The AGN family includes objects of very different luminosities. AGN which have about the same luminosity than the host galaxy are called Seyfert galaxies . They appear in the visible as usual spiral galaxies with a particularly bright centre. Quasars can emit as much as some thousand galaxies like our Milky-Way. They are thus the most powerful objects in the Universe.
Some quasars are also great radio and gamma-ray emitter.
An other essential characteristic of AGN is their great variability . Observed fluxes from AGN can vary by a factor of two or more in a few months or even in a few days for some of them. Furthermore, contrary to the stellar spectra which have absorption lines, most of the AGN spectra present broad emission-lines . To explain these characteristics, present AGN models suppose the presence of a super-massive black hole of a million to some billions of solar masses. Thus, the model usually admitted today is composed of a super-massive black hole surrounded by an accretion disk and ionized gas clouds moving randomly in the potential well of the black hole.