Coordinates: 12h 28m 24.97s, +31° 28′ 37.7″ TON 618 is a hyperluminous, broad-absorption-line, radio-loud quasar located near the border of the constellations Canes Venatici and Coma Berenices, with the projected comoving distance of approximately 18.2 billion light-years from Earth.[note 1] It possesses one of the most massive black holes ever found, with a mass of 66 billion M☉.
Because quasars were not recognized until 1963, the nature of this object was unknown when it was first noted in a 1957 survey of faint blue stars (mainly white dwarfs) that lie away from the plane of the Milky Way. On photographic plates taken with the 0.7 m Schmidt telescope at the Tonantzintla Observatory in Mexico, it appeared "decidedly violet" and was listed by the Mexican astronomers Braulio Iriarte and Enrique Chavira as entry number 618 in the Tonantzintla Catalogue.
In 1970, a radio survey at Bologna in Italy discovered radio emission from TON 618, indicating that it was a quasar. Marie-Helene Ulrich then obtained optical spectra of TON 618 at the McDonald Observatory which showed emission lines typical of a quasar. From the high redshift of the lines Ulrich deduced that TON 618 was very distant, and hence was one of the most luminous quasars known.