RECENT ADVANCES IN SCIENCE 555 



insufficient, the star will contract, causing the temperature to 

 rise and the supply to be increased. Conditions are therefore 

 stable, and a practically steady state results with a very slow 

 rate of evolution, the star shrinking only as the unknown 

 source exhausts itself. This corresponds to the slow evolu- 

 tion of the star through the various spectral types in the 

 giant stage. In the dwarf stages, the temperature and rate 

 of radiation are both decreasing, and less and less energy is 

 required to maintain the steady state. 



This hypothesis offers an easy explanation of two astro- 

 physical facts which are otherwise difficult to account for. 

 One is the abrupt commencement of the spectral series with 

 the highly luminous giant stars of type M, and the remarkable 

 infrequency of the very red giants. This is explained if it is 

 supposed that the temperature is not sufficiently high for the 

 unknown source of energy to come into play until type M is 

 reached. The earlier stages would therefore be passed through 

 very quickly, and so few stars would at any time be found in 

 them. Subsequent stages might endure for millions of years. 

 The second fact is the maintenance of variability of the Cepheid 

 type. It is fairly generally accepted that the cause of the 

 variation is some form of pulsation in the star. Eddington 

 has shown that such pulsations would be rapidly damped out 

 by the leakage of heat from the hotter to the colder regions of 

 the star's interior. Russell's hypothesis would, however, 

 supply heat to the interior at the greatest rate where it is 

 hottest, and so would tend to make good the leakage. The 

 liberation of heat would then take place when the star was 

 smallest and hottest, and would be periodic. 



Prof. Russell does not speculate as to the nature of the 

 supposed source of energy, but it will be apparent that, if a 

 source could be found which would satisfy the conditions 

 postulated, we should be provided with a rational explanation 

 of the source of stellar energy which is not incompatible with 

 the accepted age of the sun. Prof. Eddington (Observatory, 

 42, 375, 1 91 9) has suggested, as a possible source, the destruc- 

 tion of electrons by mutual annihilation at high temperatures. 



The following are some of the more important papers re- 

 cently published : 



Repsold, J. A., Friedrich Wilhelm Bessel, Ast. Nach., 1919, Bd. 210, Nos. 



5027-8. 

 Wanach, B., Die Chandlersche und die Newcombsche Periode der Polbe- 



wegung, Zcntvalbureau der Intl. Erdmessung (Neue Folge der Veroff.), 



1919, No. 34. 

 Wilkens, A., Eine Methode der Bahnbestimmung fur alle Exzentrizitaten , 



Ast. Nach., 1919, Bd. 210, Nos. 5022-3. 



