Phoenix A holds the current record. With an estimated mass around 100 billion times that of the Sun, the black hole at the center of the Phoenix Cluster’s brightest galaxy outweighs TON 618, which comes in at roughly 66 billion solar masses. Both are staggeringly large, but the comparison is less straightforward than a simple scoreboard suggests, because the two masses were measured using fundamentally different techniques, each carrying its own margin of error.
The Numbers Side by Side
TON 618 sits at the heart of a distant quasar about 10.4 billion light-years away. For decades it was the poster child for “biggest black hole ever found,” with a mass estimate of about 66 billion solar masses published in the early 2000s. That figure came from analyzing the light of superheated gas swirling around the black hole, a technique astronomers use routinely on quasars they cannot resolve in fine detail.
Phoenix A displaced TON 618 from the top spot in 2022. Located about 5.8 billion light-years away in the galaxy cluster SPT-CLJ2344-4243 (the Phoenix Cluster), this black hole was measured at roughly 100 billion solar masses using a completely different approach: tracking the motions of stars orbiting near the galaxy’s center. That makes Phoenix A about 50 percent more massive than TON 618, a gap large enough to be meaningful even accounting for the uncertainties in both measurements.
Why the Two Measurements Use Different Methods
You might wonder why astronomers do not just use the same ruler for every black hole. The reason is practical: different environments demand different tools. TON 618 is an active quasar, meaning its black hole is surrounded by a brilliant accretion disk and fast-moving clouds of gas. Astronomers estimate its mass using what is called the virial method. They measure how quickly gas clouds near the black hole are moving (from the width of emission lines in the quasar’s spectrum) and how luminous the quasar is (which serves as a stand-in for how far away those gas clouds sit from the black hole). Combine velocity and distance, and gravity gives you the mass.1Monthly Notices of the Royal Astronomical Society. Virial Black Hole Mass Estimates of Quasars in the XQ-100 Legacy Survey This approach has been refined over the years by calibrating it against objects whose distances are measured more directly through a technique called reverberation mapping, where astronomers watch for time delays in the gas clouds’ response to changes in the quasar’s brightness.2New Astronomy Reviews. Estimating black hole masses in quasars using broad optical and UV emission lines
Phoenix A, by contrast, is not a blazing quasar. It lives in a relatively quiet giant elliptical galaxy, so there is no brilliant accretion disk to analyze. Instead, astronomers measured its mass by modeling the motions of stars in the galaxy’s central region. They built a detailed gravitational model of the galaxy that includes stars, a central black hole, and a surrounding dark matter halo, then adjusted the black hole’s mass until the predicted stellar orbits matched the observed ones.3The Astrophysical Journal. The Black Hole Mass in Brightest Cluster Galaxy NGC 6086 This stellar-dynamical approach is generally considered the gold standard for nearby and moderately distant galaxies, because it relies on well-understood gravitational physics rather than assumptions about gas behavior.
How Much Uncertainty Sits Behind These Numbers
Neither measurement should be treated as exact. The virial method used for TON 618 and similar quasars carries substantial inherent scatter. Individual mass estimates can easily be off by a factor of about three in either direction, meaning TON 618’s true mass could plausibly sit anywhere from roughly 20 billion to over 200 billion solar masses.4Monthly Notices of the Royal Astronomical Society. Uncertainty quantification of the virial black hole mass with conformal prediction The uncertainty depends on which emission line is used, how the calibration relationships were derived, and various assumptions about the geometry of the gas around the black hole. Different research groups using slightly different calibrations have sometimes produced meaningfully different mass estimates for the same quasar.
Stellar-dynamical measurements like the one used for Phoenix A tend to be more precise when the data quality is high, but they are not immune to uncertainty either. The result depends on assumptions about the galaxy’s dark matter distribution, the stellar mass-to-light ratio, and the orbital structure of the stars being modeled. Still, these uncertainties are generally smaller than those plaguing virial estimates of distant quasars.
The upshot is that while Phoenix A’s best estimate sits well above TON 618’s, the error bars on TON 618’s mass are wide enough that you cannot say with absolute certainty which is larger. Most astronomers would say Phoenix A is very likely the bigger of the two, but the comparison comes with a caveat that few popular articles mention: these are not precision measurements in the way that, say, weighing something on a laboratory scale would be.
