155, 156] 
Russell's two Theories 
171 
necessity of postulating these manifold types of matter rather destroys the 
simplicity of the main conception, with the result that the whole hypothesis 
begins to look somewhat artificial. If there are about as many types of matter 
as there are giant stars, the uniformity of central temperature on the main 
branch becomes meaningless and we might as well postulate a different type 
of matter for each central temperature on the main sequence as well. Indeed 
Russell’s own calculations compel us to do this. These shew that the central 
temperatures of main sequence stars are uniform if the stars are built after 
Eddington’s model, so that they would not be uniform for stars built on any 
other model. Russell’s scheme requires a star’s whole generation of energy 
to be concentrated at its centre, and calculation shews that main sequence 
stars, built on this model, would not have anything like uniform central 
temperatures. 
The highly penetrating radiation which has already been mentioned, 
presents a further and no less serious difficulty. Its penetrating powers though 
great are inadequate to carry it through more than an exceedingly small fraction 
of the radius of a star. In whatever bodies this radiation originates, it must 
originate so near to their surfaces that no great optical thickness of matter 
lies between the point of origin and outer space. Thus the temperature of the 
matter in which the radiation originates cannot be at all comparable with that 
of stellar centres ; something of the order of 50,000 degrees would appear to 
be an upper limit. In view of this, it is difficult to believe that the normal 
radiation of the stars cannot be generated at temperatures lower than about 
30,000,000 degrees. 
The main reason which compels the abandonment of this rather fascinating 
theory is, however, that mentioned first of all, namely, that stars functioning 
in the way imagined by the theory would be violently unstable. Their 
thermodynamical properties would be those of gunpowder at its flash point, 
and gunpowder heated to its flash point does not shine with the steady light 
of the stars; it explodes. 
156. The two theories of stellar evolution just considered both recognised 
that certain parts only of the Russell diagram are tenanted by stars, and both 
tried to interpret the tenanted parts as an evolutionary sequence; these parts 
were supposed to form a system of roads along which the stars march as they 
age. 
In the last chapter, however, we found this feature of the Russell diagram 
to be adequately explained by simple considerations of stability. Some of the 
possible equilibrium configurations for a star are unstable, some are stable. 
Those parts of the Russell diagram which represent unstable configurations 
are naturally untenanted by stars (or possibly are sparsely populated by stars 
which are not in stable equilibrium); those parts which represent stable con 
figurations alone ought to be occupied, and these have in actual fact been