Every atom heavier than helium was forged in a stellar furnace — the calcium in your bones, the iron in your blood, the oxygen you breathe. Stars fuse light elements into heavier ones, but there's a wall they can't cross. Drag the slider to build elements up the periodic table and watch where fusion stops paying dividends.
Stars spend most of their lives fusing hydrogen into helium, then climb the periodic table as their cores contract and heat up. Carbon, oxygen, neon, silicon — each fusion stage is shorter and hotter than the last. But at iron-56, the game ends. Iron has the highest binding energy per nucleon: fusing it costs energy instead of releasing it. A massive star builds an iron core that can't burn, and when it collapses, the rebound shockwave tears the star apart. That supernova forges elements heavier than iron in seconds — gold, uranium, platinum — scattering them 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 require supernova explosions because fusing past iron costs energy instead of releasing it.