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.
|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 http://www.prenhall.com/chaisson
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.