See also: Nola Taylor Redd, 'Quasars: Brightest Objects in the Universe', Space.com, 23 August 2012, http://www.space.com/17262-quasar-definition.html [31.5.2017]
By 1965 it was recognized that quasars are part of a much larger population of unusually blue sources and that most of these are much weaker radio sources too faint to have been detected in the early radio surveys. This larger population, sharing all quasar properties except extreme radio luminosity, became known as “quasi-stellar objects” or simply QSOs.
Since the early 1980s most astronomers have regarded QSOs as the high-luminosity variety of an even larger population of “active galactic nuclei,” or AGNs.
