Аннотация:The hydrogen lines play an important role in diagnostics of astrophysical
plasma. In recent years, the lines of higher hydrogen series became the
objects of the research, thanks to the rapid development of the infrared
spectroscopy. In this paper, we investigate the relative intensities of the
Paschen and Brackett lines as well as the ratio of Ly alfa/H alfa over a large
range of the optical depth of gas. The calculations are ful lled on the basis of
the Sobolev approximation for collisional excitations and ionizations of atoms
for the electron temperature Te<10000 K. The behavior of the electron
density Ne at the gas thermalization is investigated. It is shown that a very
sharp jump in the degree of ionization is observed near the LTE conditions.
The results obtained can be used both for the diagnostics of emitting regions
and for determining the extinction for the objects with strong absorption.