BL Lacertae

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BL Lacertae or BL Lac is a highly variable, extragalactic AGN (active galactic nucleus or active galaxy) originally thought to be an irregular variable star in our own galaxy and so was given a variable star designation in 1941. In 1968 the "star" was identified by Schmidt at the Mount Palomar observatory as a bright, variable radio source and a faint trace of a host galaxy was found. In 1973 astronomers calculated that the galaxy was receding from the Milky Way at 20,000 km/s.

BL Lacertae is the original member of a type of blazar known as "BL Lacertae objects," also termed simply "BL Lac objects." This class is distinguished by optical spectra devoid of the broad emission lines characteristic of quasars. On occasion, however, BL Lacertae has been to display weak emission lines.

BL Lacertae changes in apparent magnitude over fairly small time periods between values of 14 and 17. Its location in the sky is at RA 22h02m07s, DEC +42°16'40".