BL Lacertae

BL Lacertae is a highly variable active galactic nucleus whose relativistic jet points nearly toward Earth. It is the namesake and prototype of BL Lacertae objects, a blazar class.

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Relativistic beaming: The concentration and amplification of radiation emitted by a source moving relativistically toward an observer. Its jet points nearly toward Earth, amplifying and rapidly varying the observed emission.
Blazar: An active galactic nucleus whose relativistic jet is oriented close to an observer's line of sight. BL Lacertae belongs to this broader class of strongly beamed sources.
Kurt W. B. Babcock: An American astronomer who studied stellar magnetic fields and identified BL Lacertae as a variable star. His early observations helped establish the unusual variability that first drew attention to the object.
Host galaxy: The galaxy containing a compact central source such as an active galactic nucleus. Imaging its host has provided evidence that BL Lacertae lies beyond the Milky Way.
Active galactic nucleus: A galaxy's exceptionally luminous central region, powered by matter accreting onto a supermassive black hole. BL Lacertae is one example of this broader class of energetic galactic cores.
BL Lacertae object: A blazar characterized by rapid variability and weak or absent broad emission lines. The source gives this subclass its name and serves as its prototype.
John L. Schmitt: An astronomer who identified BL Lacertae as a radio source associated with a star-like optical object. His identification helped connect the apparent star to the class of extragalactic radio sources.
Optical variability: Changes over time in an astronomical object's brightness at visible wavelengths. Its dramatic optical fluctuations are among the source's defining observable traits.
Relativistic jet: A narrow outflow of plasma launched from a compact astrophysical object at speeds close to light speed. The jet directed toward Earth produces BL Lacertae's defining orientation and strong variability.
Apparent superluminal motion: An observational effect in which a relativistic jet component appears to move faster than light across the sky. Relativistic motion in its jet can create this misleading appearance.