The Heavyweight Champion of the Universe

About 3.4 billion light-years away, in the general direction of the constellation Draco, lies one of the heaviest singular objects in the known universe. Designated H1821+643, it has roughly 30 billion times the mass of the sun, with an event horizon that could swallow our solar system—28 times over.

There’s been a bit of buzz lately about the smallest supermassive black hole ever discovered. While the “supermassive oxymoron” has some interesting implications for how we think about the evolution of black holes, it inspired me to wonder about the other end of the scale—the largest things out there. There are galaxy clusters so large that they defy explanation by our current theories, but I was more curious about individual bodies; those monstrous black holes that, by themselves, have as much mass as a dwarf galaxy.

My search led me to H1821+643 (referred to from here on out as ‘643’), a radio-quiet quasar surrounded by a small cluster of galaxies. While there are objects potentially larger than 643, figuring out the mass of something so distant and enormous can be tricky. As a result, we’re less certain about our estimates on some of the candidates for the “most massive body in the universe” award: right now, 643 is the largest thing that can be accurately weighed.

So what does it take to hold the heavyweight championship for the entire known universe? To give you some inkling of scale, consider the following: if we blew you up, proportionally, to the mass of this object, one of the hydrogen atoms in your body would be heavier than the entire human race.

Unfortunately, since it’s hard even to get a sense of the size of a hydrogen atom from a human perspective, this analogy falls short. However, since a black hole’s Schwarzschild radius grows proportionately with its mass, size can be a useful proxy. For scale; it takes a photon about eleven hours to get from the surface of the sun to the outer edge of pluto’s orbit. A photon crossing 643’s event horizon, heading directly for the center at the speed of light, will take almost a full week to reach it.

Beyond its mass, there’s very little that we know for sure about 643. It appears to be unique in that it’s the only slow-moving quasar couched in a galaxy cluster, but why this should be so is still a mystery. Hypotheses have been suggested as to how it could have formed, but these are difficult to test for obvious reasons.  For now, we can rest easy in the knowledge that it’s very, very far away—and getting further. Its apparent redshift is 0.29, which means it’s racing away from us at about a third of the speed of light (although we could just as easily be said to be racing away from it).

While continuous advances to the field of astronomy mean that no superlative is forever, right now it seems like H1821+634 is the go-to, if you ever need to aim a “yo mama” joke at Neil Degrasse Tyson.

You may also read these articles