Some star clusters have no business surviving. For instance, there's a tiny cluster right next to our galaxy's supermassive black hole, which should be ripped apart by its intense gravity. But there's more to this cluster than meets the eye.
This particular cluster, IRS 13E, can barely even be called one, considering it's made up of just three stars. At least, three visible stars - two researchers at the University of Bonn's Argelander Institute for Astronomy believe this represents the first known example of a dark star cluster, one composed almost entirely of black holes. There might be hundreds of former stars in the cluster now holding the entire cluster apart.
It's an ambitious explanation, but it's also the best fit for the available facts. The three stars in IRS 13E are located just 0.4 light-year away from Sagittarius A*, the supermassive black hole at the center of the Milky Way. That's just a tenth of the distance between the Sun and Alpha Centauri, and the cluster shouldn't be able to survive that sort of extreme proximity for long. And yet the cluster isn't showing any signs at all of moving closer to the black hole.
It's possible we're observing the cluster at exactly the right moment and it just happens to appear to not be moving closer in, but the odds of that are exceeding unlikely. It's far more probable that something is keeping the cluster safe, and that something must be providing a ton of gravity to counteract the huge influence of Sagittarius A*.
Black holes are the most obvious source of hidden gravity, and the researchers considered whether a single, medium-sized black hole could be at the center of IRS 13E. But the stars in the cluster aren't acting in the way they would have to if such a singularly large gravitational influence was nearby.
Instead, researchers Sambaran Banerjee and Pavel Kroupa propose the dark star cluster model. IRS 13E was once full of massive stars, but over time they ran out of fuel, went supernova, and ultimately collapsed into black holes. These would then slowly move to the center of the cluster, gobbling up other stars as they went until only a few were left - the three visible stars that make up all we can see of IRS 13E.
If we can find proof that dark star clusters really do exist, then they could have some unusual properties of great use to astrophysicists. For one thing, they'd likely be huge sources of gravitational waves, and finding those have long been on physicists' to do list. The researchers suspect dark star clusters may be extremely common throughout the galaxy - they're just so dim that they escape most detection, and it took the intense gravity of Sagittarius A* to reveal the first clear signs of one.