Opacity and Convection

The mean free path (mfp) of a photon is a measure of opacity, or the ability for light to pass through a medium.

What determines the opacity of the gas inside the Sun? All of these change with radius in the Sun. The lowered ionization of atoms in the outer regions wins and makes those layers very opaque -- mfp is very short and radiative transfer is extremely inefficient at moving energy out.

The energy MUST come out though (or else the Sun wouldn't be stable), so it does so by convection.


Convection carries the energy the last ~ 1/3 of the way to  the surface of the Sun:

From here, radiative transfer takes over again. So lets look at radiative transfer in a bit more detail...