This question already has an answer here:
Free neutrons, as we know them, are unstable and hence undergo a three-body decay into proton, electron and antineutrino with a mean lifetime of just under 15 minutes.
But there are astrophysical objects called neutron stars which are produced at the end of the lives of moderately massive stars. These objects are made entirely of neutrons in order to support the strong gravitational pulls. These are pretty stable objects in cosmological time scales lasting at least for billions of years.
So, why they are simply not decaying into proton stars due to unstable nature of neutrons? How to explain this apparent contradiction?