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active galactic nucleus  The nucleus of an active galaxy.
active galaxy  A galaxy whose nucleus is unusually bright and small.
blazar  A type of active galaxy named for BL Lacertae, the first of the type discovered. Blazars show rapid, unpredictable variations in brightness.
broad line region  The high-density region in a quasar where broad emission lines are formed.
Eddington luminosity  The maximum luminosity that a body could emit without driving away surrounding material.
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.
gamma ray burst  A brief blast of gamma rays that originates in adistant galaxy.
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.
jet  A narrow beam of gas ejected from a star or the nucleus of an active galaxy.
lookback time  The length of time that has elapsed since the light we are now receiving from a distant object was emitted.
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.
lunar eclipse  The darkening of the Moon that occurs when the Moon enters the Earth’s shadow.
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.
narrow line region  The low density region in a quasar where narrow emission lines are formed.
quasar  A distant galaxy, seen as it was in the remote past, with avery small, luminous nucleus.
radio galaxy  A galaxy that is a strong source of radio radiation.
Seyfert galaxy  A barred or normal spiral galaxy with a small, very bright nucleus.
starburst galaxy  A galaxy in which a very large number of stars have recently formed.
superluminal motion  The apparent separation of components of a quasar at speeds faster than the speed of light.
V/Vmax test  A statistical method used to determine whether quasars have changed over time.

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