What physical force halts the collapse to support the formation of a neutron star?
Neutron degeneracy pressure.
When a massive star's core collapses, gravity attempts to crush the matter down indefinitely. If the resulting remnant core mass is less than a critical upper threshold (estimated around three solar masses), the collapse stops when the density becomes so extreme that the constituent neutrons begin to resist further compression. This resistance is not thermal pressure, as fusion has ceased, but a quantum mechanical effect where neutrons refuse to occupy the same quantum state. This quantum mechanical resistance is termed neutron degeneracy pressure. This force is powerful enough to balance the crushing gravity, creating a stable, hyper-dense stellar remnant known as a neutron star, where a small volume of material achieves densities unimaginable in normal physics.
