Questions
Review Questions
Why did many nineteenth-century astronomers think that the “spiral nebulae” are part of the Milky Way?
What was the Shapley-Curtis “debate” all about? Was a winner declared at the end of the “debate”? Whose ideas turned out to be correct?
How did Edwin Hubble prove that the Andromeda “Nebula” is not a nebula within our Milky Way Galaxy?
Are any galaxies besides our own visible with the naked eye from Earth? If so, which one(s)?
An educational publication for children included the following statement: “The Sun is in fact the only star in our galaxy. All of the other stars in the sky are located in other galaxies.” How would you correct this statement?
What is the Hubble classification scheme? Which category includes the largest galaxies? Which includes the smallest? Which category of galaxy is the most common?Which is more likely to have a blue color, a spiral galaxy or an elliptical galaxy? Explain why.
Which types of galaxies are most likely to have new stars forming? Describe the observational evidence that supports your answer.
Explain why the apparent shape of an elliptical galaxy may be quite different from its real shape.
Why do astronomers suspect that the Hubble tuning fork diagram does not depict the evolutionary sequence of galaxies?
Why are Cepheid variable stars useful for finding the distances to galaxies? Are there any limitations on their use for this purpose?
Why are Type Ia supernovae useful for finding the distances to very remote galaxies? Can they be used to find the distance to any galaxy you might choose? Explain your answers.
What is the Tully-Fisher relation? How is it used for measuring distances? Can it be used for galaxies of all kinds? Why or why not?
What are masers? How can they be used to measure the distance to a galaxy?
What is the Hubble law? How can it be used to determine distances?
How did the discovery of the Hubble Law reinforce the idea that the spiral “nebulae” could not be part of the Milky Way?
Why do you suppose it has been so difficult to determine the value of H0?
Some galaxies in the Local Group exhibit blueshifted spectral lines. Why aren’t these blueshifts violations of the Hubble law?
What are the differences between regular and irregular clusters?
What is the difference between a cluster and a supercluster? Are both clusters and superclusters held together by their gravity?
What measurements do astronomers make to construct three-dimensional maps of the positions of galaxies in space?
What is the Sloan Great Wall, and why is it interesting to astronomers?
Describe what voids are and what they tell us about the large-scale structure of the universe.
Why is the intracluster gas in galaxy clusters at such high temperatures?
What are starburst galaxies? How can they be produced by collisions between galaxies?
Why do giant elliptical galaxies dominate rich clusters but not poor clusters?
What evidence is there for the existence of dark matter in clusters of galaxies?
What is gravitational lensing? Why don’t we notice the gravitational lensing of light by ordinary objects on Earth?
How do observations of the Bullet Cluster help constrain the nature of dark matter?
What is the favored hypothesis for what dark matter is made of? What properties do these particles have?
Would the existence of WIMPs be a form of exotic dark matter or “ordinary” matter? What is exotic dark matter?
What observations suggest that present-day galaxies formed from smaller assemblages of matter?
On what grounds do astronomers think that in the past, spiral galaxies were more numerous in rich clusters than they are today? What could account for this excess of spiral galaxies in the past?
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Advanced Questions
Questions preceded by an asterisk (*) involve topics discussed in the Boxes.
Problem-solving tips and tools
Box 1-1 explains the small-angle formula, and Box 17-3 discusses the relationship among apparent magnitude, absolute magnitude, and distance. As Box 22-2 explains, a useful form of Kepler’s third law is M = rv2/G, where M is the mass within an orbit of radius r, v is the orbital speed, and G is the gravitational constant. Another form of Kepler’s third law, particularly useful for two stars or two galaxies orbiting each other, is given in Section 17-9. The volume of a sphere of radius r is 4πr 3/3. The mass of a hydrogen atom (1H) is given in Appendix 7.
Hubble made his observations of Cepheids in the Andromeda Galaxy (M31) using the 100-inch (2.5-meter) telescope on Mount Wilson. Completed in 1917, this was the largest telescope in the world when Hubble carried out his observations in 1923. Why was it helpful to use such a large telescope?
The following image shows the Small Magellanic Cloud (SMC), an irregular galaxy that orbits the Milky Way. The SMC is 63 kpc (200,000 ly) from Earth and 8 kpc (26,000 ly) across, and can be seen with the naked eye from southern latitudes. What features of this image indicate that there has been recent star formation in the SMC? Explain your answer.
R I V U X G(Australian Astronomical Observatory/David Malin Images)*Astronomers often state the distance to a remote galaxy in terms of its distance modulus, which is the difference between the apparent magnitude m and the absolute magnitude M (see Box 17-3). (a) By measuring the brightness of supernova 1994I in the galaxy M51 (see Figure 23-2), the distance modulus for this galaxy was determined to be m − M = 29.2. Find the distance to M51 in megaparsecs (Mpc). (b) A separate distance determination, which involved measuring the brightnesses of planetary nebulae in M51, found m − M = 29.6. What is the distance to M51 that you calculate from this information? (c) What is the difference between your answers to parts (a) and (b)? Compare this difference with the 750-kpc distance from Earth to M31, the Andromeda Galaxy. The difference between your answers illustrates the uncertainties involved in determining the distances to galaxies!
*Suppose you discover a Type Ia supernova in a distant galaxy. At maximum brilliance, the supernova reaches an apparent magnitude of +10. How far away is the galaxy? (Hint: See Box 23-1.)
