An Etymological Dictionary of Astronomy and Astrophysics

A member of a family of → quasars, or extragalactic → Active Galactic Nuclei, which displays
a high radio emission and/or important optical variability over a short period of time. BL Lac objects appear star-like but their spectrum is flat, and partially polarized. Also called → blazars.

See also: BL Lac, from object BL in the constellation → Lacerta (BL Lacertae). The reason for this terminology is that it was originally thought to be an irregular variable star in our Galaxy; hence its variable star designation. In the 1970s the “star” was identified with a bright, variable → radio source and a very faint galaxy; → object.