Quasars or quasi-stellar radio source are sources of electromagnetic energy which are characterized with high red shifts, thus leading scientists to conclude that not only they are moving away but are also at a great distance from us. Of the over 100,000 quasars identified to-date, the greatest proportion are over one billion light-years away (the closest quasar identified to-date is 780 million light-years away whereas the most distant quasar discovered so far is 13 billion light-years away). As a result, quasars represent entities from the universe's distant past.

Given their visibility (generally as point sources of light), it follows they must be associated with tremendous amounts of energy which is only exceeded in intensity by supernovae and gamma-ray bursts. Some examples of quasars also involve the centers of (active) galaxies and which has led to the suggestion that supermassive black holes at the galaxy center and the consequent accretion of material must fuel these quasars. The rapid change in luminosity observed for some quasars also suggests they must be relatively small entities.

Note: The Third Cambridge Catalogue of Radio Sources (3C) is the most detailed collection of such radio sources and is available online as is the case for a later revision commonly referred to as 3CRR.

Note: Six-part lensed quasar (!!!): see here and here and here and here and by Roland Christen here.

Note: Extensive database of lensed quasars at CASTLEs: see here. <-------------------------

Note: For an extensive list of lensed quasars, see Table 1 here. <-------------------------