“Most of You Was Made Inside a Star” follows the life of a big star. It fuses hydrogen into helium, helium into carbon and oxygen, and then climbs on, “through neon, silicon, and on / to iron, where it stops, for when / it fuses iron, the profit’s gone”. Then its core falls in, and it explodes. Behind that one word, profit, is a single curve that explains why stars shine, why they die, and why we can run power stations on uranium.
Start with what holds a nucleus together. An atom’s nucleus is a cluster of protons and neutrons, packed into a space a hundred thousand times smaller than the atom itself. Protons are all positively charged, so they repel each other fiercely; at that tiny distance, the electric push is enormous. What holds them together anyway is the strong nuclear force, which pulls every proton and neutron towards its immediate neighbours. It’s far stronger than the electric force, but it has a very short reach: it only works between particles that are practically touching.
Now think about building nuclei bigger and bigger. In a very small nucleus, like helium, a lot of the particles are on the surface, with few neighbours to grip, so each one is held less tightly than it could be. As you add particles, more of them are surrounded on all sides, and the average grip per particle increases. Each step up releases energy, the difference between how tightly the particles were held before and after. This is what powers a star: fusing small nuclei into bigger ones lets the particles settle into a tighter grip, and the energy released comes out, by Einstein’s E equals m c squared, as a small loss of mass.
But there’s a catch, and it’s the electric force. The strong force only reaches neighbours, but the electric repulsion reaches across the whole nucleus. Every proton pushes on every other proton. As a nucleus grows, the total repulsion grows faster than the strong force’s grip can keep up with. Somewhere, these two trends cross. Up to that point, adding particles makes each one more tightly bound. Beyond it, the growing repulsion starts to win, and each particle is held a little less tightly.
That point is iron, or very close to it. Plot the energy binding each particle against the size of the nucleus, and you get a curve that rises steeply from hydrogen, levels off, peaks around iron and nickel, and then slowly falls away towards uranium. Iron-56 sits right at the top: its particles are packed into the lowest-energy arrangement of any nucleus, so it has the least mass per particle of all.
For a star, that’s the end of the road. Every fusion step from hydrogen up to iron climbs the curve and releases energy, which heats the core and creates the pressure that holds the star up against its own crushing gravity. But fusing iron into anything heavier would move down the other side of the curve. It doesn’t release energy. It takes it in. The profit’s gone. A star with an iron core has run out of fuel in the most absolute sense possible.
What happens next is fast. A massive star’s iron core is about as massive as the Sun, compressed into a ball a few thousand kilometres across, held up by the pressure of its electrons. As fusion around it keeps adding more iron, the core reaches a mass where that pressure can no longer resist gravity. In less than a second, it collapses, falling inwards at up to a quarter of the speed of light, until it is crushed into a ball of neutrons about twenty kilometres across, as dense as an atomic nucleus. The infalling layers above hit this suddenly stiff core and rebound, and, driven by a flood of neutrinos from the collapse, the star blows itself apart in a supernova, flinging out the elements it spent millions of years making.
The same curve explains the other way to get nuclear energy. If you start with something very heavy, like uranium, far down the right-hand slope, and split it into two middle-sized nuclei, those pieces are closer to the peak and more tightly bound, so splitting releases energy. That’s fission, the process in a nuclear power station. Fusion climbs the curve from the left, fission climbs it from the right, and both are trying to get to iron.
So the iron in your blood, as the poem says, was forged before the Earth existed. But it’s more than a leftover. It’s the ash at the very bottom of nature’s furnace, the most settled arrangement of protons and neutrons there is, and the reason the star that made it had to die.