BL Lac object (BL Lacertae object; Lacertid)

A type of elliptical galaxy with a bright, highly variable, compact nucleus. The first such object to be noted was BL Lacertae, which at its discovery in 1929 was thought to be a variable star (hence the form of its name). Their unique characteristics are dramatic short-term variability in light output and a featureless spectrum. The brightness can vary as much as a hundredfold over a period of a month, and day-to-day changes are sometimes observed. Many BL Lac objects are also radio sources. Intense radio bursts are seen from BL Lacertae itself, but they are apparently not correlated with the light changes.