The interstellar medium occupies the space among the stars. It is made up of cold (less than 100 K) gas, mostly atomic or molecular hydrogen and helium, and dust grains. Interstellar dust is very effective at blocking our view of distant stars, even though the density of the interstellar medium is very low. The spatial distribution of interstellar matter is very patchy. The general diminution of starlight by dust is called extinction. In addition, the dust preferentially absorbs short-wavelength radiation, leading to a distinct reddening of light passing interstellar clouds.
Interstellar dust is thought to be composed of silicates, graphite, iron, and "dirty ice." Interstellar dust particles are apparently elongated or rodlike. The polarization of starlight provides a means of studying them.
A nebula is a general term for any fuzzy bright or dark patch on the sky. Emission nebulae are extended clouds of hot, glowing interstellar gas. Associated with star formation, they result when hot O and B stars heat and ionize their surroundings. Studies of the emission lines produced by excited nebular atoms allow astronomers to measure the nebula's properties. Nebulae are often crossed by dark dust lanes—part of the larger cloud from which they formed.
Some excited atomic states take so long to emit a photon that the spectral lines associated with these transitions are never seen in terrestrial laboratories, where collisions always knock the atom into another energy state before it can emit any radiation. When these lines are seen in nebular spectra, they are called forbidden lines.
Dark dust clouds are cold, irregularly shaped regions in the interstellar medium that diminish or completely obscure the light from background stars. Astronomers can learn about these clouds by studying the absorption lines they produce in starlight that passes through them.
Another way to observe cold, dark regions of interstellar space is through 21-centimeter radiation. Such radiation is produced whenever the electron in an atom of hydrogen reverses its spin, changing its energy very slightly in the process. This radio radiation is important because it is emitted by all cool atomic hydrogen gas, even if the gas is undetectable by other means. In addition, 21—cm radiation is not appreciably absorbed by the interstellar medium, so radio astronomers making observations at this wavelength can "see" to great distances.
The interstellar medium also contains many cold, dark molecular clouds, which are observed mainly through the radio radiation emitted by the molecules they contain. Dust within these clouds probably both protects the molecules and acts as a catalyst to help them form. As with other interstellar clouds, hydrogen is by far the most common constituent, but molecular hydrogen happens to be very hard to observe. Astronomers usually study these clouds through observations of other "tracer" molecules that are less common but much easier to detect.
Astronomers believe that molecular clouds are likely sites of future star formation. Often, several molecular clouds are found close to one another, forming an enormous molecular cloud complex millions of times more massive than the Sun.
1. Interstellar matter is quite evenly distributed throughout the Milky Way Galaxy. (Hint)
2. In the vicinity of the Sun there is about as much mass in the form of interstellar matter as in the form of stars. (Hint)
3. There is a lack of heavy elements in interstellar gas because they go into making interstellar dust. (Hint)
4. The fact that starlight becomes polarized as it passes through the interstellar medium tells us that interstellar dust particles are spherical in shape. (Hint)
5. A typical region of dark interstellar space has a temperature of about 500 K. (Hint)
6. An emission nebula is a cloud of dust reflecting the light of a nearby star cluster. (Hint)
7. Emission nebulae display spectra almost identical to those of the stars embedded in them. (Hint)
8. "Forbidden" emission lines can occur in emission nebulae because the density of interstellar gas there is extremely low. (Hint)
9. Because of the obscuration of visible light by interstellar dust, we can observe stars only within a few thousand parsecs of Earth in any direction. (Hint)
10. A typical dark dust cloud is many hundreds of parsecs across. (Hint)
11. Because of their low temperatures, dark dust clouds radiate mainly in the radio part of the electromagnetic spectrum. (Hint)
12. Most interstellar matter exists in the form of molecular clouds. (Hint)
13. 21—cm radiation provides astronomers with information on the density, temperature, and internal motions of interstellar gas. (Hint)
14. 21—cm radiation can pass unimpeded through the entire Milky Way Galaxy. (Hint)
