### two-stream approximation

(approximation of a star's internal EMR's divergence from isotropic)

A **two-stream approximation** is an approximation of electromagnetic radiation
intensity as a function of direction, i.e., a description of
how intense the light is when viewed in different directions,
for example, within an atmosphere. It is a simplification
of reality that makes calculations of radiative transfer
and absorption lines tractable. It is "two-stream" for making
a distinction between "down" and "up", essentially, assuming
more light is moving up, or away from the center of the object.
There are variants, two of which are the Eddington approximation
and the Schuster-Schwarzschild model.

(*physics,EMR,stars,radiative transfer*)
**Further reading:**

http://en.wikipedia.org/wiki/Two-stream_approximation

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