An apparently star-like object with a near-featureless spectrum; also known as a Lacertid. BL Lac objects show considerable brightness variations—often by several magnitudes over days or weeks—with high (and variable) polarization, and are sometimes associated with a compact radio source. The prototype was originally classified as a peculiar 14th-magnitude variable star in the constellation of Lacerta, hence the variable-star designation. The extragalactic nature of these objects was not established until absorption lines from low-mass giant stars, characteristic of the spectra of elliptical galaxies, were identified in the spectra of the faint nebulosity surrounding some of them. From these lines a redshift could be obtained and their distances deduced. About 1500 are known. They are now thought to be high-speed jets of plasma and radiation from an active galactic nucleus, viewed nearly end-on.
Subjects: Astronomy and Astrophysics.