# Phoenix A vs TON 618 vs Holm 15A: Which Black Hole Is Really Most Massive?

There is no clean, confirmed winner. TON 618 is commonly estimated at roughly 40–70 billion times the Sun’s mass, Holm 15A’s central black hole has a direct stellar-dynamical estimate near 40 billion solar masses, and the Phoenix cluster’s central black hole is often assigned much larger numbers online without an equivalent direct measurement.

The fairest comparison is not simply “which number is largest?” It is how each mass was obtained.

Comparison at a glance

Object or system Approximate reported black-hole mass Main method What the estimate means
TON 618 About 40–70 billion solar masses Broad emission-line scaling in a luminous quasar Enormous, but model-dependent; published estimates vary
Holm 15A About 40 billion solar masses Stellar motions modelled around the galaxy’s centre A direct dynamical estimate and among the strongest measurements in this mass range
Phoenix cluster central galaxy Often around 20 billion solar masses in research estimates; larger values circulate online Indirect inference from galaxy or cluster properties No comparably secure direct dynamical measurement supports a precise viral “100 billion” figure

These values have large uncertainties and should not be ranked as though they came from the same instrument and method.

TON 618: the quasar with an enormous inferred mass

TON 618 is a distant, extremely luminous quasar. A quasar is an actively feeding supermassive black hole whose hot accretion flow can outshine its host galaxy.

Astronomers cannot watch stars orbit close to TON 618 in the way they can for nearby galaxies. Instead, they estimate its mass from the speed of gas producing broad spectral lines and an inferred size for the line-emitting region. The method is calibrated using better-studied active galaxies, but geometry, orientation and the chosen calibration introduce uncertainty.

An estimate near 66–70 billion solar masses is widely cited. Other published analyses place it closer to 40 billion. TON 618 is unquestionably in the extreme upper range of known black-hole candidates, but quoting one value to many significant figures creates false precision.

Holm 15A: a black hole weighed by stellar motion

Holm 15A is the giant central galaxy of the Abell 85 galaxy cluster. It is the galaxy—not the black hole—that is called Holm 15A.

Researchers used observations from the MUSE spectrograph on the European Southern Observatory’s Very Large Telescope to map how stars move across the galaxy’s centre. They then built orbit-based models to find the central mass needed to reproduce those motions.

The result was a black-hole mass of about 40 billion Suns. Every dynamical model has assumptions, but this is a more direct measurement than estimating mass from a quasar’s brightness and broad emission lines. Holm 15A is therefore especially important: its number is not merely an extrapolation from the size of its host galaxy.

Phoenix: why the biggest claim is the least certain

“Phoenix A” is commonly used online for the enormous central galaxy or its black hole in the Phoenix galaxy cluster. Scientific papers more often refer to the Phoenix cluster, SPT-CLJ2344−4243, and its brightest cluster galaxy.

The system is remarkable. It contains a powerful active galactic nucleus and an unusually high rate of star formation for a brightest cluster galaxy. X-ray observations show hot cluster gas cooling while energy from the central active nucleus pushes back against that cooling.

Those facts do not directly weigh the black hole. Research literature has discussed a central mass around 20 billion solar masses based on indirect relationships. Much larger figures—especially a precise 100-billion-solar-mass claim—are repeated online, but they should be described as speculative unless a peer-reviewed direct dynamical measurement is provided.

Phoenix could host an extraordinarily massive black hole. Current evidence does not justify declaring its largest viral estimate a confirmed record.

Which measurement should we trust most?

Each method answers the same question with different evidence:

  1. Stellar dynamics: measures how the black hole’s gravity changes nearby stellar motions. This is the basis of the Holm 15A result.
  2. Gas dynamics: models orbiting gas, when it moves in a sufficiently orderly way.
  3. Reverberation or broad-line scaling: estimates an active black hole’s mass from its variable light or broad emission lines. This makes distant quasars such as TON 618 measurable but adds calibration uncertainty.
  4. Host-galaxy scaling relations: infer mass from galaxy properties. These are useful for populations but can be unreliable when extrapolated to extreme objects.

A larger indirect estimate is not automatically stronger evidence than a slightly smaller dynamical measurement.

Why “temperature” is the wrong comparison

A black hole does not have a conventional surface temperature. Its theoretical Hawking temperature decreases as its mass increases and would be extraordinarily close to absolute zero for all three objects. That radiation has not been measured for astrophysical black holes.

The bright radiation astronomers observe comes from gas outside the event horizon—from the accretion disk, corona and jets. Comparing that gas can teach us about feeding and feedback, but it does not provide a simple “black-hole temperature” ranking.

Likewise, a cluster’s X-ray luminosity or a galaxy’s star-formation rate is not the luminosity of the black hole itself.

So which is the most massive?

  • Best direct dynamical case here: Holm 15A, at about 40 billion solar masses.
  • Largest commonly published range: TON 618 may reach roughly 70 billion solar masses, although lower estimates overlap Holm 15A.
  • Largest popular but unconfirmed claim: the Phoenix central black hole, where a precise 100-billion-solar-mass figure lacks an equivalent direct measurement.

The evidence therefore supports a cautious conclusion: TON 618 may be the most massive of the three, Holm 15A has the strongest direct mass measurement, and Phoenix remains too uncertain for a definitive ranking.

Update note: This article was substantially revised and fact-checked on August 23, 2026, to separate direct black-hole measurements from indirect and widely repeated estimates.

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