The nuclear fusion reaction is the source of energy in a star. In nuclear fusion reactions, the nuclei of two atoms combine to create a new atom with the release of a lot of energy. The nuclear fusion reaction takes place in the core of the star where two hydrogen atoms fuse to become one helium atom.
²₁H + ³₁H --> ⁴₂He + ¹₀n + 17.59 MeV
In the fusion reaction above deuterium fuses with tritium to form helium and releases ~18 mega electron volt of energy.