These "quasi-stellar radio sources" are among the brightest & most distant objects in the universe
.... ULAS J1120+0641 is the most distant quasar known (Credit:... [DOC] [TLE] Quasar - New World Encyclopedia [PAR] Jun 19, 2015 ... Because the nature of these objects is entirely unknown, it is hard to prepare a ... 
making quasars the most luminous objects in the known universe.