For the first few minutes after the beginning, the whole universe was hot and dense enough to fuse. Protons and neutrons found each other and built nuclei, and for a short while the universe worked the way the center of a star does now.
It did not last. The universe was expanding and thinning the whole time, and within about twenty minutes it was too cool and too sparse for fusion to continue anywhere.
What came out of it was hydrogen and helium, and by mass those are close to 75 and 25 percent. Counted by nuclei rather than by mass, hydrogen is about 92 percent, since a helium nucleus weighs four times as much.
A trace of lithium came out too, and nothing else. There is no stable nucleus of mass 5 or mass 8, so the chain had no way past helium in the time available. Every carbon atom, every oxygen atom, every atom of iron came later, and came from stars.
A star burns hydrogen to helium in its center. When the center runs out, that region contracts and heats, and at high enough temperature the helium itself starts to burn, to carbon.
In a heavy star the sequence keeps going. Carbon burns to neon and magnesium, neon to oxygen, oxygen to silicon, silicon to iron. Each stage needs a higher temperature than the last, and each is over faster: hydrogen lasts millions of years, silicon lasts about a day.
The old shells do not stop when the center moves on. Each keeps burning at its own radius, so the star ends up with layers, heaviest at the center and lightest at the surface.
The stack stops at iron, and the reason is not that the star runs out of fuel. Iron simply will not give anything back.
Take any nucleus and ask how tightly it holds each of its nucleons. Plot that against mass number and the answer is a curve that climbs steeply at the light end, flattens, peaks, and falls away slowly.
Moving to a more tightly bound nucleus releases the difference as energy. That is where the energy of fusion comes from, and it is why fusion pays only while the curve is still climbing.
The peak is at iron, at a mass number of 56. Fusing anything into iron gives energy out. Fusing iron into anything heavier takes energy in, and so does splitting it.
This one fact ends the life of every heavy star. Once the core is iron, there is no reaction left that can hold the star up, and what happens next is the collapse described in the previous article.
If fusion cannot build past iron, something else must, since gold and uranium exist. The answer is the neutron. It has no charge, so nothing repels it, and a nucleus will take one in at any energy.
A nucleus that has taken in too many neutrons is unstable, and one of them turns into a proton. The atomic number goes up by one, and the element changes. Repeat, and the elements climb.
How fast the neutrons arrive decides what gets made. Inside a giant star they trickle in over thousands of years, slowly enough that each unstable nucleus decays before the next neutron comes. This is the slow process, and it builds up along the line of stable nuclei to about bismuth.
The other way needs a flood, so many neutrons in a second that a nucleus takes in dozens before it can decay. That happens when two neutron stars collide. This is the rapid process, and it is where gold, platinum and uranium come from. Two of them were seen colliding in 2017, and the gold made in that one event outweighed the earth.
Measure how much of each element the sun holds and plot it against atomic number. The result is not smooth, and every feature of it means something.
Hydrogen and helium tower over the rest, eight orders above what follows. Those are the leftovers from the beginning, and nothing since has made a dent in them.
Lithium, beryllium and boron form a deep pit. They are fragile enough to be destroyed inside stars rather than made there, and what exists is mostly made by cosmic rays breaking up heavier nuclei in space.
There is a bump at iron, and it is the peak of the binding curve showing through. Everything on this plot above helium was assembled inside a star and returned to space when the star ended. The material in the earth, and the iron in our blood, passed through that cycle before the sun existed.