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HEASARC: Education & Public Outreach

Diagram of an Active Galactic Nuclei

Introduction to Active Galactic Nuclei

What's on This Page

What are Active Galactic Nuclei (AGN)?

In some galaxies, known as "active galactic nuclei" (AGN), the nucleus (or central core) produces more radiation than the entire rest of the galaxy! Quasars are very distant AGN - the most distant quasars mark an epoch when the universe was less than a billion years old and a sixth of its current size. In some cases, the size of the AGN is smaller than the size of our solar system. Current theory suggests that there is a supermassive black hole (millions of times the mass of the sun) at the center of AGN.

How does X-ray Astronomy Fit In?

X-ray studies are particularly useful in helping us figure out what is going on in an AGN, since they can penetrate from far within the center of a galaxy. Based on X-ray (and other) observations, a good guess is that the power source in AGN is a supermassive black hole. The picture below shows what this might look like. Although the black hole itself is invisible, gas accreting, or falling, onto a black hole becomes hot and some of this energy escapes for us to see. Many AGN also show evidence for a huge torus of gas around the black hole.

Diagram of an Active Galactic Nuclei

Further AGN Resources and References

  • Imagine the Universe! - Active Galaxies and Quasars

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    Last modified: Thursday, 26-Jun-2003 13:48:45 EDT