In astrophysics, the Tolman–Oppenheimer–Volkoff (TOV) equation constrains the structure of a spherically symmetric body of isotropic material which is in static gravitational equilibrium, as modelled by general relativity. The equation is

Here, r is a radial coordinate, and ρ(r₀) and P(r₀) are the density and pressure, respectively, of the material at r = r₀.