A basic difficulty 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, a background source must provide 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 observa-tions are impossible. As we have seen, infrared observaobserva-tions provide a means of viewing the emission from some clouds, but they do not completely solve the problem because 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.
Electron Spin
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 is analogous to 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. But bear in mind the crucial difference between plan-etary and atomic systems: A planet orbiting the Sun is free to move in any orbit and spin at any rate, but within an atom, all physical quantities, such as energy, momentum, and angular momentum (spin), are quantized—they are permitted to take on only specific, distinct values. (Sec. 4.2)
The laws of physics dictate that there are exactly two pos-sible spin configurations for a hydrogen atom in its ground state. The electron and proton can rotate in the same direc-tion, with their spin axes parallel, or they can rotate with their axes antiparallel (i.e., parallel, but oppositely oriented).
Figure 18.16 shows these two configurations. The antiparallel configuration has slightly less energy than the parallel state.
Radio Emission
All matter in the universe tends to achieve its lowest pos-sible energy state, and interstellar gas is no exception. A slightly excited hydrogen atom with the electron and pro-ton spinning in the same direction eventually drops down to the less energetic, opposite-spin state as the electron sud-denly and spontaneously reverses its spin. As with any other such change, the transition from a high-energy state to a
The emitted photon carries away energy equal to the difference in the two spin states.
A hydrogen atom has
▲ FIGURE 18.16 Hydrogen 21-cm Emission A ground-level hydrogen atom changing from a higher-energy state (top) to a lower-energy state (bottom).
25 462 CHAPTER 18 The Interstellar Medium
low-energy state releases a photon with energy equal to the energy difference between the two levels.
Because that energy difference is very small, the energy of the emitted photon is very low. (Sec. 4.2) Consequently, the wavelength of the radiation is rather long—in fact, it is 21.1 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 21-centimeter radia-tion. This spectral line provides a vital probe into any region of the universe containing atomic hydrogen gas.
Figure 18.17 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 star-light 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.
As can be seen in the figure, actual 21-cm lines are quite jagged and irregular, somewhat like nebular emission lines in appearance. The irregularities arise because there are usually numerous clumps of interstellar gas along any given line of sight, each with its own density, temperature, radial velocity, and internal motion. Thus, the intensity, width, and Doppler shift of the resultant 21-cm line vary from place to place. (Sec. 4.5) 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 “average” figures quoted earlier for the temperatures (100 K) and densities (106 atoms/m3) of the regions between the dark dust clouds are based on 21-cm measurements.
Observations of the dark clouds themselves using 21-cm radiation yield densities and temperatures in good agreement with those obtained by optical spectroscopy.
All interstellar atomic hydrogen emits 21-cm radia-tion. But if all atoms eventually fall into their lowest-energy
18.4 21-Centimeter Radiation
A basic difficulty 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, a background source must provide 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 observa-tions are impossible. As we have seen, infrared observaobserva-tions provide a means of viewing the emission from some clouds, but they do not completely solve the problem because 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.
Electron Spin
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 is analogous to 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. But bear in mind the crucial difference between plan-etary and atomic systems: A planet orbiting the Sun is free to move in any orbit and spin at any rate, but within an atom, all physical quantities, such as energy, momentum, and angular momentum (spin), are quantized—they are permitted to take on only specific, distinct values. (Sec. 4.2)
The laws of physics dictate that there are exactly two pos-sible spin configurations for a hydrogen atom in its ground state. The electron and proton can rotate in the same direc-tion, with their spin axes parallel, or they can rotate with their axes antiparallel (i.e., parallel, but oppositely oriented).
Figure 18.16 shows these two configurations. The antiparallel configuration has slightly less energy than the parallel state.
Radio Emission
All matter in the universe tends to achieve its lowest pos-sible energy state, and interstellar gas is no exception. A slightly excited hydrogen atom with the electron and pro-ton spinning in the same direction eventually drops down to the less energetic, opposite-spin state as the electron sud-denly and spontaneously reverses its spin. As with any other such change, the transition from a high-energy state to a
The emitted photon carries away energy equal to the difference in the two spin states.
A hydrogen atom has
▲ FIGURE 18.16 Hydrogen 21-cm Emission A ground-level hydrogen atom changing from a higher-energy state (top) to a lower-energy state (bottom).
