active nucleus: The center of a galaxy
that emits unusually intense
radiation. This radiation is observable
at many different wavelengths,
including visible light, ultraviolet
light, radio waves, gamma rays, and
x-rays. Astronomers theorize that the
emission from an active nucleus is
caused by an accretion disk that
surrounds a massive black hole in
the galaxy’s center. Galaxies that
exhibit active nuclei include radio
galaxies, Seyfert galaxies, and quasars.