Mass as a function of central density. The blue and magenta curves depict sequences of constant baryonic mass of 1.6 and 1.8 solar masses, which are limited by the spherically symmetric stellar sequence (Ω = 0) and the Kepler sequence (Ω = Ω_K). Such curves represent the evolutionary path taken by isolated neutron stars during their spin-evolution (and the corresponding increase in central density).