What results from the mass difference between starting hydrogen nuclei and resulting helium nucleus?

The immense energy output powering the star

The energy generation within stars, which sustains their luminosity and drives stellar evolution, is a direct consequence of mass conservation laws applied during fusion. When hydrogen nuclei fuse to form helium, the resulting helium nucleus has a slightly smaller mass than the sum of the original hydrogen nuclei that went into forming it. This minute loss of mass does not vanish; instead, it is converted into an enormous quantity of energy according to Einstein's mass-energy equivalence principle ($E=mc^2$). This massive energy output is what provides the outward pressure necessary to counteract the inward crushing force of gravity, allowing the star to maintain hydrostatic equilibrium and shine brightly over billions of years.