Unification
Unified models of AGN unite two or more classes of objects, based on the traditional observational classifications, by proposing that they are really a single type of physical object observed under different conditions. The currently favoured unified models are 'orientation-based unified models' meaning that they propose that the apparent differences between different types of objects arise simply because of their different orientations to the observer. For an overview of these see and, though some details in the discussion below have emerged since these reviews were written.
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