Ultimately, the overwhelming majority of emission-line sources in the Universe are “galactic sources” - meaning discrete objects located within a particular galaxy (rather than some global property of a galaxy or some source not located in a galaxy). However, the most common of these, HII regions, are so ubiquitous that they are being covered elsewhere in this volume as the “baseline” source of emission lines. In addition, most of the other chapters are devoted to line emission either integrated over entire galaxies (or significant portions thereof) or from active galactic nuclei.
Given that coverage, I will focus this chapter primarily on “stellar” sources of line emission in the Milky Way other than HII regions - including young stellar objects, massive and/or evolved stars, and stellar remnants (planetary nebulae, supernova remnants, and accreting compact objects in binary systems). I will also put considerable emphasis on emission lines with rest wavelengths in the near-infrared waveband, due to the importance of this waveband for probing the dusty planar regions of the Milky Way where most of these sources are to be found.
In the sections below, I will begin with a review of important diagnostic optical emission lines and a more-detailed overview of key (rest-wavelength) infrared emission lines. I will then move on to “nebular” sources of emission lines (omitting HII regions, but including planetary nebulae and supernova remnants).