TON 618: How Massive Is This Monster Black Hole?

A quasar so bright it drowns out its own galaxy, powered by a black hole that shouldn’t have had time to grow.

What would it take for a single object to outshine an entire galaxy of a hundred billion stars? Welcome, curious reader. Today we’re travelling to one of the strangest addresses in the sky, and we’d love you to stay with us to the end, because the most interesting part of this story isn’t how big TON 618 is. It’s how little we can actually pin that number down.

The TON 618 black hole is one of the most massive ever measured, weighing somewhere between 40 and 66 billion times the mass of our Sun. It sits at the heart of a brilliant quasar in the constellation Canes Venatici, and its light left when the universe was roughly a quarter of its present age. That single spread of figures, from 40 to 66 billion, is the whole story, so let’s walk through it together.

What exactly is TON 618?

TON 618 is a quasar, the blazing core of a distant galaxy where gas spirals into a giant black hole and heats up until it glows brighter than everything around it. The name is short for Tonantzintla 618. Mexican astronomers Braulio Iriarte and Enrique Chavira first catalogued it in 1957 from photographic plates taken with a small Schmidt telescope, where it showed up as a faint, oddly violet dot. They assumed it was a dead white dwarf star in our own galaxy’s halo.

They had no way of knowing better. Quasars weren’t recognised as a class until 1963. The real breakthrough came in 1970, when a radio survey at Bologna picked up strong emissions from the same spot, and Marie-Helene Ulrich later took spectra at the McDonald Observatory in Texas. Those spectra carried a huge redshift, the fingerprint of an object very far away and, given how bright it still looked, extraordinarily luminous. You can read more in our guide to how quasars and active galactic nuclei light up.

How far away is TON 618?

TON 618’s light has been travelling for roughly 10.8 billion years, so we see it as it was when the universe was young. That “look-back” figure is the one worth holding onto. Our 2021 write-up quoted 10.4 billion light-years; the small difference comes down to which cosmological parameters you plug in, and 10.8 billion years is the value most current sources settle on.

Here’s the part that trips people up. Because space itself has stretched during that long journey, the quasar is now much farther away than 10.8 billion light-years. Measured as it sits today, its comoving distance is about 18.2 billion light-years. Both numbers are correct; they just answer different questions. One asks how long the light travelled, the other asks where the object is right now. If that distinction feels slippery, our explainer on the true size of the observable universe unpacks it properly.

Is it 66 billion Suns, or closer to 40?

Both figures come from real, careful studies, and the honest answer is that we don’t measure the mass directly at all. A black hole this distant is a point of darkness we can never resolve. What astronomers actually measure is how fast gas whips around near the black hole, read from the width of emission lines in its spectrum, and then convert that speed into a mass using the physics of orbits.

The famous 66-billion-solar-mass figure comes from a 2004 study by Ohad Shemmer and colleagues, based on the width of the Hβ emission line. In 2019, Xu Ge and colleagues reworked the same object using a different line, C IV, and found the gas moving more slowly, around 2,760 km/s rather than the faster Hβ velocities. Slower gas means a weaker gravitational grip, and so a lighter black hole: about 40.7 billion Suns.

Which is right? Nobody can say with certainty. C IV is known to be distorted by powerful outflows in quasars, which can inflate or deflate the apparent velocity. Hβ is cleaner but harder to observe at this redshift. This is exactly the kind of genuine uncertainty we’d rather show you than hide. The single black hole may be more massive than every star in the Milky Way combined (about 64 billion solar masses), or it may be roughly two-thirds of that. The measurement, not the object, is what’s fuzzy.

How big is the TON 618 black hole’s event horizon?

The event horizon, the point of no return, has a radius of roughly 1,300 astronomical units if the black hole really is 66 billion Suns. That’s about 195 billion kilometres, or more than 40 times the distance from Neptune to the Sun. Drop TON 618 into our solar system and its horizon would swallow every planet many times over. You don’t need to take that on faith, though. You can work it out yourself.

How could it grow so large, so early?

That’s the deepest puzzle TON 618 hands us, and it remains unsolved. At redshift 2.2, we’re seeing the quasar as it looked only about three billion years after the Big Bang. Ordinary accretion, where a black hole feeds on surrounding gas at a steady, radiation-limited pace, struggles to build tens of billions of solar masses in that little time. Something had to speed things up.

Two ideas lead the field. In the first, these giants skip the slow start by forming from the direct collapse of enormous primordial gas clouds, beginning life already heavy. In the second, they feed in bursts that briefly break the usual brightness limit, gorging faster than the textbook allows. The James Webb Space Telescope has since found other black holes that look impossibly heavy for their age, which tells us TON 618 was an early warning, not a one-off. Our overview of how supermassive black holes are built goes deeper into both routes.

Is TON 618 still the biggest we know?

As of 2026, TON 618 still holds the crown for the most massive black hole with a widely accepted estimate, but the title is genuinely contested. It has company at the top, and some of the challengers were measured with methods that don’t rely on quasar spectral lines at all.

Phoenix A*, at the centre of the Phoenix Cluster, is sometimes floated as larger, but its mass estimates swing wildly and stay poorly constrained. The 2025 discovery of a 36-billion-solar-mass black hole in the Cosmic Horseshoe lens is notable precisely because lensing gives a cleaner weigh-in than quasar lines do. None of these clearly beats TON 618 yet. For a sense of scale, compare it with the first black hole ever imaged in our piece on the Event Horizon Telescope’s portrait of M87.

One more marvel before we go: TON 618 isn’t just a black hole. It’s wrapped in a Lyman-alpha blob, a cloud of glowing hydrogen roughly 330,000 light-years across, more than twice the width of the Milky Way. The quasar’s radiation lights it up like a lantern in fog. The host galaxy itself? Still invisible, completely outshone by the monster at its core.

The takeaway

TON 618 gives us a rare kind of honesty. Its distance splits into two right answers depending on the question you ask. Its mass splits into two right answers depending on the spectral line you trust. And its very existence, so massive so early, still outruns our best formation models. The monster is real; our certainty about its dimensions is not, and saying so out loud is what science looks like.

Frequently asked questions

Sources

1. Shemmer, O. et al. (2004). Near-infrared spectroscopy of high-redshift luminous quasars, The Astrophysical Journal. Hβ-based mass estimate (~66 billion M☉).
2. Ge, X. et al. (2019). Reanalysis of TON 618 using the C IV emission line, giving a lower mass (~40.7 billion M☉).
3. Iriarte, B. & Chavira, E. (1957). Tonantzintla Catalogue of faint blue stars, Tonantzintla Observatory, Mexico.
4. Ulrich, M.-H. (1976). Optical spectroscopy of TON 618, McDonald Observatory.
5. Melo-Carneiro, C. R. et al. (2025). Unveiling a 36-billion-solar-mass black hole in the Cosmic Horseshoe gravitational lens, arXiv:2502.13788.
6. Guinness World Records & NASA Goddard reference materials on the most massive observed black hole.
7. BBC Sky at Night Magazine (2025). Feature on TON 618 and the Lyman-alpha blob (~330,000 light-years).