Nearly every atom heavier than helium was forged in a star — the calcium in your bones, the iron in your blood, the oxygen you breathe. Stars fuse light nuclei into heavier ones, but there's a wall they can't cross. Move the nucleus along the binding-energy curve and watch where fusion stops paying.
Stars spend most of their lives fusing hydrogen into helium. A Sun-like star goes one step further, fusing helium into carbon and oxygen, and stops there: its core never gets hot enough to go on. Only massive stars, more than about eight times the mass of the Sun, keep climbing the curve as their cores contract and heat up: oxygen burning makes silicon, and silicon burning makes iron. Each stage is shorter and hotter than the last. At iron-56 the curve peaks: iron has about the highest binding energy per nucleon, so fusing it costs energy instead of releasing it. A massive star builds an iron core that cannot burn, and when it collapses the rebound tears the star apart. Elements heavier than iron, such as uranium and gold, are not made by fusion: they are built by neutron capture in ageing giant stars, supernovae and neutron-star mergers, and scattered into space to seed the next generation of stars and planets.
Fusion builds elements up to iron, which has the highest binding energy per nucleon; heavier elements are built by neutron capture (in dying stars, supernovae and neutron-star mergers) because fusing past iron costs energy instead of releasing it.