The Milky Way Galaxy, or simply the Galaxy, is a typical example of a galaxy, a large, independent system of stars, star clusters, and interstellar material. By studying the Milky Way, we can better understand galaxies as a whole. The Milky Way can be identified as a spiral galaxy because of the pinwheel‐shaped interstellar material that traces out a spiral pattern in the plane of the Galaxy. The Galaxy is actually made up of two distinctly different spatial distributions of stars, one of which forms a flat disk and the other a centrally condensed, slightly flattened sphere that surrounds the disk.
About 3 percent of the mass of the Milky Way exists in the form of interstellar matter, or diffuse material that floats between the stars. These extremely thin clouds of dust and gas average about 1 atom (mostly hydrogen) per cubic centimeter compared to air, which has a standard pressure and temperature of 10 22 particles/cm 3.
The role of interstellar matter in astronomy is threefold. First, it provides the source of material for the formation of new stars. In turn, the material is partially replenished by mass loss from aging stars. Second, direct observation of interstellar matter provides scientists with information about the Galaxy that complements evidence obtained from studying stars. Finally, the presence of interstellar material along the line of sight to an object may dramatically affect the observation of the light coming from that object.
The two components of the interstellar material—gas and dust—must be clearly distinguished. Gas and dust are not only detected in different ways, but their distinct characteristics also cause them to affect observations of other objects differently. Gas, which makes up about 98 percent of the interstellar material, consists of individual atoms, primarily hydrogen and helium, and in dense regions there may also be molecules (more than 100 kinds of simple molecules, for example, water, formaldehyde, and alcohol, composed of up to 14 atoms have been identified so far). Gas has little effect on the observation of objects seen through it; it will add a few absorption lines to the spectrum of the light detected by the observer, but overall, interstellar absorption lines added to the spectrum of a distant star have a negligible effect on the star's observed brightness and color. The 21‐cm wavelength of neutral hydrogen (HI) is especially important for studying of the Galaxy because this long wavelength passes through the dust without being absorbed. If it were not for this 21‐cm radiation, most of the Galaxy could not be observed and studied by astronomers. Gas produces detectable, visible emission lines only in relatively dense, hot regions around hot stars.
The remaining 2 percent of the interstellar material is dust. Dust can be detected by the thermal radiation it produces, characteristic of its temperature in interstellar space (typically around 100 K). Its effect on the observation of other objects, however, cannot be ignored. Dust absorbs light that passes through it and scatters that light into other directions, thus dimming objects located in or behind it. In the plane of the Galaxy, dust effectively prohibits visually observing stars beyond about 3 Kpc (10,000 light‐years) distance. The absorption is greatest for short‐wavelength blue light; thus, the dust also makes distant objects appear redder (see Figure ).
Interstellar absorption. Blue light from a distant object is preferentially scattered by dust along the line of sight, hence the object appears both redder and dimmer.