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. Thermonuclear Bombs Could thermonuclear reactions be made to take place on earth? The conditions that exist in the center of the sun would be extremely difficult to duplicate on the earth, so there was a natural search for any kind of nuclear fusion that would produce similar energies to those going on in the sun but which would be easier to bring about. There are 3 hydrogen isotopes known to exist. Ordinary hydrogen is almost entirely hydrogen-1, with a nucleus made up of a single proton. Small quantities of hydrogen-2 (deuterium) with a nucleus made up of a proton plus a neutron also exist and such atoms are perfectly stable. In 1934 Rutherford, along with the Australian physicist Marcus Laurence Elwin Oliphant (1901- ) and the Austrian chemist Paul Harteck (1902- ) sent hydrogen-2 nuclei flying into hydrogen-2 targets and formed hydrogen-3 (also called “tritium” from the Greek word for “third”) with a nucleus made up of a proton plus 2 neutrons. Hydrogen-3 is mildly radioactive. Hydrogen-2 fuses to helium more easily than hydrogen-1 does and, all things being equal, hydrogen-2 will do so at lower temperatures than hydrogen-1. Hydrogen-3 requires lower temperatures still. But even for hydrogen-3 it still takes millions of degrees. Hydrogen-3, although the easiest to be forced to undergo fusion, exists only in tiny quantities.