German Physicists Reach Milestone In Nuclear Fusion Energy Quest

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11. Beta decay takes care of that.

Usually, in a basic small stellar core (Sun sized or smaller), two hydrogen (simple protons) bang together and fall apart very quickly. Diproton (helium-2) is incredibly unstable. This happens most of the time, with no net energy absorbed or released.

Rarely, one of the protons decays to a neutron, and the diproton becomes deuterium. That's then free to fuse with another hydrogen atom to produce helium-3. Two helium-3 atoms will then fuse to form a helium-4, and release two hydrogen atoms again as well.

That happens often enough to maintain hydrostatic equilibrium of the star. If it didn't, pressures and temperatures would increase until enough diproton had the chance to decay and release enough energy to do so.

Larger stellar cores (just a tiny bit more massive than our Sun and upward) use the CNO (carbon, nitrogen, and oxygen) process primarily instead.