Phoenix A black hole: mass estimates, distance and size

Phoenix A has no measured mass: published estimates for the black hole at the heart of the Phoenix galaxy cluster range from about 10 billion to possibly more than 100 billion times the Sun’s mass (roughly 2 × 10⁴⁰ to 2 × 10⁴¹ kg), and none comes from a direct measurement. The popular claim that it is “the biggest black hole in the universe” rests on the top of that range, which comes from a theoretical growth scenario.

What is firmly measured is the black hole’s extraordinary environment. The galaxy around it forms stars hundreds of times faster than the Milky Way, and jets from the black hole have carved cavities in the surrounding hot cluster gas.

Key facts about Phoenix A

• Host: the brightest central galaxy of the Phoenix cluster, catalogued as SPT-CLJ2344-4243, in the southern constellation Phoenix.
• Distance: redshift z = 0.596; its light has travelled about 5.7–5.9 billion years to reach us.
• Mass estimates: about 1 × 10¹⁰ (2025 energy-balance argument), about 2 × 10¹⁰ (2012 Chandra release) and “might exceed” 1 × 10¹¹ solar masses (2016 modelling scenario).
• Event horizon, if non-spinning: about 395 AU across at 10 billion solar masses, or about 3,950 AU at 100 billion.
• Not Holm 15A: Holm 15A is a different galaxy in a different cluster, Abell 85, about eight times closer in light-travel time.

What is Phoenix A?

“Phoenix A” is an informal name for the supermassive black hole in the central galaxy of the Phoenix cluster. Research papers usually refer instead to the Phoenix cluster, SPT-CLJ2344-4243, and its brightest cluster galaxy. Some websites write “Phoenix A*” by analogy with Sagittarius A*, the Milky Way’s central black hole.

That asterisk does not mean “active galactic nucleus.” Radio astronomer Robert Brown coined the name Sgr A* in 1982, borrowing the atomic-physics notation for an excited atom because he saw the source as the “exciting” source of nearby gas clouds. “Phoenix A*” has no official status.

Phoenix A is also not the same object as Holm 15A, the central galaxy of the Abell 85 cluster, whose black hole was weighed at about 40 billion solar masses using stellar motions. The two are sometimes merged online, including on an earlier SciQuest page.

Where is Phoenix A, and how far away is it?

The Phoenix cluster lies in the constellation Phoenix at a redshift of 0.596. NASA’s Chandra X-ray Center describes it as about 5.7 billion light-years away, meaning its light has travelled for about 5.7 billion years. SciQuest’s calculation with Planck 2018 cosmological parameters gives about 5.9 billion years.

You may also see figures of 7–9 billion light-years. Those describe how far away the cluster is now, after the universe has expanded while the light was in transit. With Planck 2018 parameters that present-day (comoving) distance is about 7.4 billion light-years. Both kinds of number are legitimate, but they answer different questions.

The cluster was identified by the South Pole Telescope through the Sunyaev–Zel’dovich effect, a faint distortion of the cosmic microwave background caused by hot cluster gas. A 2011 catalogue of the survey’s most massive clusters listed SPT-CL J2344-4243 as having the highest X-ray luminosity in its sample and noted a bright Type 2 Seyfert galaxy—one with an obscured active nucleus—close to its centre.

How massive is Phoenix A?

Every published figure for Phoenix A is indirect. None comes from tracking stars or gas orbiting the black hole, or from gravitational lensing—the methods used for the best-measured giant black holes.

Source note: see the Sources list. No uncertainty ranges were published for these figures.

The spread is a factor of ten. Any article that gives Phoenix A a single precise mass, or calls it a confirmed record holder, is overstating the evidence.

Where does the 100-billion-solar-mass figure come from?

It traces to a 2016 paper by Michael Brockamp and colleagues in Astronomy & Astrophysics. The team converted the energy that black holes pump into the most X-ray-luminous clusters into possible growth rates, then ran computer simulations of how that growth would reshape stars in the central galaxy.

