TON 618 is the quasar noted for hosting an ultramassive black hole often estimated at about 66 billion solar masses.
TON 618 is a hyperluminous, radio-loud quasar near the constellations Canes Venatici and Coma Berenices. Its intense light comes from an active galactic nucleus, where gas and dust orbit a supermassive black hole, forming a brilliantly hot accretion disk and broad emission-line region.
The object was first catalogued in 1957 at Mexico’s Tonantzintla Observatory as a faint blue source. Radio observations in 1970 revealed its quasar nature, and later spectroscopy showed that it is extremely distant, with a redshift of about 2.219. Because the host galaxy is overwhelmed by the quasar’s brightness, astronomers infer the black hole’s mass from the velocity and size of surrounding emitting gas.
The frequently quoted 66-billion-Sun figure is an estimate, not a direct weighing. A later analysis produced a lower estimate of roughly 40.7 billion solar masses, so the exact value is method-dependent. That uncertainty does not change the object’s canonical identification.