What is a Blazar?

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  • Written By: Michael Anissimov
  • Edited By: Bronwyn Harris
  • Last Modified Date: 16 October 2019
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Blazars are among the most luminous phenomena in the universe besides the Big Bang. They are a subtype of active galactic nuclei (AGN), which are created when large clouds of dust and gas generate tremendous friction as they are sucked into a supermassive black hole. This accretion of matter forms a ringlike structure called an accretion disc. Perpendicular to the plane of the disc, powerful relativistic (near light speed) plasma jets are released, which can be observed from Earth using optical/radio telescopes if the jet happens to be pointing towards us. If not, the blazar may be unobservable from our perspective.

As a group, objects with the above properties are called active galaxies. The light released by the relativistic jets is so powerful that it can be observed from billions of light years away. Blazars are a subclass of active galaxies, including two types of objects — OVV (optically violent variable) quasars and BL Lacertae objects. Both are characterized by polarized light and high variability in energy output. This variability is caused by the "chunkiness" of the matter falling into the central black hole in the blazar.


Blazars were first incorrectly identified as variable stars in our own galaxy. Measuring their redshift proved otherwise — these objects are seen to be billions of light years away, also meaning they are billions of years old. Blazars and other active galaxies are much more common in the early universe than today, presumably because the matter in galaxies has either settled into stable orbits around the central supermassive black hole (which is suspected to exist in every galaxy) or was sucked in long ago.

Blazars are made even brighter by an effect called relativistic beaming. As Einstein demonstrated, time slows down at speeds close to that of light. The plasma jets are moving at a substantial percentage of light speed, so from our perspective, more plasma is ejected in a given time frame, and as a result the beam appears brighter. Another highly important factor is the orientation of the jet towards us &mndash; even a few degrees can make orders of magnitude difference as a factor of brightness.


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I was wondering what the difference between a quasar and a blazar is.

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