Questions

Review Questions

Questions preceded by an asterisk (*) involve topics discussed in Box 24-1.

1. When quasi-stellar radio sources were first discovered and named, why were they called “quasi-stellar”?

2. How were quasars first discovered? How was it known that they are very distant objects?

3. Suppose you saw an object in the sky that you suspected might be a quasar. What sort of observations might you perform to test your suspicion?

4. How does the spectrum of a quasar differ from that of an ordinary galaxy? How do spectral lines help astronomers determine the distances to quasars?

5. Since quasars lie at the centers of galaxies, why don’t we see strong absorption lines from a galaxy’s stars when we look at the spectrum of a quasar (like that shown in Figures 24-3)?

6. It was suggested in the 1960s that quasars might be compact objects ejected at high speeds from the centers of nearby ordinary galaxies. Explain why the absence of blueshifted quasars disproves this hypothesis. (We see only quasars with redshifted lines.)

7. How do astronomers know that quasars are located in galaxies? Are the host galaxies easy to observe for distant quasars?

8. What is a radio galaxy?

9. What is the main difference between a Type 1 and a Type 2 AGN?

10. Some quasars appear to be ejecting material at speeds faster than light. Is the material really moving that fast? If so, how is this possible? If not, why does the material appear to be traveling so fast?

11. What do the brightness fluctuations of a particular active galaxy tell us about the size of the energy-emitting region within that galaxy?

12. How could a supermassive black hole, from which nothing—not even light—can escape, be responsible for the extraordinary luminosity of a quasar?

13. What is the Eddington limit? Explain how it can be used to set a limit on the mass of a supermassive black hole, and explain why this limit represents a minimum mass for the black hole.

14. How does matter falling inward toward a central black hole find itself being ejected outward in a high-speed jet?

15. In the unified model, why do some AGN have broad spectral lines while others do not?

16. *How does the unified model of active galaxies explain that quasars and blazars are the same kind of object viewed from different angles?

17. What is it about fueling of quasars that there are no quasars relatively near to our Galaxy?

Advanced Questions

Questions preceded by an asterisk (*) involve topics discussed in the Boxes in Chapters 4, 21, 23.

18. When we observe a quasar with redshift z = 1.0, about how far into the past are we looking (see Figure 24-4)? If we could see that object as it really is right now (that is, if the light from the object could somehow reach us instantaneously), would it still look like a quasar? Explain why or why not.

19. In the image that opens this chapter, the close-up view of the Cygnus A jet shows that different wavelengths are preferentially emitted at different locations along the jet’s length. Explain why, using the following principle: As an individual particle moves in a magnetic field, the greater its speed, the shorter the wavelength of the radiation that it emits.

20. Suppose the distance from point A to point B in Figure 24-11a is 26 light-years and the blob moves at 13/15 of the speed of light. As the blob moves from A to B, it moves 24 light-years toward Earth and 10 light-years in a sideways, transverse direction. (a) How long does it take the blob to travel from A to B? (b) If the light from the blob at A reaches Earth in 2020, in what year does the light from B reach Earth? (c) As seen from Earth, at what speed does the blob appear to move across the sky?

21. *Suppose a blazar at z = 1.00 goes through a fluctuation in brightness that lasts one week (168 hours) as seen from Earth. (a) At what speed does the blazar seem to be moving away from us? (b) Using the idea of time dilation, determine how long this fluctuation lasted as measured by an astronomer within the blazar’s host galaxy. (c) What is the maximum size (in AU) of the region from which this blazar emits energy?

22. (a) Calculate the maximum luminosity that could be generated by accretion onto a black hole of 3.7 × 10⁶ solar masses. (This is the size of the black hole found at the center of the Milky Way.) Compare this to the total luminosity of the Milky Way, about 2.5 × 10¹⁰ L_⊙. (b) Speculate on what we might see if the center of our Galaxy became an active galactic nucleus with the luminosity you calculated in (a).

23. *Observations of a certain galaxy show that stars at a distance of 16 pc from the center of the galaxy orbit the center at a speed of 200 km/s. Use Newton’s form of Kepler’s third law to determine the mass of the central black hole.

24. *Calculate the Schwarzschild radius of a 10⁹ solar mass black hole. How does your answer compare with the size of our solar system (given by the diameter of Pluto’s orbit)?

25. *Figure 24-15 shows the double radio source Centaurus A. Is it possible that somewhere in the universe there is an alien astronomer who observes this same object as a blazar? Explain your answer with a drawing showing the relative positions of Earth, the alien astronomer, and Centaurus A.

26. * What is a blazar? What is unique about its spectrum? How is it related to other active galaxies?

27. *What is a Type 1 Seyfert galaxy? Does the active nucleus of a Type 1 Seyfert galaxy differ from that of a quasar?

Discussion Questions

28. The accompanying image from the Very Large Array (VLA) shows the radio galaxy 3C 75 in the constellation Cetus. This galaxy has several radio-emitting jets. High-resolution optical photographs reveal that the galaxy has two nuclei, which are the two red spots near the center of the VLA image. Propose a scenario that might explain the appearance of 3C 75.

    

    R I V U X G

    (NRAO)

29. Some quasars show several sets of absorption lines whose redshifts are less than the redshifts of their emission lines. For example, the quasar PKS 0237-23 has five sets of absorption lines with redshifts in the range from 1.364 to 2.202, whereas the quasar’s emission lines have a redshift of 2.223. Propose an explanation for these sets of absorption lines.

30. The Milky Way Galaxy is in the process of absorbing the satellite galaxy called the Canis Major Dwarf (see Section 23-6). Discuss whether this process could cause the Milky Way to someday become an active galaxy.

31. Figure 24-14 shows ionized gas streaming away from the “central engine” of the radio galaxy Cygnus A. Instead of spreading outward equally in all directions, the gas appears to be funneled into two oppositely directed cones. Discuss how this could be caused by a dusty torus surrounding a supermassive black hole at the center of Cygnus A.

Web/eBook Questions

32. Search the World Wide Web for information about “micro-quasars.” These are objects that are found within the Milky Way Galaxy. How are they detected? What are the similarities and differences between these objects and true quasars? Are they long-lasting or short-lived?

33. The Lockman hole is a region in the constellation Ursa Minor where the Milky Way’s interstellar hydrogen is the thinnest. By observing in this part of the sky, astronomers get the clearest possible view of distant galaxies and quasars. Search the World Wide Web for information about observations of the Lockman hole. What has been learned through X-ray observations made by spacecraft such as ROSAT and XMM-Newton?