  The present work proposes a self-consistent reduced-order NLTE kinetic model
for radiating plasmas such as are found in the outer layers of stellar
atmospheres. Starting from the most up-to-date set of ab-initio and
experimental data, the highly complex collisional-radiative kinetic mechanism
is simplified by lumping the bound energy states in groups. Different grouping
strategies are investigated, such as uniform and Maxwell-Boltzmann. The reduced
set of governing equations for the material gas and the radiation field is
obtained based on a moment method. Applications consider the steady flow across
a shock wave in partially ionized hydrogen. The results show that adopting a
Maxwell-Boltzmann grouping allows, on the one hand, for a substantial reduction
of the number of unknowns and, on the other, to maintain accuracy for both gas
and radiation quantities. It is observed that, when neglecting line radiation,
the use of two groups already leads to a very accurate resolution of the
photo-ionization precursor, internal relaxation and radiative cooling regions.
The inclusion of line radiation requires adopting just one additional group to
account for optically thin losses in the alpha, beta and gamma lines of the
Balmer and Paschen series. This trend has been observed for a wide range of
shock wave velocities.
