Fusion fuels are fuels to use in hypothetical Fusion power reactors. They include deuterium (H) and tritium (H) as well as helium-3 (He). Many other elements can be fused together, but the larger electrical charge of their nuclei means that much higher temperatures are required. Only the fusion of the lightest elements is seriously considered as a future energy source.  Fusion of the lightest atom, H hydrogen, as is done in the Sun and other stars, has also not been considered practical on Earth.  Although the energy density of fusion fuel is even higher than fission fuel, and fusion reactions sustained for a few minutes have been achieved, utilizing fusion fuel as a net energy source remains only a theoretical possibility.