The existence of interstellar material is most obvious where it is densest, in the form of clouds or nebulae. They form three basic types, those that emit their own radiation, those that are identified by absorption of the light from distant stars, and those that shine by reflected star light.
Emission nebulae (HII regions)
Emission nebulae, or nebulae that glow with their own light, exist where relatively dense interstellar material is found in the vicinity of hot stars. Such material may be left over from star formation (for example, the Orion nebula, which encloses several hundred young O and B stars) or from stellar mass loss in supernovae, planetary nebulae, or novae. Ultraviolet light from the hot stars is absorbed by the gas, ionizing the hydrogen (also termed HII, hence the alternative name HII regions) at a high temperature, around 10,000 K. As ionized hydrogen recombines with electrons, hydrogen emission lines in the visible part of the spectrum are produced. The net effect is conversion of stellar ultraviolet radiation into emission lines of hydrogen, primarily the reddish Hα line in hydrogen's Balmer series. Hence, emission nebulae have a characteristic red or pinkish color. The extent of the ionization around a star depends upon its ultraviolet output. An O5 star with a surface temperature of 50,000 K produces sufficient ultraviolet radiation to ionize hydrogen out to a distance of several parsecs (depending on the density of the gas), whereas a cooler star like the Sun is able to ionize surrounding hydrogen only within the confines of the solar system (∼40 AU).
Absorption nebulae (dark nebulae)
If no hot stars are present, the interstellar gas will be cold, and dust may condense from the heavier elements. Interstellar dust has a size and structure similar to soot particles, and in the densest regions the dust may completely obscure the light of stars behind it, producing an absorption nebula, or dark nebula. Typical dust cloud sizes are about 10 pc, with representative masses of about 50 solar masses. Within these dusty regions, globules of denser material (Bok globules, of 0.1–1 pc diameter) may be found where self‐gravity has led to a collapse of the interstellar material, leading to eventual star formation.
Reflection nebulae
The obscuring effect of dust is primarily the result of scattering or reflecting light into different directions. A dust cloud near stars but not close enough to destroy the dust through heat or ultraviolet radiation will preferentially reflect the stars' blue light; hence a bluish‐looking nebula can be observed (the blueness of the daytime sky is produced in this same manner from scattering sunlight). The bluish wisps seen in the Pleiades star cluster are reflection nebulae.
Because young, hot stars are intermixed with older, cooler stars, both of which may be associated with interstellar material, it is not uncommon to find complicated structures in which all three types of nebulae are seen in close proximity.
Reddening and extinction of starlight
In addition to the obvious interstellar material, there is also a general interstellar dust that pervades all regions of space. This dust has the same effects as in the denser regions—it dims and reddens the light of stars. Nearby stars are relatively unaffected, but more distant stars are progressively fainter, not only because of the distance effect but also because of the dimming (technically termed extinction) of their starlight by the dust. Again because the dust affects blue light preferentially, distant stars appear progressively redder than they would if the dust were not present. In order to obtain correct measurements of stellar luminosities and colors, all observations must be corrected for this interstellar extinction and reddening.
The diffuse dust effectively hides stars that are more distant than 3 kpc in the galactic plane. Prior to 1920, however, astronomers did not recognize the existence of this dust. As a result, counts of stars in all directions led to the erroneous conclusion that the Sun was at the center of a stellar distribution—the so‐called Kapteyn universe—whose equatorial plane measured roughly 17,000 pc in diameter and approximately 4,000 pc perpendicularly. With the recognition that dust hides the most distant stars, astronomers realized that this conclusion was erroneous, and they discarded it.
Interstellar absorption lines
Interstellar absorption lines are superimposed on the absorption features already present in stellar spectra. They are best studied by observation of O and B stars, which have the fewest intrinsic absorption lines, or in the spectra of unresolved binary stars. In these binary star spectra, pairs of stellar absorption lines (one from each star) will oscillate in wavelength as the stars alternatively move toward or away from observer (the Doppler effect), but non‐stellar absorption lines produced along the line of sight will have fixed wavelengths. Interstellar absorption lines are also much narrower than stellar features because the gas temperature is much colder. Features attributable to elements such as H, He, Na, K, Ca, Be, Fe, Ca, and Ti and molecules of CN and CH have been identified and show that the interstellar chemistry is like that of stars. Often multiple interstellar absorption features are found, suggesting that the interstellar mate‐rial is not uniformly distributed, but has a wispy cloudlike structure.
Interstellar molecules and giant molecular clouds
In dense regions where dust shields interstellar material from the ultraviolet radiation of surrounding stars, molecules can form. Scientists believe that the chemical reactions that produce molecules occur on the surfaces of dust grains from which the molecules subsequently escape. Such molecules (some 120 or so have been identified) are made up of the most common elements (hydrogen, oxygen, carbon, and nitrogen). Molecules, unlike individual gas atoms, have vibrational and rotational states that allow radiation in the infrared and radio part of the spectrum. Such long wavelengths are unaffected by the interstellar dust and thus can be detected coming from within these dense regions, allowing astronomers to study the structures and the dynamics of dense interstellar clouds. The detection of such radiation also led to the discovery of the giant molecular clouds, huge assemblages of up to 10 6 solar masses of gas that otherwise are invisible to observation.
21-cm radio radiation from neutral hydrogen
The most important radio radiation coming from the interstellar material is the 21‐centimeter radiation (a wavelength that corresponds to a frequency of 1,421 MHz) coming from hydrogen, the most common element. This long wavelength is not affected by dust and can be observed coming from every region in the Galaxy. Radio telescopes are extremely sensitive, so even very weak signals may be detected.