A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter with a density of about 2×109 kg/m3. They are thought to be the final evolutionary state of stars that become red giants and whose mass is not high enough to become a neutron star or black hole. Over billions of years, a white dwarf will cool and it will become a cold black dwarf.
Subrahmanyan Chandrasekhar discovered in 1931 that non-rotating white dwarfs have a maximum mass of 1.4 solar masses. As small or intermediate stars collapse, the electrons in the stars are crushed together and reach a state in which they can be no denser due to the Pauli Exclusion Principle that creates an outward electron degeneracy pressure. However, above 1.4 solar masses, electron degeneracy could no longer counteract that crushing gravitational force, and the star would continue to collapse (e.g. into a neutron star) or blow away excess mass from it surface in a supernova explosion. In 1983, Chandrasekhar won the Nobel Prize for his studies on stellar evolution.