Universe 8e Lecture Chapter 25 Quasars and Active Galaxies

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Roger A. Freedman William J. Kaufmann III
Universe Eighth Edition
CHAPTER 25 Quasars and Active Galaxies
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By reading this chapter, you will learn

• 25-1 The distinctive features of quasars
• 25-2 The connection between quasars and distant
  galaxies
• 25-3 What Seyfert galaxies and radio galaxies are
  and how they compare to quasars
• 25-4 The properties of active galactic nuclei
• 25-5 How supermassive black holes can power
  active galactic nuclei
• 25-6 Why many active galaxies emit ultrafast jets
  of material

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Key Ideas

• Quasars A quasar looks like a star but has a
  huge redshift. These redshifts show that quasars
  are several hundred megaparsecs or more from the
  Earth, according to the Hubble law.
• To be seen at such large distances, quasars must
  be very luminous, typically about 1000 times
  brighter than an ordinary galaxy.
• About 10 of all quasars are strong sources of
  radio emission and are therefore called
  radio-loud the remaining 90 are
  radio-quiet.
• Some of the energy emitted by quasars is
  synchrotron radiation produced by high-speed
  particles traveling in a strong magnetic field.

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Key Ideas

• Seyfert Galaxies Seyfert galaxies are spiral
  galaxies with bright nuclei that are strong
  sources of radiation. Seyfert galaxies seem to be
  nearby, low-luminosity, radio-quiet quasars.
• Radio Galaxies Radio galaxies are elliptical
  galaxies located midway between the lobes of a
  double radio source.
• Relativistic particles are ejected from the
  nucleus of a radio galaxy along two oppositely
  directed beams.

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Key Ideas

• Blazars Blazars are bright, starlike objects
  that can vary rapidly in their luminosity. They
  are probably radio galaxies or quasars seen
  end-on, with a jet of relativistic particles
  aimed toward the Earth.
• Active Galaxies Quasars, blazars, and Seyfert
  and radio galaxies are examples of active
  galaxies. The energy source at the center of an
  active galaxy is called an active galactic
  nucleus.
• Rapid fluctuations in the brightness of active
  galaxies indicate that the region that emits
  radiation is quite small.

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Key Ideas

• Black Holes and Active Galactic Nuclei The
  preponderance of evidence suggests that an active
  galactic nucleus consists of a supermassive black
  hole onto which matter accretes.
• As gases spiral in toward the supermassive black
  hole, some of the gas may be redirected to become
  two jets of high-speed particles that are aligned
  perpendicularly to the accretion disk.
• An observer sees a radio galaxy when the
  accretion disk is viewed nearly edge-on, so that
  its light is blocked by a surrounding torus. At a
  steeper angle, the observer sees a quasar. If one
  of the jets is aimed almost directly at the
  Earth, a blazar is observed.