l>Chapter 18, thing Review


The interstellar medium occupies the space among the stars. The is made up of cold (less 보다 100 K) gas, mainly atomic or molecule hydrogen and also helium, and also dust grains. Interstellar dust is an extremely effective in ~ blocking our view of remote stars, also though the density of the interstellar medium is really low. The spatial circulation of interstellar issue is really patchy. The basic diminution of starlight by dust is called extinction. In addition, the dust preferentially absorbs short-wavelength radiation, bring about a distinctive reddening of light passing interstellar clouds.

Interstellar dust is believed to be created of silicates, graphite, iron, and "dirty ice." Interstellar dust corpuscle are apparently elongated or rodlike. The polarization of starlight provides a method of studying them.

A nebula is a general term for any kind of fuzzy shining or dark job on the sky. Emission nebulae are extended clouds the hot, glow interstellar gas. Associated with star formation, they an outcome when hot O and also B stars heat and also ionize their surroundings. Research studies of the emission lines produced by excited nebular atoms enable astronomers to measure the nebula"s properties. Nebulae are regularly crossed by dark dust lanes—part of the larger cloud from which castle formed.

Some excited atomic claims take so lengthy to emit a photon the the spectral lines connected with these transitions are never seen in terrestrial laboratories, wherein collisions constantly knock the atom into one more energy state prior to it have the right to emit any radiation. As soon as these lines are seen in nebular spectra, lock are called forbidden lines.

Dark dust clouds are cold, irregularly shaped regions in the interstellar tool that diminish or totally obscure the light from lift stars. Astronomers can learn about these clouds by researching the absorb lines they create in starlight the passes with them.

Another means to watch cold, dark regions of interstellar space is v 21-centimeter radiation. Such radiation is developed whenever the electron in an atom the hydrogen reverses that is spin, an altering its energy an extremely slightly in the process. This radio radiation is important since it is emitted by all cool atom hydrogen gas, even if the gas is undetectable by various other means. In addition, 21—cm radiation is not appreciably soaked up by the interstellar medium, therefore radio astronomers making observations at this wavelength deserve to "see" to an excellent distances.

The interstellar medium additionally contains plenty of cold, dark molecular clouds, which space observed mostly through the radio radiation emitted through the molecule they contain. Dust within this clouds more than likely both protects the molecules and also acts as a catalyst to aid them form. Similar to other interstellar clouds, hydrogen is by much the most common constituent, however molecular hydrogen wake up to be very hard to observe. Astronomers usually study these clouds through observations of various other "tracer" molecules the are less common but much much easier to detect.

Astronomers think that molecular clouds are likely sites of future star formation. Often, number of molecular clouds are discovered close come one another, developing an massive molecular cloud complex millions of times more massive 보다 the Sun.



You are watching: What information does 21 cm radiation provide about the gas clouds?

Interstellar issue is quite evenly dispersed throughout the Milky method Galaxy. (Hint)

2. In the vicinity of the sunlight there is around as lot mass in the type of interstellar matter as in the kind of stars. (Hint)

3. there is a absence of heavy aspects in interstellar gas because they get in making interstellar dust. (Hint)

4. The truth that starlight becomes polarized as it passes with the interstellar tool tells united state that interstellar dust particles space spherical in shape. (Hint)

5. A typical region of dark interstellar an are has a temperature of about 500 K. (Hint)

6. An emissions nebula is a cloud of dust showing the light of a surrounding star cluster. (Hint)

7. emission nebulae screen spectra virtually identical to those that the stars installed in them. (Hint)

8. "Forbidden" emission lines can take place in emission nebulae due to the fact that the thickness of interstellar gas over there is exceptionally low. (Hint)

9. since of the obscuration of visible light by interstellar dust, we have the right to observe stars just within a few thousand parsecs of planet in any type of direction. (Hint)

10. A typical dark dust cloud is many hundreds that parsecs across. (Hint)

11. due to the fact that of your low temperatures, dark dust clouds radiate mainly in the radio component of the electromagnetic spectrum. (Hint)

12. many interstellar issue exists in the form of molecular clouds. (Hint)

13. 21—cm radiation offers astronomers with info on the density, temperature, and internal activities of interstellar gas. (Hint)

14. 21—cm radiation deserve to pass unimpeded through the entire Milky way Galaxy. (Hint)

15. Water, formaldehyde, carbon monoxide, and numerous essential molecules are all generally found in molecule clouds.


1. The interstellar medium is comprised of _____ and _____. (Hint)

2. to scatter a beam of radiation, a particle must be _____ in size to the wavelength the the radiation. (Hint)

3. extinction is the _____ the starlight by interstellar _____. (Hint)

4. The thickness of interstellar matter deserve to be characterized as being really _____. (Hint)

5. Interstellar gas is created of 90 percent _____ and 9 percent _____. (Hint)

6. The temperature the a typical emission nebula is around ____ K. (Hint)

7. The procedure in i beg your pardon clouds that cool thick gas (such as in the Eagle Nebula) are consumed away by radiation from nearby hot young stars is dubbed ____. (Hint)

8. an HII an ar is one more name because that an _____. (Hint)

