BL Lacertae object (BL Lac object)
A BL Lacertae object (BL Lac object) is a type of active galactic nucleus (AGN) characterized by a nearly featureless spectrum (i.e., virtually devoid
of spectral lines), rapid, marked variability at radio, infrared, and optical
wavelengths (often by several magnitudes over a few days or weeks), and
strong, variable polarization at radio
and optical wavelengths. The prototype, in the constellation Lacerta,
was originally classified as a variable star.
|BL Lacertae. Credit: Zsófia Nagy and Szilárd Csizmadia,
1m RCC telescope of the Konkoly Observatory
BL Lac objects belong to the category of AGN now known as blazars and are believed to be essentially quasars viewed directly down the axis of their powerful jets.