If the sun and stars are supported by gas pressure, what supports a white dwarf?

One type of pressure that results from the Pauli Exclusion Principle, a purely quantum mechanical effect, is called the degeneracy pressure.

I'll attempt to describe this degeneracy pressure, but doing so will require some basic knowledge of quantum mechanics.

The system can be modeled as fermionic particles in a box because a white dwarf can be thought of as a gravitational potential well that confines free electrons, which are known as fermions. In quantum mechanics, particles such as electrons are quantized, and the permitted discrete values of energies are arranged into energy states. Fermions are subject to the Pauli Exclusion Principle, which states that there can only be two electrons in an energy state (corresponding to two different spin states).

The quantum law that prohibits two fermions from occupying the same quantum state would manifest as a pressure known as the degeneracy pressure, which prevents any further gravitational compaction and supports a white dwarf against gravitational collapse, if we tried to pack electrons into lower energy states. Therefore, electrons cannot all go into lower energy states.

A white dwarf is supported by electron degeneracy pressure.