| http://www.w3.org/ns/prov#value | - Astron. a compact region at the centre of a galaxy which emits an abnormally large amount of radiation in one or more parts of the electromagnetic spectrum.Such regions are usually hypothesized to contain supermassive black holes.1965 Astrophysical Jrnl. 142 1086 Violent phenomena such as novae, supernovae, active galactic nuclei, formation of hot stars, turbulence, etc., excite waves (hydromagnet
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