They concluded that Phoenix “could be one of the few objects” where their most extreme model applies, so its black hole “might exceed” 10¹¹ solar masses. They presented this as a candidate worth testing, not as a measurement. Popular articles later rounded that conditional statement into a definite “100 billion Suns.”

What have astronomers actually measured in the Phoenix cluster?

The Phoenix cluster is remarkable for what can be observed directly:

• Rapid cooling: a 2012 study in Nature inferred that hot gas in the cluster core is cooling at about 3,820 ± 530 solar masses per year, an extremely strong cooling flow.
• A starburst: the same study measured star formation of 740 ± 160 solar masses per year in the central galaxy. A 2015 analysis gave a time-averaged 610 ± 50, and JWST data give 740 ± 80 averaged over 100 million years. The Milky Way forms about 1.65 ± 0.19 solar masses of stars per year, so Phoenix is forming stars roughly 370–450 times faster.
• A buried quasar: Suzaku X-ray observations classify the central nucleus as a type 2 quasar hidden behind a column of about 3.2 × 10²³ hydrogen atoms per square centimetre, with an intrinsic 2–10 keV luminosity of about 4.7 × 10⁴⁵ erg/s (4.7 × 10³⁸ W).
• Jet-blown cavities: deep Chandra images show cavities in the hot gas within about 10 kiloparsecs of the centre, implying jet power of 2–7 × 10⁴⁵ erg/s, plus older “ghost” cavities from an outburst about 100 million years ago.

These observations show that the black hole is feeding and pushing back on its surroundings. They do not reveal its mass directly. Even its growth rate is disputed: the 2012 Chandra release estimated about 60 solar masses per year, while the 2025 JWST analysis estimated roughly 5 solar masses per year.

How big is Phoenix A’s event horizon?

For a non-spinning black hole the event-horizon radius is r = 2GM/c², about 2.95 km per solar mass. For 2 × 10¹⁰ solar masses: 2.95 km × 2 × 10¹⁰ ≈ 5.9 × 10¹⁰ km, or about 395 AU (1 AU = 149.6 million km).

Calculated by SciQuest using GM☉ = 1.327 × 10¹¹ km³/s² and Pluto’s mean distance of 5,906.4 million km (39.5 AU), both from NASA NSSDCA fact sheets. A spinning black hole would have a somewhat smaller horizon.

How many Suns or Earths could fit inside Phoenix A?

A mass of 100 billion Suns does not mean 100 billion Suns would fill its volume. Volume grows with the cube of radius. Treating the horizon as an ordinary sphere—a simplification, because space inside a black hole is not ordinary—a 10¹¹-solar-mass horizon of radius 2.95 × 10¹¹ km could hold about 7.7 × 10¹⁶ Sun-sized spheres (Sun radius 695,700 km) or about 1 × 10²³ Earth-sized spheres (Earth radius 6,371 km). At 2 × 10¹⁰ solar masses the figures fall to about 6 × 10¹⁴ Suns and 8 × 10²⁰ Earths.

Is Phoenix A bigger than TON 618?

It cannot be settled with current data. TON 618’s mass is estimated at about 41–66 billion solar masses from quasar emission lines. Phoenix A could be smaller or larger depending on which estimate is right. Our Phoenix A vs TON 618 comparison sets the evidence side by side, and the guide to the biggest black holes in the universe ranks the leading candidates by measurement quality.

Earlier versions of this page said Phoenix A was “40 times bigger” than TON 618. That was an arithmetic error: even 100 billion divided by 66 billion is only about 1.5.

Is Phoenix A a primordial black hole, and how old is it?

There is no evidence that Phoenix A is primordial—that is, formed directly in the early universe rather than grown from a smaller seed. The standard picture is that supermassive black holes grow by swallowing gas and merging with other black holes. A 2021 theoretical study of hypothetical black holes above 10¹¹ solar masses noted they might have been seeded by primordial black holes, but it did not identify Phoenix A as one.

