At such high temperatures two nuclei may occasionally approach each other in spite of the electrical repulsion between them. Sometimes they may even get close enough to each other to undergo a nuclear reaction. This, of course, happens with least difficulty if the nuclear charge is small. Hydrogen nuclei, which carry charge 1, participate in such reactions most easily. In the interior of stars temperatures range from about 10 to 100 million degrees, and nuclear reactions do occur. The reaction responsible for the production of energy in the stars is: 4H¹ → He⁴ + energy Four protons combine to make an alpha particle with a release of energy. Actually this reaction does not take place all at once but several steps are required. That energy should be released, one expects from the fact that the alpha particle is very stable. Any process in which light nuclei combine to form a heavier nucleus with a release of energy is known as “fusion.” The particular fusion process that goes on in the stars releases its energy in many forms: as positrons, neutrinos, electromagnetic radiation, and motion of the reacting particles. The positrons also carry off the excess charge of the reaction. The neutrinos fly through the star without interacting, carrying their energy away into outer space, probably never again to make contact with the material universe. The remainder of the fusion energy is deposited within the star’s interior, which is thus kept hot enough so that the fusion reaction can keep going. The name “thermonuclear” is appropriately applied to this type of reaction. A lot of effort and imagination is being devoted to the problem of making a controlled thermonuclear reaction. The motivation for this project comes from the fact that good thermonuclear fuels, such as deuterium (H²), are abundant and cheap. There is enough deuterium in the oceans of the world to supply man’s energy needs for many millions of years. One difficulty, of course, is to find a container for the reaction. Even under stellar conditions the rate of fusion reactions is not very great. It takes approximately a billion years for only one per cent of the nuclei to react. Consequently even higher temperatures than those found in stars are required to produce large amounts of energy in a short time. But no known materials can withstand temperatures of more than a few thousand degrees centigrade. One idea is to keep the “burning” fuel away from material walls by means of magnetic fields.