Approximately 5000 quasars have been found to date.
Quasars are small, powerful, extremely remote objects.
They are thought to be very distant, very powerful active galactic nuclei. They are therefore very compact objects. At the same time, they are superluminous, approximately 10 - 1000 times more luminous than a normal galaxy. Compact, superluminous objects like quasars can be understood in terms of the unified model. If about one solar mass of material per year flowed into the black hole, the energy released could power a quasar.