‣As colisões atômicas são suficientes para elevar a energia do HI para o estado excitado com spins paralelos. A energia necessária para produzir essa transição, pode ser obtida da energia térmica do gás kT; com T= 100 K .
‣O e l é t r o n p o d e d e c a i r espontaneamente para o estado de menor energia (spins anti-paralelos) emitindo um fóton na frequência de rádio.
26
‣ FIGURA: Linhas de 21 cm típicas observadas em várias regiões diferentes do meio interestelar.
‣ Por causa da alta abundância de hidrogênio, apesar da longa vida média, ela é observada em todas as direções do céu.
‣ O s p i c o s n ã o o c o r r e m t o d o s n o comprimento de onda de exactamente 21,1 cm (1420 MHz), porque o gás em nossa Galáxia está em movimento em relação à terra.
‣ Essa emissão em rádio atravessa facilmente o meio interestelar, mesmo regiões com muita poeira e chega até nós.
SECTION 18.5 Interstellar Molecules 463
CONCEPT Check
4 Why is 21-cm radiation so useful as a probe of galactic structure?
18.5 Interstellar Molecules
In some particularly cold (typically, 10–20 K) interstellar regions, 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 mol-ecules in these dense interstellar regions, they are known as molecular clouds. They literally dwarf even the largest emis-sion nebulae, which were previously thought to be the most massive residents of interstellar space.
Molecular Spectral Lines
As noted in Chapter 4, much like atoms, molecules can become excited through collisions or by absorbing radiation.
(Sec. 4.4) Furthermore, again like atoms, molecules even-tually return to their ground states, emitting radiation in the process. The energy states of molecules are much more com-plex than those of atoms, however. Once more like atoms, mol-ecules 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 18.18 depicts a simple molecule rotating rapidly—that is, a molecule 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.
configuration, then why isn’t all the hydrogen in the Galaxy in the lower energy state by now? Why do we see 21-cm radia-tion 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 main-tain comparable numbers of hydrogen atoms in either state.
At any instant, any sample of interstellar hydrogen will con-tain many atoms in the upper level, so conditions will always be favorable for 21-cm radiation to be emitted.
Of great importance, the wavelength of this character-istic radiation is much larger than the typical size of inter-stellar dust particles. Accordingly, 21-cm 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 impor-tant and useful in all astronomy. We will see in Chapters 23 through 25 how such observations are indispensable in allowing astronomers to map out the large-scale structure of our Galaxy and many others.
Intensity
Frequency 1420 MHz
(Wavelength = 21.1 cm)
▲ FIGURE 18.17 21-cm Lines 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 21.1 cm, corresponding to a frequency of 1420 MHz, because the gas in our Galaxy is moving with respect to Earth.
This is a spinning formaldehyde molecule, H2CO.
▲ FIGURE 18.18 Molecular Emission 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. The lengths of the curved arrows are proportional to the spin rate of the molecule.
27
Observação de nuvens interestelares
‣ Regiões HII: gás ionizado por estrelas quentes; as temperaturas chegam até 104 K (nebulosas de emissão). Podem ser detectadas no óptico.
‣ Regiões HI: gás neutro, T = 50 a 100 K, são detectadas pela linha de 21 cm.
28
‣ As nuvens moleculares estão compostas predominantemente de moléculas.
‣ As primeiras moléculas interestelares foram descobertas em 1937-1938, na forma de metilidina CH, CH+, e cianogênio CN, aparentes nos espectros de algumas estrelas, mas causadas por absorção interestelar.
‣ Hidrogênio molecular H2 foi descoberto no início dos anos 1970, junto com monóxido de carbono CO. Muitos outros tipos de moléculas têm sido encontradas desde então, desde amônia NH3, até as mais complexas como etanol C2H5OH.
‣ Baseado principalmente nas observações de CO, nota-se que as moléculas estão concentradas em nuvens moleculares, com massas de poucas vezes até um milhão de massas solares, e se estendem de alguns até cerca de 600 anos-luz. As temperaturas são da ordem de 10 K.
‣ As estrelas se formam nas partes mais densas destas nuvens moleculares.