Ionized hydrogen or H II regions and gaseous nebulae are generally low-density objects that appear as extended and diffuse clouds. Typical electron temperatures are of the order of 104 K, or ~ 1 eV, and densities are between 102 and 106 cm-3. But ionizing sources of H II regions in general are quite diverse. Among the most common variety are those found in giant molecular clouds photoionized by newly formed hot stars with sufficient UV flux to ionize hydrogen and several other elements to low ionization states. Similar H II regions are commonplace in astronomy, as part of otherwise unrelated objects, such as active galactic nuclei (Chapter 13) and supernova remnants. Such regions are also easily observable, since they are largely optically thin. Furthermore, a number of nebular ions are commonly observed from a variety of gaseous objects. In fact, in Chapter 8 we had developed the spectral diagnostics of optical emission lines, as observed from the Crab nebula in Fig. 8.3. That nebula is the remnant of a supernova explosion, in the constellation of Taurus, witnessed in AD 1054 by Arab and Chinese astronomers. The central object is a fast spinning neutron star – pulsar – energizing the surrounding nebula. Nebular spectroscopy therefore forms the basis of most spectral analysis in astrophysics.

We describe the essentials of nebular astrophysics with emphasis on spectroscopic analysis, and address the pervasive problem of atomic data sources of varying accuracy.