Active galaxies are much more luminous than normal galaxies and have nonstellar spectra, emitting most of their energy in the radio and infrared parts of the electromagnetic spectrum.

A Seyfert galaxy looks like a normal spiral except that the Seyfert has an extremely bright central galactic nucleus. Spectral lines from Seyfert nuclei are very broad, indicating rapid internal motion, and rapid variability implies that the source of the radiation is much less than one light-year across. Radio galaxies emit most of their energy in the radio part of the spectrum. They are generally comparable to Seyferts in total energy output. The corresponding visible galaxy is usually elliptical. In a core-halo radio galaxy, most of the energy is emitted from a small central nucleus. In a lobe radio galaxy, the energy comes from enormous radio lobes that lie far beyond the visible portion of the galaxy.

Many active galaxies have high-speed, narrow jets of matter shooting out from their central nuclei. In lobe radio galaxies, astronomers believe that the jets transport energy from the nucleus, where it is generated, to the lobes, where it is radiated into space. The jets often appear to be made up of distinct "blobs" of gas, suggesting that the process generating the energy is intermittent.

The generally accepted explanation for the observed properties of active galaxies is that the energy is generated by accretion of galactic gas onto a supermassive (billion-solar-mass) black hole lying at the center of the nucleus. The small size of the accretion disk explains the compact extent of the emitting region, and the high-speed orbit of gas in the black hole’s intense gravity accounts for the rapid motion observed. Typical active galaxy luminosities require the consumption of about one solar mass of material every few years. Some of the infalling matter is blasted out into space, producing magnetized jets that create and feed the radio lobes. Charged particles spiraling around the magnetic field lines produce synchrotron radiation whose spectrum is consistent with the nonstellar radiation observed in radio galaxies and Seyferts.

Quasars, or quasi-stellar objects, are starlike radio sources with spectra redshifted to wavelengths much longer than normal. All quasars have large redshifts, indicating that we see them as they were in the distant past. Even the closest quasars lie at great distances from us. Quasars are the most luminous objects known. They exhibit the same basic features as active galaxies, and astronomers believe that their power source is also basically the same—a supermassive black hole consuming many stars per year. The brightest quasars consume so much fuel that their energy-emitting lifetimes must be quite short.

Some quasars have been observed to have double or multiple images due to gravitational lensing, where the gravitational field of a foreground galaxy or galaxy cluster bends and focuses the light from the more distant quasar. Analysis of this bending provides a means of determining the masses of galaxy clusters—including the dark matter—far beyond the region defined by the optical images of the galaxies.

Quasars, active galaxies, and normal galaxies may represent an evolutionary sequence. Quasars probably represent a brief early phase of violent galactic activity. As the fuel supply diminishes, the quasar dims, and the host galaxy becomes visible as an active galaxy. When the fuel supply declines to the point where the nucleus becomes virtually inactive, a normal galaxy is all that remains.

Review and Discussion

1. Name two basic differences between normal galaxies and active galaxies. HINT

2. Describe some of the basic properties of Seyfert galaxies. HINT

3. What distinguishes a core-halo radio galaxy from a lobe radio galaxy? HINT

4. What is the evidence that the radio lobes of some active galaxies consist of material ejected from the galaxy’s nucleus? HINT

5. What conditions result in a head-tail radio galaxy? HINT

6. Briefly describe the leading model for the central engine of an active galaxy. HINT

7. How do astronomers know that the energy-producing region of an active galaxy must be very small? HINT

8. What is synchrotron radiation, and what does it tell us about energy emission from active galaxies? HINT

9. What evidence do we have that the energy production in active galaxies is intermittent? HINT

10. What was it about the spectra of quasars that was so unexpected and surprising? HINT

11. Why do astronomers prefer to speak in terms of redshifts rather than distances to faraway objects? HINT

12. How do we know that quasars are extremely luminous? HINT

13. How are the spectra of distant quasars used to probe the space between us and them? HINT

14. What evidence do we have that quasars represent an early stage of galactic evolution? HINT

15. If many galaxies were once quasars, what has happened to the energy sources at their centers? HINT

True or False?

____ 1. Active galaxies can emit thousands of times more energy than our own Galaxy. HINT

____ 2. The spectrum of a typical active galaxy is well described by a blackbody curve. HINT

____ 3. The "extra" radiation emitted by active galaxies is due to the tremendous number of stars they contain. HINT

____ 4. Active galaxies emit most radiation at visible wavelengths. HINT

____ 5. Most radio galaxies are ellipticals. HINT

____ 6. The diameter of a billion-solar-mass black hole is about 20 A.U. HINT

____ 7. Nearby active galaxies are most likely the result of galaxy interactions. HINT

____ 8. A redshift greater than 1 means a recessional velocity greater than the speed of light. HINT

____ 9. All active galaxies are far away. HINT

____ 10. All quasars are far away. HINT

____ 11. Other than a small amount of visible light, quasars emit all of their radiation at radio wavelengths. HINT

____ 12. Many nearby normal galaxies may once have been quasars. HINT

____ 13. Quasars emit about as much energy as normal galaxies. HINT

____ 14. Astronomers have no direct evidence that quasars are found in the hearts of young galaxies. HINT

____ 15. The quasar stage of a galaxy ends because the central black hole uses up all the matter around itself. HINT

Fill in the Blank

1. Active galaxies generally emit most of their radiation at _______ wavelengths. HINT

2. A Seyfert galaxy looks like a normal spiral, but with a very bright galactic _______. HINT

3. In a core-halo radio galaxy, most of the radio radiation is emitted from the _______. HINT

4. Lobe radio galaxies emit radio radiation from regions that are typically much _______ in size than the visible galaxy. HINT

