The cold neutral medium (CNM) represents gas at temperature T ∼ 80 K and number density n ∼ 40 cm-−3, where heating by photoelectrons ejected from dust grains balances cooling by fine-structure line emission from C+. The cold neutral medium is studied by looking at the absorption lines caused by the CNM along the line of sight to bright background stars. Interpreting these absorption lines requires solving the equation of radiative transfer. In particular, the curve of growth for an absorption line yields the relation between the observed equivalent width of a line and the underlying column density of the atom or ion giving rise to the absorption.

The warm ionized medium (WIM) represents gas at T ~ 8000 K and n ~ 0.2 cm−3, where heating by a variety of mechanisms balances cooling by fine-structure line emission from oxygen and Lyman alpha emission from hydrogen. Ionized nebulae, such as H ii regions around hot stars and planetary nebulae around newly unveiled white dwarfs, have temperatures similar to the WIM, but much higher density. Ionized nebulae can be idealized as spherical Strömgren spheres. The physics of an ionized nebula is made more complex (and interesting!) by the presence of helium and “metals.” Emission lines from oxygen, nitrogen, sulfur, and other metals both help to cool an ionized nebula and provide useful diagnostic tools to determine observationally the density and temperature of the nebula.

The warm neutral medium (WNM) represents gas at T ∼ 6000 K and n ∼ 0.4 cm−3, where heating by photoelectrons from dust grains balances cooling by fine-structure line emission from oxygen. The warm neutral medium is studied by looking at 21 cm emission from the hyperfine transition of the ground state of hydrogen. The upper hyperfine level is excited and de-excited primarily by collisions with gas particles. The relatively rare radiative de-excitations, however, produce 21 cm photons that are a useful diagnostic of neutral hydrogen. All-sky maps of 21 cm intensity (commonly expressed as an “antenna temperature”) can be translated into a map of the column density of neutral hydrogen.

The circumgalactic medium (CGM) is the gas that lies outside the main stellar distribution of a galaxy, but inside its virial radius. The first part of our own galaxy’s CGM to be discovered was a population of high-velocity clouds, discovered through the 21 cm emission of their neutral hydrogen. The high-velocity clouds, however, are embedded within hotter components of the CGM, with temperatures ranging from 104 K to 106 K. These hotter components can be detected through absorption and emission lines of ionized metals such as oxygen. The intracluster medium (ICM) is the gas that lies inside the virial radius of a cluster of galaxies, but which is not associated with any individual galaxy. The ICM can be detected and studied through its free--free emission, which indicates temperatures as high as 108 K.

The hot ionized medium (HIM) represents gas at T ∼ 106 K and n ∼ 0.004 cm−3. It constitutes gas that has been shock-heated by supernova explosions, and which has not yet had time to cool by free--free emission. The properties of a spherically expanding shock front are described by the Sedov–Taylor solution; when radiative losses from the post-shock gas are large, the expanding supernova remnant transitions to the snowplow solution. The hot gas inside a supernova-blown bubble is in collisional ionization equilibrium, which permits a calculation of the ionization state of each element as a function of temperature. Emission lines from ionized iron and absorption lines of ionized oxygen (seen in absorption toward hot white dwarfs) provide information about the density and temperature of the hot gas in the Local Bubble within which the Sun lies.

The warm-hot intergalactic medium (WHIM) is the hottest portion of the intergalactic medium; its temperature 105 K < T < 107 K is the result of shock heating as gas flows along the filaments of the cosmic web. Numerical simulations indicate that the WHIM is only now overtaking the cooler DIM as the more massive component of intergalactic gas. The WHIM is difficult to detect – to the point where astronomers long complained of a “missing baryon problem.” However, the cooler portions of the WHIM can be detected by looking for absorption lines of O vi along lines of sight to bright quasars. The portion of the WHIM at T ∼ 106 K can be detected from absorption lines of O vii. The very hottest portion of the WHIM, it is hoped, will be detected from absorption lines of iron, which still clings to its innermost electrons at T ∼ 107 K.

The diffuse intergalactic medium (DIM) is photoionized gas at temperature T < 105 K that lies outside galaxies and clusters. The absence of the Gunn–Peterson effect (optically thick absorption by Lyman alpha) at low redshifts indicates that the DIM is almost entirely ionized today. The hydrogen gas filling the universe was also almost entirely ionized soon after the Big Bang; however, at a redshift z ∼ 1400, the hydrogen went from being ionized to being neutral. The end of the era of neutrality came at a redshift z ∼ 7, when the earliest hot massive stars had emitted enough UV photons to reionize the intergalactic gas. Today, within the mostly ionized DIM, there exist regions of higher neutral hydrogen density; these regions give rise to the Lyman alpha forest of absorption lines seen in the spectra of relatively low-redshift quasars.

Molecular clouds contain gas at T ∼ 15 K and at densities of n ∼ 100 cm^-−3 and upward. Although ${\mathrm{H}}_2$ provides the overwhelming majority of molecules in a molecular cloud, the absence of an electric dipole in the symmetric ${\mathrm{H}}_2$ molecule means it is difficult to observe. The 2.6 mm emission from the $J=1\to 0$ rotational transition in ${}^{12}\mathrm{CO}$ is among the most useful ways to observe molecular gas. Most molecular clouds, however, are optically thick to the 2.6 mm emission from ${}^{12}\mathrm{CO}$; seeing to their centers requires observing the scarcer isotopologue ${}^{13}\mathrm{CO}$ rather than the more abundant ${}^{12}\mathrm{CO}$. Within dusty, optically thick molecular clouds, molecules are made by dust grain catalysis. In the surface layers of molecular clouds, molecules are destroyed through photodissociation by ultraviolet light. However, molecular clouds are self-shielded from UV outside the cloud.