Emission nebulae are only one small component of interstellar space. Most of space—in fact, more than 99 percent of it—is devoid of nebular regions and contains no stars. It is simply dark. Look again at Figure 18.5, or just ponder the evening sky. The dark regions are by far the most representative regions of interstellar space.
Within these dark voids among the nebulae and the stars lurks another type of astronomical object—the dark dust cloud. These clouds are cooler than their surroundings (with temperatures as low as a few tens of kelvins), and thousands or even millions of times denser. Along any given line of sight, cloud densities can range from 107 atoms/m3 to more than 1012 atoms/m3 (106 atoms/cm3). These latter clouds are generally called dense interstellar clouds by researchers, but even these densest interstellar regions are about as tenuous as the best laboratory vacuum. Still, it is because their density is much larger than the average value of 106 atoms/m3 that we can distinguish clouds from the surrounding expanse of interstellar space.
These clouds bear little resemblance to terrestrial clouds. Most of them are bigger than our solar system, and some are many parsecs across. (Yet even so, they make up no more than a few percent of the entire volume of interstellar space.) Despite their name, these clouds are made up primarily of gas, just like the rest of the interstellar medium. However, their absorption of starlight is due almost entirely to the dust they contain.
Figure 18.12(a) is an optical photograph of a typical interstellar dust cloud. Pockets of intense blackness mark regions where the dust and gas are especially concentrated and the light from background stars is completely obscured. This cloud takes its name from a nearby star, Rho Ophiuchi, and resides at the relatively nearby distance of about 300 pc. Measuring several parsecs across, this cloud is only a tiny part of the grand mosaic shown in Figure 18.5. Note especially the long "streamers" of (relatively) dense dust and gas. This cloud clearly is far from spherical. Indeed, most interstellar clouds are very irregularly shaped. The bright patches within the dark region are emission nebulae in the foreground. Some of them are part of the cloud itself, where newly formed stars near the surface have created a "hot spot" in the cold, dark gas. Others have no connection to the cloud and just happen to lie along our line of sight.
Figure 18.12 (a) A typical dark dust cloud, known as Rho Ophiuchi, is "visible" only because it blocks the light coming from stars lying behind it. The approximate outline of the cloud is indicated by the dashed line. (b) An infrared map of the same region, to roughly the same scale. The infrared emission, and therefore the dust that produces it, displays a structure similar to the cloud's visual image. The very bright source of infrared radiation near the top of the cloud comes from a hot emission nebula, which can also be seen in the optical image (The black diagonal streak at right is an instrumental effect).
Like all dark dust clouds, the Rho Ophiuchi cloud is too cold to emit any visible light. However, it does radiate strongly at longer wavelengths. Figure 18.12(b) shows an infrared view of the same region, captured by sensitive detectors aboard the Infrared Astronomy Satellite. 
(Sec. 5.6)
These dark and dusty interstellar clouds are sprinkled throughout our Galaxy. We can study them at optical wavelengths only if they happen to block the light emitted by more distant stars or nebulae. The dark outline of Rho Ophiuchi and the dust lanes visible in Figures 18.8 and 18.9 are good examples of this obscuration. The dust is apparent only because it blocks the light coming from behind it. Figure 18.13 shows another striking example of a dark cloud—the Horsehead Nebula in Orion. This curiously shaped finger of gas and dust projects out from the much larger dark cloud in the bottom half of the image and stands out clearly against the red glow of a background emission nebula.
Figure 18.13 The Horsehead Nebula in Orion is a striking example of a dark dust cloud, silhouetted against the bright background of an emission nebula. The "neck" of the horse is about 0.25 pc across. The nebular region is roughly 1500 pc from Earth.
Astronomers first became aware of the true extent of dark interstellar clouds in the 1930s as they studied the optical spectra of distant stars. In addition to the wide absorption lines normally formed in stars' lower atmospheres, much narrower absorption lines were also detected. Figure 18.14(a) illustrates how light from a star may pass through several interstellar clouds on its way to Earth. These clouds need not be close to the star, and indeed they usually are not. Each absorbs some of the stellar radiation in a manner that depends on its own temperature, density, velocity, and elemental abundance. Figure 18.14(b) depicts part of a typical spectrum produced in this way.
Figure 18.14 (a) A simplified diagram of some interstellar clouds between a hot star and Earth. Optical observations might show an absorption spectrum like that traced in (b). The wide, intense lines are formed in the star's hot atmosphere; narrower, weaker lines arise from the cold interstellar clouds. The smaller the cloud, the weaker the lines. The redshifts or blueshifts of the narrow absorption lines provide information on cloud velocities. The widths of all the spectral lines depicted here are greatly exaggerated for the sake of clarity.
The narrow absorption lines contain information about dark interstellar clouds, just as stellar absorption lines reveal the properties of stars, and nebular emission lines tell us about conditions in hot nebulae. By studying these lines, astronomers can probe the cold depths of interstellar space. In most cases the elemental abundances detected in interstellar clouds mirror those found in other astronomical objects—perhaps not surprising, since interstellar clouds are the regions that spawn emission nebulae and stars.
