BL Lacertae

BL Lacertae or "BL Lac" is a highly variable, extragalactic AGN (active galactic nucleus or active galaxy). It was first discovered by Cuno Hoffmeister in 1929 [cite journal | last= Hoffmeister |first= Cuno |year= 1929 |title=354 neue Veränderliche|journal= Astronomische Nachrichten |volume= 236 |pages= 233 | url=http://adsabs.harvard.edu/abs/1929AN....236..233H |doi=10.1002/asna.19292361502] , but was originally thought to be an irregular variable star in our own galaxy and so was given a variable star designation. In 1968 the "star" was identified by John Schmitt at the David Dunlap Observatory as a bright, variable radio source. A faint trace of a host galaxy was also found. [cite journal | last= Schmitt |first= John L. |year= 1968 |month= May |title= BL Lac identified as a Radio Source |journal= Nature |volume= 218 |pages= 663 |url= http://www.nature.com/nature/journal/v218/n5142/abs/218663a0.html;jsessionid=42293521D95FBBFB98B58E8AE6D11FEE |doi= 10.1038/218663a0 ] In 1974, Oke and Gunn measured the redshift of BL Lacertae as z = 0.07, corresponding to a recession velocity of 21,000 km/s with respect to the Milky Way. [cite journal | last = Oke | first = J. B. | coauthors = Gunn, J. E.
year = 1974 | title = The Distance of BL Lacertae | journal = Astrophysical Journal Letters | volume = 189 | pages = 5 | url = http://adsabs.harvard.edu/abs/1974ApJ...189L...5O | doi = 10.1086/181450 ]

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 observed 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".

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