Chapter 12

Which of the following characteristics is most likely to be the same among stars in a given cluster? a. age b. spectral type c. temperature d. mass e. presence of planets

Which phase of the ISM is most opaque to visible light? a. H I region b. H II region c. molecular cloud d. WISM e. coronal gas

Which of the following is/are not a source of turbulence in the ISM? a. rotation of the galaxy b. supernova explosions c. ultraviolet radiation from young stars d. jets from young stars e. infrared radiation from young stars

Given three forms of hydrogen, rank them correctly according to increasing temperature of the environments in which they are usually found. a. neutral, ionized, molecular b. molecular, neutral, ionized c. ionized, neutral, molecular d. molecular, ionized, neutral e. neutral, molecular, ionized

Which phase of the ISM is found near hot, young stars? a. H I region b. H II region c. molecular cloud d. WISM e. coronal gas

Which of the following is not a quality that all phases of the ISM share? a. absence of free electrons as part of gas b. more gas than dust c. more hydrogen than other elements d. turbulence e. much lower density than that of Earth's atmosphere

What kinds of photons are absorbed by typical dust grains? a. infrared b. visible c. ultraviolet d. radio e. microwave

Why does interstellar dust play a vital role in the chemistry of the galaxy? a. It allows gamma rays to penetrate into dark clouds. b. When it breaks apart, it provides new elements for the ISM. c. There is so much more of it than there is gas. d. Chemistry can occur on its surface. e. Its color changes the appearance of stars.

How do molecular clouds provide protection from ultraviolet photons that can dissociate molecules? a. Their thermal energy deflects the ultraviolet photons. b. Their temperature is too low for the photons to be effective. c. Water within the cloud absorbs the photons. d. Their turbulence keeps molecules in rapid motion. e. The dense gas and dust in their interiors shield molecules from interstellar ultraviolet radiation.

Which of the following is not a characteristic of molecular clouds? a. uniform structure throughout b. presence of several kinds of molecules c. high density, relative to other phases of the ISM d. presence of dust e. mass many times greater than the Sun's

Many astronomical objects have dense cores. Which of the following do not? a. stars b. protostars c. molecular clouds d. globular clusters e. coronal gas clouds

Which of the following is not a typical outcome for material in an accretion disk around a protostar? a. It accretes onto the protostar. b. It becomes part of a planet. c. It gets eroded by photoevaporation. d. It remains in the disk indefinitely. e. It returns to the ISM.

Two protostars have evolved to the point of nuclear fusion. One has a temperature of 12 million \(\mathrm{K}\); the other, a temperature of 17 million K. Which of the following statements is/are true about the stars? Choose all that apply. a. The 17 -million-K star burns nuclear fuel faster. b. The 17 -million-K star may be using the CNO cycle of nuclear fusion. c. The 17 -million-K star will die sooner. d. The 17 -million-K star can be using only the P-P process of nuclear fusion. e. The stars may have identical masses.

A star has a mass of \(0.7 M_{\text {suat }} .\) Which statement about it is true? a. It will live less than 10 billion years. b. Its temperature is greater than 16 million \(\mathrm{K}\). c. It may be using the CNO cycle for nuclear fusion. d. It is a brown dwarf. e. It is using the \(P\) -P process of nuclear fusion.

From a \(1,000-M_{\text {Sun }}\) molecular cloud, one or more stars form. Which of the following is likely to be true? a. Many stars form that are almost all \(1-M_{\mathrm{Su} \text { a }}\) stars. b. One \(1,000-M_{\text {sua }}\) star forms. c. since molecular clouds exist near massive young stars, none of the stars will have accretion disks. d. A number of stars will form with a range of sizes, and most will be less- massive stars. e. Astronomers will be able to observe all the stars that form in visible wavelengths throughout the star formation process.

