Unhelpfully Explained New post How big is Phoenix A, the largest known black hole in 2026?

It’s 3am, and I’ve been rolling from one side of the bed to the other for what feels like a geological era. So I do what I always do when sleep refuses to show up: I open Wikipedia and read until my brain gives up before I do. My history from that night reads like the shopping list of a very confused serial killer with a soft spot for botany – bone saw, promenades, leap second, Toyoko kids, Urpflanze (Goethe’s archetypal plant, the one every other plant was supposedly a variation of) – and, wedged somewhere in between, supermassive black holes. Don’t ask; that’s how my brain works at 3 in the morning.

Supermassive black holes led to a list, the list had a top, and the top is called Phoenix A: the black hole at the heart of the Phoenix Cluster and, as of October 2026, the most massive one anybody knows of. Probably.

The catch: nobody has ever actually weighed it. The headline figure of ≈100,000,000,000 [100 billion] solar masses comes out of a calorimetric model of how its host galaxy’s core regrew – detective work with equations, not a scale – and another recent estimate lands at a comparatively humble ≈12,600,000,000 [12.6 billion], roughly eight times less; I’m going with the big one, obviously. Ahem.

The podium isn’t exactly settled either. IC 1101 sits in second place at ≈97,700,000,000 [97.7 billion] solar masses, a gap of ≈2.3% with error bars about a hundred times wider than the gap itself; and astronomy has form here: a quasar with the catchy name SDSS J140821.67+025733.2 was once announced at ≈196,000,000,000 [196 billion] solar masses, until somebody noticed a mismeasured emission line and corrected it down to ≈8,000,000,000 [8 billion]. A 24× nerf, patch notes and all. On top of that, black holes with typical properties shouldn’t be able to grow past ≈50,000,000,000 [50 billion] solar masses in the first place (only one spinning at the theoretical maximum stretches that ceiling to ≈270,000,000,000 [270 billion]), so if the big estimate holds, Phoenix A is twice as heavy as a black hole is supposed to get away with, presumably by spinning like a lunatic.

So: largest known black hole as of October 2026, estimated, never measured, and liable to shrink by a factor of eight the next time somebody points a better telescope at it. Everything below is back-of-the-envelope math – Wolfram Alpha, one formula from 1915 and a generous amount of rounding – and exists purely for entertainment.

Weighing the unweighable

Let’s start sane. In Suns, Phoenix A weighs those ≈100 billion, which almost sounds manageable, because “billion” is a word the news throws at us every day. In Earths, it’s ≈33,294,600,000,000,000 [33.3 quadrillion]. In kilograms, it’s ≈198,847,000,000,000,000,000,000,000,000,000,000,000,000 kg [198.8 duodecillion]: 42 digits and a name that sounds like a minor Roman emperor.

Sagittarius A*, the black hole at the center of our own galaxy, weighs a respectable ≈4,300,000 [4.3 million] Suns; Phoenix A could swallow ≈23,256 of them and still have room for dessert. Then there’s the Milky Way itself: every star in it combined adds up to ≈60,800,000,000 [60.8 billion] solar masses according to a 2015 meta-analysis, or ≈26,070,000,000 [26.07 billion] according to a 2025 recount that caught our galaxy padding its résumé by a factor of two. Either way, Phoenix A outweighs every fucking star in the Milky Way, by ≈1.64× or ≈3.84× – pick your paper.

A few years back I tried to put the mass of the observable universe into perspective, and the ordinary matter in it came to ≈2 × 10⁵³ kg. Divide that by Phoenix A and you get ≈1,006,000,000,000 [1 trillion], so this one object weighs about a trillionth of all the ordinary matter there is. One hole, one trillionth of everything.

Earths, threaded

Mass is one thing; size is where Wolfram starts sweating: a non-rotating black hole’s event horizon sits at its Schwarzschild radius, named after Karl Schwarzschild, who worked out the solution in 1915 while serving as an artillery officer on the Russian front – between shells, more or less. For Phoenix A, that radius is ≈295,333,938,207 km [295.3 billion], which makes the whole thing ≈590,667,876,413 km [590.7 billion] across.

That’s ≈46,355,978 Earths threaded, at ≈12,742 km each. Or ≈1,534,202 trips from Earth to the Moon. Or ≈50.002 complete orbits of Pluto laid side by side, which is so suspiciously round it feels like the universe is trolling me. In astronomical units it’s ≈3,948 AU, and that’s where it starts to hurt: Voyager 1, the most distant thing humanity ever built, will be exactly one light-day from Earth on November 18, 2026, after ≈49.2 years of flying. Phoenix A is ≈22.8 light-days wide; at Voyager’s pace, crossing it would take ≈1,122 years, and the probe’s batteries are expected to quit sometime in the 2030s.

