Assume a star of radius R has a density profile equal to p(r)=p_c (1- r/R) and a nuclear production rate per unit mass equal to E(r) =E_c (1- r/(0.2R)) for r</= 0.2R and E(r) = 0 for r > 0.2R. Calculate the luminosity of the star at its surface in terms of R, p_c, and E_c