Its age is also unknown. We see the system as it was about 5.8 billion years ago, but that light-travel time says nothing about when the black hole first formed.

What temperature and colour is Phoenix A?

The black hole itself emits no light that astronomers can detect. Its theoretical Hawking temperature, about 6.2 × 10⁻⁸ kelvin divided by its mass in solar masses, would be between about 10⁻¹⁸ and 10⁻¹⁹ K for the published mass range—far colder than any measured temperature. The X-rays and other radiation observed from Phoenix come from gas and jets outside the event horizon. Artists’ glowing “colours” are illustrations, not observations.

For a size comparison with the largest known stars, see Stephenson 2-18 vs Phoenix A.

What scientists don’t know yet

• The mass: estimates differ by a factor of ten, and none is a direct measurement.
• Whether it is undermassive or ultramassive: the 2025 JWST study suggests the black hole may be small for such an extreme cluster, the 2016 modelling study the opposite.
• The growth rate: estimates range from about 5 to about 60 solar masses per year.
• Its future: the JWST team expects cooling and star formation to slow if the black hole has already grown enough to balance the cooling, but confirming that needs a better mass measurement.

Related reading

• What is the biggest black hole in the universe?
• How big is TON 618? Mass, event horizon and distance
• What is at the centre of the Milky Way?

Update note: This article was substantially revised on October 12, 2026, to replace the unsupported claims that Phoenix A is a confirmed 100-billion-solar-mass record holder, a primordial black hole and an object whose “A*” means active nucleus; to correct the “40 times bigger than TON 618” and “77 times bigger than Abell 1201” arithmetic and the confusion between mass and volume; to remove an unsupported 2020 discovery claim; and to add the range of published mass estimates with their methods. It was drafted with AI assistance and checked against the sources listed below.

Sources

• Brockamp, Baumgardt, Britzen and Zensus, Unveiling Gargantua: a new search strategy for the most massive central cluster black holes, Astronomy & Astrophysics 585, A153 (2016)
• Reefe et al., Cold gas and star formation in the Phoenix cluster with JWST, accepted by The Astrophysical Journal (2025)
• NASA Chandra X-ray Center, Phoenix Cluster Sets Record Pace at Forming Stars (August 15, 2012)
• NASA Chandra X-ray Center, Phoenix Cluster: A Fresh Perspective on an Extraordinary Cluster of Galaxies (September 30, 2015)
• McDonald et al., A massive, cooling-flow-induced starburst in the core of a highly luminous galaxy cluster, Nature 488, 349 (2012)
• McDonald et al., Deep Chandra, HST-COS, and Megacam observations of the Phoenix cluster, The Astrophysical Journal 811, 111 (2015)
• Ueda et al., Suzaku observations of the type 2 QSO in the central galaxy of the Phoenix cluster, The Astrophysical Journal 778, 33 (2013)
• Williamson et al., An SZ-selected sample of the most massive galaxy clusters in the 2500-square-degree South Pole Telescope survey, The Astrophysical Journal 738, 139 (2011)
• Licquia and Newman, Improved estimates of the Milky Way’s stellar mass and star formation rate, The Astrophysical Journal 806, 96 (2015)
• Mehrgan et al., A 40 billion solar mass black hole in the extreme core of Holm 15A, The Astrophysical Journal 887, 195 (2019)
• Carr, Kühnel and Visinelli, Constraints on stupendously large black holes, Monthly Notices of the Royal Astronomical Society 501, 2029 (2021)
• Goss, The discovery of Sgr A* (2003)
• Paris Observatory, Hawking temperature, Etymological Dictionary of Astronomy and Astrophysics
• Planck Collaboration, Planck 2018 results. VI. Cosmological parameters, Astronomy & Astrophysics 641, A6 (2020)
• NASA NSSDCA, Sun Fact Sheet, Earth Fact Sheet and Planetary Fact Sheet