554 SCIENCE PROGRESS 



times more energy than would be liberated by their gravi- 

 tational contraction. 



It follows that there exists within the stars some unknown 

 source of energy of enormous magnitude which continually 

 supplies the heat lost by radiation. Russell points out that 

 the process of transformation of this energy must satisfy the 

 following conditions : 



(i) It must generate heat in the interiors of the stars, but 

 little or none in the earth or under laboratory conditions. 



(2) Since the stars are in general very stable, it must not 

 be liable to accelerate its rate so as to end in an explosive 

 catastrophe. 



(3) It must be regulated in such a way as to supply heat 

 to each star at almost exactly the rate at which the star radiates 

 heat to space, so ensuring that the rate of transformation is 

 slow, as we know to be the case from the slowness of stellar 

 evolution. 



(4) It must ultimately die down so that a stage will finally 

 result in which the radiation is very slow. 



(5) Since the " dwarf " stage of a star's life is very long, a 

 sufficient amount of energy must be available even in the 

 later stages to permit them to be of long duration. 



Russell points out that (1) indicates a source of energy 

 liberated only under the extreme conditions of temperature 

 or pressure prevailing in a star ; (4) suggests that the store 

 is limited, and that the rate of transformation falls off as the 

 remaining supply diminishes : complete exhaustion would be 

 approached gradually, thus satisfying (5). 



Conditions (2) and (3) can be satisfied as follows : the 

 star at first is a diffuse mass of gas at comparatively low tem- 

 perature. It contracts rapidly with loss of heat, but, accord- 

 ing to Lane's law, with rise of temperature, until at length the 

 unknown supply of energy begins to be liberated. The rate 

 of contraction then slows down ; but the rate of radiation of 

 energy, which Eddington has shown to be determined mainly 

 by the mass and opacity, scarcely changes. The temperature 

 continues to increase, and with it the rate of supply of energy 

 from the unknown source. A stage at length arrives at which 

 this rate balances the loss due to radiation, and the contrac- 

 tion is then checked. What then happens ? Will the tem- 

 perature go on increasing with the further supply until at 

 length there is an explosion ? Russell points out that this 

 cannot be so because the specific heat of a gaseous star is 

 negative. When heat is added to it, it expands and the tem- 

 perature falls. Thus the further supply of heat reduces the 

 temperature, and therefore the rate of supply from the un- 

 known source. On the other hand, if the supply begins to be 



