Neutron stars: compact objects with relativistic gravity


Abstract: General properties of neutron stars are briefly reviewed with an emphasis on the indispensability of general relativity in our understanding of these fascinating objects. In Newtonian gravity the pressure within a star merely plays the role of opposing self-gravity. In general relativity all sources of energy and momentum contribute to the gravity. As a result, the pressure not only opposes gravity but also enhances it. The latter role of pressure becomes more pronounced with increasing compactness, $M/R$, where $M$ and $R$ are the mass and radius of the star, and sets a critical mass beyond which collapse is inevitable. This critical mass has no Newtonian analogue; it is conceptually different from the Chandrasekhar limit in Newtonian gravity, which is attained asymptotically for ultra-relativistic fermions. For white dwarfs the general relativistic critical mass is very close to the Chandrasekhar limit. For neutron stars the maximum mass-so called Oppenheimer-Volkoff limit-is significantly smaller than the Chandrasekhar limit. This follows from the fact that the general relativistic correction to hydrostatic equilibrium within a neutron star is significant throughout the star, including the central part, where the mass contained within the radial coordinate, $m(r)$, and the Newtonian gravitational acceleration, $Gm(r)/r^2$, is small.

Keywords: Neutron stars, gravity, general relativity

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