Process by which nuclear reactions between light elements form heavier ones, releasing huge amounts of energy. In 1939 Hans Bethe suggested that the energy output of the sun and other stars is a result of fusion reactions among hydrogen nuclei. In the early 1950s American scientists produced the hydrogen bomb by inducing fusion reactions in a mixture of the hydrogen isotopes deuterium and tritium, forming a heavier helium nucleus. Though fusion is common in the sun and other stars, it is difficult to produce artificially and is very difficult to control. If controlled nuclear fusion is achieved, it might provide an inexpensive energy source because the primary fuel, deuterium, can be extracted from ordinary water, and eight gallons of water could provide the energy equivalent to 2,500 gallons of gasoline.
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