Phoenix A Isn't a Black Hole — It's Something Worse (And It Started Hunting Again)

Added: 2026-05-25

[00:00:00]There is a black hole at the center of a galaxy cluster nearly 6 billion light years from Earth. It is very large. By the best current estimates, [music] it weighs somewhere between 18 and 100 billion times the mass of our sun.

[00:00:12]That places it among the largest black holes we have ever measured, possibly the largest. The data is still being refined. For most of the recent history of the universe, this object was quiet.

[00:00:21]Black holes the size we are talking about typically run out of fuel. The galaxies around them are stripped of gas, the inflow stops, and the black hole goes into a long sleep that can last billions of years. Most of the ultra-massive black holes scattered through the universe are in that state right now, dormant, invisible, waiting.

[00:00:38]This one is not not anymore. Sometime in the past tens of millions of years hole at the center of the Phoenix cluster began feeding again. We do not know exactly when it started. We can see only the consequences. Gas is falling onto it from a vast reservoir of hot plasma that surrounds it. Stars are forming around it at a rate close to a thousand times higher than the star formation rate of our own galaxy. The light from these events left the cluster nearly 6 billion years ago and is reaching our telescopes now. Astronomers have called this black hole Phoenix A. [music] It sits inside the Brightest Cluster at the heart of a structure cataloged as SPT-CLJ2344-243, the Phoenix Cluster. The cluster itself contains the mass of roughly 2 quadrillion suns. The galaxy at its core is one of the most active stellar nurseries known to us, and the black hole at the center of that galaxy is, by every measure, breaking the rules. The textbook version of how black holes form does not produce something like Phoenix A. The math runs out before it gets this big.

[00:01:33]The standard models of black hole feedback do not predict cooling flows of the magnitude observed in the Phoenix Cluster. Something here is happening that we do not yet have a complete theory for. And right now, in the present, it is still happening. This is the story of a quiet object that recently became loud. A black hole large enough that we have to use new categories for it. An ongoing cosmic event we are watching unfold across nearly 6 billion years of light travel time. What we know about it and what we do not comes from a sequence of careful observations beginning in 2010 and continuing through papers published earlier this year.

[00:02:07]If you find yourself drawn to these slow accumulations of evidence about objects so large the universe itself seems [music] strained by them, subscribing helps this channel keep building these explorations. Now, back to what is actually happening 6 billion light years away. The discovery did not happen the way most astronomical discoveries do.

[00:02:24]There was no single telescope pointing at a star, no dramatic image of a galaxy. Phoenix A was found because of an indirect signature it left in the oldest light in the universe. In 2010, the South Pole Telescope was operating on the Antarctic Plateau at an elevation of nearly 3,000 m where the air is thin and dry and the sky is cold enough to make precision microwave measurements possible. The telescope's primary mission was to map the cosmic microwave background, the relic radiation left over from the Big Bang, the afterglow of the universe's first moments.

[00:02:54]But hot gas, when it sits between us and the cosmic microwave background, leaves a distortion in the radiation, a specific signature called the Sunyaev-Zel'dovich effect. The photons that pass through hot [music] intergalactic gas get scattered slightly, their energy bumped up, leaving a measurable shadow against the background. The denser and hotter the gas, the stronger the [music] signature.

[00:03:14]The South Pole Telescope found a very strong signature in one particular direction. It was followed up with optical observations with X-ray observatories, Chandra, XMM-Newton, with radio dishes, with ground-based imaging telescopes. The picture that emerged was a galaxy cluster of extraordinary mass and brightness, two quadrillion solar masses bound together by gravity, the brightest X-ray cluster ever found of its kind, a swarm of galaxies, hundreds of them, embedded in a sea of plasma heated to tens of millions of degrees.

[00:03:41]The team named it the Phoenix Cluster after the constellation it sits in. The discovery paper was led by Michael McDonald at the Massachusetts Institute of Technology, published in the journal Nature in August of 2012, what McDonald and his collaborators found inside the cluster turned out to be more interesting than the cluster itself. At the heart of the brightest galaxy in the cluster, gas was cooling and falling inward at staggering rates.

