The name of these objects comes from the prototype, a star-like object long believed to be a quite ordinary irregularly variable star. Following traditional variable star naming conventions, it was known as "BL Lac". BL is a progressive ID not unlike car plates, and Lacerta is the constellation where it resides.
In 1968, a radio source coincident with the (supposed) star was discovered, and a spectrum was taken. Surprisingly, none of the usual spectral lines was present, ruling out the star interpretation for BL Lac.
In the following years, a few similar objects were found. But not many, because the quest was difficult: these objects appear as point-like as any star, and only with laborious cross-checking between different catalogues one can single out a potential BL Lac object.
The real nature of BL Lac objects remained unknown: if near to us, they were totally enigmatic and featureless. If extra-galactic, given their relatively high apparent magnitude, they would have been among the most luminous objects in the Universe.
In 1974, J. B. Oke and J. E. Gunn proved that the point-like source is only a little part of a larger, dimmer object. In each and every case where detection was technically possible, a surrounding galaxy was found, proving that BL Lac objects were extra-galactic.
As of 2003, a few hundreds BL Lac objects are known.
Blazars are thought to be active galaxy nuclei, not very different from quasars, with jets directly pointing to the observer.
The special jet orientation explains most of their peculiar characteristics: the high luminosity, the very rapid variation, the sometimes superluminal motions found in high-resolution images, the very faint lines found in the spectrum.
The jet orientation argument requires that a low number of BL Lac objects exist together with a much higher number of their active nuclei cousins, such as quasars. And indeed, this is supported by observations.
From this interpretation follows that blazars are in the center of an otherwise normal galaxy, and are probably powered by a supermassive black hole.
This relatively simple picture is certainly not complete, and recently (as of 2003) alternative explanations can be found in scientific literature. The main one is that at least some blazar are the result of gravitational lensing, where a massive nearby object acts as a lens on a distant one. Anyway, it is clear that the BL Lac class is far from understood, and that it will possibly be divided into different classes of similar-appearing, but physically different objects.
Blazars include 3C279.History
Current vision