iron peak
/ EYE-urn /
Stars build heavier and heavier elements by fusion, climbing a ladder from hydrogen to helium to carbon, oxygen, neon, silicon, and onward. But the ladder has a top rung. Around iron and its neighbors (nickel, cobalt), the climb stops paying for itself. The iron peak is that summit — the cluster of elements where nuclei are bound most tightly of all, and beyond which fusion can no longer release energy. It is the literal end of the line for a star's nuclear furnace.
The reason is the binding-energy-per-nucleon curve, which rises to a maximum right at the iron group. Iron-56 and nickel-62 are the most tightly bound nuclei in nature, the most stable possible arrangements of protons and neutrons. Fusing anything lighter toward iron releases energy; fusing iron into anything heavier costs energy. So when a massive star's core finally fuses its way to iron, there is simply no further reaction that can give back energy to keep the core hot and pressurized.
The iron peak is why the most massive stars die so violently. Through their lives such stars burn fuel in nested shells, building an inert iron core at the center. The moment that core grows too heavy to support itself, with no fusion left to prop it up, it collapses in less than a second — the catastrophe that drives a core-collapse supernova. The iron peak also explains why iron is so common in the universe and in your blood: it is the natural endpoint at which stellar fusion piles up, the ash of the cosmic fire.
A massive star can fuse silicon into iron in just a day or so near the end of its life — and then the music stops. With an iron core that cannot release energy by fusing, the star has run out of fuel in the deepest sense, and gravity takes over within seconds.
Reaching an iron core is the true end of fusion fuel, and the trigger for collapse.
Elements heavier than iron are not made by ordinary energy-releasing fusion — they form by neutron capture, mostly during and after supernovae and in neutron-star mergers. 'Iron is the end' means the end of energy-releasing fusion, not the end of element-building.