Overview

Abstract

Neutron stars are compact stars which contain matter of supranuclear density in their interiors (presumably with a large fraction of neutrons). They have typical masses M ∼ 1.4M⊙ and radii R ∼ 10 km. Thus, their masses are close to the solar mass M⊙ = 1.989 × 10³³ g, but their radii are ∼ 10⁵ times smaller than the solar radius R⊙ = 6.96×10⁵ km. Accordingly, neutron stars possess an enormous gravitational energy E_grav and surface gravity g,

$$ \begin{gathered} E_{grav} \sim GM^2 /R \sim 5 \times 10^{53} erg \sim 0.2Mc^2 , \hfill \\ g \sim GM/R^2 \sim 2 \times 10^{14} cm{\text{ }}s^{ - 2} , \hfill \\ \end{gathered} $$

((1.1))

where G is the gravitational constant and c is the speed of light. Clearly, neutron stars are very dense. Their mean mass density is

$$ \bar \rho \simeq 3M/(4\pi R^3 ) \simeq 7 \times 10^{14} g cm^{{\text{ - 3}}} \sim (2 - 3)\rho _0 , $$

((1.2))

where ρ₀ = 2.8 × 10¹⁴ g cm⁻³ is the so called normal nuclear density, the mass density of nucleon matter in heavy atomic nuclei. The central density of neutron stars is even larger, reaching (10–20) ρ₀. By all means, neutron stars are the most compact stars known in the Universe.