### local thermodynamic equilibrium

**(LTE)**
(state where temperature variation is too small to matter)

**Local thermodynamic equilibrium** (**LTE**) is a condition,
e.g., of some gas, such that the variation in temperature
is small enough that "constant temperature" is a useful approximation,
i.e., it can be usefully treated as if in thermodynamic equilibrium.
If processes under study directly affect matter only within
sufficiently short distances, then assuming a
constant temperature can be a useful approximation.
The term is also used for the model that assumes the condition.

The term **NLTE** (for **not** or **non-LTE** or
**non local thermodynamic equilibrium**)
is used for an amended LTE-like model that accommodates
a common discrepancy from true LTE,
specifically, excess electromagnetic radiation invading the locality.
Like LTE, it is an approximation used to make stellar models
(and other models) tractable.
The terms **kinetic equilibrium** and **statistical equilibrium**
are sometimes used as an alternative to "NLTE" since
the term NLTE might literally be taken to mean
"global thermodynamic equilibrium"
or to mean any other concept that isn't LTE.

(*thermodynamics,astrophysics*)
**Further reading:**

http://en.wikipedia.org/wiki/Thermodynamic_equilibrium

https://arxiv.org/abs/1406.3553

**Referenced by pages:**

CMFGEN

gray atmosphere

photosphere

Rosseland mean opacity

equation of radiative transfer (RTE)

stellar structure

temperature

thermodynamic equilibrium (TE)

Index