ABSTRACT
So far, we have been restricting attention to the upper parts of the photosphere in the Sun, where energy is transported through the gas almost entirely by means of radiation. The gas in the photosphere (at least in all parts except the very lowest regions) does not move, but simply “processes” the photons, absorbing, emitting, and scattering them in such a way that there is a net transport of energy in the outward (radial) direction. Because of the existence of radiative equilibrium, the equation of radiative transfer allows us to extract reliable physical properties of the gas in the photosphere, where there are no systematic gas motions.
