eli5 where does the neutron come from when creating helium in fusion?

Question

I keep seeing things about how protons turn into neutrons but that makes no sense to me. How can the quarks of a proton seemingly change and turn into a neutron? I have a project and I need to know this for both neutron stars and nuclear fusion

In: 3

Answer 1

if we are thinking about the *sun*, the answer is “complicated physics stuff happens”.

if you want more detail on fusion in stars, you should check out the somewhat-amusingly-named [pp chain](https://en.wikipedia.org/wiki/Proton%E2%80%93proton_chain). the answer to your question specifically involves the weak interaction. in the first step of the chain, for example, two protons fuse together. a neutrino and a positron are emitted. the weak interaction creates the neutrino, and it causes one of the *quarks* to change.

aside: protons and neutrons are made of quarks. protons comprise two up quarks and a down quark, while neutrons comprise two down quarks and an up quark. the weak interaction here causes an up quark to change to a down quark. this is the weird physics stuff, and I don’t have an explanation for how the quark itself changes beyond this.

the positron then carries away the extra positive charge that was lost from one of the protons changing to a neutron

in non-sun environments there are different ways fusion can happen, but this is how stars do it