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: Lensed quasar SBS 0909+532 in Ursa Major lies at a distance of nearly 9 billion light-years away at a visual magnitude of 17.24. Studies have revealed this lensed quasar to have two components separated by 1.1 arc-seconds and with the primary component having a magnitude of 17.24 whereas the second component is much dimmer. Due to its notable redshift (z=1.375000), this quasar is receeding away from us at 69.9% the speed of light (ie. 209,500 km/sec).