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: Quasar 3C 191 in Cancer has the distinction of being the first quasar to be discovered. It lies at a distance of 9.973 billion light-years away and is believed to be 3.326 billion years old. Located just northwest of β-Cancri (mag 3.52), this QSO is characterized with a visual magnitude of 18.65. Quasar 3C 191 is associated with a sizeable redshift (z=1.956) and which suggests that it is receeding away from us at 79.5% the speed of light (ie. 238,220 km/sec)!

Note: Further details for this quasar are available here.