[00:04:05]Most cool core clusters have rapid cooling flows in theory, but in practice the central black hole's energetic feedback heats the gas back up before it can fall to the center. In the Phoenix cluster, that feedback was not working.

[00:04:16]The gas was reaching the center and it was producing stars. The central galaxy of the Phoenix cluster, the brightest cluster galaxy, was forming new stars at a rate of approximately 700 to 1,000 solar masses per year. For perspective, the Milky Way produces about one solar mass of new stars per year. This galaxy was building stellar mass close to a thousand times faster than ours.

[00:04:37]McDonald and his team used the relationship between galaxy mass and central black hole mass, a scaling relationship that holds remarkably well across many galaxies, to estimate the mass of the black hole at the very center. Their initial estimate was around 18 billion solar masses.

[00:04:52]Other approaches, based on different assumptions, have produced [music] estimates ranging up to 100 billion. The truth is somewhere in that range. The uncertainty is still large, but even at the lower end, the black hole was extraordinary. It was big enough to belong to a category astronomers had only recently begun discussing, a category that did not exist in textbooks, ultramassive. To understand what an ultramassive black hole means physically, you have to think about what mass does to the event horizon. The event horizon is the boundary of a black hole, the region from which nothing, not even light, can escape. The radius of that boundary scales linearly with the mass. Double the mass of a black hole and you double the radius of its event horizon. Triple the mass, triple the radius. There is no compression that happens with size. Larger black holes are not denser. They are just larger. A black hole made from compressing the sun into a point would have an event horizon less than 4 m across. The black hole at the center of our own galaxy, Way A, has a mass of about 4 million solar masses and an event horizon roughly 24 million kilometers across. That sounds enormous, but it is small enough to fit comfortably inside the orbit of Mercury.

[00:05:57]Our solar system, if you place Sagittarius A at its center, would still extend much farther than the event horizon. Phoenix A is operating on a different scale entirely. If the higher mass estimates are correct, its event horizon would stretch about 300 billion kilometers from center to edge. The total diameter would be roughly 600 [music] billion kilometers, which is about 100 times the distance from our sun to Pluto. Light, the fastest thing in the universe, >> [music] >> would need approximately 3 weeks to cross from one side of this event horizon to the other. A photon traveling at 300,000 kilometers per second, starting at the boundary, would not reach the opposite boundary for almost a month. This is not a hole in space the way a stellar mass black hole is. It is something closer to a region, a territory, a volume larger than most star clusters, [music] fenced off from the rest of the universe. Anything that crosses the boundary does not just fall in. It stops existing as far as the outside universe is concerned. No information from that volume can ever return. There is a counterintuitive consequence of this scale that is worth mentioning. Near a small black hole, the gravity at the event horizon is intense, and so are the tidal forces.

[00:07:02]The pull on the front of an object would be much stronger than the pull on the back, stretching anything that approached into a long thin filament.

[00:07:09]The classical description of being torn apart by a black hole, the term spaghettification, applies to small black holes very well. For an ultramassive black hole like Phoenix A, this does not happen. The event horizon is so far from the center that the gravitational gradient across an approaching object is very gentle. A person, if somehow placed at the boundary of Phoenix A, would not be stretched. There would be no shudder, no warning, no obvious moment of crossing.

[00:07:33]The gravity would [music] simply hold, the universe behind would visibly compress into a small bright circle, and the rest of reality [music] would slip away. The traveler would not feel anything change at the moment of crossing. The destruction would come later, hours or days inward, somewhere far below the surface, when [music] the tidal forces finally rose to lethal levels. This is part of why ultramassive black holes have a different texture in the imagination than smaller ones. They are not predators. They are not hunters lurking in cosmic shadows. They are regions. They are domains. A place where the universe simply stops responding.

