Active Galaxies and Quasars
http://imagine.gsfc.nasa.gov/science/objects/active_galaxies1.html
Active galaxies are galaxies which have a small core of emission embedded in an otherwise typical galaxy. This core may be highly variable and very bright compared to the rest of the galaxy... For “normal” galaxies, we can think of the total energy they emit as the sum of the emission from each of the stars found in the galaxy. For the “active” galaxies, this is not true. There is a great deal more emitted energy than there should be... and this excess energy is found in the infrared, radio, UV, and X-ray regions of the electromagnetic spectrum.
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