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degenerate matterMatter in a highly dense form that can exert a pressure as a result of quantum mechanical effects. Degenerate matter occurs in white dwarfs and neutron stars. During the gravitational collapse of a dying star, the electrons are stripped from their atomic nuclei, and nuclei and electrons exist in a closely packed, highly dense mass. As the density increases, the number of electrons per unit volume increases to a point when the electrons can exert a considerable pressure, called degeneracy pressure. This pressure is a result of the laws of quantum mechanics. Unlike normal pressure, degeneracy pressure is essentially independent of temperature, depending primarily on density. At the immense densities typical of white dwarfs (107 kg m–3 or more) it becomes sufficiently large to counteract the gravitational force and thus prevents the star from collapsing further. The gross properties of a white dwarf are therefore described in terms of a gas of degenerate electrons.
Above a certain stellar mass (the Chandrasekhar limit) equilibrium cannot be attained by a balance of gravitational force and electron degeneracy pressure. The star must collapse further to become a neutron star. It is then the degeneracy pressure exerted by the tightly packed neutrons that balances the gravitational force. See also black hole.