Neutron star structure and the equation of state

The structure of neutron stars is considered from theoretical and observati onal perspectives. We demonstrate an important aspect of neutron star struc ture : the neutron star radius is primarily determined by the behavior of t he pressure of matter in the vicinity of nuclear matter equilibrium density . In the event that extreme softening does not occur at these densities, th e radius is virtually independent of the mass and is determined by the magn itude of the pressure. For equations of state with extreme softening or tho se that are self-bound, the radius is more sensitive to the mass. Our resul ts show that in the absence of extreme softening, a measurement of the radi us of a neutron star more accurate than about 1 km will usefully constrain the equation of state. We also show that the pressure near nuclear matter d ensity is primarily a function of the density dependence of the nuclear sym metry energy, while the nuclear incompressibility and skewness parameters p lay secondary roles. In addition, we show that the moment of inertia and th e binding energy of neutron stars, for a large class of equations of state, are nearly universal functions of the star's compactness. These features c an be understood by considering two analytic, yet realistic, solutions of E instein's equations, by, respectively, Buchdahl and Tolman. We deduce usefu l approximations for the fraction of the moment of inertia residing in the crust, which is a function of the stellar compactness and, in addition, the pressure at the core-crust interface.