Neutron stars: compact objects with relativistic gravity

Abstract

General properties of neutron stars are brie y reviewed with an emphasis on the indispensability of generalrelativity in our understanding of these fascinating objects. In Newtonian gravity the pressure within a star merely playsthe role of opposing self-gravity. In general relativity all sources of energy and momentum contribute to the gravity. As aresult, the pressure not only opposes gravity but also enhances it. The latter role of pressure becomes more pronouncedwith increasing compactness,M=R, whereMandRare the mass and radius of the star, and sets a critical massbeyond which collapse is inevitable. This critical mass has no Newtonian analogue; it is conceptually different from theChandrasekhar limit in Newtonian gravity, which is attained asymptotically for ultra-relativistic fermions. For whitedwarfs the general relativistic critical mass is very close to the Chandrasekhar limit. For neutron stars the maximummass{so called Oppenheimer-Volkoff limit{is signi cantly smaller than the Chandrasekhar limit. This follows from thefact that the general relativistic correction to hydrostatic equilibrium within a neutron star is signi cant throughoutthe star, including the central part, where the mass contained within the radial coordinate,m(r), and the Newtoniangravitational acceleration,Gm(r)=r2, is small.