Editing Compact object (section)

==White dwarfs==
{{Main|White dwarf}}
[[Image:Ngc2392.jpg|upright|thumb|The [[Eskimo Nebula]] is illuminated by a white dwarf at its center.]]
The stars called [[white dwarf|white or degenerate dwarf]]s are made up mainly of [[degenerate matter]]; typically carbon and oxygen nuclei in a sea of degenerate electrons. White dwarfs arise from the cores of [[main-sequence star]]s and are therefore very hot when they are formed. As they cool they will redden and dim until they eventually become dark [[black dwarf]]s. White dwarfs were observed in the 19th century, but the extremely high densities and pressures they contain were not explained until the 1920s.

The [[equation of state]] for degenerate matter is "soft", meaning that adding more mass will result in a smaller object. Continuing to add mass to what begins as a white dwarf, the object shrinks and the central density becomes even greater, with higher degenerate-electron energies. After the degenerate star's mass has grown sufficiently that its radius has shrunk to only a few thousand kilometers, the mass will be approaching the [[Chandrasekhar limit]] – the theoretical upper limit of the mass of a white dwarf, about 1.4&nbsp;times the [[mass of the Sun]] ({{Solar mass|link=y}}).

If matter were removed from the center of a white dwarf and slowly compressed, electrons would first be forced to combine with nuclei, changing their [[proton]]s to [[neutron]]s by [[inverse beta decay]]. The equilibrium would shift towards heavier, neutron-richer nuclei that are not stable at everyday densities. As the density increases, these nuclei become still larger and less well-bound. At a critical density of about 4{{e|14}} kg/m<sup>3</sup> – called the [[neutron drip line]] – the atomic nucleus would tend to dissolve into unbound protons and neutrons. If further compressed, eventually it would reach a point where the matter is on the order of the density of an atomic nucleus – about 2{{e|17}}&nbsp;kg/m<sup>3</sup>. At that density the matter would be chiefly free neutrons, with a light scattering of protons and electrons.