Most active galaxies have an optical spectrum rich in emission lines. These are superposed on a continuum spectrum which normally has essentially a black-body (thermal) form determined by the temperature of an object. In some active galaxies, however, the continuum spectrum appears to be of a non-thermal origin, in that it cannot be due to an object with a single temperature or, indeed to the superposition of a large number of objects of different temperature. It is likely that in these cases the continuum is produced by synchrotron radiation, generated when energetic electrons move through a region containing a weak magnetic field.

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