A basic problem with the optical technique just described is that we can examine interstellar clouds only along the line of sight to a distant star. To form an absorption line, there has to be a background source of radiation to absorb. The need to see stars through clouds also restricts this approach to relatively local regions, within a few thousand parsecs of Earth. Beyond that distance, stars are completely obscured, and optical observations are impossible. As we have seen, infrared observations provide a means of viewing the emission from some clouds, but they do not completely solve the problem, as only the denser, dustier clouds emit enough infrared radiation for astronomers to study them in that part of the spectrum.
To probe interstellar space more thoroughly, we need a more general, more versatile observational method—one that does not rely on conveniently located stars and nebulae. In short, we need a way to detect cold, neutral interstellar matter anywhere in space through its own radiation. This may sound impossible, but such an observational technique does in fact exist. The method relies on low-energy radio emissions produced by the interstellar gas itself.
Recall that a hydrogen atom has one electron orbiting a single-proton nucleus. Besides its orbital motion around the central proton, the electron also has some rotational motion—that is, spin—about its own axis. The proton also spins. This model parallels a planetary system, in which, in addition to the orbital motion of a planet about a central star, both the planet (electron) and the star (proton) rotate about their own axes.
The laws of physics dictate that there are exactly two possible spin configurations for a hydrogen atom in its ground state. The electron and proton can rotate in the same direction, with their spin axes parallel, or they can rotate with their axes antiparallel (that is, parallel, but oppositely oriented). Figure 18.15 shows these two configurations. The antiparallel configuration has slightly less energy than the parallel state.
Figure 18.15 Diagram of a ground-level hydrogen atom changing from a higher—energy state (electron and proton spins are parallel) to a lower—energy state (spins are antiparallel). The emitted photon carries away an energy equal to the energy difference between the two spin states.
All matter in the universe tends to achieve its lowest possible energy state, and interstellar gas is no exception. A slightly excited hydrogen atom with the electron and proton spinning in the same direction eventually drops down to the less energetic, opposite—spin state as the electron suddenly and spontaneously reverses its spin. As with any other such change, the transition from a high—energy state to a low—energy state releases a photon with energy equal to the energy difference between the two levels.
Because the energy difference between the two states is very small, the energy of the emitted photon is very low. Consequently, the wavelength of the radiation is rather long—in fact, about 21 cm, roughly the width of this book. That wavelength lies in the radio portion of the electromagnetic spectrum. Researchers refer to the spectral line that results from this hydrogen—spin—flip process as the 21-centimeter radiation. It provides a vital probe into any region of the universe containing atomic hydrogen gas. Figure 18.16 shows typical spectral profiles of 21—cm radio signals observed from several different regions of space. These tracings are the characteristic signatures of cold, atomic hydrogen in our Galaxy. Needing no visible starlight to help calibrate their signals, radio astronomers can observe any interstellar region that contains enough hydrogen gas to produce a detectable signal. Even the low—density regions between the dark clouds can be studied.
Figure 18.16 Typical 21-cm radio spectral lines observed from several different regions of interstellar space. The peaks do not all occur at a wavelength of exactly 21 cm, corresponding to a frequency of 1.4 GHz (gigahertz), because the gas in the Galaxy is moving with respect to Earth.
As can be seen in Figure 18.16, actual 21—cm lines are quite jagged and irregular, somewhat like nebular emission lines in appearance. These irregularities arise because there are usually numerous clumps of interstellar gas along any given line of sight. Each has its own density, temperature, radial velocity, and internal motion, so the intensity, width, and Doppler shift of the resultant 21—cm line vary from place to place. All these different lines are superimposed in the signal we eventually receive at Earth, and sophisticated computer analysis is generally required to disentangle them. The figures quoted earlier for the temperatures (100 K) and densities (106 atoms/m3) of the regions among the dark dust clouds are based on 21—cm measurements; observations of the dark clouds themselves yield densities and temperatures in good agreement with those obtained by optical spectroscopy.
All interstellar atomic hydrogen emits 21—cm radiation. But if all atoms eventually fall into their lowest-energy configuration, why isn't all the hydrogen in the Galaxy in the lower-energy state by now— Why do we see 21—cm radiation today? The answer is that the energy difference between the two states is comparable to the energy of a typical atom at a temperature of 100 K or so. As a result, atomic collisions in the interstellar medium are energetic enough to boost the electron into the higher-energy configuration and so maintain comparable numbers of hydrogen atoms in either state. At any instant, any sample of interstellar hydrogen will contain many atoms in the upper level, and 21—cm radiation will always be emitted.
Of great importance, the wavelength of this characteristic radiation is much larger than the typical size of interstellar dust particles. Accordingly, this radio radiation reaches Earth completely unscattered by interstellar debris. The opportunity to observe interstellar space well beyond a few thousand parsecs, and in directions lacking background stars, makes 21—cm observations among the most important and useful in all astronomy.
