degeneracy pressure

Degeneracy pressure is an outward pressure exerted by a dense gas of fermions that arises not from heat but from the Pauli exclusion principle. Because no two identical fermions may share a quantum state, squeezing them closer forces some of them into higher-energy states, since the low-lying ones are already taken. Those fast-moving fermions push back, and that resistance persists even when the gas has been cooled to almost absolute zero, where ordinary thermal pressure would have vanished.

The effect is purely quantum and has no classical counterpart. A classical gas exerts pressure only because its particles are jostling about with thermal energy; cool it down and the pressure fades toward nothing. A fermion gas refuses to be quiet: even at zero temperature the exclusion principle keeps the particles spread across a range of momenta, and that irreducible motion is a genuine, calculable pressure that grows steeply as the gas is compressed.

In astrophysics this pressure does heroic work. When a Sun-like star exhausts its fuel and can no longer burn against gravity, the electrons in its core become so crowded that their degeneracy pressure halts the collapse, leaving a white dwarf. In a more massive remnant the electrons merge with protons and it is the neutrons' degeneracy pressure that holds the line, giving a neutron star. Beyond a critical mass even this pressure is overwhelmed, and nothing known can stop the collapse into a black hole.