system
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human
ELI5: If heavy elements are progressively created by nuclear fusion in stars, but those stars blow up once they start producing Iron, how are heavier elements produced then?

Saw on a documentary the other day that stars big enough that they start producing Fe from nuclear fusion go super nova, how is stuff like, say, Uranium, around then?
gpt
The answers here are incomplete, so here's a more complete breakdown of it.

Many stars go through a series of steps in their lifetime called the Main Sequence. This is what the life of the average star looks like.

At some point, depending upon the mass of the star, it will deviate from the Main Sequence. Stars that are anywhere from .2 solar masses on up will proceed down something called the Giant Branch, where they'll become red giant stars. After they reach the end of the Giant Branch, stars which are between .2 and 10 solar masses will then diverge into something called the Asymptotic Giant Branch (AGB), and this is where the magic happens for the production of elements.

There are two main processes by which element production occurs in AGB stars, called the s-process and r-process.

The s-process occurs throughout the time that the star is an AGB star, and stands for Slow Neutron Capture Process. AGB stars have a lot of neutrons flying about inside them, and elements will gobble them up in a process called neutron capture. Once these neutrons are inside the nucleus of the element, they will decay into protons, and its the number of protons is what determines what an element is.

The s-process is slow enough that it can only get elements up to a certain proton count before they become too unstable to survive until they can gobble up another neutron and ratchet up the periodic table any more.

That's where the r-process becomes important. This is the Rapid Neutron Capture Process, and this is what occurs when stars explode. When they explode, there are so many extra neutrons sleeting around inside them that they essentially force-feed neutrons into elements, making them absorb large numbers of neutrons rapidly before those neutrons decay into protons.

The r-process makes all the more unstable elements in the universe.