blazar
/ BLAY-zar /
Imagine standing not beside a fire hose but directly in front of its nozzle, looking straight down the stream. Everything the hose blasts comes hurtling at you. A blazar is an AGN whose relativistic jet happens to point almost exactly at Earth, so we are staring down the barrel of a black hole's near-light-speed beam. The result is one of the most extreme and erratic kinds of light source in the sky.
Because the jet is aimed at us, its light is dramatically intensified by relativistic beaming, sometimes brightened by a factor of hundreds, and its features appear to change with breathtaking speed. Blazars flare unpredictably across the whole spectrum, from radio to the highest-energy gamma rays, and their brightness can double in hours or even minutes. The jet's glow, mostly synchrotron radiation boosted by the beaming, swamps almost everything else, so blazars show few or weak emission lines and look like featureless, wildly variable points. They come in two main flavors: BL Lacertae (BL Lac) objects, with almost no emission lines, and flat-spectrum radio quasars, which still show some.
Blazars are the most extreme face of AGN unification: take a radio galaxy and swing its jet around to point at us, and the same object becomes a blazar. They are prime targets for high-energy and multi-messenger astronomy — one blazar, TXS 0506+056, was even pinpointed as a likely source of a high-energy cosmic neutrino, tying jet physics to particle astrophysics. The blinding variability is not the black hole itself changing wildly, but our front-row, beamed view amplifying every flicker in the jet.
The blazar BL Lacertae was first catalogued as a 'variable star' because it flickered so much. Only later did astronomers realize it is the beamed jet of a distant AGN aimed almost straight at us — its erratic brightness changes come from a relativistic jet, not a pulsating star.
An AGN jet aimed straight at us — beaming makes it blindingly bright and wildly variable.
A blazar is not intrinsically the brightest type of AGN — it just looks that way because its jet beams light straight at us. Swing the same jet sideways and the object would appear as an ordinary radio galaxy; the spectacular variability is geometry plus relativity, not the engine misbehaving.