Questions

Review Questions

  1. If no one has ever seen a star go through the complete formation process, how are we able to understand how stars form?

  2. Why is it more difficult to observe the life cycles of stars than the life cycles of planets or animals?

  3. If an interstellar medium fills the space between the stars, how is that we are able to see the stars at all?

  4. Summarize the evidence that interstellar space contains (a) gas and (b) dust.

  5. What are H II regions? Near what kinds of stars are they found? Why do only these stars give rise to H II regions?

  6. What are stationary absorption lines? In what sort of spectra are they seen? How do they give evidence for the existence of the interstellar medium?

  7. In Figure 18-2, what makes the Horsehead Nebula dark? What makes IC 434 glow?

  8. Why is the daytime sky blue? Why are distant mountains purple? Why is the Sun red when seen near the horizon at sunrise or sunset? In what ways are your answers analogous to the explanations for the bluish color of reflection nebulae and the process of interstellar reddening?

  9. To see the constellation Coma Berenices (Berenice’s Hair) you must look perpendicular to the plane of the Milky Way. By contrast, the Milky Way passes through the constellation Cassiopeia (named for a mythical queen). Would you expect H II regions to be more abundant in Coma Berenices or in Cassiopeia? Explain your reasoning.

  10. The interior of a dark nebula is billions of times less dense than the air that you breathe. How, then, are dark nebulae able to block out starlight?

  1. Why are low temperatures necessary in order for protostars to form inside dark nebulae?

  2. Compare and contrast Barnard objects and Bok globules. How many Sun-sized stars could you make out of a Barnard object? Out of a Bok globule?

  3. Describe the energy source that causes a protostar to shine. How does this source differ from the energy source inside a main-sequence star?

  1. What is an evolutionary track? How can evolutionary tracks help us interpret the H-R diagram?

  2. What happens inside a protostar to slow and eventually halt its gravitational contraction?

  3. Why are the evolutionary tracks of high-mass stars different from those of low-mass stars? For which kind of star is the evolution more rapid? Why?

  4. Why are protostars more easily seen with an infrared telescope than with a visible-light telescope?

  5. In what ways is the internal structure of a 1-M⊙ main-sequence star different from that of a 5-M⊙ main-sequence star? From that of a 0.5-M⊙ main-sequence star? What features are common to all these stars?

  6. What sets the limits on the maximum and minimum masses of a main-sequence star?

  7. What are T Tauri stars? How do we know that they eject matter at high speed? How does their rate of mass loss compare to that of the Sun?

  8. What are Herbig-Haro objects? Why are they often found in pairs?

  9. Why do disks form around contracting protostars? What is the connection between disks and bipolar outflows?

  1. Young open clusters like those shown in Figure 18-20a and Figure 18-21 are found only in the plane of the Galaxy. Explain why this should be.

  1. Why are observations at millimeter wavelengths so much more useful in exploring interstellar clouds than observations at visible wavelengths?

  2. What are giant molecular clouds? What role do these clouds play in the birth of stars?

  3. Giant molecular clouds are among the largest objects in our Galaxy. Why, then, were they discovered only relatively recently?

  4. Consider the following stages in the evolution of a young star cluster: (i) H II region; (ii) dark nebula; (iii) formation of O and B stars; (iv) giant molecular cloud. Put these stages in the correct chronological order and discuss how they are related.

  5. Briefly describe four mechanisms that compress the interstellar medium and trigger star formation.

Advanced Questions

Problem-solving tips and tools

You may find it helpful to review Box 17-4, which describes the relationship among a star’s luminosity, radius, and surface temperature. The small-angle formula is described in Box 1-1. Orbital periods are described by Kepler’s third law, which we discussed in Box 4-2 and Box 4-4. Remember that the Stefan-Boltzmann law (Box 5-2) relates the temperature of a blackbody to its energy flux. Remember, too, that the volume of a sphere of radius r is 4πr3/3.

  1. If you looked at the spectrum of a reflection nebula, would you see absorption lines, emission lines, or no lines? Explain your answer. As part of your explanation, describe how the spectrum demonstrates that the light was reflected from nearby stars.

  2. In the direction of a particular star cluster, interstellar extinction allows only 15% of a star’s light to pass through each kiloparsec (1000 pc) of the interstellar medium. If the star cluster is 3.0 kiloparsecs away, what percentage of its photons survive the trip to Earth?

  3. The visible-light photograph below shows the Trifid Nebula in the constellation Sagittarius. Label the following features on this photograph: (a) reflection nebulae (and the star or stars whose light is being reflected); (b) dark nebulae; (c) H II regions; (d) regions where star formation may be occurring. Explain how you identified each feature.

    R I V U X G
    (Australian Astronomical Observatory/David Malin Images)

  4. Find the density (in atoms per cubic centimeter) of a Bok globule having a radius of 1 ly and a mass of 100 M⊙. How does your result compare with the density of a typical H II region, between 80 and 600 atoms per cm3? (Assume that the globule is made purely of hydrogen atoms.)

