Neutrons are indeed unstable when they are isolated outside of atomic nuclei. On their own, free neutrons have a half-life of about 14 minutes before they decay into a proton, an electron, and an antineutrino through a process known as beta decay. However, in the extreme conditions found within neutron stars, the stability of neutrons is supported by different factors.
Neutron stars are incredibly dense objects that form when a massive star undergoes a supernova explosion and its core collapses. The gravitational pressure in the core of a neutron star is so intense that it counteracts the decay process of individual neutrons. The immense gravitational force crushes protons and electrons together, converting protons into neutrons through a process called inverse beta decay or neutronization. As a result, the core of a neutron star is primarily composed of neutrons packed closely together.
The pressure generated by the enormous mass of the star prevents the neutrons from decaying into protons and stabilizes them. The exact mechanism that sustains this stability is complex and depends on various factors, including the equation of state for dense matter and the interplay of different particles and forces under extreme conditions.
It's important to note that while neutron stars are stable in the sense that their core maintains a predominantly neutron composition, they are not completely immune to changes. Neutron stars can undergo various dynamic processes, such as accretion of matter, mergers with other compact objects, or occasional starquakes, which can lead to different phenomena like X-ray bursts, gamma-ray bursts, or the formation of black holes.
In summary, while free neutrons outside of atomic nuclei are unstable and decay relatively quickly, the extreme gravitational pressures within neutron stars stabilize neutrons, preventing their decay and allowing the stars to exist in a stable state over long periods of time.