(!) At temperatures of a few million degrees, certain light nuclei fuse, for example:
$_1^2H$ $+$ $_1^3H$ $\longrightarrow$ $_2^4He$ $+$ $n^o$
Deuterium and tritium → helium and neutron
Deuterium and tritium are both isotopes of hydrogen. Physicists have given them special names (which is not the rule for isotopes) because their masses are very different. The energy produced by these reactions is much greater than that released by nuclear fusion.
The sun is a huge fusion reactor which radiates its energy.
In hydrogen bombs (H-bombs), fusion is triggered by the explosion of fission bombs (A-bombs) that produce the required temperature exploding.
Physicists have already managed to create ministars with the size of a car in the reactor "tokamak" where they created temperatures of 23 million degrees. But the time has not yet come when the energy drawn from it exceeds the energy invested therein.