The process that fuels our sun and allows it to give off that much energy is called nuclear fusion. Nuclear fusion is a reaction where two atomic nuclei fuse together to create a larger nucleus and in the process release energy.

When deuterium atoms are heated to an extremely high temperature under extremely high pressure, then two deuterium nuclei combine together to form a heavy nucleus of helium, and a neutron is emitted. A tremendous amount of energy is liberated in the process.