What force resists the collapse of a core remaining between 1.4 and 3 solar masses?

Neutron degeneracy pressure

When the core of a massive star collapses after fusing elements up to iron, if the core's mass falls within a specific range (approximately 1.4 to 3 solar masses), the collapse is powerful enough to overcome electron degeneracy pressure. This extreme pressure forces electrons to merge with protons, creating a ball composed almost entirely of neutrons. At this density, the neutrons themselves exert a tremendous outward resistance against further compression, known as neutron degeneracy pressure. This force is what halts the collapse, resulting in a compact object about the size of a city: a Neutron Star. If this pressure were insufficient, the object would continue collapsing towards a singularity.