Tags: Star.
The source function is a characteristic of a stellar atmosphere and in the case of no scattering of photons describes the ratio of the emission coefficient to the absorption coefficient. It is a measure of how photons in a light beam are removed and replaced by new photons by the material it passes through. Its units in the cgs-system are erg s-1 cm-2 sr-1 Hz-1 and in SI are W m-2 sr-1 Hz-1 .