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If black holes were T-shirts, then scientists rummaging through the clothes rack that is our universe have managed to pull out only two sizes, small and XXL. They would love to find a black hole in a size medium, and they may have just nabbed one. NPR's Nell Greenfieldboyce explains why this is such a big deal.
NELL GREENFIELDBOYCE, BYLINE: Black holes are weird cosmic beasts that are so massive, their gravity sucks in everything, including light. Sometimes they form when a star dies and collapses in on itself. This produces a black hole that can be, say, 10 times as massive as our sun. Then there's a kind of black hole that's way bigger, supermassive black holes, which can be millions of times more massive than our sun. So there's these two extremes, and scientists have looked for black holes that would fall somewhere in the middle.
MAXIMILIAN HABERLE: But it has been very difficult to find them. And so people have wondered, is it difficult to find them because they are just not there?
GREENFIELDBOYCE: Maximilian Haberle is with the Max Planck Institute for Astronomy in Germany. He and some colleagues recently went searching for an intermediate-sized black hole. They looked in a relatively nearby star cluster. It's a large ball-shaped cloud made of millions of stars, like a swarm of insects. And it turns out the Hubble Space Telescope has looked at this cluster a lot.
HABERLE: So every year, they take some images of the central region. And this is actually for technical reasons, to calibrate the instrument.
GREENFIELDBOYCE: Having so many images, going back over 20 years, let the researchers precisely track the motion of 1.4 million stars. They found seven stars in the center of the cluster that were moving super fast, so fast that you'd expect them to shoot out of this ball of stars and be gone forever.
HABERLE: The fact that we can see them, they are concentrated in the center means that there must be something that is pulling on them gravitationally, such that they don't escape.
GREENFIELDBOYCE: He says the only thing that can be is an invisible black hole, about 8,000 times more massive than our sun - a black hole that is not small and not supermassive, but something in between. A report on the find in the journal Nature has other astronomers thrilled. Jenny Green studies black holes at Princeton University.
JENNY GREENE: This is really exciting, right? This is only the second black hole where you can see individual stars whizzing around the black hole.
GREENFIELDBOYCE: She says the only other place astronomers have seen this is at the center of our Milky Way around the supermassive black hole that lives there. Such supermassive black holes are mysterious. Scientists aren't sure when and how they grew so large. Greene says being able to study less massive ones, like this new one, could provide clues about how these enormous monsters came to be.
Nell Greenfieldboyce, NPR News.
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