### Eddington approximation

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

The **Eddington approximation** is a
relatively-easy-to-work-with description of electromagnetic radiation's variation by
direction (anisotropy, departure from isotropic),
used in modeling stellar structure.
Assuming isotropy could make modeling even simpler,
but would fail to (realistically?) transfer energy toward the surface
where it is emitted as starlight.
The Eddington approximation assumes there is a general
transfer of energy away from the middle of the star ("up"),
and models the intensity of radiation
in each direction such as to do that,
with tractable complication to equations that would presume isotropy.
It is one of the approximations developed to make the modeling
of stars practical and can be considered an example of a
**two-stream approximation**.
I believe the Eddington approximation represents a simplified model
yielding the net effect of the slow outward diffusion of
inherent in the photons' random walks.
The approximation-method has been refined over time, and
variants have been developed that trade away some of the simplicity
for more realism.

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

https://en.wikipedia.org/wiki/Radiative_transfer#The_Eddington_approximation

**Referenced by pages:**

gray atmosphere

photosphere

equation of radiative transfer (RTE)

stellar structure

two-stream approximation

Index