In certain interstellar regions of cold (typically 20 K) neutral gas, densities can reach as high as 10¹² particles/m³. Until the late 1970s, astronomers regarded these regions simply as abnormally dense interstellar clouds, but it is now recognized that they belong to an entirely new class of interstellar matter. The gas particles in these regions are not in atomic form at all; they are molecules. Because of the predominance of molecules in these dense interstellar regions, they are known as molecular clouds. Only within recent years have astronomers begun to appreciate the vastness of these clouds. They literally dwarf even the largest emission nebulae, which were previously thought to be the most massive residents of interstellar space.

MOLECULAR SPECTRAL LINES

As noted in Chapter 4, molecules can become collisionally or radiatively excited, much like atoms. (Sec. 4.3) Furthermore, again like atoms, molecules eventually relax back to their ground states, emitting radiation in the process. The energy states of molecules are much more complex than those of atoms, however. Molecules, like atoms, can undergo internal electron transitions, but unlike atoms, they can also rotate and vibrate. They do so in specific ways, obeying the laws of quantum physics. Figure 18.17 depicts a simple molecule rotating rapidly—that is, in an excited rotational state. After a length of time that depends on its internal makeup, the molecule relaxes back to a slower rotational rate (a state of lower energy). This change causes a photon to be emitted, carrying an energy equal to the energy difference between the two rotational states involved. The energy differences between these states are generally very small, so the emitted radiation is usually in the radio range.

Figure 18.17 As a molecule changes from a rapid rotation (left) to a slower rotation (right), a photon is emitted that can be detected with a radio telescope. Depicted here is the formaldehyde molecule, H₂CO. The length of the curved arrows is proportional to the spin rate of the molecule.

We are fortunate that molecules emit radio radiation, because they are invariably found in the densest and dustiest parts of interstellar space. These are regions where the absorption of shorter-wavelength radiation is enough to prohibit the use of ultraviolet, optical, and most infrared techniques that might ordinarily detect changes in the energy states of the molecules. Only low-frequency radio radiation can escape.

Why are molecules found only in the densest and darkest of the interstellar clouds? One possible reason is that the dust serves to protect the fragile molecules from the normally harsh interstellar environment—the same absorption that prevents high-frequency radiation from getting out to our detectors also prevents it from getting in to destroy the molecules. Another possibility is that the dust acts as a catalyst that helps form the molecules. The grains provide both a place where atoms can stick and react and a means of dissipating any heat associated with the reaction, which might otherwise destroy the newly formed molecules. Probably the dust plays both roles. The close association between dust grains and molecules in dense interstellar clouds argues strongly in favor of this picture, but the details are still being debated.

MOLECULAR TRACERS

In mapping molecular clouds, radio astronomers are faced with a problem. Molecular hydrogen (H₂) is by far the most common constituent of these clouds, but unfortunately, despite its abundance, this molecule does not emit or absorb radio radiation. It emits only short-wavelength ultraviolet radiation, so it cannot easily be used as a probe of cloud structure. Nor are 21-cm observations helpful—they are sensitive only to atomic hydrogen, not to the molecular form of the gas. Theorists had expected H₂ to abound in these dense, cold pockets of interstellar space, but proof of its existence was hard to obtain. Only when spacecraft measured the ultraviolet spectra of a few stars located near the edges of some dense clouds was the presence of molecular hydrogen confirmed.

With hydrogen effectively ruled out as a probe of molecular clouds, astronomers must use observations of other molecules to study the dark interiors of these dusty regions. Molecules such as carbon dioxide (CO), hydrogen cyanide (HCN), ammonia (NH₃), water (H₂O), methyl alcohol (CH₃OH), formaldehyde (H₂CO), and about 60 others, some of them quite complex, are now known to exist in interstellar space. *

(Some remarkably complex organic molecules have been found in the densest of the dark interstellar clouds, including formaldehyde (H₂CO), ethyl alcohol (CH₃CH₂OH), methylamine (CH₃NH₂), and formic acid (H₂CO₂). Their presence has fueled speculation about the origins of life, both on Earth and in the interstellar medium—especially since the recent report by radio astronomers of evidence that glycine (NH₂CH₂COOH), one of the key amino acids that form the large protein molecules in living cells, may also be present in interstellar space.)

These molecules are found only in very small quantities—they are generally 1 million to 1 billion times less abundant than H₂—but they are important as tracers of a cloud's structure and physical properties. They are thought to be produced by chemical reactions within molecular clouds. When we observe them, we know that the regions under study must also contain high densities of molecular hydrogen, dust, and other important constituents.

The rotational properties of different molecules often make them suitable as probes of regions with different physical properties. Formaldehyde may provide the most useful information on one region, carbon monoxide on another, and water on yet another, depending on the densities and temperatures of the regions involved. These data equip astronomers with a sophisticated spectroscopic "toolbox" for studying the interstellar medium.

For example, Figure 18.18 shows some of the sites where formaldehyde molecules have been detected near M20. At practically every dark area sampled between M16 and M8, this molecule is present in surprisingly large abundance (although it is still far less common than H₂). Analyses of spectral lines at many locations along the 12°-wide swath shown in Figure 18.6 indicate that the temperature and density are much the same in all the molecular clouds studied (50 K and 10¹¹ molecules/m³, on average). Figure 18.19 shows a contour map of the distribution of formaldehyde molecules in the immediate vicinity of the M20 nebula. It was made by observing radio spectral lines of formaldehyde at various locations and then drawing contours connecting regions of similar abundance. Notice that the amount of formaldehyde (and, we assume, the amount of hydrogen) peaks in a dark region, well away from the visible nebula.

Figure 18.18 Spectra indicate that formaldehyde molecules exist around M20, as indicated by the arrows. The lines are most intense both in the dark dust lanes trisecting the nebula and in the dark regions beyond the nebula.

Figure 18.19 Contour map of the amount of formaldehyde near the M20 nebula, demonstrating how formaldehyde is especially abundant in the darkest interstellar regions. Other kinds of molecules have been found to be similarly distributed. The contour values increase from the outside to the inside, so the maximum density of formaldehyde lies just to the bottom right of the visible nebula. The green and red contours outline the intensity of the formaldehyde absorption lines at different rotational frequencies. The nebula itself is about 4 pc across.