CONSTITUTION OF THE STARS RUSSELL 153 



dington puts it, we would expect " something to happen " in this 

 critical interval, and " what happens is the stars." It is only when 

 the radiation pressure and the gas pressure share the gravitational 

 food that we get anything that can fairly be called a star. Smaller 

 masses do not give out light enough to make them visible at inter- 

 stellar distances, while the great ones, in which the radiation pres- 

 sure is almost sufficient to counteract gravitation, would be in an 

 almost unstable condition, so that a small disturbance, such as might 

 be produced by a moderate rotation, would cause them to break up 

 into parts. Hence smaller masses do not shine, and bigger ones 

 break up, and only those in the critical intervening range of mass 

 remain as luminous stars. We have seen that this should occur for 

 masses comparable with those of spheres 33 and 34 of the series. 

 Now the first of these is of half the mass of the sun, and the second 

 has five times the sun's mass, so that the actual masses of the stars 

 fall very exactly into the range indicated by the theory. Since the 

 constants of this theory are derived from those which are the most 

 fundamental in modern physics, we may truthfully say that the 

 masses, and hence the sizes and brightness of the stars are deter- 

 mined directly by the fundamental properties of the very atoms 

 of which they are composed. It may be shown, for example, from 

 Eddington's equation, that a mass of gas will shine as a giant star 

 when, and only when, the ratio of the diameter of the star to the 

 average distance between the atoms which compose it is about 20 

 times -the ratio between the charge of an electron and the average 

 mass of an atom (provided that this mass is measured, not in the 

 ordinary way, but, as in the electrical case, by its power of attract- 

 ing a similar mass at a given distance). The latter ratio is very 

 large, about 4X10 17 , so that the number of atoms in the star is enor- 

 mous, and the star itself a very large mass. 



One of the most impressive consequences of the whole theory is 

 that the masses of the stars are determined by the interplay of 

 the two forces, gravitation and radiation pressure, which, among all 

 those in nature, are so feeble, under the conditions of ordinary ex- 

 periment, that it taxes the skill of the experimenter to build an 

 apparatus delicate enough to measure the effects of either one. Were 

 we confined to experiments in enclosed laboratories, isolated in space, 

 without the earth's attraction to prove to us the existence of gravi- 

 tation, it would probably have been long before the very existence 

 of either of these forces would have been suspected ; yet these forces, 

 and these alone, when working on the grand scale, are powerful 

 enough to shape the stars. 



One question still remains. How long a time is required for 

 this sequence of evolutionary changes? What is the life of a star? 



