system
Given the following post, respond as if you're commenting on Reddit:
human
Where do neutrons come from? If the whole universe started as hydrogen which is just a proton and an electron, where did the neutrons come from when they fused into heavier elements?
gpt
After the Big Bang, the universe eventually cooled enough for quarks to form bound hadrons. Protons and neutrons both would've been produced as a result.

Protons are stable (as far as we know), but free neutrons decay with a half-life on the order of ten minutes. So any neutrons produced during this process would not have lasted very long.

Luckily neutrons can be produced in stars as well. What happens is that two protons fuse together, forming a diproton resonance (helium-2, if you wish). Since the diproton is unbound, the vast majority of the time, it will immediately decay back into two protons. But some small fraction of the time, it can beta decay instead, forming a deuteron (hydrogen-2). The deuteron is a bound system consisting of a proton and a neutron.