[00:08:04]The question that most directly bothers astronomers about Phoenix A is not how big it is, or even what it is doing [music] now. It is how it got this big in the first place. The standard theory of black hole formation says that supermassive black holes grow from much smaller seeds. A massive star collapses at the end of its life, leaving behind a stellar [music] mass black hole, somewhere between 3 and 100 solar masses. Over time, that black hole pulls in surrounding gas. Other black holes form nearby. Galaxies collide. Black holes merge. The seed grows, doubles, doubles again, slowly assembling itself into something larger over hundreds of millions to billions of years. This process works for most supermassive black holes we observe. The math runs, the growth curves match. Sagittarius A at 4 million solar masses, M87 central black hole at 6 and 1/2 billion solar masses. These can be assembled by the standard process within the lifetime of the universe. For Phoenix A, the math does not run. The universe is approximately 13.8 billion years old. To grow a black hole of 100 billion solar masses through standard accretion, you would need the seed black hole to feed it very close to the maximum theoretical rate, the Eddington limit, continuously [music] for nearly the entire age of the universe. That does not happen in the real universe. Real black holes radiate so much energy when they feed that the radiation pressure pushes incoming gas away. They throttle their own growth.

[00:09:21]They take breaks. They go through long quiet periods when nothing is falling in. The average accretion rate over cosmic time is far below the maximum.

[00:09:29]So, either Phoenix A is the result of an unusually long, unusually efficient feeding history that exceeded what we believe is physically possible, or it started out much larger than a stellar mass seed. The leading alternative is something called the direct collapse hypothesis.

[00:09:43]In the very early universe, before the first stars had formed, certain regions of primordial gas may have collapsed directly into massive black holes without ever passing through a stellar phase. These so-called direct collapse seeds could have started life at tens of thousands or even hundreds of thousands of solar masses. From a much larger starting point, the path to 100 billion solar masses becomes more plausible.

[00:10:05]Another possibility involves galaxy cluster mergers. If multiple galaxies, each with their own central supermassive black hole, fall together into the same gravitational well, their central black holes can eventually merge. In the densest regions of the early universe, where massive clusters were forming, this merger pathway may have funneled the mass of many separate supermassive black holes into one ultramassive one.

[00:10:27]Neither explanation is settled. Both have unresolved problems. What is clear is that the textbook story of slow accretion from stellar seeds, repeated across cosmic time, cannot explain Phoenix A. Something either different or extreme has to be invoked. The cleanest interpretation is that the universe first billion years did things we have not fully reconstructed. And the consequences are still sitting at the cores of galaxy clusters, 6 billion light-years away, occasionally waking up to remind us of the gap in our understanding. The hot gas filling the space between galaxies in a cluster like Phoenix is not air or fluid in any familiar sense. It is plasma, mostly ionized hydrogen, heated to temperatures of about 10 million degrees Kelvin. It brightly in x-rays. It is held together by the gravity of the cluster itself.

[00:11:10]And it should in theory cool over time.

[00:11:12]Hot gas loses energy through radiation.

[00:11:14]As it loses energy, its pressure drops, and it should fall inward toward the deepest part of the gravitational well, the cluster's center, where the central galaxy sits. As it falls, it should compress, cool further, condense, and ultimately form new stars. This is the cooling flow scenario. In the 1980s and 1990s, before high-resolution X-ray observatories were available, theorists predicted that most massive [music] cluster cores should be producing stars at extraordinary rates. When better X-ray observatories came online, the prediction did not match observations.

[00:11:44]Most clusters were not forming stars at the predicted rates. The cooling flows, theoretically, should have been there.

[00:11:50]In practice, they were missing. The resolution turned out to involve the central black hole. As gas cools [music] and starts to fall inward, some of it reaches the supermassive black hole at the center of the cluster's brightest galaxy. The black hole then becomes active, launching powerful jets and outbursts of energy that heat the surrounding gas, pushing it back outward, preventing the rest of it from cooling further. The black hole functions as a thermostat. Cooling triggers feeding, which triggers heating, which prevents further cooling.

