### two-stream approximation

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

A **two-stream approximation** is a type of 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, such as within an atmosphere. Physical reality
can be subtle and complicated and a two-stream approximation is a
simplification that makes calculations of radiative transfer
and absorption lines tractable. It is "two-stream" for making
a distinction between "down" and "up", essentially, assuming there
is a portion of the light that is moving up, or away from the center
of the object. I gather this simulates the effect of diffusion
of photons in their random walk and/or the effect of the
temperature gradient, both being overall effects that models
focused on local phenomena would miss. There are variants, two of
which are the Eddington approximation and the
Schuster-Schwarzschild model.

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

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

**Referenced by pages:**

Eddington approximation

Schuster-Schwarzschild model

Index