Questions
Review Questions
Why do the stars of the Galaxy appear to form a bright band that extends around the sky?
How did interstellar extinction mislead astronomers into believing that we are at the center of our Galaxy?How did observations of globular clusters help astronomers determine our location in the Galaxy?
What are RR Lyrae stars? Why are they useful for determining the distance from our solar system to the center of the Galaxy?
Why are infrared telescopes useful for exploring the structure of the Galaxy? Why is it important to make observations at both near-infrared and far-infrared wavelengths?
The galactic halo is dominated by Population II stars, whereas the galactic disk contains predominantly Population I stars. In which of these parts of the Galaxy has star formation taken place recently? Explain your answer.
O or B main-sequence stars are found in the galactic disk but not in globular clusters. Why is this so?
What must happen within a hydrogen atom for it to emit a photon of wavelength 21 cm?
Most interstellar hydrogen atoms emit only radio waves at a wavelength of 21 cm, but some hydrogen clouds emit profuse amounts of visible light (see, for example, Figure 18-1b and Figure 18-2). What causes this difference?
How do astronomers determine the distances to H I (neutral hydrogen) clouds?
The radio map in Figure 22-14 has a large gap on the side of the Galaxy opposite to ours. Why is this?
In a spiral galaxy, are stars in general concentrated in the spiral arms? Why are spiral arms so prominent in visible-light images of spiral galaxies?
Many old black-and-white photographs of spiral galaxies were taken using film that was most sensitive to blue light. Explain why the spiral arms were particularly prominent in such photographs.
What kinds of objects (other than H I clouds) do astronomers observe to map out the Galaxy’s spiral structure? What is special about these objects? Which of these can be observed at great distances?
Why don’t astronomers detect 21-cm radiation from the hydrogen in giant molecular clouds?
In what way are the orbits of stars in the galactic disk different from the orbits of planets in our solar system? What does this difference imply about the way that matter is distributed in the Galaxy?
How do astronomers determine how fast the Sun moves in its orbit around the Galaxy? How does this speed tell us about the amount of mass inside the Sun’s orbit? Does this speed tell us about the amount of mass outside the Sun’s orbit?
How do astronomers conclude that vast quantities of dark matter surround our Galaxy? How is this dark matter distributed in space?
Another student tells you that the Milky Way Galaxy is made up “mostly of stars.” Is this statement accurate? Why or why not?
What is the difference between dark matter and dark nebulae?
What proposals have been made to explain the nature of dark matter? What experiments or observations have been made to investigate these proposals? What are the results of this research?
What is the winding dilemma? What does it tell us about the nature of spiral arms?
Do density waves form a stationary pattern in a galaxy? If not, do they move more rapidly, less rapidly, or at the same speed as stars in the disk?
In our Galaxy, why are stars of spectral classes O and B found only in or near the spiral arms? Is the same true for stars of other spectral classes? Explain why or why not.
Compare the kinds of spiral arms produced by density waves with those produced by self-propagating star formation. By examining Figure 22-16, cite evidence that both processes may occur in our Galaxy.
What is the evidence that there is a supermassive black hole at the center of our Galaxy? How is it possible to determine the mass of this black hole?What is the evidence that material has been falling into the supermassive black hole at the galactic center?
Advanced Questions
Questions preceded by an asterisk (*) involve topics discussed in the Boxes.
Problem-solving tips and tools
Several of the following questions make extensive use of Newton’s form of Kepler’s third law, and you might find it helpful to review Box 4-4. Another useful version of Kepler’s third law is given in Section 17-9. Box 17-2 discusses the relationship between luminosity, apparent brightness, and distance. We discussed the relationship between the energy and wavelength of a photon in Section 5-5. According to the Pythagorean theorem, an isosceles right triangle has a hypotenuse that is longer than its sides by a factor of 
= 1.414. The formula for the Schwarzschild radius of a black hole is given in Box 21-2. You will find the small-angle formula in Box 1-1 useful. It is also helpful to remember that an object 1 AU across viewed at a distance of 1 parsec has an angular size of 1 arcsecond. Remember, too, that the volume of a cylinder is equal to its height multiplied by the area of its base, the area of a circle of radius r is πr2, and the volume of a sphere of radius r is 4πr3/3. You can find other geometrical formulae in Appendix 8.
Discuss how the Milky Way would appear to us if the Sun were relocated to (a) the edge of the Galaxy; (b) the galactic halo; (c) the galactic bulge.
Explain why globular clusters spend most of their time in the galactic halo, even though their eccentric orbits take them close to the galactic center.
The disk of the Galaxy is about 50 kpc in diameter and 600 pc thick. (a) Find the volume of the disk in cubic parsecs. (b) Find the volume (in cubic parsecs) of a sphere 300 pc in radius centered on the Sun. (c) If supernovae occur randomly throughout the volume of the Galaxy, what is the probability that a given supernova will occur within 300 pc of the Sun? If there are about three supernovae each century in our Galaxy, how often, on average, should we expect to see one within 300 pc of the Sun?
*An RR Lyrae star whose peak luminosity is 100 L⊙ is in a globular cluster. At its peak luminosity, this star appears from Earth to be only 1.47 × 10−18 as bright as the Sun. Determine the distance to this globular cluster (a) in AU and (b) in parsecs.
A typical hydrogen atom in interstellar space undergoes a spin-flip transition only once every 107 years. How, then, is it at all possible to detect the 21-cm radio emission from interstellar hydrogen?
Calculate the energy of the photon emitted when a hydrogen atom undergoes a spin-flip transition. How many such photons would it take to equal the energy of a single Hα photon of wavelength 656.3 nm?
