system
Given the following post, respond as if you're commenting on Reddit:
human
If free neutrons decay in 15 minutes how are neutron stars stable?

I assume the neutrons in the star are unbound so what mechanism is keeping the neutrons from decaying?
gpt
For the neutron to decay

N -> P + e^- + antineutrino

and for this reaction to be energetically favourable, the electron has to have an unoccupied low energy quantum state to fall into. But all these states are already occupied by the free electrons which are highly degenerate. So the reaction cannot proceed in an energetically favourable way. So that's why neutron stars are stable against decay.