Opacity
We are familiar with the absorption of light by an atmosphere. For example,
in a fog we can see only a short distance relative to the distance we can see on a clear day.
Because the efficiency of an atmosphere at absorbing light is so important, we
define a quantity called the opacity to specify how "opaque" an atmosphere is.
If we can see large distances the opacity is small and if we can see only a short distance the opacity
is large.
Radiation Transport
Generally, understanding the properties of a star's
atmosphere is a difficult problem in radiative transport, a discipline that seeks to
describe how electromagnetic radiation moves through an atmosphere. This is a complex mathematical
problem that requires extensive analysis and
large-scale computer simulations for its solution.