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Active galaxy, artwork. An active galaxy is thought to have a black hole (black, centre) at its centre. This can account for the energetic radiation and other phenomena associated with such objects. The super-massive black hole accretes gas and dust from the galaxy, here emitting some of it in two fast beams (upper and lower centre), that emerge from either side. Active galaxies include Seyfert galaxies, quasars and radio galaxies. (KEYSTONE/SCIENCE PHOTO LIBRARY/JON LOMBERG/SCIENCE PHOTO LIBRARY)