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.
- the mfp of (optical) light through glass is
large -- glass is transparent.
- the mfp of light through a brick is small -- bricks
are opaque.
What determines the opacity of the gas inside the Sun?
- density
- temperature
- ionization/excitation
states of atoms -- a biggie
- wavelength of light
All of these change with radius in the Sun.
- inner regions: dense, high temp, high
ionization, gamma rays
- outer regions: lower density, lower temp,
lower ionization, optical light
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:
- hot gases rise, cool
gases fall.
- bulk motion of "cells"
of gas.
- net transport of energy
to the upper layers.
- no net transport of gas.
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...