When a star uses up its available fusion fuel, its mass can no longer be supported by internal gas pressure. While outer layers are blown away, the resulting collapsed core will result in either of a white dwarf, a neutron star, or a black hole, depending on its final mass.

If the core mass is less than about 1.4 times that of the Sun, the result is a white dwarf. It is supported by "electron degeneracy" pressure, a quantum mechanical concept based on the Pauli exclusion principle which restricts the number of electrons occupying the same energy state.

If the mass is greater than 1.4 times that of the Sun, electron degeneracy is not enough to overcome the force of gravity and a neutron star can result. A neutron star is basically a giant neutron, with all matter compressed to a maximum density allowed for matter. A single teaspoon of neutron matter would weigh billions of tons.