The Hubble Space Telescope has found an elusive black hole in a star cluster: NPR

Astronomers have used the Hubble Space Telescope to find evidence of an elusive type of black hole that is about 8,000 times more massive than our Sun.

What makes this black hole special is its size, according to the discovery report in the journal Nature.

It is much more massive than a garden-variety black hole, the type that is created when a dead star collapses in on itself. But it’s also nowhere near as big as the supermassive black hole that lurks at the center of galaxies and holds hundreds of thousands to millions of suns.

Scientists have long searched for medium-sized black holes like this new one, because finding them could shed light on the myriad ways black holes can form and why some grow into huge monsters.

However, despite much effort over the years, scientists have not been able to find solid examples of black holes in the so-called intermediate size range, which includes any black hole with a mass between 100 and 100,000 times the mass of the Sun.

“So people were asking, are they hard to find because they’re just not there or because they’re hard to detect?” says Maximilian Häberle of the Max Planck Institute for Astronomy, Heidelberg, Germany.

He and some colleagues recently decided to look for one in a large bright star cluster known as Omega Centauri. This densely packed globular cloud of millions of stars is about 17,000 light-years away.

Black holes cannot be observed directly because their gravity attracts everything, including light. However, scientists can see if the black hole’s gravity affects nearby objects, including stars.

And scientists knew that the stars in this particular cluster had been continuously observed by the Hubble Space Telescope, which takes pictures of the cluster’s central region every year.

“It’s actually for technical reasons, due to the calibration of the instruments,” says Häberle.

Because the telescope made high-quality observations spanning more than two decades, Häberle and his colleagues were able to precisely measure the motion of the cluster’s 1.4 million stars.

“Our list of stars for which we have measured motions is much, much larger than any previous effort,” he says, adding that “all the stars move in random directions and like a swarm of insects.”

In the end, scientists were able to pick out seven stars in the center that move much faster than the others. These stars are actually moving so fast that they really should shoot out of the cluster and disappear forever.

The fact that they remain stuck and concentrated in the center, says Häberle, “means that there must be something gravitationally pulling them to keep them from escaping. And the only object that can be that massive is an intermediate mass black hole with a minimum mass of at least 8,000 solar masses.

A black hole is unlikely to be more than about 50,000 times the mass of the Sun, he says, because if it were, scientists would expect many more stars to be affected by its gravity.

He notes that there was previously a claim to find a medium-sized black hole candidate in this cluster dating back to 2008, but that was disputed.

This time, he says, “I think our evidence is very robust” because of the years’ worth of data.

In addition, future observations with the James Webb Space Telescope are already planned, and this powerful telescope will be able to look for telltale signs of gas that heats up as it falls into a black hole.

“This is really exciting, isn’t it? This is only the second black hole where you can see individual stars whizzing around a black hole,” says Jenny Greene, an astrophysicist at Princeton University.

He notes that the only similar observation was Nobel Prize-winning work that saw stars flying around the black hole at the center of our Milky Way galaxy, a supermassive that is about four million times more massive than our Sun.

“So I think it’s a really big deal. And it’s a black hole with a much lower mass,” he says.

No one knows how a black hole of this size formed.

One possibility is that small black holes can merge into larger ones. Evidence for this comes from the detection of gravitational waves from the collision of two black holes, an event that spawned a black hole about 150 times more massive than the Sun.

Another possible way for medium-sized black holes to grow, recently suggested by astronomers, is that many stars can collide in a dense cluster like Omega Centauri to become one very massive star. Later, this massive star would collapse into a medium-sized black hole.

Understanding where medium-sized black holes lie and how they grow can help scientists understand what role they might play in the development of even larger ones found at the heart of galaxies.

The newly discovered black hole “is really going to tell us important information about how these large black holes first formed and grew,” says Greene.

Such supermassive black holes appear to have appeared surprisingly soon after the universe began, just a few hundred million years after the big bang.

That’s according to new observations made with the James Webb Space Telescope, which have left astronomers baffled as to how the black hole could have expanded so quickly.

Before these observations, Greene says, she thought galaxies first grew and then black holes formed at their centers. “I’m less sure now,” she says. “There is now some exciting evidence that black holes grew before their galaxies.

The medium-sized black holes that exist today could be remnants of that early black hole creation process, Greene says, and could provide clues about how it happened.

“Ultimately, to get the full picture, we need more than just one,” he says, “but this really opens the door.”