The calculated properties of the interior of a star. Although only the properties of a star’s surface can be observed directly, these can be used to calculate how temperature, pressure, density, and so on vary throughout the star. The equations used assume that the star is in equilibrium: that at each point within it, the outward pressure of the star’s radiation balances the inward pull of gravity, and that the rate at which energy is generated matches the rate at which energy is transported outwards.