Some of the brightest galaxies were first catalogued as flickering local stars
Astronomers once listed several objects as irregular variable stars inside the Milky Way, brightening and fading without pattern. They turned out to be colossal galaxies billions of light years away, looking straight down a jet of matter moving at nearly light speed. Seen head-on like that, these galactic cores are called blazars.
A blazar is an active galactic nucleus: a supermassive black hole swallowing gas, dust and the occasional star, which heats a disk around it to extreme temperatures. Perpendicular to that disk, magnetic fields and winds squeeze plasma into a pair of jets that can stretch tens of kiloparsecs. The plasma can move at 99.5 percent of light speed. What makes a blazar special is simply geometry: one jet happens to point almost directly at Earth.
That alignment dramatically amplifies what we see, an effect called relativistic beaming. A jet angled five degrees from our line of sight, travelling at 99.9 percent of light speed, looks about 70 times brighter than it really is; aimed dead on, around 600 times. Time dilation compresses its flickers, so outbursts seem to arrive far faster than they occur, and blobs in the jet can appear to cross the sky faster than light. Meanwhile the opposite jet, speeding away, looks much fainter. The same kind of galaxy seen from a different angle would look like an ordinary radio galaxy.
Unmasking them took decades. Radio telescopes of the late 1950s could match radio sources to visible objects, revealing quasars, and one of the first measured, 3C 273, is itself a blazar some 2.5 billion light years away. In 1968 the supposed star BL Lacertae was linked to a powerful radio source, and in 1974 faint traces of a surrounding galaxy confirmed it was nothing of the sort. The astronomer Edward Spiegel coined the word blazar in 1978, blending BL Lacertae with quasar.
Blazars now anchor research into black holes, cosmic rays and high-energy particles, and over three thousand BL Lac-type sources had been confirmed by 2015. In July 2018, the IceCube Neutrino Observatory in Antarctica traced a neutrino detected in 2017 to the blazar TXS 0506+056, 3.7 billion light years away. It was the first time a neutrino detector had pinpointed an object in space.
Source: Blazar
