Highly compressed matter in which the normal atomic structure has broken down and which, because of quantum-mechanical effects, exerts a pressure that is independent of temperature. Bodies with masses less than Chandrasekhar's limit (1.4 solar masses) are supported by electron degeneracy pressure and have densities of about 10^6 kg/m^3. In collapsed stars of mass above 1.4 solar masses, gravity will overwhelm electron degeneracy and further collapse ensues. Electrons combine with protons to form neutrons, so producing a neutron star. Because neutrons, like electrons, are fermions and therefore subject to the Pauli exclusion principle, at high enough densities, about 10^14 kg/m^3, neutron degeneracy pressure prevents further collapse of the star. For masses larger than 2-3 solar masses, even neutron degeneracy cannot prevent further collapse, and a black hole is formed.