[00:12:18]The cluster sits in equilibrium. In the Phoenix Cluster, the thermostat is failing. Earlier this year, in February of 2025, a team led by Michael Reefe and Michael McDonald at MIT published a paper in the journal Nature presenting new James Webb Space Telescope observations of the Phoenix Cluster.

[00:12:34]They mapped the emission of an ionized neon line that traces gas at temperatures around 300,000 degrees Kelvin. Gas that is in the process [music] of cooling from the hot intracluster medium down towards star-forming temperatures. The cooling flow was visible. The map showed gas in a transitional phase, spread across a region of the central galaxy that matches exactly the regions where new stars are forming most intensely. The team estimated that gas is being cooled and pulled inward at a rate of somewhere between 5,000 and 23,000 solar masses per year. This is many times higher than the rates measured in any other cluster of comparable mass. The central galaxy of Phoenix is forming new stars at approximately 700 to 1,000 solar masses per year. Some calculations push this higher. Either way, it is at least several hundred times the star formation rate of the Milky Way. New stars are being assembled in this galaxy on geological time scales rather than cosmological ones. Whole stellar populations are emerging on time frames shorter than the rise and fall of mountain ranges. And in the middle of all of this newborn fury sits Phoenix A.

[00:13:35]The black hole is feeding. The jets are punching outward. The energy output is enormous, but the gas keeps falling.

[00:13:41]Whatever combination of cluster geometry, accretion zone physics, and feedback dynamics is supposed to throttle this process is for now failing to do so. The standard model suggests the cluster will eventually reestablish equilibrium. The black hole's outburst will catch up.

[00:13:55]>> [music] >> The cooling will slow. Star formation will fall back to ordinary rates. The team estimates this transition may take another 100 million years. We are watching in slow so motion a rare moment of imbalance. What makes this even more striking is that the kind of process happening in the Phoenix cluster right now is the kind of process that was probably common in the very early universe when supermassive black holes were first assembling themselves.

[00:14:18]Phoenix may be offering us a live view, 6 billion light years away, of how the largest objects in the universe [music] got that way. In 2020, three astronomers, Bernard Carr, Florian Kühnel, and Luca Visinelli, published a paper proposing a new category of objects, black holes with masses above 100 billion solar masses. The authors called them stupendously large black holes. The name was deliberately informal. The point was that the usual categories, stellar mass, intermediate, supermassive, ultramassive, did not have a label for objects this big, and that the authors believed such objects must exist somewhere in the universe. The argument was theoretical. There is no fundamental upper limit to how large a black hole can grow, only practical limits set by accretion physics and the available mass in any given environment.

[00:15:02]Galaxy clusters contain enough mass to sustain growth of central black holes to scales well beyond 100 billion solar masses, given enough time. The early universe, during periods when accretion may have been unusually efficient, may have produced objects much larger than anything in our local cosmic neighborhood. Most of these would be dormant. They would sit at the centers of massive galaxy clusters whose gas reservoirs have long since been depleted or where feedback has shut down inflow.

[00:15:27]They would be invisible to current telescopes because they are not actively feeding. The only signatures they leave are gravitational through their effect on the motion of nearby stars and gas, through gravitational lensing, through their contribution to the overall mass budget of clusters. Estimating how many stupendously large black holes might exist is difficult. The theoretical calculation suggests there could be many. Maybe thousands scattered through the observable universe. Maybe more.

[00:15:54]What this means for Phoenix A is that it is probably not unique. It is unusual both in its size and in the fact that it is actively feeding right now. The underlying population it belongs to may be much larger than we currently know.

[00:16:05]Phoenix is visible because the cluster geometry concentrated mass densely enough at its core that cooling could break through the black hole's feedback.

