Published online by Cambridge University Press: 14 August 2015
The theory of radiative and collisional processes in gaseous nebulae makes possible precise predictions of line and continuum intensities as functions of density, electron temperature, and degree of ionization of the emitting gases. It predicts, for example, the relative intensities of successive members of the Balmer series (Balmer decrement), and of corresponding transitions in the Balmer and Paschen series. Hence accurate line intensity measurements are not only of astrophysical interest; they also serve to assess the amount of space absorption.