To stick with silly comparisons: ≈331,835,885,625,492 [331.8 trillion] Chuck Norris standing on top of each other measuring 1.78 meters. Back in 2022, the largest known star took ≈1,687,000,000,000 [1.687 trillion] of him; Phoenix A is ≈196.69 Stephenson 2-18s across. The star got demoted to a rounding error, and Chuck Norris got a promotion.

The commute

Around the equator, Phoenix A measures ≈1,855,637,861,252 km [1.86 trillion]. Light does a lap around Earth in ≈0.134 seconds; around Phoenix A it needs ≈71 days 15 hours 22 minutes. Light! A Zalo message sent around its equator today would arrive between Christmas and New Year, and the “seen” would make it back only after Tết.

The ISS circles Earth every ≈92.9 minutes at ≈7.66 km/s. One lap around Phoenix A would take it ≈7,677 years – the time it needs for ≈43,460,777 laps around Earth, give or take a few resupply missions. The Parker Solar Probe, the fastest object humans have ever built, hit ≈690,000 km/h at its closest pass to the Sun in 2024; at that speed, one lap takes ≈306.8 years. Faster, sure, but still three centuries of staring at nothing but horizon.

Thinner than air

Divide the mass by the volume, though, and things stop making sense. Treated as a plain sphere (which it isn’t – inside a horizon, “volume” stops meaning much, and a spinning Phoenix A would have a smaller horizon anyway; the envelope doesn’t care), Phoenix A holds ≈107,902,000,000,000,000,000,000,000,000,000,000 km³ [107.9 decillion] behind a surface of ≈1,096,066,000,000,000,000,000,000 km² [1.096 septillion]. That gives it an average density of ≈0.00184 kg/m³, while air at sea level comes in at ≈1.225 kg/m³. The heaviest black hole anybody knows of – and this is easily my favorite number of the night – is, on average, ≈665× less dense than the air you’re breathing right now; it would lose a density contest to a, well, burp.

That thinness has a weird consequence: nothing dramatic happens at the edge. Falling through the horizon, the tidal force stretching a 2 m body comes to ≈0.000000000002 m/s², about six times the tug the Moon puts on you right now, so you’d cross the point of no return without noticing a thing (neat blueprint for a “ur mum” joke btw). Spaghettification comes later, much deeper in, when it’s far too late to file a complaint. Gravity at the horizon, measured from far away, is ≈152 m/s² or ≈15.5 g – you’d weigh fifteen and a half times what you do now (and hovering there, from your own point of view, would take infinite thrust; minor detail).

Real estate

Coruscant, Star Wars’ planet-sized city, has a diameter of ≈12,240 km and ≈2,981,780,000,000 [2.98 trillion] inhabitants. That works out to ≈6,335.24 people per km² (with thousands of stacked levels, it really should be per km³, but let’s stick to tradition) – less than Hong Kong at ≈6,801 per km², a territory where roughly 40% of the land is country parks. A planet-wide city, out-crowded by a place that’s mostly hiking trails.

Now say Phoenix A had a surface – which, as far as we know, or rather don’t, it might; see Planck stars – and say we paved it Coruscant-style. That’s room for ≈6,944,000,000,000,000,000,000,000,000 [6.944 octillion] people, or ≈2,328,755,368,038,780 [2.33 quadrillion] Coruscants.

Well, except: if Phoenix A really is a Planck star, its surface isn’t out at the horizon at all. The collapse stops at Planck density, ≈5.1 × 10⁹⁶ kg/m³, and for Phoenix A’s mass that’s a ball with a radius of ≈0.00000000000000000021 m, roughly 4,000× smaller than a proton. At Coruscant density, that surface holds ≈0.0000000000000000000000000000000000000035 people: not one Coruscanti, not one atom of one.

The long goodbye

Black holes don’t last forever, though. Stephen Hawking figured out that they leak, ever so slowly, and that the bigger they are, the colder they glow. Phoenix A’s temperature: ≈0.000000000000000000617 K. The cosmic microwave background, the leftover glow of the Big Bang, sits at ≈2.725 K, which is ≈4,400,000,000,000,000,000 [4.4 quintillion] times warmer; so right now, Phoenix A soaks up more than it radiates, and it can’t even start dying until the whole universe has cooled down below its own temperature (and since it sits in a cluster happily eating gas, the point is moot anyway). Once it finally does start, it’ll take

≈20,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 years [2 googol]

or ≈1.5 × 10⁹⁰ times the current age of the universe – yep, a googol, the actual one, twice.

I didn’t fall asleep after that, by the way. Hawking radiation led to Karl Schwarzschild, Schwarzschild to the event horizon, the event horizon to the Event Horizon Telescope, and at some point the tabs won. The next morning, the numbers were still there, refusing to fit into my head, so I did the only reasonable thing and translated them into Chuck Norrises, Zalo messages and planet-sized cities: comparisons you can picture perfectly well and that help with absolutely nothing. Explaining something without actually being helpful – that’s the whole service.

Sweet dreams 🕳️