Every fusion reaction in a star's core trades a tiny bit of mass for energy — climbing the binding energy curve toward iron-56, one of the most tightly bound nuclei there is. Below that peak, light nuclei release energy by fusing. Above it, heavy nuclei release energy by splitting. Change the mass number and watch where fusion stops being profitable.
The Sun fuses hydrogen into helium via the proton-proton chain: four protons collide over multiple steps to form one helium-4 nucleus, releasing 26.7 MeV per cycle. The core's 15 million kelvin temperature gives protons enough kinetic energy to overcome electrostatic repulsion, and immense gravitational pressure — 250 billion atmospheres — confines them long enough for the strong nuclear force to bind them. Each second, the Sun converts 600 million tonnes of hydrogen into helium, losing 4 million tonnes as pure energy. It's been doing this for 4.6 billion years and will continue for another 5 billion, then fuse helium into carbon and oxygen. That is as far up the binding energy curve as the Sun gets: it will end as a white dwarf. Only stars above about 8 times the mass of the Sun keep fusing all the way to an iron-56 core.
Fusion releases energy only when climbing toward iron-56 on the binding energy curve — beyond that peak, only fission pays out, which is why massive stars die when their cores turn to iron.