Here κ κ is the opacity of the stellar material, defined as the fraction of radiation energy flux absorbed by the medium per unit density and unit length. For ionized hydrogen, κ κ = σ σ T / m p , where σ σ T is the Thomson scattering cross-section for the electron and m p is the mass of a proton. Note that F r a d = d 2 E / d A d t is defined as the energy flux over a surface, which can be expressed with the momentum flux using E = p c for radiation. Therefore, the rate of momentum transfer from the radiation to the gaseous medium per unit density is κ κ F r a d / c , which explains the right-hand side of the above equation.