If stars can’t fuse iron, where do it and all other heavier elements come from?
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When stars go supernova and explode, the pressures and temperatures of the explosion itself are higher than what the star had when it was a regular star. The supernova explosion is very fast, but still in that brief moment atoms are squeezed with enough energy to go past iron.
Also stars, just bigger ones. The key is that they can only *produce energy* up until iron. But, if you have a ~~very large star exploding,~~ or two “smaller” (still very big) stars colliding together, you have a lot of energy there already. And that energy is used to explode and fuse into the elements heavier than iron.
Edit: looking up the topic, recent evidence has actually shown that the exploding star hypothesis doesn’t actually work out, it turns out that about half comes from the colliding neutron stars, and the other half comes from a slower process where neutrons hit stable nuclei, and then they beta decay into heavier elements.
You can fuse atoms (technically, atomic nuclei) and get a product that is heavier than iron. People do this in labs, which is how we create new elements. What you can’t do is fuse two atoms together to make an atom heavier than iron *and get fusion energy out of it*.
Fusion that gives off energy can be a self sustaining process. You fuse hydrogen into helium, which gives off energy, and you can use some of that energy to fuse more hydrogen into helium. This is what happens in the center of the Sun. Fusing heavier elements doesn’t give off energy, so it can’t be a self sustaining process. That doesn’t mean it can’t happen, but it’s hard to make it happen. You can’t have a star fusing iron into tellurium, because fusing iron doesn’t give off energy. The reaction would stop pretty quickly if you didn’t keep putting energy in to make it go.
If you’re willing to put a lot of energy into it and not get energy out, you can fuse pretty much whatever you want. People can do this with particle accelerators. When a star explodes and becomes a supernova, there’s a lot of energy in that explosion, some of which can go into fusing elements heavier than iron.
The fact that it’s really hard to create elements heavier than iron is why there’s so much more silicon and iron around than, say, gold or uranium.
They can fuse iron. But it consumes energy, instead of releasing energy. That means that the star can’t use iron fusion as fuel, so when the star tries to use it as fuel, it dies.
Along with what others have said, I’ll explain a bit more about two specific processes: the rapid (r) process and the slow (s) process.
First, just so we’re on the same page: the thing that defines what an element is is the number of protons. For example, hydrogen has 1 proton, helium has 2, etc. But there can be different numbers of neutrons for each element, and different numbers of neutrons are called isotopes. For example, hydrogen with 0 neutrons is just normal hydrogen, but if there’s 1 neutron, it’s called deuterium. Still hydrogen, just a different isotope.
Some isotopes are stable (like hydrogen). Some are unstable, like uranium. If they’re unstable, they’ll decay into other elements.
Ok. Imagine a square grid of all the isotopes of all the elements. The horizontal axis is the number of neutrons, and the vertical axis is the number of protons. So each row is a different element, so as you go up and down, you change element; as you go left and right within a row, you change isotope.
The r process occurs in stars when an atom captures a whole bunch of neutrons at once. (I.e., there’s a rapid capture of neutrons.) So on our grid, that corresponds to sliding way to the right. If the isotope you’ve arrived at is stable, we’re done, but we haven’t made a new element. But what if it’s unstable? Well, one of the neutrons will decay into a proton — that’s called beta decay. What does this decay look like on our grid? Well, we’re losing a neutron (moving left on the grid), and gaining a proton (moving up). So this process moves up and left on the grid. If this resulting element is stable, we’re done, and we’ve created a new element, because we’re in a new row in the grid. If it’s unstable, we can decay and move up and left on the grid again, creating a new element. This keeps going until we hit a stable element. For example, if we wanted to move 5 rows up, we could capture 5 neutrons (move right 5), then let them all decay into protons (move up and left 5).
The s process is the same, except instead of capturing a bunch of neutrons at once, an atom only captures one. Then decay happens, moving up grid. Then neutron capture. Then decay. You can zigzag your way up the grid in this way, a little at a time.
They can fuse it, it just *consumes energy* instead of producing it. And thats exactly what seems to happen. In the late stages of a star’s life, there is a lot of neutrons around due to some of the other fusion reactions, and over thousands of years, these neutrons can combine with iron and then it beta decays into cobalt, then it can happen with the cobalt, and so on.
Then there’s the neutron star fusion method, which *can’t* just be a supernova. There are not enough neutrons around. It has to be the combination of neutron stars, which does have the neutron density necessary.
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