Question 17.1

What is a double-radio source seen along one axis of a jet called?

1. a BL Lac object
2. a quasar
3. a binary star system
4. a double-radio source
5. a pulsar

Question 17.2

What two things does the engine of a quasar contain?

1. a supermassive black hole and a binary companion
2. a stellar-mass black hole and a binary companion
3. a supermassive black hole and an accretion disk
4. a stellar-mass black hole and an accretion disk
5. a supergiant star and an accretion disk

Question 17.3

Suppose you suspected a certain object in the sky to be a quasar. What sort of observations would you perform to confirm your hypothesis?

Question 17.4

Explain why astronomers do not use any of the standard candles described in Chapter 16 to determine the distances to quasars.

Question 17.5

Explain how the rate of variability of a source of light can be used to place an upper limit on the size of the source.

Question 17.6

What is an active galaxy? List the different kinds of active galaxies. How do they differ from one another?

Question 17.7

Why do astronomers believe that the energy-producing region of a quasar is very small?

Question 17.8

How is synchrotron radiation produced?

Question 17.9

What evidence indicates that quasars are extremely distant active galaxies?

Question 17.10

What is a double-radio source?

Question 17.11

What is a supermassive black hole? What observational evidence suggests that supermassive black holes are located at the centers of many galaxies?

Question 17.12

Why do many astronomers believe that the engine at the center of a quasar is a supermassive black hole surrounded by an accretion disk?

Question 17.13

How does the orientation of the jets that emanate from the center of a galaxy (relative to our line of sight) relate to the type of active galaxy that we observe?

Question 17.14

In the 1960s, some astronomers suggested that quasars might be compact objects ejected at high speeds from the centers of nearby ordinary galaxies. Why do astronomers now believe that is not correct? (Hint: Consider the directions in which those objects would be ejected.)

Question 17.15

When quasars were first discovered, many astronomers were optimistic that these extremely luminous objects could be used to probe distant regions of the universe. For example, it was hoped that quasars would provide high-redshift data from which the Hubble constant could be more accurately determined. Why have these hopes not been realized?

Question 17.16

Explore the belief that quasars, double-radio sources, and giant elliptical galaxies represent an evolutionary sequence.

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Question 17.17

Some quasars show several sets of absorption lines whose redshifts are less than the redshift of the quasars’ emission lines. For example, the quasar PKS 0237-23 has five sets of absorption lines, all with redshifts somewhat less than the redshift of the quasar’s emission lines. Propose an explanation for these sets of absorption lines.

Question 17.18

Earth passed through a jet emitted by a radio galaxy? What would we see and what might happen to Earth?

Question 17.19

A jet of gas suddenly appeared across the night sky? What might it indicate has recently happened?

Question 17.20

Sirius suddenly became ring-shaped for a few hours and then returned to normal? What might cause such an event?

Question 17.21

To test your understanding of active galaxies, do Interactive Exercise 17.1 on the assigned Web site. You can print out your answers, if required.

Question 17.22

What is a quasar?

Question 17.23

What does a quasar look like through an optical telescope?