  5. The Becklin-Neugebauer object is a newly formed star within the Orion Nebula. It is substantially more luminous than the other newly formed stars in that nebula. Assuming that all these stars began the process of formation of the same time, what can you conclude about the mass of the Becklin-Neugebauer object compared with those of the other newly formed stars? Does your conclusion depend on whether or not the stars have reached the main sequence? Explain your reasoning.

  6. The two false-color images opposite show a portion of the Trifid Nebula (see Question 31). The reddish-orange view is a false-color infrared image, while the bluish picture (shown to the same scale) was made with visible light. Explain why the dark streaks in the visible-light image appear bright in the infrared image.

  7. At one stage during its birth, the protosun had a luminosity of 1000 L⊙ and a surface temperature of about 1000 K. At this time, what was its radius? Express your answer in three ways: as a multiple of the Sun’s present-day radius, in kilometers, and in astronomical units.

    (ESA/ISO, ISOCAM, and J. Cernicharo et. al.; IAC, Observatorio del Teide, Tenerife)

  8. A newly formed protostar and a red giant are both located in the same region on the H-R diagram. Explain how you could distinguish between these two.

  9. (a) Determine the radius of the circumstellar accretion disk in Figure 18-15. (You will need to measure this image with a ruler. Note the scale bar in this figure.) Give your answer in astronomical units and in kilometers. (b) Assume that the young star at the center of this disk has a mass of 1 M⊙. What is the orbital period (in years) of a particle at the outer edge of the disk? (c) Using your ruler again, determine the length of the jet that extends to the right of the circumstellar disk in Figure 18-15. At a speed of 200 km/s, how long does it take gas to traverse the entire visible length of the jet?

  10. The star cluster NGC 2264 (Figure 18-20) contains numerous T Tauri stars, while the Pleiades (Figure 18-21) contains none. Explain why there is a difference.

  11. The concentration or abundance of ethyl alcohol in a typical molecular cloud is about 1 molecule per 108 cubic meters. What volume of such a cloud would contain enough alcohol to make a martini (about 10 grams of alcohol)? A molecule of ethyl alcohol has 46 times the mass of a hydrogen atom (that is, ethyl alcohol has a molecular weight of 46).

  12. From the information given in the caption to Figure 18-26, calculate the angular diameter in arcminutes of Cassiopeia A as seen from Earth.

  13. From the information given in the caption for Figure 18-26, calculate the average speed at which the shock wave has spread away from the site of the supernova explosion. Give your answer in kilometers per second and as a fraction of the speed of light. (Hint: There are 3.16 × 107 seconds in a year and the speed of light is 3.00 × 105 km/s.)

Discussion Questions

  1. Some science-fiction movies show stars suddenly becoming dramatically brighter when they are “born” (that is, when thermonuclear fusion reactions begin in their cores). Discuss whether this is a reasonable depiction.

  2. Suppose that the electrons in hydrogen atoms were not as strongly attracted to the nuclei of those atoms, so that these atoms were easier to ionize. What consequences might this have for the internal structure of main-sequence stars? Explain your reasoning.

  3. What do you think would happen if our solar system were to pass through a giant molecular cloud? Do you think Earth has ever passed through such clouds?

  4. Many of the molecules found in giant molecular clouds are organic molecules (that is, they contain carbon). Speculate about the possibility of life-forms and biological processes occurring in giant molecular clouds. In what ways might the conditions existing in giant molecular clouds favor or hinder biological evolution?

  5. Speculate on why a shock wave from a supernova seems to produce relatively few high-mass O and B stars, compared to the lower-mass A, F, G, and K stars.

Web/eBook Questions

  1. In recent years astronomers have been able to learn about the character of the interstellar medium in the vicinity of the Sun. Search the World Wide Web for information about aspects of the nearby interstellar medium, including features called the Local Interstellar Cloud and the Local Bubble. How do astronomers study the nearby interstellar medium? What makes these studies difficult? Is the interstellar medium relatively uniform in our neighborhood, or is it clumpy? If the latter, is our solar system in a relatively thin or thick part of the interstellar medium? How is our solar system moving through the interstellar medium?

  2. Search the World Wide Web for recent discoveries about how brown dwarfs form. Do they tend to form in the same locations as “real” stars? Do they form in relatively small or relatively large numbers compared to “real” stars? What techniques are used to make these discoveries?

  3. Measuring a Stellar Jet. Access the animation “A Stellar Jet in the Trifid Nebula” in Chapter 18 of the Universe Web site or eBook. (a) The Trifid Nebula as a whole has an angular diameter of 28 arcmin. By stepping through the animation, estimate the angular size of the stellar jet shown at the end of the animation. (b) The Trifid Nebula is about 2800 pc (9000 ly) from Earth. Estimate the length of the jet in light-years. (c) If gas in the jet travels at 200 km/s, how long does it take to traverse the length of the jet? Give your answer in years. (Hint: There are 3.16 × 107 seconds in a year.)