[00:16:13]Most ultra-massive black holes do not have that geometry. They sit quietly at the centers of older, more diffuse clusters with no flow to trigger feeding. They are there. We just cannot see them. If even a small fraction of massive galaxy clusters contain ultra-massive black holes comparable to Phoenix A, the total population is enormous. The map of the universe's largest objects is mostly empty on our current charts. Not because there are few such objects, but because most of them are silent. Phoenix is the exception that lets us measure one. For most of human history, the largest objects we could imagine were the things we could see. Mountains, oceans, continents. Then we discovered that the Earth was a sphere drifting in space, that the Sun was a hundred times larger, that the galaxy held hundreds of billions of stars, that there were hundreds of billions of galaxies. Each step outward made our scale smaller.

[00:17:00]Each step taught us that what we thought was big was a footnote on something larger. Phoenix A is the next footnote.

[00:17:06]A black hole so large that the standard concept of a black hole strains to contain it. A region of space so wide that light needs weeks to cross it. A presence so heavy that that organizes a galaxy cluster bigger than anything in our local neighborhood, and it is one object among many in a universe full of structures we have only begun to map.

[00:17:23]The light reaching our telescopes from the Phoenix cluster started its journey nearly 6 billion years ago. That is a length of time difficult to feel in any direct way. When the photons that are now arriving at Chandra and at the James Webb Space Telescope left their source galaxy, the Earth did not yet exist. Our planet would not begin to form for another half billion years. The Sun was still assembling itself from a collapsing cloud of gas and dust. There were no continents. There were no oceans. There were no organisms. The entire chemical history of life on Earth, from the first replicating molecules to whoever is reading these words, has unfolded in the time it took those photons to cross the space between Phoenix A and us. And in that time, Phoenix A simply continued. It did what it has always done. It sat in its cluster, slowly accumulating, slowly waiting, occasionally feeding. The interval that contains the rise of single-celled life, the Cambrian explosion, the age of dinosaurs, every mass extinction, every era of mammalian evolution, and every step of human history is, for Phoenix A, a brief stretch of routine cosmic existence.

[00:18:22]This is not a perspective most of us can hold for long. Our minds are not built for it. We measure time against the cycles of our own lives, a day, a year, a generation. The idea of an object that has been operating unchanged in any meaningful sense for the entire duration of life on Earth is something the imagination can describe in words, but cannot easily inhabit. Phoenix A is one of those objects. It is not the only one. The universe is full of structures that operate on time scales we have to translate into careful arithmetic just to compare against our own existence.

[00:18:51]Black holes are perhaps the most extreme example. They are slow. They are patient. They do not respond to disturbances quickly. And the largest of them, the ultra-massive ones, are slow in a way that makes everything around them, galaxies, clusters, even the cosmic background itself, >> [music] >> look hurried by comparison. What Phoenix A is doing right now in our cosmic present is feeding again after a long quiet period. The thermostat at the center of its cluster has failed. The gas is reaching it faster than it can throw the gas [music] back. Stars are being born around it at extraordinary rates. The whole system is out of balance. In a hundred million years, the balance will likely return. The black hole will quiet again, the starburst will fade, the cluster will resume its equilibrium. But for the moment, in the small window of cosmic time in which we are alive and watching, the largest single object we know about is awake.

[00:19:38]The data that will eventually tell us how it formed, what the upper mass really is, whether there are larger ones hiding clusters we have not yet looked at carefully, that data is being collected now, year by year by the James Webb Space [music] Telescope, by Chandra, by the next generation of X-ray observatories being designed for the 2030s. The answers will arrive in pieces over decades. The universe is in no hurry to deliver them. Neither is Phoenix A. It will still be there when whatever telescopes we eventually build are obsolete and replaced [music] by their successors.

[00:20:08]It will still be there when the questions we are asking today have been replaced by different questions. It will still be there long after the last person who watched this video has stopped watching anything at all. That is the part of these objects that lingers. Not the size, not the gravity.

[00:20:23]The continuity. The fact that something so large can be so patient that it can wait through the entire history of life on Earth and still be hungry at the end of it.

[00:20:31]Sometimes the most unsettling thing about the [music] universe is not what is loud. It is what has been waiting.