If stars can’t fuse iron, where do it and all other heavier elements come from?
In: 7
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.
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