15. Water, formaldehyde, carbon monoxide, and numerous organic molecules are all commonly found in molecular clouds.
1. The interstellar medium is made up of _____ and _____. (Hint)
2. To scatter a beam of radiation, a particle must be _____ in size to the wavelength of the radiation. (Hint)
3. Extinction is the _____ of starlight by interstellar _____. (Hint)
4. The density of interstellar matter can be characterized as being very _____. (Hint)
5. Interstellar gas is composed of 90 percent _____ and 9 percent _____. (Hint)
6. The temperature of a typical emission nebula is about ____ K. (Hint)
7. The process in which clouds of cool dense gas (such as in the Eagle Nebula) are eaten away by radiation from nearby hot young stars is called ____. (Hint)
8. An HII region is another name for an _____. (Hint)
9. Dark dust clouds can have temperatures as low as _____ K. (Hint)
10. 21-cm radiation is emitted by _____ hydrogen. (Hint)
11. 21-cm radiation results from a change in the _____ of the electron in a _____ atom. (Hint)
12. Molecular clouds typically have temperatures of about _____ K. (Hint)
13. Emissions from molecular clouds are in the _____ part of the electromagnetic spectrum. (Hint)
14. The most common constituent of molecular clouds is molecular _____. (Hint)
15. A molecular cloud complex may contain as much as _____ solar masses of gas. (Hint)
1. Give a brief description of the interstellar medium. (Hint)
2. What is the composition of interstellar gas? What about interstellar dust? (Hint)
3. How dense is interstellar matter between the stars, on average? (Hint)
4. If space is a near-perfect vacuum, how can there be enough dust in it to block light? (Hint)
5. How is interstellar matter distributed throughout space? (Hint)
6. What are some methods that astronomers use to study interstellar dust? (Hint)
7. What is an emission nebula? (Hint)
8. Why do emission nebulae appear red in color photographs? (Hint)
9. Give a brief description of a dark dust cloud. (Hint)
10. What is 21—cm radiation? With what element is it associated? (Hint)
11. Why can't 21—cm radiation be used to probe the interiors of molecular clouds? (Hint)
12. How does a molecular cloud differ from other interstellar matter? (Hint)
13. If our Sun were surrounded by a cloud of gas, would this cloud be an emission nebula? Why or why not? (Hint)
14. Compare the reddening of stars by interstellar dust with the reddening of the setting Sun. (Hint)
15. Explain what it means for a star's light to be polarized. How does the polarization of starlight provide a means of studying the interstellar medium? (Hint)
1. The average density of interstellar gas within about 100 pc of the Sun (in the Local Bubble) is much lower than the value mentioned in the text—in fact, it is roughly 103 hydrogen atoms/m3. Given that the mass of a hydrogen atom is 1.7 
10-27 kg, calculate the total mass of interstellar matter contained within a volume equal to that of Earth.
2. Assuming the same average density as in the previous question, calculate the total mass of interstellar hydrogen contained within a cylinder of cross-sectional area 1 cm2, extending from Earth to Alpha Centauri. (Hint)
3. Assuming a density of 3000 kg/m3, estimate the mass of the dust particle illustrated in Figure 18.3. (Hint)
4. A beam of light shining through a dense molecular cloud is diminished in intensity by a factor of 2.5 for every 3 pc it travels. By what total factor is it reduced if the total thickness of the cloud is 60 pc?
5. Interstellar extinction is sometimes measured in magnitudes per kiloparsec (1 kpc = 1000 pc). Light from a star 250 pc away is observed to be diminished in intensity by a factor of 100 over and above the effect of the inverse-square law. What is the average interstellar extinction along the line of sight, in mag/kpc?
6. In order to carry enough energy to ionize a hydrogen atom, a photon must have a wavelength of less than 9.12 10-8 m (91.2 nm). Using Wien's law (Section 3.4), calculate the temperature a star must have for the peak wavelength of its blackbody curve to equal this value.
7. Estimate the escape speeds near the edges of the four emission nebulae listed in Table 18.1, and compare them with the average speeds of hydrogen nuclei in those nebulae (see More Precisely 8-1). Do you think it is possible that the nebulae are held together by their own gravity? (Hint)
8. A roughly spherical dark dust cloud has a radius of 10 pc and a density of 109 atoms/m3. If there is one dust particle per trillion (1012) atoms, the average mass of a dust particle is as in problem 3, and the gas contains 1 helium atom (of mass four times the mass of a hydrogen atom) for every 9 hydrogen atoms, calculate the total mass of gas and dust in the cloud. (Hint)
9. Calculate the frequency of 21-cm radiation. If a group of interstellar clouds along the line of sight have radial velocities in the range 75 km/s (receding) to 50 km/s (approaching), calculate the range of frequencies and wavelengths over which the 21—cm line will be observed.
10. Calculate the radius of a spherical molecular cloud whose total mass equals the mass of the Sun. Assume a cloud density of 1012 hydrogen atoms per cubic meter. (Hint)
1. The constellation Orion the Hunter is prominent in the evening sky of winter. Its most noticeable feature is a short, straight row of three medium-bright stars: the famous belt of Orion. A line of stars extends from the eastmost star of the belt, toward the south. This line represents Orion's sword. Towards the bottom of the sword is the sky's most famous emission nebula, M42, the Orion Nebula.
Observe the Orion Nebula with your eye, with binoculars, and with a telescope. What is its color? How can you account for this? With the telescope, try to find the Trapezium, a grouping of four stars in the center of M42. These are hot, young stars; their energy causes the Orion Nebula to glow.
2. Observe the Milky Way on a dark, very clear night. Is it a continuous band of light across the sky or is it mottled? The parts of the Milky Way that appear missing are actually dark dust clouds that are relatively near the Sun. Identify the constellations in which you see these clouds. Make a sketch and compare with a star atlas. Find other small clouds in the atlas and try to find them with your eye or with binoculars.