The masers that orbit the center of the spiral galaxy M106 travel at an orbital speed of about 1000 km/s. Astronomers observed these masers at intervals of 4 months. (a) What distance does a single maser move during a 4-month period? Give your answer in kilometers and in AU. (b) During this period, a maser moving across the line of sight (like the maser shown in green in Figure 23-15) appeared to move through an angle of only 10−5 arcsec. Calculate the distance to the galaxy.
The average radial velocity of galaxies in the Hercules cluster pictured in Figure 23-18 is 10,800 km/s. (a) Using H0 = 73 km/s/Mpc, find the distance to this cluster. Give your answer in megaparsecs and in light-years. (b) How would your answer to (a) differ if the Hubble constant had a smaller value? A larger value? Explain your answers.
A certain galaxy is observed to be receding from the Sun at a rate of 7500 km/s. The distance to this galaxy is measured independently and found to be 1.4 × 108 pc. Using these data, what is the value of the Hubble constant?
*In the spectrum of the galaxy NGC 4839, the K line of singly ionized calcium has a wavelength 403.2 nm. (a) What is the redshift of this galaxy? (Hint: See Box 23-2.) (b) Determine the distance to this galaxy using the Hubble law with H0 = 73 km/s/Mpc.
*The galaxy RD1 has a redshift of z = 5.34. (a) Determine its recessional velocity v in km/s and as a fraction of the speed of light. (b) What recessional velocity would you have calculated if you had erroneously used the low-speed formula relating z and v? Would using this formula have been a small or large error? (c) According to the Hubble law, what is the distance from Earth to RD1? Use H0 = 73 km/s/Mpc for the Hubble constant, and give your answer in both megaparsecs and light-years.
It is estimated that the Coma cluster (see Figure 23-21) contains about 1013 M⊙ of intracluster gas. (a) Assuming that this gas is made of hydrogen atoms, calculate the total number of intracluster gas atoms in the Coma cluster. (b) The Coma cluster is roughly spherical in shape, with a radius of about 3 Mpc. Calculate the number of intracluster gas atoms per cubic centimeter in the Coma cluster. Assume that the gas fills the cluster uniformly. (c) Compare the intracluster gas in the Coma cluster with the gas in our atmosphere (3 × 1019 molecules per cubic centimeter, temperature 300 K); a typical gas cloud within our own Galaxy (a few hundred molecules per cubic centimeter, temperature 50 K or less); and the corona of the Sun (105 atoms per cubic centimeter, temperature 106 K).
*Two galaxies separated by 600 kpc are orbiting each other with a period of 40 billion years. What is the total mass of the two galaxies?
*Figure 23-31 shows the rotation curve of the Sa galaxy NGC 4378. Using data from that graph, calculate the orbital period of stars 20 kpc from the galaxy’s center. How much mass lies within 20 kpc from the center of NGC 4378?
How might you determine what part of a galaxy’s redshift is caused by the galaxy’s orbital motion about the center of mass of its cluster?
The accompanying images show the unusual elliptical galaxy NGC 5128 in visible and infrared wavelengths. Explain how the properties of this galaxy seen in the infrared image can be explained if NGC 5128 is the result of a merger of an elliptical galaxy and a spiral galaxy.
(Visible image: Eric Peng, Herzberg Institute of Astrophysics and NOAO/AURA/NSF;Infrared image: Jocelyn Keene, NASA/JPL and Caltech)Explain why the dark matter in galaxy clusters could not be neutral hydrogen.
According to Figure 23-36c, elliptical galaxies continue to form stars for about a billion years after they form. Give an argument why we might expect to find some Population I stars in an elliptical galaxy. (Hint: Table 19-1 gives the main–sequence lifetimes for stars of different masses.)
Discussion Questions
Earth is composed principally of heavy elements, such as silicon, nickel, and iron. Would you be likely to find such planets orbiting stars in the disk of a spiral galaxy? In the nucleus of a spiral galaxy? In an elliptical galaxy? In an irregular galaxy? Explain your answers.
Discuss what observations you might make to determine whether or not the various Hubble types of galaxies represent some sort of evolutionary sequence.
Discuss the advantages and disadvantages of using the various standard candle distance indicators to obtain extragalactic distances.
How would you distinguish star images from unresolved images of remote galaxies on a CCD?
Describe what sorts of observations you might make to search for as-yet-undiscovered galaxies in our Local Group. How is it possible that such galaxies might still remain to be discovered? In what part of the sky would these galaxies be located? What sorts of observations might reveal these galaxies?
Web/eBook Questions
When galaxies pass close to one another, as should happen frequently in a rich cluster, tidal forces between the galaxies can strip away their outlying stars. The result should be a loosely dispersed sea of “intergalactic stars” populating the space between galaxies in a cluster. Search the World Wide Web for information about intergalactic stars. Have they been observed? If so, where are they found? What would our nighttime sky look like if our Sun were an intergalactic star?
The Hubble Space Telescope (HST) has made extensive observations of very distant galaxies. Visit the HST Web site to learn about these investigations. How far back in time has HST been able to look? What sorts of early galaxies are observed? What is the current thinking about how galaxies formed and evolved?
Radiation from a Rotating Galaxy. Access and view the animation “Radiation from a Rotating Galaxy” in Chapter 23 of the Universe Web site or eBook. Describe how the animation would have to be changed for a spiral galaxy of the same size but (a) greater mass and (b) smaller mass.