The Primary Sources of Continuous Opacity
1. Electron (Thompson) scattering - This is a truly gray (λ independent) source of opacity. It is the dominant source in
many stellar interiors and also in the atmospheres of O- and early B-type stars. This is so for two reasons: (1) at high
temperatures, free electrons are abundant. Even more importantly, (2), at high temperatures the other (competing)
processes that contribute to continuous opacity become negligible.2. He II photoionization - important, but less so than electron scattering, in O-star atmospheres. Unimportant in cooler
atmospheres since the concentration of He II is nil. See He II energy-level diagram.3. He I photoionization - important in B atmospheres, less so in O stars where most of the He is ionized. Unimportant in
cooler stars where most of the radiation is of too long wavelength to photoionize He I. See He I energy-level diagram.4. H photoionization - dominant in B through A stars, but affects only λ < 912 Å in the Lyman continuum, λ < 3647 Å in the
Balmer continuum, λ < 8862 Å in the Paschen continuum, λ < 19445 Å in the Brackett continuum, etc. See H energy-
level diagram.5. H- photoionization - dominant in late A through K stars. This is the dominant source of continuous opacity in the solar
atmosphere. The H- ion need only be abundant in low concentrations to dominate opacity. (In the sun only about
1/ 3.3´107 hydrogen atoms is in the H- ionization state [inferred from the Saha equation], but only about 1/ 1.7´109
hydrogen atoms is in the n = 3 level of the neutral state [inferred from the Boltzmann equation] such that it can
contribute to the Paschen continuum. Therefore atoms which contribute to the H- continuum outnumber those which
contribute to the competing Paschen continuum by about 1.7´109 / 3.3´107 » 50.) The limiting wavelength for H-
photoionization is λ < 17000 Å.
