| This dissertation considers five problems of current interest in studies of nebulae and active galactic nuclear emission-line regions.; First we examine the effects of electron collisions on the populations of the highly excited states (n from 10 to 100) of ions. We demonstrate that 3-body recombinations to these states enhance the total recombination rate in cold nova shells and broad-line regions of active galactic nuclei (AGN).; Second, we consider the treatment of the hydrogen atom in photoionization models. We discuss the applicability and accuracy of l -mixing and the published collision rates. We present a general method to treat a 100-level atom as a 10-level atom, increasing accuracy but not calculation time.; Third, we predict the helium singlet recombination spectrum for cases of finite optical depth (i.e., intermediate between cases A and B), and allow for the destruction of He I resonance photons through photoionizations of neutral hydrogen. This prevents the resonance photons from scattering often enough to attain case B. We demonstrate that it is possible to measure the temperature of an ionizing star from the deviation of these lines from their case B emissivities.; Fourth, we predict the H {dollar}beta{dollar} emissivity for cases of finite optical depth, and allow for the destruction of Lyman photons on dust grains before they can attain case B. We demonstrate that for dusty H II regions, helium abundance measurements made relative to H {dollar}beta{dollar} have been overestimated by 5-15%. This is an important error source for measurements of the primordial helium abundance or of the effects of galactic chemical evolution.; Finally, we demonstrate through photoionization modeling of its fine structure line spectrum, that the center of our galaxy, Sgr A West, is ionized by a soft spectrum like that of a 35,000 K star. We rule out an AGN-like ionizing spectrum. This suggests that our galaxy's nucleus is similar to starburst nuclei. |