9. Dark dust clouds deserve to have temperatures together low together _____ K. (Hint)

10. 21-cm radiation is emitted by _____ hydrogen. (Hint)

11. 21-cm radiation outcomes from a change in the _____ that the electron in a _____ atom. (Hint)

12. molecule clouds commonly have temperatures of about _____ K. (Hint)

13. Emissions from molecule clouds room in the _____ component of the electromagnetic spectrum. (Hint)

14. The most usual constituent of molecule clouds is molecular _____. (Hint)

15. A molecular cloud complex may save as much as _____ solar masses the gas. (Hint)


1. provide a brief description of the interstellar medium. (Hint)

2. What is the ingredient of interstellar gas? What around interstellar dust? (Hint)

3. How thick is interstellar matter in between the stars, top top average? (Hint)

4. If space is a near-perfect vacuum, how deserve to there be sufficient dust in it come block light? (Hint)

5. how is interstellar matter distributed throughout space? (Hint)

6. What space some methods that astronomers use to examine interstellar dust? (Hint)

7. What is an emissions nebula? (Hint)

8. Why do emission nebulae appear red in color photographs? (Hint)

9. offer a short description that a dark dust cloud. (Hint)

10. What is 21—cm radiation? v what facet is that associated? (Hint)

11. Why can"t 21—cm radiation be offered to probe the interiors of molecule clouds? (Hint)

12. just how does a molecular cloud differ from other interstellar matter? (Hint)

13. If our sun were surrounding by a cloud of gas, would this cloud be an emission nebula? Why or why not? (Hint)

14. to compare the reddening of stars by interstellar dust through the reddening of the setup Sun. (Hint)

15. explain what it means for a star"s irradiate to be polarized. Exactly how does the polarization of starlight carry out a method of researching the interstellar medium? (Hint)


1. The average thickness of interstellar gas within about 100 computer of the sun (in the regional Bubble) is much reduced than the value discussed in the text—in fact, it is approximately 103 hydrogen atoms/m3. Offered that the mass of a hydrogen atom is 1.7 10-27 kg, calculation the total mass that interstellar matter included within a volume equal to the of Earth.

2. presume the exact same average density as in the vault question, calculate the full mass the interstellar hydrogen included within a cylinder of cross-sectional area 1 cm2, prolonging from planet to Alpha Centauri. (Hint)

3. assuming a thickness of 3000 kg/m3, estimate the mass of the dust particle shown in figure 18.3. (Hint)

4. A beam of light shining through a dense molecular cloud is decreased in strongness by a variable of 2.5 for every 3 computer it travels. Through what complete factor is it lessened if the complete thickness that the cloud is 60 pc?

5. Interstellar die out is occasionally measured in magnitudes every kiloparsec (1 kpc = 1000 pc). Light from a star 250 pc away is observed to be decreased in strongness by a factor of 100 end and over the impact of the inverse-square law. What is the mean interstellar extinction along the line of sight, in mag/kpc?

6. In stimulate to bring enough power to ionize a hydrogen atom, a photon must have a wavelength of less than 9.12 10-8 m (91.2 nm). Utilizing Wien"s law (Section 3.4), calculate the temperature a star must have actually for the top wavelength that its blackbody curve to equal this value.

7. calculation the to escape speeds close to the edge of the 4 emission nebulae detailed in Table 18.1, and also compare them v the typical speeds of hydrogen nuclei in those nebulae (see much more Precisely 8-1). Carry out you think that is possible that the nebulae are held together by their own gravity? (Hint)

8. A roughly spherical dark dust cloud has actually a radius of 10 pc and also a thickness of 109 atoms/m3. If there is one dust particle per sunshine (1012) atoms, the average mass the a dust particle is together in problem 3, and also the gas includes 1 helium atom (of mass 4 times the fixed of a hydrogen atom) for every 9 hydrogen atoms, calculate the total mass of gas and also dust in the cloud. (Hint)

9. calculation the frequency that 21-cm radiation. If a team of interstellar clouds follow me the heat of sight have actually radial velocities in the variety 75 km/s (receding) come 50 km/s (approaching), calculation the variety of frequencies and wavelengths end which the 21—cm line will certainly be observed.

10. calculate the radius the a spherical molecular cloud whose complete mass equals the mass of the Sun. Assume a cloud density of 1012 hydrogen atoms every cubic meter. (Hint)


1. The constellation Orion the Hunter is influential in the evening sky of winter. Its many noticeable feature is a short, right row of three medium-bright stars: the famous belt of Orion. A line of stars extends indigenous the eastmost star the the belt, towards 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 through your eye, with binoculars, and also with a telescope. What is the color? How deserve to you account because that this? through the telescope, shot to find the Trapezium, a group of four stars in the facility of M42. These are hot, young stars; their energy causes the Orion Nebula to glow.


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observe the Milky way on a dark, very clear night. Is it a consistent band of light across the sky or is the mottled? The components of the Milky way that appear lacking are in reality dark dust clouds that are fairly near the Sun. Determine the constellations in which you view these clouds. Make a sketch and also compare through a star atlas. Find other tiny clouds in the atlas and try to discover them v your eye or v binoculars.