As it happens, though, nuclear fission is not the only route to useful
nuclear energy.
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.
More and more was learned about the exact manner in which nuclei
interacted and about the quantity of energy given off in particular
nuclear reactions. It became possible to calculate what might be going
on inside the sun by considering the densities and temperatures present,
the kind and number of different nuclei available, and the quantity of
energy that must be produced. In 1938 the German-American physicist Hans
Albrecht Bethe (1906- ) and the German astronomer Carl Friedrich von
Weizsäcker (1912- ) independently worked out the possible reactions,
and hydrogen fusion was shown to be a thoroughly practical way of
keeping the sun going.
Thanks to the high rate of energy production by thermonuclear reactions
and to the vast quantity of hydrogen in the sun, not only has it been
possible for the sun to have been radiating energy for the last
5,000,000,000 years or so, but it will continue to radiate energy in the
present fashion for at least 5,000,000,000 years into the future.
[Illustration: _Hans Bethe_]
Even so, the sheer quantity of what is going on in the sun is staggering
in earthly terms. In the sun 650,000,000 tons of hydrogen are converted
into helium every second, and in the process each second sees the
disappearance of 4,600,000 tons of mass.