life as a giant, relatively cold. Gravitation 

 leads to its contraction, the energy due to the compression raising 

 the star's temperature. Thus in measure as its radiating surface 

 decreases, its intrinsic brightness increases in such a manner that its 

 apparent brightness remains practically constant. As another result 

 of the increasing temperature the spectrum type alters and the star 

 mounts from one class to another in the order, M, K, G, F, etc. 6 



The density of a star, however, tends to a limit and the contraction 

 becomes slower and slower. When the giant star enters the hot 

 classes A and B, its diameter will be scarcely greater than the sun's. 

 The converted gravitational energy then ceases to compensate for the 

 radiation losses. The temperature passes a maximum and commences 

 to decrease. The star becomes a dwarf whose diameter from then on 

 decreases almost insensibly and retrogrades down the series of spec- 

 trum classes in the order, B, A, K, M. During their evolution 



stars pass twice through each spectrum class, first as a giant and 

 finally as a dwarf. The second time its surface brightness decreases 

 rapidly. The star grows fainter and fainter and redder as it passes 

 to its final extinction. Thus Betelgeuse is a young star, Sirius has 

 attained a ripe age, and the sun is in its decline. 



8 It was thought evident before the data Just given was obtained that stars like 

 Betelgeuse and Aldebaran were in the process of cooling off and extinction. We see now 

 how risky such a priori conclusions are. 



