# Proton + Electron = Neutron?

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1. May 15, 2015

### Stephanus

Dear PF Forum,
I have a question to ask.
Supernovae produce neutron star (or Black Hole).

This is what I summarize from wikipedia.
1. Is P + e = N? Is it that simple?
Judging by its mass, altough slightly off.
2. Is Up Quark + e = Down Quark?

Thanks for any answer

2. May 15, 2015

### Staff: Mentor

No. As you note, p = uud and n = udd, in terms of quarks.

No. Up and down quarks are both fundamental particles, not composed of other particles, as far as we know.

3. May 15, 2015

### Stephanus

So, what makes neutron star?
Matter is we know it, consist atom. Proton, Neutron, Electron and I read somewhere, there are abundant neutrinos, too. Altough their combined mass are still much less than combined baryon mass in the universe.
So, if there are Proton, Neutron and Electron, and the result is Neutron star, what makes it?

Thanks.

4. May 16, 2015

### Orodruin

Staff Emeritus
Mostly neutrons ...

That particles are fundamental does not mean that they cannot be converted into each other. An up quark and an electron may be converted into a down quark and a neutrino by exchanging a W boson. This is what happens in beta decay and when a neutron star is formed (many many many times of course).

5. May 16, 2015

### Stephanus

As I know it. The process of producing neutron star is supernova, which happens in 1 seconds. How long is "beta decay"?

Thanks.

6. May 16, 2015

### ChrisVer

The exact name of the interaction would be "electron capture" or one of the inverse beta decays. But still time of interaction won't tell you much. An energy approach can help. In particular the interaction is:
$p + e^- \rightarrow n + \nu_e -0.8~MeV$
This becomes energetically favorable around at the energies/momenta of the electrons when the white dwarf becomes gravitationally unstable to collapse. And by the end of the collapse, the protons in the core have been converted to neutrons. Neutrons then can't decay back because there is an electron/proton atmosphere in a degenerate state and so the resulting electrons/protons from a neutron beta decay can't find any state to exist in (Pauli-block due to Pauli exclusion principle)

Last edited: May 16, 2015