AstroJargon of the Week – AGN

Jeffrey Bennett, astronomer and author, once told us that a typical astronomy textbook has about as many vocabulary words as a typical foreign language textbook. So, in addition to teaching physical and astronomical concepts, we’re teaching a whole new language! Jargon is incredibly useful for making detailed communication within a specific field efficient and convenient, but you have to be aware of it when teaching students or talking with people outside your sub-field. So, I’m going to attempt a weekly series of astrophysics jargon, inspired by the Geology Word of the Week on Skepchick. I’ll try to demystify some terms that you may hear astronomers bandying about!


This week’s jargon is one of my favorites, and it’s one that gets the most quizzical looks when it slips out in a tutoring session. AGN stands for Active Galactic Nucleus (or Nuclei). It is literally when the nucleus, or center, of a galaxy, or vast collection of stars, is active, or really, really bright.

There is a whole zoo of subclassifications within the grouping of AGN, mainly due to the various methods by which these galaxies were discovered. However, the basic principle lies with the giant that is at the heart of every major galaxy, a black hole that is millions (or billions) of times the mass of the sun! One of the fascinating results to come from the Hubble Space Telescope is that, everywhere we look, every galaxy (with an appreciable central bulge) has a black hole in its center.

Not only that, but the larger the bulge of stars, the larger the black hole! But that’s a story for another day. From hubblesite.org.

Although we don’t yet know where these super-massive black holes first came from, we do know that they can get bigger if mass falls onto them and becomes part of the black hole. Turns out, this is a really, REALLY energetic process. Black holes have a lot of mass packed into a (relatively) tiny space, so any gas that gets close enough to fall in doesn’t do so right away, but settles into a disk around the black hole to spin around a bunch of times before finally crossing the “point of no return.” The material in the disk gives off a LOT of energy, thus powering the active galactic nucleus.

Artist’s conception of a black hole, disk, and jet. M. Weiss, Chandra X-Ray Observatory

How much energy? A moderate AGN gives off approximately 20 times the light output of a galaxy like the Milky Way! And the light is spread out over all wavelengths, from x-ray and ultraviolet through visible and infrared, and even some in the radio. Because they are so bright, they are powerful probes of the universe at large distances. They may also be intimately involved in the life history of a galaxy, as this bright light from the galaxy’s center has an impact on the environment around it. Also, powerful jets of particles moving at almost the speed of light may be generated near the black hole and affect the galaxy and the galaxy’s environment in a violent way.

One of my favorite radio sources, 3C84 (pink) has jets that have carved out a hole in the x-ray gas (blue) in the Perseus Galaxy Cluster. (Chandra image by A. Fabian, VLA image by G. Taylor)

There are so many cool and interesting subclasses of AGN, and so many structures within and related to the AGN, that it’ll probably take a whole sub-series itself! But I hope this introduction gives a clearer picture of what astronomers means when they say, “That’s my favorite AGN!”


Have an astrophysics jargon suggestion? Email me, and I’ll try and include it!

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