| active galactic nucleus | The nucleus of an active galaxy.
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| active galaxy | A galaxy whose nucleus is unusually bright and small.
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| blazar | A type of active galaxy named for BL Lacertae, the first of the type discovered. Blazars show rapid, unpredictable variations in brightness.
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| broad line region | The high-density region in a quasar where broad emission lines are formed.
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| Eddington luminosity | The maximum luminosity that a body could emit without driving away surrounding material.
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| Einstein ring | The ring or near ring into which the image of a distant quasar is distorted if the quasar lies directly behind a galaxy or cluster of galaxies producing a gravitational lens.
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| gamma ray burst | A brief blast of gamma rays that originates in adistant galaxy.
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| gravitational lens | A massive body that bends light passing near it. A gravitational lens can distort or focus the light of background sources of electromagnetic radiation.
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| jet | A narrow beam of gas ejected from a star or the nucleus of an active galaxy.
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| lookback time | The length of time that has elapsed since the light we are now receiving from a distant object was emitted.
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| luminosity function | The distribution of stars or galaxies according to their luminosities. A luminosity function is often expressed as the number of objects per unit volume of space that are brighter than a given absolute magnitude or luminosity.
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| lunar eclipse | The darkening of the Moon that occurs when the Moon enters the Earth’s shadow.
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| Lyman _ forest | The large number of absorption lines seen at wavelengths just longer than the wavelength of the Lyman _ line of hydrogen in the spectrum of a quasar. The Lyman _ forest is caused by absorption by gas clouds lying between the quasar and the Earth.
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| narrow line region | The low density region in a quasar where narrow emission lines are formed.
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| quasar | A distant galaxy, seen as it was in the remote past, with avery small, luminous nucleus.
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| radio galaxy | A galaxy that is a strong source of radio radiation.
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| Seyfert galaxy | A barred or normal spiral galaxy with a small, very bright nucleus.
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| starburst galaxy | A galaxy in which a very large number of stars have recently formed.
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| superluminal motion | The apparent separation of components of a quasar at speeds faster than the speed of light.
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| V/Vmax test | A statistical method used to determine whether quasars have changed over time.
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