Aston’s studies in the 1920s had shown that it was the middle-sized nuclei that were most tightly packed. Energy would be given off if middle-sized nuclei were produced from either extreme. Not only would energy be formed by the breakup of particularly massive nuclei through fission, but also through the combination of small nuclei to form larger ones (“nuclear fusion”). In fact, from Aston’s studies it could be seen that, mass for mass, nuclear fusion would produce far more energy than nuclear fission. This was particularly true in the conversion of hydrogen to helium; that is, the conversion of the individual protons of 4 separate hydrogen nuclei into the 2-proton—2-neutron structure of the helium nucleus. A gram of hydrogen, undergoing fusion to helium, would deliver some fifteen times as much energy as a gram of uranium undergoing fission. As early as 1920, the English astronomer Arthur Stanley Eddington (1882-1944) had speculated that the sun’s energy might be derived from the interaction of subatomic particles. Some sort of nuclear reaction seemed, by then, to be the most reasonable way of accounting for the vast energies constantly being produced by the sun. The speculation became more plausible with each year. Eddington himself studied the structure of stars, and by 1926 had produced convincing theoretical reasons for supposing that the center of the sun was at enormous densities and temperatures. A temperature of some 15,000,000 to 20,000,000°C seemed to characterize the sun’s center. At such temperatures, atoms could not exist in earthly fashion. Held together by the sun’s strong gravitational field, they collided with such energy that all or almost all their electrons were stripped off, and little more than bare nuclei were left. These bare nuclei could approach each other much more closely than whole atoms could (which was why the center of the sun was so much more dense than earthly matter could be). The bare nuclei, smashing together at central-sun temperatures, could cling together and form more complex nuclei. Nuclear reactions brought about by such intense heat (millions of degrees) are called “thermonuclear reactions”. As the 1920s progressed further studies of the chemical structure of the sun showed it to be even richer in hydrogen than had been thought. In 1929 the American astronomer Henry Norris Russell (1877-1957) reported evidence that the sun was 60% hydrogen in volume. (Even this was too conservative; 80% is considered more nearly correct now.) If the sun’s energy were based on nuclear reactions at all, then it had to be the result of hydrogen fusion. Nothing else was present in sufficient quantity to be useful as a fuel.