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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 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-type stars heat and ionize their surroundings. Studies of the emission lines produced by excited nebular atoms allow astronomers to measure the nebula’s properties. 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. Nebulae are often crossed by dark dust lanes, part of the larger cloud from which they formed.

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. 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, the amount of mass in the form of interstellar matter is comparable to the amount of mass 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 group of stars. 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 far infrared 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 have all been found in molecular clouds. HINT


1. The interstellar medium is made up of _____ and _____. HINT

2. To scatter a beam of radiation, a particle’s size must be _____ the wavelength of the radiation. HINT

3. Extinction is the _____ of starlight by interstellar _____. HINT

4. The density of interstellar matter is 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 _____. 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 _____. HINT

12. Emissions from molecular clouds are in the _____ part of the electromagnetic spectrum. HINT

13. _____ is thought to play an important role in the formation of molecules in molecular clouds. 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. Why is interstellar dust so much more effective than interstellar gas at absorbing starlight? HINT

4. How dense is interstellar matter on average? 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. What is photoevaporation, and how does it change the structure and appearance of an emission nebula? HINT

9. Why are some spectral lines observed in emission nebulae not normally seen in laboratories on Earth? HINT

10. Describe some ways in which we can “see” a dark interstellar cloud. HINT

11. Give a brief description of a dark dust cloud. HINT

12. What is 21-cm radiation? With what element is it associated? HINT

13. Why is 21-cm radiation useful to astronomers? HINT

14. Why can’t 21-cm radiation be used to probe the interiors of molecular clouds? HINT

15. How does a molecular cloud differ from other interstellar matter? HINT

16. Why can’t astronomers use observations of hydrogen to explore the structure of molecular cloud complexes? HINT

17. How do astronomers explore the structure of molecular cloud complexes? HINT

18. If our Sun was surrounded by a cloud of gas, would this cloud be an emission nebula? Why or why not? HINT

19. Compare the reddening of stars by interstellar dust with the reddening of the setting Sun. HINT

20. 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

PROBLEMS Algorithmic versions of these questions are available in the Practice Problems Module of the Companion Website.

The number of squares preceding each problem indicates the approximate level of difficulty.

1. The average density of interstellar gas within 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 Bubble volume equal in size to planet Earth. HINT

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 m2, extending from Earth to Alpha Centauri. HINT

3. Given the average density of interstellar matter stated in Section 18.1, calculate how large a volume of space would have to be compressed to make a cubic meter of gas equal in density to air on Earth (1.2 kg/m3). HINT

4. Assuming a density of 3000 kg/m3, estimate the mass of the dust particle illustrated in Figure 18.3 (a). HINT

5. A beam of light shining through a dense molecular cloud is diminished in intensity by a factor of two for every 5 pc it travels. By how many magnitudes is the light from a background star dimmed, if the total thickness of the cloud is 60 pc? HINT

6. Interstellar extinction is sometimes measured in magnitudes per kiloparsec (1 kpc = 1000 pc). Light from a star 1500 pc away is observed to be diminished in intensity by a factor of 20 over and above the effect of the inverse-square law. What is the average interstellar extinction along the line of sight, in mag/kpc? HINT

7. Spectroscopic observations of a certain star reveal it to be a B2II giant, with absolute magnitude –6. (Secs. 17.4, 17.7) The star’s apparent magnitude is 14. Neglecting the effects of interstellar extinction, calculate the distance to the star. If the star’s distance is known (by other means) to be 5000 pc, calculate the average extinction along the line of sight, in mag/kpc. (More Precisely 17-1) HINT

8. A star of apparent magnitude 10 lies 500 pc from Earth. If interstellar absorption results in an average extinction of
2 mag/kpc, calculate the star’s absolute magnitude and luminosity. HINT

9. A star of known absolute magnitude 25 has apparent magnitude 10. If interstellar absorption results in an average extinction of 2 mag/kpc, calculate the star’s distance. (Note: This problem does not have an algebraic solution. You wil have to solve it by numerical means—essentially trial and error on a calculator.) HINT

10. 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, calculate the temperature a star must have for the peak wavelength of its blackbody curve to equal this value. (Sec. 3.4) HINT

11. 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. (More Precisely 8-1) Do you think it is possible that the nebulae are held together by their own gravity? HINT

12. What would the mass of M8 have to be in order for its escape speed to equal its average molecular speed? HINT

13. 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 21.1-cm (1420 MHz) line of hydrogen will be observed. (Sec. 3.5) HINT

14. 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

15. A cloud of atomic hydrogen has a radius of 1 pc and an average density 106 hydrogen atoms per cubic meter. Collisions between atoms ensure that, at any instant, 3/4 of all atoms are in the upper (parallel spin) state, as discussed in Section 18.4. The transition producing the 21-cm line is very unlikely—the probability that any given atom in the upper state will make the transition during any given second is about 3 10-15 (compare with 108 for the Ha transition). Use these figures, together with the Planck formula for the energy of the photon emitted in the transition, to estimate the total radio luminosity of the cloud. (Sec. 4.2) HINT


1. Exploring Density. The interstellar medium has a very low density of about 1000 per cubic kilometer. Estimate the population density of students in the tallest dormitory on campus using units of students per cubic feet and compare to the population density of the classroom. Explain your reasoning.

RESEARCHING ON THE WEB To complete the following exercises, go to the online Destinations Module for Chapter 18 on the Companion Website for Astronomy Today 4/e.

1. Access the "Messier Catalog" pages and determine which objects constitute the majority of objects M1 through M10 and which objects for M90 through M100.

2. Access the "Nebulae: Fuzzy Patches In Space" page and define the four types of nebulae.


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.

SKYCHART III PROJECTS The SkyChart III Student Version planetarium program on which these exercises are based is included as a separately executable program on the CD in the back of this text.

1. One of the most interesting nebula is the Great Orion Nebula, M42. Use SkyChart III to locate Orion, and determine what time of the year it is overhead at a convenient time for viewing. While the nebula is visible to the unaided eye, it is much better with binoculars, and only gets more interesting as you use larger and larger telescopes to observe it.

In addition to the Practice Problems and Destinations modules, the Companion Website at provides for each chapter an additional true-false, multiple choice, and labeling quiz, as well as additional annotated images, animations, and links to related Websites.