Compact Objects

At the end of a star's life, the outer regions of the star are blown off into the interstellar medium, leaving behind a compact core. These compact cores can be of three kinds: white dwarfs, neutron stars, or black holes.

In compact objects gas pressure is ineffective in maintaining hydrostatic equilibrium. Another kind of pressure, called degeneracy pressure comes into play instead.



image credit: Harvey Richer (Univ. of British Columbia) and NASA

White dwarfs are composed of ordinary matter compressed as tightly as possible. These objects have the mass of the Sun, but only the size of the Earth, with densities of as much as a ton per cubic centimeter. Many of these objects have been observed. In the 1930s, Subramanyan Chandrasekhar, then a graduate student at Cambridge University, proved that white dwarfs with mass greater than 1.4 times that of the Sun would not be able to support themselves, and would continue to collapse. This was such a bizarre concept that Chandrasekhar's thesis advisor, the famous astrophysicist Arthur Eddington remarked that "there ought to be a law of nature to prevent stars from behaving in this foolish way". Eddington's prominent opposition to Chandrasekhar's Limit slowed research on compact objects. It may be that if Eddington had believed his student rather than his intuition our understanding of black holes might be decades further advanced.

Neutron stars are not composed of ordinary matter. In these exotic objects all of the electrons and protons contained in ordinary matter have been squashed together to form neutrons. In essence a neutron star is a single gigantic atomic nucleus, containing only neutrons. A typical neutron star has 1.5 times the mass of the Sun, but a radius of only 10 kilometers. Neutron stars have been observed in the form of pulsars.

Neutron stars more massive than three times the mass of the Sun fall within their own Schwarzschild radii, and thus are black holes. This conclusion is inescapable unless one either assumes that Einstein's Theory of General Relativity is not the right theory of gravity, or that there are stars made up of subatomic particles other than those in ordinary matter. There is no evidence for either of these assumptions, so the most conservative conclusion is that a compact object with a mass greater than three times that of the Sun must be a black hole.