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:
“Things happen to us, all the time. It was like that for a century, and it is again. It’s not like here: People always do things, because you are born with it; you are brought up in this spirit, the active approach to life: “Stand up and go.” We were not. We were always passive in our lives.”
—Natasha Dudinska (b. c. 1967)
“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)
“Just see how it glints and sparkles. Of course it is a nucleus and focus of crime. Every good stone is. They are the devil’s pet baits. In larger and older jewels every facet may stand for a bloody deed....”
—Sir Arthur Conan Doyle (1859–1930)