An active galactic nucleus (AGN) is a compact region at the centre of a galaxy that has a much higher than normal luminosity over at least some portion, and possibly all, of the electromagnetic spectrum. Such excess emission has been observed in the radio, infrared, optical, ultra-violet, X-ray and gamma ray wavebands. A galaxy hosting an AGN is called an active galaxy. The radiation from AGN is believed to be a result of accretion of mass by the supermassive black hole at the centre of the host galaxy. AGN are the most luminous persistent sources of electromagnetic radiation in the universe, and as such can be used as a means of discovering distant objects; their evolution as a function of cosmic time also provides constraints on models of the cosmos.
Read more about Active Galactic Nucleus: Discovery, Models of The Active Nucleus, Observational Characteristics, Types of Active Galaxy, Unification, Cosmological Uses and Evolution
Famous quotes containing the words active, galactic and/or nucleus:
“All is changed. All looks strange to me and gives me a feeling which I would rather get away from, although I know it to be the carrying out of natural laws. And I am not complaining. I am doing the same as many old people have done, I suppose, who have led an active life and suddenly find themselves living without a purpose. Oh, my heart is so full. I could write a big book on the subject of going out of this world gracefully.”
—Maria D. Brown (1827–1927)
“In sci-fi convention, life-forms that hadn’t developed space travel were mere prehistory—horse-shoe crabs of the cosmic scene—and something of the humiliation of being stuck on a provincial planet in a galactic backwater has stayed with me ever since.”
—Barbara Ehrenreich (b. 1941)
“I could not undertake to form a nucleus of an institution for the development of infant minds, where none already existed. It would be too cruel.”
—Henry David Thoreau (1817–1862)