A star radiates energy into space because it is hot. The energy lost into interstellar space is replenished by fusion. When fusion stops, the lost energy is no longer replaced, and the temperature and pressure within the star drop. When the radius and masses of white dwarfs such as Sirius B were first computed, astronomers were flabbergasted. Stars with masses comparable to that of the Sun were scrunched down into a volume comparable to that of the Earth.
White dwarfs are compact objects supported by degeneracy pressure.
Thus, white dwarfs must be very dense compared to an everyday main sequence star. That's 3 tons per cubic centimeter. A teaspoonful of white dwarf stuff would be as massive as an elephant. Under the extreme conditions which prevail within a white dwarf, the laws of quantum mechanics become important. Subatomic particles do not always obey the same laws as large objects.