The new wave-mechanics of Schrödinger draws a very different picture of atomic interiors from the simpler theory of Bohr which it is rapidly superseding. Even the electron is something very different from the electron of Bohr’s old theory. It is the old-fashioned electron only when it is at an infinite distance from the nucleus. As it gradually approaches this nucleus, it undergoes a metamorphosis of a kind which no one has yet succeeded in describing, and it is utterly impossible to say what form it may have assumed by the time it is doing what we call “describing a _K_-ring orbit.” All we know about the _K_-ring orbit is its energy, and it seems impossible to predict the amount of space occupied by such an orbit until we have a better knowledge of the qualities of the article which is describing it. We have of course to admit that the physical evidence, such as it is, seems to point to _K_-ring atoms being substantially smaller than is needed for the liquid-star hypothesis. But the astronomical evidence seems to me stronger and more reliable, and to point in exactly the opposite direction. And here we must leave the puzzle until further pieces come to light. Until we know the kind of atoms of which a particular star is composed, we cannot calculate the extent to which they will be broken up by the temperature of the star’s interior. As a consequence, the theoretical curves of demarcation between stable and unstable configurations cannot be calculated without assuming definite atomic numbers for the stellar atoms. The curves shewn in fig. 24 have been drawn for an atomic number of about 95, this being slightly higher than the atomic number, 92, of uranium. This atomic number was selected because it was found to produce the best agreement between theory and observation, but we shall see that other considerations justify our choice. STELLAR STRUCTURE A star, like a house or a pile of sand, is a structure which would collapse under its own weight were it not that each layer is held up against gravity by the pressure which the next inner layer of the star exerts upon it. This pressure is not, like ordinary gas-pressure, the result of the impacts of complete molecules. It is produced in part by the impact of a certain number of atoms which have been stripped of electrons almost or quite down to their nuclei, but to a far greater extent by the impact of a hail of free electrons. In massive stars, an additional pressure is produced by the impact of radiation which, as we have seen, carries weight about with it, and so exerts pressure on any obstacle it encounters. The combined impacts of free electrons, of atoms (or bare nuclei), and of radiation prevent the star from falling in under its own gravitational attraction.