Could There Be Even Bigger Black Holes Out There
Almost certainly. The universe is enormous, and the census of ultramassive black holes is far from complete. Astronomers have only been able to measure black hole masses in a small fraction of the galaxies that exist, and the brightest cluster galaxies where the biggest black holes tend to live are spread across the cosmos in numbers we have barely sampled.
There is, however, a theoretical ceiling. For a black hole that is actively accreting material through a disk (like those powering quasars), physics sets an upper limit somewhere around 50 billion solar masses under standard conditions. Beyond that mass, the innermost stable orbit around the black hole extends so far out that a coherent accretion disk cannot form, and the feeding process shuts itself off.5Monthly Notices of the Royal Astronomical Society: Letters. How big can a black hole grow? This does not mean a black hole cannot exceed 50 billion solar masses; it means it probably cannot get there solely through steady disk accretion. Mergers between galaxies, where two already-massive black holes combine, can push the total above that limit. Phoenix A likely reached its extreme mass through some combination of accretion and mergers over billions of years.
That theoretical limit applies specifically to quasar-mode growth. A black hole that has already surpassed 50 billion solar masses could still gain mass by swallowing stars, merging with other black holes, or accreting gas in less organized ways. It just would not look like a traditional quasar while doing so, which is consistent with Phoenix A’s relatively quiet current state.
How Black Holes Get This Large in the First Place
Growing a black hole to tens of billions of solar masses requires a head start and the right environment. The leading models suggest that the most massive black holes began as “seeds” that were already unusually heavy early in cosmic history. In dense nuclear star clusters, stellar-mass black holes can sink toward the center and undergo repeated mergers with one another, rapidly building up a single dominant black hole. When this process occurs in a gas-rich environment, the growing black hole can also feed on surrounding gas at rates up to the Eddington limit, the maximum rate at which radiation pressure and gravitational pull balance out.6Monthly Notices of the Royal Astronomical Society. Supermassive black holes from runaway mergers and accretion in nuclear star clusters
Once a seed black hole reaches a few million solar masses, it can grow further through galaxy mergers. When two galaxies collide and merge, their central black holes eventually spiral toward each other and combine. For a galaxy sitting at the center of a massive cluster like the Phoenix Cluster, this has happened many times over cosmic history. Each merger adds mass directly and also funnels new gas toward the center, fueling further accretion. The biggest black holes live in the biggest galaxies in the densest environments, and that is not a coincidence.
TON 618’s host is harder to study because the quasar’s brilliance drowns out the surrounding galaxy’s light. But at a redshift that places it about 10 billion years in the past, we are seeing it during a period when quasar activity was far more common and gas was more plentiful. It may have been growing rapidly at the time we observe it, while Phoenix A, seen at a somewhat later cosmic epoch, may have already settled into a quieter phase after most of its growth was complete.
Why TON 618 Still Dominates the Headlines
If Phoenix A is larger, why does TON 618 still appear in nearly every “biggest black hole” listicle? Partly it is a matter of timing. TON 618’s mass estimate has been widely cited since 2004, giving it almost two decades of pop-science momentum before Phoenix A’s measurement was published. It became the go-to example in YouTube videos, Wikipedia articles, and size comparison graphics, and that cultural inertia is hard to dislodge even after a new record-holder arrives.
There is also the drama of the quasar itself. TON 618 is spectacularly luminous, one of the brightest objects in the known universe, and it sits at a staggering distance. That combination of extreme luminosity and extreme distance makes for a compelling narrative. Phoenix A, while more massive, lives in a comparatively understated galaxy cluster and lacks the visual fireworks of an active quasar. In popular science, spectacle sells, and a blazing quasar is simply more photogenic than a dormant giant.
A subtler factor is that Phoenix A’s measurement has not yet been replicated or independently confirmed by multiple groups using different data sets. TON 618’s mass, despite its wide uncertainty range, has been re-estimated multiple times with broadly consistent results. Science tends to move cautiously: a single measurement, even from excellent data, invites a degree of wait-and-see. If future observations confirm Phoenix A’s mass near 100 billion solar masses, it will likely become the new standard reference in popular accounts. For now, both names circulate, sometimes with conflicting claims about which is bigger.
