Where gravity
stops negotiating.

TON 618 is the brilliant core of a very distant galaxy, a hyperluminous quasar sitting near the border of the constellations Canes Venatici and Coma Berenices. The light reaching us tonight left about 10.8 billion years ago, so we are seeing it while the universe was still young. Space has kept stretching in the meantime, and on today's map the same object sits roughly 18.2 billion light-years away.

Its name is only a catalogue number. In 1957, Mexican astronomers Braulio Iriarte and Enrique Chavira were photographing faint blue stars well away from the plane of the Milky Way with the 0.7-metre Schmidt telescope at the Tonantzintla Observatory when one speck looked "decidedly violet" to them, and they wrote it in as entry 618. Nobody could have guessed what it was, because quasars were not recognised as a class until 1963. Thirteen years later a radio survey at Bologna picked up radio waves coming from it, and optical spectra taken by Marie-Hélène Ulrich at the McDonald Observatory in Texas showed the broad emission lines that identified it as a quasar.

The engine behind those lines is one of the most massive black holes ever measured: about 40.7 billion Suns, with older and still widely quoted estimates near 66 billion, because the answer shifts depending on how astronomers read its spectrum — which is why modern figures are usually given as a range rather than one final number. At that mass the event horizon would be some 2,600 astronomical units across, so all the planets of our Solar System could fit inside it many times over, and the whole system shines at around 140 trillion times the Sun's brightness as gas falls in at roughly 7,000 kilometres per second. It is also wrapped in an enormous cloud of glowing hydrogen called a Lyman-alpha blob. Even then, at magnitude 15.9 it is far too dim for the naked eye; we only know it exists because of how hard it is working.