The discovery of AGN began with the development of radio astronomy after World War II when hundreds of sources of radio waves on the sky were detected and catalogued (e.g. Third Cambridge Catalogue (3C) - Edge et al. 1959 and its revision (3CR) - Bennett 1961), but the nature of these strong radio emitters was unknown. Astronomers at Palomar attempted to optically identify some of the catalogued radio sources; Baum & Minkowski (1960) discovered optical emission from a faint galaxy at the position of the radio source 3C295 and, on studying the galaxy's spectrum, or cosmic bar-code, measured its redshift and inferred a distance of 5000 million light years, making it the most distance object known at that time. Distances can be inferred from Hubble's law whereby the more distant an object, the faster it appears to be receding from us, due to the expansion of the Universe. Chemical elements present in these objects emit or absorb radiation at known characteristic frequencies and when observed in a receding object, the observed frequency is reduced or redshifted due to the Doppler effect; the same physical process that causes a receding ambulance siren to be lowered in pitch after it passes the observer.
Attempts to find visible galaxies associated with other strong radio sources such as 3C48, 3C196 and 3C286 failed and only a faint blue, star-like object at the position of each radio source was found - thus leading to their name `quasi-stellar radio sources', or `quasars' for short. The spectrum of these quasars resembled nothing that had previously been seen for stars in our Galaxy and these blue points remained a mystery until Maarten Schmidt (1963) concentrated on 3C273, for which an accurate radio position was known (Hazard, Mackey & Shimmins 1963). The optical spectrum of the blue source associated with the radio emitter seemed unidentifiable until Schmidt realised that the spectrum could be clearly identified with spectral lines emitted from hydrogen, oxygen and magnesium atoms if a redshift corresponding to 16% of the speed of light was applied. The same technique was applied successfully to 3C48 (Greenstein & Matthews 1963) and demonstrated that these objects are not members of our own galaxy but lie at vast distances and are super-luminous. Indeed, the radiation emitted from a quasar (L 1013 L, where the Sun's luminosity is L = 3.8 × 1026 Watt) is bright enough to outshine all the stars in its host galaxy. Such energies cannot be produced by stars alone and it was quickly realised that the release of gravitational potential energy from material falling towards, or being accreted by, a supermassive black hole at the galaxy centre, ~ 100 times more energy efficient than nuclear fusion, was the only effective way to power such prodigious outputs (Lynden-Bell 1969).
A black hole is a region of space inside which the pull of gravity is so strong that nothing can escape, not even light. Two main kinds of black holes are thought to exist in the Universe. Stellar-mass black holes arise from the the collapsed innards of a massive star after its violent death when it blows off its outer layers in a spectacular supernova explosion; these black holes have mass slightly greater than the Sun but are compressed into a region only a few kilometres across. In contrast, supermassive black holes, which lurk at the centres of galaxies, are 10 million to 1000 million times more massive than the Sun and contained in a region about the size of the Solar System. The emission of radiation from a supermassive black hole appears at first to be contradictory; however, the energy generating processes take place outside the black hole's point-of-no-return, or event horizon. The mechanism involved is the conversion of gravitational potential energy into heat and light by frictional forces within a disk of accreting material, which forms from infalling matter that still possesses some orbital energy, or angular momentum, and so cannot fall directly into the black hole.
Radiation from AGN is detected across the electromagnetic spectrum and today, nuclear activity in galaxies has been detected over a wide range of luminosities, from the most distant and energetic quasars, to the weaker AGN seen in nearby galaxies, such as Seyferts (Seyfert 1943), and even the nucleus of our own Milky Way.