AstroGroup logo

MSSL Astrophysics Group Beginner's Guides

A Beginner's Guide to AGN

rainbow strip

The nucleus of a galaxy is said to be `active' if it appears that the light which we measure is greater than that which can be emitted by stars alone.


Quasars, very high luminosity AGN, are the most distant objects in the Universe. They have been discovered out to redshifts of about 5, when the Universe was only one-tenth of its present age (about 10 billion years ago).

The amount of light emitted from AGN is immense; a typical object is approximately one million, million times brighter than our own Sun. This radiation is produced over the full range of the electromagnetic spectrum, from low-energy radio waves to high-energy gamma-rays.

By measuring the luminosity of an AGN, we can find out a minimum level for its mass: usually around one billion times the mass of the Sun. The rate at which the light from an AGN can change indicates that this enormous mass is contained within a region less than one light-day across. Only a `supermassive' black hole could have such a high density.

In order to produce such vast quantities of light, AGN consume fuel at an enormous rate; the equivalent of about a few Suns every year. The fuel, surrounding gas and dust, and probably infalling stars, is dragged in by the huge gravitational force of the central black hole.

Explore the structure of a galactic nucleus with these Quasar Snapshots

Weak, or low-luminosity, AGN are sometimes powered by regions which are dominated by intensive bursts of star formation - these are known as `Starburst Galaxies' - see the section on Narrow Emission Line Galaxies

Other Extragalactic Astronomy pages:

Also - why not try the public pages of the Hubble Space Telescope for more pictures and information?

Visitor Centre logo Return to the MSSL Astrophysics Group Visitor Centre

AstroGroup Intraweb logo Mullard Space Science Laboratory
Astrophysics Group

MSSL logo Back to the Mullard Space Science Laboratory homepage

This page last updated on the 11th September 1999.