We've long known that black holes come in two flavors. There are stellar black holes, which form when a giant star collapses in on itself, compressing the mass of five or more our suns into an infinitely small point. They just float around the universe, sucking up anything that gets in their way.

And then there are supermassive black holes. These are a lot more mysterious, and their mass can be as high as millions or even billions of our suns. They don't go anywhere, but stay at the center of their galaxies like the hubs of a cosmic wheel. But it always seemed strange to astronomers that there were itty-bitty black holes and mega-gigantic black holes, but nothing in between. Until now.

Filling in the Gaps

Supermassive black holes are sort of the linchpin of the galaxy. In 1971, astronomers Donald Lynden-Bell and Martin Rees hypothesized that instead of being empty, the blank spaces at the center of every galaxy might actually contain these insatiable giants, and in 1974, we proved there was one at the middle of the Milky Way. It's called Sagittarius A*, because that's what happens when you let scientists name things.

Since then, we've discovered the effect that giant black holes have on their galaxies. The bigger the black hole, the bigger the galaxy, and the faster it spins. So that suggested that the black holes might actually play a role in how galaxies are formed — but if they were there before the galaxy was, then where did they come from?

You might think that stellar black holes could cast some light on the mystery, but it turns out they are notoriously bad at casting light on anything. These things are way tinier than their supermassive counterparts, like billions of times tinier, making it very unlikely that stellar black holes have anything to do with supermassive black holes creation. So what gives?

In 2016, observers at Japan's Keio University noticed something that just might link the two types of black holes. A gas cloud near the center of the galaxy (but not inside it) was found to be accelerating in a strange way that suggested it was being influenced by a giant black hole. But not too giant — they're calling this one an "intermediate-mass black hole". And if it really is the black hole they think it is, it's still about a million times smaller than ol' Sagittarius A*. That means that it could finally reveal how the supermassives form, growing slowly by sucking up errant gas until they get big enough to either form the core of a new galaxy, or crash into the central supermassive black hole of the galaxy they're currently in.

How Big Are Black Holes?

Ready for a head trip? We've been talking about how big black holes are, but in fact, the stellars, the intermediates, and the supermassives are all the exact same size. That's because a black hole is just any non-zero amount of matter condensed into a one-dimensional point — and more matter means high density.

Still, you can judge the size of a black hole by the size of its event horizon. That's the radius surrounding the black hole at which you would have to be moving at the speed of light in order to escape its gravitational pull. The denser the hole, the larger the event horizon. The radius encompassing the event horizon of a black hole is called a Schwarzchild radius, and to the naked eye it would appear to be a completely black sphere, since no light can escape its grasp.

Curious about what your Schwarzchild radius would be? You can calculate it right here. If the sun were to collapse into a black hole tomorrow (which, just so you know, is totally impossible), then its event horizon would be about 2.95 kilometers (about 1.83 miles) from its center. The smallest black hole we know about has the mass of 3.8 suns, making its Schwarzchild radius about 11 kilometers (7 miles). Even the largest stellar black hole would only have an event horizon extending out 46 kilometers (28 miles) — that's just a 30-minute drive on solid ground. And then there's Sagittarius A*. Its event horizon extends 13.3 million kilometers (8.26 million miles) in every direction. We wouldn't recommend that as a commute, even if there weren't any black holes around.

The Positive Side Of Black Holes

Written by Reuben Westmaas September 18, 2017