where μ μ H is the molecular weight per particle and H is (to a very good approximation) the mass of a hydrogen atom. This equation represents a polytrope of index 1.5, since a polytrope is defined by P = K ρ ρ 1 + 1 / n , where n=1.5 is the polytropic index. Applying the equation to the center of the star gives: P c = ( k ρ ρ c μ μ H ) γ γ C .