Neutron stars

Summary

The structure of neutron stars is discussed with a view to explore (1) the extent to which stringent constraints may be placed on the equation of state of dense matter by a comparison of calculations with the available data on some basic neutron star properties; and (2) some astrophysical consequences of the possible presence of strangeness, in the form of baryons, notably the Λ and Σ⁻, or as a Bose condensate, such as a K⁻ condensate, or in the form of strange quarks.

Introduction

Almost every physical aspect of a neutron star tends to the extreme when compared to similar traits of other commonly observed objects in the universe. Stable matter containing A ∼ 10⁵⁷ baryons and with a mass in the range of (1 − 2) M_⊙ {M_⊙ ≅ 2 × 10³³ g) confined to a sphere of radius R ∼ 10 km (recall that R_⊙ = 6.96 × 10⁵ km) represents one of the densest forms of matter in the observable universe. Depending on the equation of state (EOS) of matter at the core of a neutron star, the central density could reach as high as (5 − 10)p₀, where p₀ ≅ 2.65 × 10¹⁴ g cm⁻³ (corresponding to a number density of n₀ ≅ 0.16 fm⁻³) is the central mass density of heavy laboratory nuclei (compare this to P_⊙= 1.4 g cm⁻³).