Other Contenders for the Largest Known Black Hole
TON 618 and Phoenix A are not the only ultramassive black holes worth knowing about. Holm 15A*, at the center of the galaxy Holm 15A in the Abell 85 cluster, was measured at roughly 40 billion solar masses using stellar dynamics, making it smaller than either TON 618 or Phoenix A but still extraordinarily large. IC 1101, one of the largest known galaxies, is sometimes cited as likely hosting a black hole of comparable or greater mass, but no reliable dynamical measurement has been published for it, so that remains speculative.
The quasar SDSS J140821.67+025733.2 has occasionally appeared in lists with a mass estimate exceeding 100 billion solar masses, but that figure comes from virial estimates with very large uncertainties, and some researchers have argued the line measurements were unreliable. This illustrates a recurring pattern: the most extreme mass claims for quasar black holes tend to come from the noisiest data, precisely because the quasars are so distant and faint that their spectra are hard to analyze cleanly.
Among black holes with well-established masses, M87* is probably the most famous thanks to the Event Horizon Telescope’s 2019 image, but at about 6.5 billion solar masses, it is roughly a tenth the mass of TON 618 and a fifteenth of Phoenix A. It is enormous by ordinary standards but modest by ultramassive standards.
What Future Telescopes Will Change
The Extremely Large Telescope (ELT), currently under construction in Chile, is expected to push stellar-dynamical black hole mass measurements much farther into the universe. Simulations suggest it will be able to recover black hole masses with roughly 10 percent accuracy in galaxies at redshifts around 1 to 2, a range that corresponds to looking back 8 to 10 billion years.7Monthly Notices of the Royal Astronomical Society. Extending the frontier of spatially resolved supermassive black hole mass measurements to at 1 ≲ z ≲ 2: simulations with ELT/MICADO high-resolution mass models and HARMONI integral-field stellar kinematics That would allow astronomers to apply the more precise stellar-dynamical method to galaxies that currently can only be studied with the cruder virial approach, and it could produce much tighter mass estimates for objects in TON 618’s distance range.
If ELT-era measurements confirm or refine the masses of the current record-holders, the ranking could shift again. It is also plausible that the new telescope will find even larger black holes lurking in massive galaxies that have not yet been surveyed in sufficient detail. The census of ultramassive black holes is still in its early stages, and every time astronomers have gained access to better instruments, they have found things they did not expect. Whether Phoenix A retains the top spot for another decade or gets displaced by something discovered in the 2030s is genuinely an open question.
The Event Horizon Size Comparison
Mass is not the only way to think about “bigger.” A black hole’s event horizon, the boundary beyond which nothing escapes, scales directly with mass. For a non-spinning black hole, the event horizon’s radius is proportional to the mass: double the mass and the radius doubles. TON 618’s event horizon has a radius of roughly 1,300 astronomical units (where one AU is the distance from Earth to the Sun), which would extend well past Pluto’s orbit if you dropped it into our solar system. Phoenix A’s event horizon, being about 50 percent more massive, would stretch to roughly 2,000 AU, engulfing a region larger than even the most distant known objects orbiting our Sun.
In practice, both of these black holes almost certainly spin, which changes the event horizon’s size and shape. A rapidly spinning black hole has a smaller event horizon than a non-spinning one of the same mass, and the region around it where spacetime is dragged along with the rotation (the ergosphere) adds geometric complexity. Spin measurements for ultramassive black holes are extremely difficult, and neither TON 618 nor Phoenix A has a reliable spin estimate, so the event horizon sizes quoted in popular comparisons assume the simplified non-spinning case. The true horizons could be somewhat smaller.
Even setting aside spin, the sheer scale is hard to internalize. Light itself takes hours to cross the event horizon of either black hole. A beam of light entering one side of Phoenix A’s event horizon and traveling straight across would need about 22 hours to reach the other side, roughly the same time it takes light from the Sun to reach Uranus. These are objects whose “surfaces” are measured in light-hours, occupying a volume that dwarfs most things humans have a conceptual vocabulary for.