A blazar is a very compact and highly variable energy source associated with a
supermassive black hole at the center of a
host galaxy. Blazars are among the most violent phenomena in the universe and are an important topic in
extragalactic astronomy. Blazars are members of a larger group of
active galaxies, also termed active galactic nuclei (AGN). However, blazars are not a homogeneous group and can be divided into two: highly variable
quasars, sometimes called
Optically Violently Variable (OVV) quasars (these are a small subset of all
quasars) and BL Lacertae objects ("
BL Lac objects" or simply "BL Lacs"). A few rare objects may be "intermediate blazars" that appear to have a mixture of properties from both OVV quasars and BL Lac objects. The name "blazar" was originally coined in 1978 by astronomer Ed Spiegel to denote the combination of these two classes.
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Arguably the most active of active galaxies. The name is a concatenation of
BL Lacertae object and quasar; both can be described as blazars if they show violent variability in the optical region of the electromagnetic spectrum. According to some unified theories of active galaxies, the activity in blazars is caused by jets of gas being expelled from the active galactic nucleus, at speeds close to the speed of light, almost directly into the line of sight from Earth.
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