The last supernovas were observed in the Milky Way Galaxy more than 400 years ago. Which of the following statements related to these events is/are true? Choose all that apply. a. The rarity of these events is explained, at least in part, by the IMF. b. Each one may trigger the formation of new stars. c. Turbulence will increase in the ISM surrounding these superno- vas. d. Coronal gas will have been created in the process. e. Many more would have been observed in our galaxy with today's observing techniques.

Which of the following statements about interstellar dust is not true? a. It provides a platform for the formation of molecules. b. It emits blackbody radiation. c. It makes distant objects appear closer than they are. d. It shields star-forming regions from ultraviolet light. e. It is much rarer than gas in the ISM.

Carbon monoxide is useful as a tracer for molecular clouds because a. it is found in high concentrations. b. it can be excited to emit photons at lower temperatures than many other molecules do. c. it is an organic molecule. d. the light it emits is mostly ultraviolet light. e. it is found at higher temperatures than other molecules within molecular clouds.

An object has a mass that is less than 8 percent of the Sun's mass, and its elemental composition includes carbon, nitrogen, and oxygen. Which of the following is/are true? Choose all that apply a. It could be a planet. b. It could be a brown dwarf. c. It must be on or moving toward the main sequence. d. It must not have reached 16 million \(\mathrm{K}\). e. It must be fusing protons into deuterium.

What two measurement techniques provide astronomers with information about the temperature and composition of interstellar dust?

Astronomers believe most molecular clouds are made up of primarily molecular hydrogen, even though they locate most molecular clouds by using observations of \(\mathrm{CO}\) emission lines. Why don't they just look for the molecular hydrogen?

Why do molecular clouds have limited lifetimes?

What is the birth line on the HR diagram, and where is it?

Describe the components of a protostar.

How does a disk form around a protostar?

Describe the differences in the formation of a brown dwarf versus a planet; then describe the differences in the formation of a brown dwarf versus a main-sequence star.

How do live, massive stars affect nearby stars and clouds? What about dying massive stars?

Define proplyds and elepbant trunks as they relate to star formation and the ISM.

Describe the cause, location, and direction of jets in young stars.

Molecular cloud A has a pressure of \(P .\) Cloud B has a temperature half that of cloud \(A\) and a density 3 times as great. What is the ratio of the pressure in cloud \(B\) to that in cloud \(A\) ?

Two regions of the ISM have identical pressure, but the temperature of region 1 is 2.5 times that of region \(2 .\) How does the density of region 1 compare with that of region \(2 ?\)

Radiation from a nearby star has heated a cloud in the ISM from \(100 \mathrm{K}\) to \(550 \mathrm{K},\) and its density has dropped to 0.25 times its earlier density. What is the ratio of its current pressure to its pressure before these changes?

Region 2 of an interstellar cloud has 1.7 times the density of region 1 Assuming the pressure of the two regions is equal, what is the ratio of the temperature in region 2 to that of region 1 ?

The temperature in a region of coronal gas is \(10^{6} \mathrm{K},\) while a nearby H II region has a temperature of \(10,000 \mathrm{K}\). Assuming the pressure of the two regions is equal, what is the ratio of the density of the coronal gas to that of the H II region?

The free-fall time of a gravitationally collapsing cloud is \(T\). What would be its free-fall time in terms of \(T\) if its density were 3.2 times as high?

A cloud \(\mathrm{C}_{1}\) of density \(D\) has a calculated free-fall time of \(T_{1}\). For a similar cloud \(\mathrm{C}_{2}\) whose free-fall time is twice as long, what is the ratio of its density to that of \(\mathrm{C}_{1}\) ?

Two separate cores of equal density in a molecular cloud have radii of 1 light-year and 1.7 light-years, respectively. How does the free-fall time of the larger cloud compare with that of the smaller one?

Free-fall time for region 1 of an interstellar cloud is \(1.3 \times 10^{6}\) years; for region \(2,\) free-fall time is \(0.9 \times 10^{6}\) years. What is the ratio of the density of region 2 to that of region \(1 ?\)