Definition of BL Lac object in US English:

BL Lac object

noun

Astronomy
  • A type of remote elliptical galaxy with a very compact starlike nucleus, remarkable for its considerable short-term variations in brightness and radio emissions.

    • ‘However, BL Lac objects are the active cores of elliptical galaxies.’
    • ‘A number of BL Lac objects are known in the vicinity of clusters of galaxies so this provides indirect evidence in support of their extragalactic nature.’
    • ‘The bright reddish object to the lower left of the center is a BL Lac object at the center of a cluster of galaxies.’
    • ‘This result supports the interpretation of H2155 304 as a highly luminous BL Lac object.’
    • ‘Active elliptical galaxies are called BL Lac objects if their jets are viewed end-on; they look very different, having giant radio lobes, if their jets are viewed from the side.’

Origin

BL Lac, short for BL Lacertae, the designation of the first such object discovered (originally thought to be a variable star).