What fundamental quantum mechanical principle provides the ultimate resistance against complete gravitational collapse in a Neutron Star?

The Pauli Exclusion Principle enforcing neutron degeneracy pressure

The process of gravitational collapse in the stellar core is halted only when the inward crush of gravity is successfully counteracted by an outward pressure. In a neutron star, this resistance comes from the neutrons themselves, which are packed so tightly that they are essentially touching. This state is governed by quantum mechanics, specifically the Pauli Exclusion Principle. This principle dictates that two identical fermions, such as neutrons, cannot occupy the exact same quantum state simultaneously. This inherent resistance manifests as neutron degeneracy pressure, which provides the final, necessary support mechanism to keep the star from collapsing further into a black hole, provided the core mass remains below the critical upper limit.