  We study the approximation of the radiative transfer equation with a
relatively few moments in the spherically symmetric case. We propose a
three-moment model based on choosing the beta distribution as the ansatz for
the specific intensity. This ansatz enables our model to capture the anisotropy
in the distribution function. The characteristic structure of the Riemann
problem of the model is studied in detail. Numerical simulations demonstrate
its validity in approximating the radiative transfer equation in the
spherically symmetric case and its advantage in approximating highly
anisotropic distribution functions in comparison to the $P_n$ method.
