Active galactic nuclei (AGN) are compact regions at the center of galaxies that emit a significant amount of energy across the electromagnetic spectrum. They are the most luminous persistent sources of electromagnetic radiation in the universe, and can be used to discover distant objects. Their evolution as a function of cosmic time also puts constraints on models of the cosmos.
The non-stellar radiation from an AGN is theorized to result from the accretion of matter by a supermassive black hole at the center of its host galaxy.
The observed characteristics of an AGN depend on several properties such as the mass of the central black hole, the rate of gas accretion onto the black hole, the orientation of the accretion disk, the degree of obscuration of the nucleus by dust, and presence or absence of jets.
Numerous subclasses of AGN have been defined on the basis of their observed characteristics; the most powerful AGN are classified as quasars. A blazar is an AGN with a jet pointed toward Earth, in which radiation from the jet is enhanced by relativistic beaming.
AGNs are very energetic due either to the presence of a black hole or star formation activity at the core of the galaxy. Astronomers generally divide AGNs into two groups based on emission line features.
An active galactic nucleus refers to the center of an active galaxy where there is intense radiation activity and not to the black hole only. AGN are usually just the size of our solar system or may be even smaller. They are the most luminous parts of the universe and the biggest sources of electromagnetic radiation.