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Text File  |  1995-11-04  |  477b  |  13 lines
  1. active nucleus: The center of a galaxy
  2. that emits unusually intense
  3. radiation. This radiation is observable
  4. at many different wavelengths,
  5. including visible light, ultraviolet
  6. light, radio waves, gamma rays, and
  7. x-rays. Astronomers theorize that the
  8. emission from an active nucleus is
  9. caused by an accretion disk that
  10. surrounds a massive black hole in
  11. the galaxyΓÇÖs center. Galaxies that
  12. exhibit active nuclei include radio
  13. galaxies, Seyfert galaxies, and quasars.