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The sun works by fusing hydrogen to helium (at the moment). While hydrogen is a single proton (I'm ignoring electrons here... not sure if they are relevant since it's probably a plasma anyway) helium consists of two protons and two neutrons.

To get fusion to work on earth we therefore use deuterium and tritium (getting us one extra neutron in the process, but it's easier to fuse them).

I don't think that the hydrogen in the core of the sun is mostly deuterium (or tritium for that matter). So where do the neutrons in the sun come from?

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At high enough temperatures (or in other words interaction energies) you can produce pretty much any particle. This is also the logic behind particle colliders that allow us to produce various other particles by colliding protons or electron-positron. This can explain how the seemingly non-existent neutrons you are referring to suddenly appear. To be exact the weak force allows neutron formation from proton only initial states.

The sun's surface temperature in natural units is $T_\odot = 0.46 \text{eV} \ll 1\text{MeV} \approx m_n - m_p$ which makes the production of neutrons or more importantly Deuterium pretty rare. However, through proton-proton chain interactions Deuterium can be formed. Once you have Deuterium you can fuse it to produce Helium.

This of course is just the tip of the iceberg, the full dynamics of the sun also include Tritium production amongst other processes.

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  • $\begingroup$ Thanks. Nicely explained for a simple mind like me. So it's quite a "messy" process if we look at all the radiation it produces. So much for "clean fusion energy"? $\endgroup$
    – kruemi
    Feb 3, 2022 at 11:12
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    $\begingroup$ Yes, in this case it is quite messy because it involves complex interactions to get sufficient energy. However, this shouldn't be interpreted as a bound on terrestrial production of clean fusion energy. If you can produce deuterium and tritium + give enough interaction energy you might be able to produce "pure" nuclear fusion. This is also just the beginning of making "clean" energy as the next step is finding an efficient way to store and/or capture the radiated energy from the fusion. $\endgroup$
    – Rotem Ovadia
    Feb 3, 2022 at 11:23

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