Just the concept of black holes is captivating, isn’t it? No wonder they’re central to the plotlines of so many movies and sci-fi stories. The idea of an object so dense that light can’t travel fast enough to escape from it is pretty mind boggling. It’s well known that galaxies contain supermassive black holes at their hearts. But exactly how supermassive can a black hole grow?

Well, the theoretician’s answer would be ‘infinitely big’. There’s no upper limit to how big a black hole could become. White dwarfs and neutron stars are constrained by how much punishment matter can take before it collpses into a denser form. Having compacted into a singularity, black holes are beyond the constraints of matter. Keep feeding a black hole and it will keep eating, it’s as simple as that.

In the real Universe though, infinity is just a concept. The Universe isn’t infinite, therefore no black hole will ever attain infinite size. That said, we can make a pretty good prediction of how big a black hole could have grown by now, given the age of the Universe. Also, regardless of the voracity of one of these beasts, their feeding habits are self-regulating. Matter falling into a black hole releases energy as it falls. Lots of energy. Actually, the extreme forces at play around black holes can convert matter to energy more efficiently than the nuclear fusion that goes on inside stars! The radiation emitted by a feeding black hole can actually blow away other material, slowing the feeding rate. Apparently, black holes still have table manners.

It’s recently been calculated that the largest size a black hole could possibly have grown to in our Universe is 50 billion solar masses. That’s terrifyingly big. 50 000 000 000 Suns. 100 duodecillion kilograms. These numbers are so big as to be practically meaningless to us mere humans.

The largest black hole yet discovered is a true leviathan. Lying around 3.5 billion light years away from us is a quasar known as OJ 287. In it’s heart lies not one but two black holes. A monstrous binary pair. The smaller of the two is “only” 100 million solar masses. Hurtling around the quasar’s interior, it orbits it’s larger companion once every 12 years, punching holes in the larger black hole’s accretion disk as it goes and causing violently bright outbursts. It’s the orbit of this satellite black hole that lets us measure the mass of the beast it orbits at around 18 billion solar masses. Big enough to make Sagittarius A* look positively feeble by comparison!

A system like this probably won’t be too stable. It’s pretty likely that the two black holes will merge in a few thousand years. When they do, the fireworks will probably be unimaginable…

Image Credit: NASA/Dana Berry, SkyWorks Digital

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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