NLTE (non-local thermodynamic equilibrium)

NLTE (non-local thermodynamic equilibrium) refers to a condition in a gas or plasma where the energy distribution of particles, like atoms and electrons, does not follow the typical balance assumed in local thermodynamic equilibrium. In simpler terms, the energy states of particles are influenced by factors beyond their immediate surroundings, such as radiation from distant regions, leading to deviations from standard temperature-based behavior. This is important in astrophysics, especially when analyzing stellar atmospheres or interstellar space, because it affects how we interpret light spectra and understand the physical conditions there.