A central region of an active galaxy, which is a light-year or less in diameter and
has an abnormally high luminosity. The nucleus emits high energy radiation (gamma
rays, X-rays, ultraviolet) and shows variability over various time-scales, sometimes
very short (hours to weeks). Emission line spectra reveal high velocity motions up
to 10^4 km s^-1. AGNs are divided into two main types. Type I refers to an AGN whose
nucleus is visible (the spectra has both narrow and broad emission lines), while in
type II AGN, the broad line region (BLR) is obscured and the lines are very narrow.
This may be due either to the viewing angle or some intrinsic difference in structure.