If we have been right (p. 149) in attributing the observed highly penetrating radiation in the earth’s atmosphere to the annihilation of matter in distant astronomical bodies, then the first hypothesis is confirmed. For the radiation could not retain its observed high penetrating power if it had already penetrated any great thickness of matter. The struggle of passing through matter lengthens the wave-length of all kinds of radiation (the quanta get weaker all the time), and so diminishes its penetrating power. Thus, wherever the highly penetrating radiation originated, it must have got out into empty space without much of a struggle, and this is the same thing as saying that it must have originated in matter at a comparatively low temperature. Thus the existence of the highly penetrating radiation proves that matter can be annihilated in great quantities at quite low temperatures; the high temperatures of stellar interiors are not needed, as Russell’s theory asserts, for annihilation to occur. A simple calculation shews that there can be no appreciable annihilation of the earth’s substance. In the sun about one atom in every 10¹⁹ is annihilated every minute; if even a ten-thousandth part as many atoms as this were annihilated in the earth, its surface would be too hot for human habitation. We can no longer explain this by saying that the sun is hot and the earth cool, so that annihilation goes on in the former but not in the latter. We must rather suppose that the atoms in the sun are of a different type from those on earth. Solar atoms spontaneously annihilate themselves, terrestrial atoms do not, or at least do not to any appreciable extent. The second of our hypotheses, that the central regions of the stars are more like a liquid than a gas, is, as we have seen, a necessary consequence of the first, but independent evidence in its favour is found in the formation of binary systems by fission. For in Chapter IV (p. 222) we saw that a star could only break up by fission if it had a liquid, or a nearly liquid, centre. THE STABILITY OF STELLAR STRUCTURES For the present, let us tentatively accept the hypothesis that the generation of stellar energy occurs spontaneously, like the disintegration of radio-active atoms. The atoms which are responsible for the light and heat of the stars may be regarded as super-radio-active atoms which spontaneously annihilate themselves and so change their substance into radiation.