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The Question

(Submitted January 24, 1998)

I am a high school student and I am interested in this subject. Why do some galaxies have an active nucleus and others don`t?

The Answer

According to current theories, a galactic nucleus is active if it has a large black hole which is consuming large quantities of matter.

If a nucleus doesn't have a large black hole, then it is not an AGN. If it has a black hole, but no stars etc. are falling into it, then it is also not an AGN. If all of the stars that had been in orbits that get very near the black hole have already been eaten, then the remaining stars are safe, so that nucleus is no longer active.

David Palmer
for Ask an Astrophysicist

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