Why are white dwarfs so dense despite having masses similar to the Sun?

White dwarfs are exceptionally dense because they represent the compact, collapsed core of a star that has exhausted its nuclear fuel. While they possess a mass comparable to that of the Sun, their physical dimensions are reduced to a radius similar to that of Earth, resulting in average densities that can reach 1,000,000 times that of water.12

Unlike main-sequence stars, which are supported against gravitational collapse by the pressure generated from nuclear fusion, white dwarfs have no remaining nuclear energy sources. Instead, they are supported by degeneracy pressure—a quantum mechanical effect where the electrons in the star's interior exert an increased resistance to further compression. This state of matter, known as a degenerate gas, prevents the star from contracting further under its own gravity. Because of this unique equilibrium, white dwarfs follow a specific mass-radius relationship: the more massive the star, the smaller its radius, which further increases its density.123