the opposite of a black hole..

id say a black hole is a lot more active than a galactic nucleus

"An active galactic nucleus (AGN) is a compact region at the centre of a galaxy which has a much higher than normal luminosity over some or all of the electromagnetic spectrum (in the radio, infrared, optical, ultra-violet, X-ray and/or 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 on to 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 cosmological models." Source and further information: http://en.wikipedia.org/wiki/Active_galactic_nucleus

In some galaxies, known as "active galactic nuclei" (AGN), the nucleus (or central core) produces more radiation than the entire rest of the galaxy! Quasars are very distant AGN - the most distant quasars mark an epoch when the universe was less than a billion years old and a sixth of its current size. In some cases, the size of the AGN is smaller than the size of our solar system. Current theory suggests that there is a supermassive black hole (millions of times the mass of the sun) at the center of AGN. http://heasarc.gsfc.nasa.gov/docs/objects/agn/agntext.html

Here are some images, http://search.msn.com/images/results.aspx?q=active%20galactic%20nucleus A galactic nucleus that gives off much more energy than can be explained purely in terms of its star content. AGN are found at the heart of active galaxies, including quasars, Seyfert galaxies, blazars, and radio galaxies. In addition to their great energy output, they can be highly variable. Some quasars vary in brightness over a few weeks or months, while some blazars show changes in X-ray output over as little as three hours. These fluctuations place strict limits on the maximum size of the energy source, because an object cannot vary in brightness faster than it takes light to travel from one side of its energy-producing region to the other. The rapid flickering of AGN means that they draw their energy from a small volume, in some cases less than one light day across. Furthermore, observations of the orbital motion of stars and other material around AGN show that a large mass, ranging up to several billion solar masses, is concentrated within its "engine room." This leads to the almost unavoidable conclusion that the central engine is a supermassive black hole. Since a black hole, by definition, emits nothing, the radiation from an AGN is believed to come from material heated to several million degrees in an accretion disk before tumbling into the black hole or, in some cases, being shot away in twin jets along the central engine’s spin axis. http://www.daviddarling.info/encyclopedia/A/AGN.html