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.

A special group of quasars commonly referred to as blazars are characterized with very high luminosities and display variability in their magnitude which range from a few tenths to 3-4 units over the course of a few minutes to a few days. Blazars differ from regular quasars only because of perspective, for blazars have their relativistic jet pointing at earth, thus allowing the observer to peer directly into the jet itself and the ensuing emission activity that is present. The current belief is that blazars harbour a supermassive black hole with a blazer situated at each of the two opposing poles. To-date, 66 highly-probable blazars and another 27 suspected blazars have been discovered by the Compton Gamma-Ray Observatory and which are available in the Third EGRET catalog.

Note: For an extensive list of blazars with finder charts and light curves, see the Perugia Blazar List here.