The central region of a galaxy that contains a supermassive black hole which is accumulating material from a surrounding accretion disk. Active galactic nuclei were named from the fact that their central regions emit far more radiation than can be accounted for by stars alone. A galaxy with such a nucleus is known as an active galaxy. The first active galaxies were discovered in 1943 by C. K. Seyfert, who noticed that some spirals have an exceptionally bright core; these are now termed Seyfert galaxies. Quasars, discovered in the 1960s, are a more luminous version of Seyferts in which the nucleus is so bright that the surrounding galaxy can hardly be seen at all. About 10% of active galaxies have the additional characteristic of being strong radio sources, or ‘radio-loud’. The radio emission from such radio galaxies is often from regions well outside the galaxy itself known as lobes, although in some cases there are radio jets pointing back towards the centre of the galaxy, indicating that it is the nucleus which is the ultimate source of the energy. Active galactic nuclei are often highly variable over timescales as short as days or even hours, meaning that the radiation must come from a region not much bigger than the Solar System.

The black holes at the centres of AGN have masses up to a billion solar masses or more. The radiation is produced not by the black hole itself but by gas in an accretion disk around it. The viscosity in the disk causes gas gradually to spiral towards the black hole. As it does so, gravitational energy is converted into heat, and the hot gas then emits large amounts of radiation. However, there are still many unanswered questions, including how the jets in radio galaxies are produced.

In addition to Seyfert galaxies, quasars, and radio galaxies, the range of active galaxies now includes blazars and BL Lacertae objects, plus many subspecies. However, it seems likely that at heart most active galactic nuclei are physically similar. Their diversity in outward appearance has two main causes. One is differences in the energy output of the ‘central engine’ itself—quasars, for example, are particularly powerful. The second factor is the angle from which the AGN is viewed, and how obscured the central black hole is by gas and dust around it, which is thought to be in the form of a torus or ring. If we view the AGN along the axis of the ring, we see the radiation emitted by the accretion disk; the gas in the disk moves so fast it produces broad spectral lines. However, if we view the AGN through the ring, this radiation and the broad spectral lines are hidden and we can see only narrow spectral lines produced by gas clouds farther out. The appearance of a ‘radio-loud’ AGN will also depend on the viewing angle. Viewed along the jet (i.e. along the axis of the ring) a radio-loud AGN may be classified as a blazar and the radio jet will often appear to be moving faster than the speed of light (superluminal velocity); viewed from close to the plane of the ring, the AGN will be classified as a radio galaxy, with not many signs of activity in the nucleus but with huge lobes of radio-emitting plasma on either side of the galaxy.

Most galaxies including our own are thought to contain supermassive black holes at their centres and are likely to have passed through an AGN phase at some stage. Observations show that there were many more active galactic nuclei in the past than there are today. It seems likely that the inactive nearby galaxies once contained an AGN, but for some reason the central engine in these galaxies has now run out of fuel. Possibly, all the gas in the vicinity of the black hole has now been consumed.