Astronomers Discover Mysterious Black Hole Nestled in Star Cluster

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

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

It’s much more massive than a regular black hole, the kind that forms when a dead star collapses in on itself. But it’s also not as big as the kind of supermassive black hole that lurks at the centers of galaxies and can contain hundreds of thousands or even millions of suns.

Scientists have long been on the lookout for medium-sized black holes like this one, as their discovery could shed light on the myriad ways black holes can form and why some grow into gargantuan monsters.

However, despite many efforts over the years, scientists have failed to find any solid examples of black holes in the so-called intermediate size range, which would include any black hole with a mass between 100 and 100,000 times that of the Sun.

“So people ask themselves, is it hard to find them because they’re just not there, or is it hard to detect them?” says Maximilian Häberle of the Max Planck Institute for Astronomy in Heidelberg, Germany.

He and some colleagues recently decided to look for such a star in a large, bright star cluster called Omega Centauri. This dense, spherical cloud of millions of stars lies about 17,000 light-years from Earth.

Black holes cannot be observed directly because their gravity attracts everything, including light. But researchers can observe whether a black hole’s gravity affects nearby objects, including stars.

The researchers knew that the stars in this particular cluster were being continuously observed by the Hubble Space Telescope, which takes images of the cluster’s central region every year.

“Actually, it’s for technical reasons, to calibrate the instruments,” Häberle explains.

Thanks to high-quality observations made by the telescope over more than two decades, Häberle and his colleagues were able to precisely measure the motion of 1.4 million stars in the cluster.

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

Eventually, the researchers were able to identify seven stars in the center that are moving much faster than the others. These stars are moving so fast that they should simply disappear from the star cluster and be gone forever.

The fact that they remain stuck and concentrated in the center, Häberle explains, “means that there must be something gravitationally attracting them so that they cannot escape. 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.”

It’s unlikely that the black hole is larger than about 50,000 times more massive than the Sun, he said, because if it were, scientists would expect many more stars to be affected by its gravity.

He notes that there had previously been a claim that an intermediate-sized black hole candidate had been found in this cluster, dating back to 2008, but this has been disputed.

This time, he said, “I think our evidence is very strong” because of the additional years of data.

Additionally, 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 heating up as it falls into the black hole.

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

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

“So I think it’s a really big problem. And it’s a much lower mass black hole,” she said.

No one knows how a black hole of this size is created.

It is possible that small black holes can merge to form a larger one. Evidence for this comes from the detection of gravitational waves from the collision of two black holes, an event that gave birth to a black hole about 150 times more massive than the Sun.

Another possibility for the formation of medium-sized black holes, recently suggested by astronomers, is that many stars collide in a dense cluster like Omega Centauri and become a single, very massive star. Later, this large star collapses to form an intermediate-sized black hole.

Understanding where medium-sized black holes are located and how they develop could help scientists understand the role they might play in the evolution of even larger black holes that reside at the heart of galaxies.

The newly discovered black hole “will give us important information about how these large black holes formed and grew,” Greene says.

Such supermassive black holes appear to have appeared surprisingly early after the beginning of the universe, only 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 a black hole could grow so large so quickly.

Before these observations, Greene says, she thought galaxies grew first and then black holes formed at their centers. “Now I’m less sure,” she says. “There’s fascinating evidence now that black holes grew earlier than their galaxies.”

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

“To get the whole picture, we need more than one,” she said, “but this really opens the door.”