Nuclear Fusion

Two protons both carry positive charge, so they repel each other hard. To fuse, they need to get close enough for the strong nuclear force to take over, a distance far smaller than the reach of electric repulsion. The only way to force that approach is speed: heat the nuclei until their thermal kinetic energy is big enough to punch through most of the Coulomb barrier. That is why the Sun's core sits near 15 million kelvin. A higher nuclear charge means a bigger barrier, which is why fusing heavier elements demands even hotter, denser cores.

Inside the Sun, the proton-proton chain does the work. Two protons fuse into a deuteron, releasing a positron and a neutrino as one proton converts to a neutron. The deuteron then fuses with another proton to make helium-3, releasing a gamma ray. Finally two helium-3 nuclei fuse into helium-4, spitting out two leftover protons. The helium-4 product has less mass than the four original protons combined, and that missing mass becomes the energy radiated as sunlight, through E = mc^2.

This only pays off while nuclei are light. The binding-energy-per-nucleon curve climbs steeply from hydrogen up to iron, then slowly falls for heavier elements. Fusing light nuclei moves nucleons up that rising slope toward iron's peak, releasing energy. Fission of heavy nuclei moves nucleons up the falling side toward that same peak. Iron sits at the top, so fusing it further would mean sliding back down the curve, which costs energy instead of releasing it. That is why stars build elements up to iron and stop.