BL Lacertae object (BL Lac object)
BL Lacertae. Credit: Zsófia Nagy and Szilárd Csizmadia, 1m RCC telescope of the Konkoly Observatory.
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.