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The compact region at the centre of a galaxy that exhibits higher than normal levels of emission of electromagnetic radiation. The excess radiation can occur over the complete electromagnetic spectrum and has been observed at radio, microwaves, infrared, optical, ultra-violet, X-ray and gamma ray wavelengths.

Such objects are believed to be powered by matter falling on to a supermassive black hole. Supergiant elliptical galaxy M87 in Virgo has an active galactic nucleus.

Messier 87 contains an AGN (credit:- NASA, ESA, and The Hubble Heritage Team (STScI/AURA))

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