026 — IRON

Iron is the twenty-sixth element, and it is where fusion stops paying. Every step up from hydrogen releases energy because the product nucleus is more tightly bound than the fuel that made it. Somewhere in the mid-fifties of mass number that stops being true. Past the iron peak, fusing nuclei together costs more energy than it returns, so a star cannot afford the reaction — and the reaction was the only thing holding the core up against its own weight.

What a massive star actually manufactures in its last day is not iron. Silicon burning, at temperatures of a few billion kelvin, runs on alpha-particle captures and therefore lands on nickel-56: twenty-eight protons, twenty-eight neutrons, the alpha-conjugate nuclide at that mass. Nickel-56 is radioactive. It decays to cobalt-56 with a half-life of about 6.1 days, and cobalt-56 decays to iron-56 with a half-life of about 77 days. The iron in a supernova remnant, and the iron in your blood, arrived as nickel and became iron afterwards. That same decay chain is what keeps a type Ia supernova bright for months after the explosion has ended.

The iron peak is also slightly untidy, and the untidiness is worth knowing. Iron-56 is not, strictly, the most tightly bound nucleus there is: nickel-62 holds that record, at 8.7945 MeV of binding energy per nucleon against iron-56's 8.7903. Iron-56 wins a different contest — it has the lowest mass per nucleon of any nuclide, because a nucleus pays for every neutron with the neutron-proton mass difference. Both records sit within two units of each other, and both say the same thing: around mass 56 to 62, fusion has nothing left to give.

So the elements heavier than iron were never fused in a working star at all. They are assembled by neutron capture — slowly, over thousands of years, in the shells of dying red giants, or in seconds, in the collapse of a massive star and in the collision of two neutron stars. Copper, silver, gold, uranium: none of them came out of a star that was still running. Everything past 26 required a dying star.

Everything past 26 required a dying star.