The large quantity of energy released by the sun and the stars is the result of the conversion of matter into energy. This occurs when the lightest atom, hydrogen, is heated to very high temperatures forming a special gas called "plasma". In this plasma, hydrogen atoms combine, or "fuse", to form a heavier atom, helium. In the process of fusing, some of the hydrogen involved is converted directly into large amounts of energy.

 

A fusion reaction occurs when nuclei of light elements, specifically hydrogen and its isotopes (deuterium, or "heavy water," and tritium), are forced together at extremely high temperatures and densities until they fuse into nuclei of heavier elements and release enormous amounts of energy. If fusion is to yield net energy, the fuel must be heated in the form of plasma (a highly ionized gas) to a very high temperature and the plasma must then be held together for a sufficiently long time such that the number of fusion reactions occurring releases more energy than was required to heat the fuel.