5. Radio lobes are always found aligned with the _______ of the visible galaxy. HINT

6. For all types of active galaxies, the original source of the tremendous energy emitted is the galactic _______. HINT

7. The energy source of an active galaxy is unusual in that a large amount of energy is emitted from a region much less than _______ in diameter. (Give size and unit.) HINT

8. The mass of the black hole responsible for energy production in the active galaxy M87 is thought to be approximately equal to _______ solar masses. HINT

9. The amount of mass that must be consumed by a supermassive black hole to provide the energy for an active galaxy is about _______ per _______. HINT

10. Quasars are also known as quasi-stellar objects because of their _______ appearance at visible wavelengths. HINT

11. Quasar spectra were understood when it was discovered that their radiation is _______ by an unexpectedly large amount. HINT

12. The distance to a quasar in light-years is not simply equal to the time in years since the quasar emitted the light we see because of the _______ of the universe. HINT

13. The fact that a typical quasar would consume an entire galaxy’s worth of mass in 10 billion years suggests that quasar lifetimes are relatively _______. HINT

14. The image of a distant quasar can be split into several images by gravitational lensing, produced by a foreground _______ along the line of sight. HINT

15. Quasar host galaxies are hard to see because they are so much _______ than the quasar itself. HINT


1. A Seyfert galaxy is observed to have broadened emission lines indicating a speed of 1000 km/s at a distance of 1 pc from its center. Assuming circular orbits, use Kepler’s laws (Section 14.6) to estimate the mass within this 1-pc radius. (Sec. 14.5) HINT

2. Centaurus A—from one radio lobe to the other—spans about 1 Mpc. It lies at a distance of 4 Mpc from Earth. What is the angular size of Centaurus A? Compare this value with the angular diameter of the Moon. HINT

3. Assuming a jet speed of 0.75 c, calculate the time taken for material in Cygnus A’s jet to cover the 500 kpc between the galaxy’s nucleus and its radio-emitting lobes. HINT

4. Assuming the same efficiency as indicated in the text, calculate the amount of energy an active galaxy would generate if it consumed one Earth mass of material every day. Compare this value with the luminosity of the Sun. HINT

5. Based on the data presented in the text, calculate the orbital speed of material orbiting at a distance of 0.5 pc from the center of M87. HINT

6. A certain quasar has a redshift of 0.25 and an apparent magnitude of 13. Using the data from Table 16.1, calculate the quasar’s absolute magnitude, and hence its luminosity. (More Precisely 10-1) Compare the apparent brightness of the quasar, viewed from a distance of 10 pc, with that of the Sun as seen from Earth. HINT

7. Assuming an energy-generation efficiency (that is, the ratio of energy released to total mass-energy available) of 10 percent, calculate how long a 1041-W quasar can shine if a total of 1010 solar masses of fuel is available. HINT

8. The spectrum of a quasar with a redshift of 0.20 contains two sets of absorption lines, redshifted by 0.15 and 0.155, respectively. If H0 = 65 km/s/Mpc, estimate the distance between the intervening galaxies responsible for the two sets of lines. HINT

9. Light from a distant quasar passes 200 kpc from the center of an intervening galaxy cluster before being deflected to a detector on Earth. If Earth, the cluster, and the quasar are all aligned, the quasar is 2500 Mpc away, and the cluster lies midway between Earth and the quasar, calculate the angle through which the galaxy cluster bends the quasar’s light. HINT

10. Light from a distant star is deflected by 1.75" as it grazes the Sun. (More Precisely 13-2) Given that the deflection angle is proportional to the mass of the gravitating body and is inversely proportional to the minimum distance between the light ray and the body, calculate the mass of the galaxy cluster in the previous question. HINT


Here are three observing projects that are increasingly challenging. If you can find the three objects listed here, you have started to become an accomplished observer!

1. In Project 2 of Chapter 15, you were given directions for finding the Virgo Cluster of galaxies. (Chapter 15, Project 2) M87, in the central part of this cluster, is the core-halo radio galaxy nearest Earth and has coordinates RA = 12h 30.8m, declination = +12° 249. At magnitude 8.6, it should not be difficult to find in an 8-inch telescope. Its distance is roughly 20 Mpc. Describe its nucleus; compare what you see with the appearance of other nearby ellipticals in the Virgo Cluster.

2. NGC 4151 is the brightest Seyfert galaxy. Its coordinates are RA = 12h 10.5m, dec = +39° 24', and it can be found below the Big Dipper in Canes Venatici. At magnitude 10-12 (it is variable), it should be visible in an 8-inch telescope but will be challenging to find. Its distance from Earth is 13.5 Mpc. Describe its nucleus and compare its appearance with what you have seen for other galaxies.

3. 3C 273 is the nearest and brightest quasar. However, that does not mean it will be easy to find and see. Its coordinates are RA = 12h 29.2m, dec = +12° 03'. It is located in the southern part of the Virgo Cluster but is not associated with that cluster. At magnitude 12-13, it may require a 10- or 12-inch telescope to see, but try first with an 8-inch. 3C 273 should appear as a very faint star. The significance of seeing this object is that it is 640 Mpc distant. The light you are seeing left this object over two billion years ago. 3C 273 is the most distant object observable with a small telescope.