This is very cool news: Astronomers may have found the closest known intermediate black hole to Earth!

First, don’t panic. It’s still over 17,000 light-years away, and outside our galaxy. There may be some closer, but this is the closest we’ve seen so far.

OK, yeah. Let me explain.

When you think black hole, you probably think of the stellar mass kind, ones with masses similar to those of stars. These range from a minimum of about 3 times that of the Sun up to maybe a hundred solar masses. The upper limit isn’t rigorously defined. We know of lots of these, and there are likely tens of billions of them in the Milky Way galaxy.

There are also supermassive black holes, with millions or billions of times the mass of the Sun. These monsters form in the centers of all big galaxies. The one in the Milky Way, called Sgr A*, is 26,000 light-years away and is about 4.3 million solar masses. We know of ones much, much bigger.

But in between these two extremes are black holes with a few hundred to a few hundred thousand times the Sun’s mass. We call these intermediate-mass black holes, or IMBHs. We have a lot of evidence for them, but at the moment nothing super conclusive. They don’t tend to blast out high-energy light like the other kinds. Most of the candidates we have are seen in dwarf (small) galaxies, or sometimes in distant galaxies when a star wanders too close and gets torn apart in a tidal disruption event.

The best hunting ground for IMBHs is in star clusters, especially globular clusters. These are collections of hundreds of thousands or even millions of stars in a tightly packed ball usually a hundred or so light-years wide. We see them orbiting big galaxies, and the Milky Way has about 160 we know of so far. Some of them are likely clusters that formed along with the galaxy about 13 billion years ago, but some are actually the leftover cores of small galaxies that had their outer stars stripped away as they passed close to our much bigger galaxy. Weirdly they both wind up looking very similar, so it’s hard to tell the apart without careful analysis.

But either way, the idea is massive stars in the cluster eventually explode and form stellar-mass black holes, which then tend to fall to the center of the cluster — this process is called mass segregation; more massive objects with higher gravity pass by lower-mass ones and give them more energy, flinging them farther out in the cluster while the more massive black hole drops down to the core. Eventually you wind up with a lot of black holes in the cluster core, which can collide and merge, forming a bigger black hole, and, maybe, an IMBH.

In the case of a globular that used to be a galaxy in its own right, the central black hole may have grown naturally as the galaxy formed, with a lot of material falling to the center and collapsing to form the IMBH. We really aren’t hugely sure how big black holes get their start, and it’s a hot topic in astronomy right now.

Either way, you get an IMBH in a cluster core. But the thing about black holes is, they’re black. They don’t emit light. So how can you find one?

While they’re dark, they still have gravity, and the cluster core is a crowded place. It can be packed with stars, with several thousand jammed into just a few light-years. Mind you, the closest star to the Sun, Proxima Centauri, is a little over 4 light-years from us, but in a cluster that same volume would have thousands of stars. It’s a bit terrifying to think about. But what a view^*! 

These stars all orbit around the center of the cluster due to the overall gravitational field, plus the combined gravity of all the objects closer in to the center. If there’s a big ol’ black hole sitting there, then we should see stars very close to the center moving a whole (haha! “hole”) lot faster than they would be otherwise, due to the IMBH’s intense gravity.

 And that’s just what astronomers looked for [link to journal paper].

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