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 104 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. See also
→ broad-line region,
→ narrow-line region,
→ quasar.
See also: → active; → galactic;
→ nucleus.
