= Solution
In the Sun's cool outer layers, partial ionization lowers the adiabatic temperature gradient, while large opacity makes radiative transport inefficient. The required radiative gradient
$$
\nabla_{\rm rad}=\frac{3\kappa L_rP}{16\pi acGm_rT^4}
$$
can exceed $\nabla_{\rm ad}$, so <convection> carries part of the outward <luminosity>. The Sun therefore has an outer <convective envelope>, above its primarily radiative interior. Here $a$ is the radiation constant, $\kappa$ opacity per unit mass and $m_r,L_r$ the enclosed mass and luminosity. Near the visible surface, radiative losses are again important and the flow need not be perfectly adiabatic.
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