Suppose you were to use a radio telescope to measure the Doppler shift of 21-cm radiation in the plane of the Galaxy. (a) If you observe 21-cm radiation from clouds of atomic hydrogen at an angle of 45° from the galactic center, you will see the highest Doppler shift from a cloud that is as far from the galactic center as it is from the Sun. Explain this statement using a diagram. (b) Find the distance from the Sun to the particular cloud mentioned in (a).
Sketch the rotation curve you would obtain if the Galaxy were rotating like a rigid body.
The mass of our Galaxy inside the Sun’s orbit is calculated from the radius of the Sun’s orbit and its orbital speed. By how much would this estimate be in error if the calculated distance to the galactic center were off by 10%? By how much would this estimate be in error if the calculated orbital velocity were off by 10%? Explain your reasoning.
A gas cloud located in the spiral arm of a distant galaxy is observed to have an orbital velocity of 400 km/s. If the cloud is 20,000 pc from the center of the galaxy and is moving in a circular orbit, find (a) the orbital period of the cloud and (b) the mass of the galaxy contained within the cloud’s orbit.
According to the Galaxy’s rotation curve in Figure 22-18, a star 16 kpc from the galactic center has an orbital speed of about 270 km/s. Calculate the mass within that star’s orbit.
Speculate on the reasons for the rapid rise in the Galaxy’s rotation curve (see Figure 22-18) at distances close to the galactic center.
*Show that the form of Kepler’s third law stated in Box 22-2, P2 = 4π2a3/G (M + M⊙), is equivalent to M = rv2/G, provided the orbit is a circle. (Hint: The mass of the Sun (M⊙) is much less than the mass of the Galaxy inside the Sun’s orbit (M).)
The accompanying image shows the spiral galaxy M74, located about 55 million light-years from Earth in the constellation Pisces (the Fish). It is actually a superposition of two false-color images. The red portion is an optical image taken at visible wavelengths, while the blue portion is an ultraviolet image made by NASA’s Ultraviolet Imaging Telescope, which was carried into orbit by the space shuttle Columbia during the Astro-1 mission in 1990. Compare the visible and ultraviolet images and, from what you know about stellar evolution and spiral structure, explain the differences you see.
RIVUXG(NASA, UIT)The accompanying figure shows infrared images of two spiral galaxies. Explain which of these is a grand-design spiral galaxy and which is a flocculent spiral galaxy. Explain your reasoning.
R I V U X G(NASA; JPL-Caltech; R. Kennicutt [University of Arizona]; and the SINGS Team)*(a) Calculate the Schwarzschild radius of a supermassive black hole of mass 4.1 × 106 M⊙, the estimated mass of the black hole at the galactic center. Give your answer in both kilometers and astronomical units. (b) What is the angular diameter of such a black hole as seen at a distance of 8 kpc, the distance from Earth to the galactic center? Give your answer in arcseconds. Observing an object with such a small angular size will be a challenge indeed! (c) What is the angular diameter of such a black hole as seen from a distance of 45 AU, the closest that the star S0-16 comes to Sagittarius A*? Again, give your answer in arcseconds. Would it be discernible to the naked eye at that distance? (A normal human eye can see details as small as about 60 arcseconds.)
*(a) The scale bar in Figure 22-28 shows that at the distance of Sagittarius A*, a length of 1600 AU has an angular size of 0.2 arcsecond. Use this information to calculate the distance to Sagittarius A*. (b) The star S0-16 moves around Sagittarius A* in an elliptical orbit with semimajor axis 1680 AU. Use this and the information given in the caption to Figure 22-28 to find the orbital period of S0-16. (c) Given the period and the semimajor axis of the star’s orbit, is it possible to calculate the mass of S0-16 itself? If it is, explain how this could be done; if not, explain why not.
*The stars S0-2 and S0-19 orbit Sagittarius A* with orbital periods of 14.5 and 37.3 years, respectively. (a) Assuming that the supermassive black hole in Sagittarius A* has a mass of 4.1 × 106 M⊙, determine the semimajor axes of the orbits of these two stars. Give your answers in AU. (b) Calculate the angular size of each orbit’s semimajor axis as seen from Earth. (See Section 22-1 for the distance from Earth to the center of the Galaxy.) Explain why extremely high-resolution infrared images are required to observe the motions of these stars.
Consider a star that orbits around Sagittarius A* in a circular orbit of radius 530 AU. (a) If the star’s orbital speed is 2500 km/s, what is its orbital period? Give your answer in years. (b) Determine the sum of the masses of Sagittarius A* and the star. Give your answer in solar masses. (Your answer is an estimate of the mass of Sagittarius A*, because the mass of a single star is negligibly small by comparison.
Discussion Questions
From what you know about stellar evolution, the interstellar medium, and the density-wave theory, explain the appearance and structure of the spiral arms of grand-design spiral galaxies.
What observations would you make to determine the nature of the dark matter in our Galaxy’s halo?
Describe how the appearance of the night sky might change if dark matter were visible to our eyes.
Discuss how a supermassive black hole could have formed at the center of our Galaxy.
Web/eBook Questions
Some scientists have suggested that the rotation curve of the Galaxy can be explained by modifying the laws of physics rather than by positing the existence of dark matter. Search the World Wide Web for information about these proposed modifications, called MOND (for MOdified Newtonian Dynamics). Under what circumstances do MOND and conventional physics predict different behavior? What evidence is there that MOND might be correct?
Fast-Moving Stars at the Galactic Center. Access and and view the video “Stars Orbiting Sagittarius A*” in Chapter 22 of the Universe Web site or eBook. Explain how you can tell which of the stars in the video are actually close to Sagittarius A* and which just happen to lie along our line of sight to the center of the Galaxy.