EVOLUTION OF THE UNIVERSE. 577 
weight than terrestrial atoms. Many considerations suggest that the 
stars produce their radiation by the annihilation of their substance. 
Perhaps the strongest, and certainly the most clear-cut, is that the stars 
which appear to be the youngest (such as binary stars describing close 
circular orbits) are statistically the most massive. It looks as though the 
stars lost a large part of their mass in the course of their lives, and this 
can only mean that a large number of their atoms, or a large part of these 
atoms, undergo annihilation. If so, our terrestrial atoms may well be a 
sort of indestructible ash, the relics of more complex atoms after all that 
can be annihilated has been—just as lead and helium are the relics of 
radio-active atoms of greater atomic weight. 
There is, however, an alternative possibility. If matter is capable 
of annihilation, the general principles of the quantum theory show that the 
rate of this annihilation must consist of two parts. The first is an 
annihilation which goes on steadily regardless of external conditions of 
temperature and pressure, just as radio-active disintegration does. The 
_ second is an annihilation which is incited by high temperature, and whose 
rate increases with an increase of temperature, Calculation shows that 
this will only come into play when the temperature begins to approach a 
million million degrees. 
Clearly, if the cosmic radiation we receive on earth proceeds from 
annihilation, it must be annihilation of the first kind; if it had been 
produced by annihilation of the second kind, and so at a very hot place, 
it would have all been absorbed by matter before it could climb down the 
very long temperature gradient and emerge into outer space. But it is 
possible that it is mainly radiation of this second kind which, after in- 
numerable absorptions and re-emissions, or repeated softenings, appears as 
the ordinary light of the sun and stars. Thus, many astronomers think 
that matter is annihilated in appreciable amount at high temperatures 
only, and that the annihilation at low temperatures is negligible. In 
their view, terrestrial atoms escape annihilation, not because they are 
specially indestructible, but because they are specially cool. 
Yet this view is confronted by many difficulties. The cosmic radiation 
cannot be produced by this high temperature annihilation, and yet this 
is of vast amount—comparable at least with the radiation of all the stars. 
Also, for the heat of the stars’ interiors to produce annihilation, their 
temperatures must approach a million million degrees. This requires 
what so far seems to be an impossibly high temperature-gradient inside 
the star. Also such high temperatures, and such a mode of generation 
of energy, would, so far as we can see, make the stars unstable, and indeed 
explosive, structures. The best simple analogy to such a structure 
would be a keg of gunpowder, with its centre raised to the flash-point of 
the powder. Nevertheless, it must be admitted that we are far from 
definite knowledge on these questions, which are still in the stage of very 
heated discussion. 
There is yet another possibility, which seems far more likely to-day 
than even a year or two ago. It is that stellar light and heat do not 
result from annihilation of matter at all, but from some less fundamental 
change in atomic structure. If so, unduly high temperatures are no longer 
needed at the centre of the stars; this difficulty disappears. And if the 
1931 Be 
