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The Question

(Submitted June 08, 1998)

I have absolutely no experience in astrophysics but a friend of mine did a a thesis on AGN. She could not explain it to me because she is Polish and I am an American. so my question comes in two parts. What is AGN exactly? What implications does it have for understanding how the universe works?

The Answer

An AGN is short for "Active Galactic Nucleus." Some galaxies have nuclei (centers) that are 'active', meaning they emit large amounts of radiation (radio, optical, X-rays, gamma-rays, particle jets, etc.), and/or are highly variable. (For example, a galactic nucleus starting at 30 billion times as bright as the Sun, then growing to 45 billion times as bright as the Sun in just half an hour).

Since they vary so rapidly, the important region must be small, no larger than the inner solar system (since the time over which something can vary is limited to the time it takes light to get from one side to the other). Since they are so bright, that small region must have unbelievable energies in it.

Gigantic black holes, billions of times as massive as the Sun, swallowing stars and gas clouds, are the only reasonable theories that seem to fit the data.

Do a search on our website for 'active' to find more information

David Palmer
for Ask an Astrophysicist

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