Hubble Space Telescope Finds Elusive Black Hole in Star Cluster : NPR

The Omega Centauri star cluster contains millions of stars. The motion of some of the stars suggests that there is a medium-sized black hole at its center.

The Omega Centauri star cluster contains millions of stars. The motion of some of the stars suggests that there is a medium-sized black hole at its center.


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Astronomers using the Hubble Space Telescope have found evidence for the existence 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 is much more massive than a typical black hole, the type created when a dead star collapses in on itself. But it is also nowhere near as big as the kind of supermassive black hole found at the centre of galaxies, which can contain hundreds of thousands to millions of suns.

Scientists have long been searching for medium-sized black holes like this new one. Finding them would help them understand the myriad ways black holes can form and why some grow into gigantic monsters.

Despite much effort over the years, scientists have not yet been able to find solid examples of black holes in the so-called intermediate size range. This includes all black holes that are 100 to 100,000 times more massive than the sun.

“People have wondered whether it is hard to find them because they are simply not there, or because they are hard to detect?” says Maximilian Häberle of the Max Planck Institute for Astronomy in Heidelberg, Germany.

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

Black holes can’t be observed directly because their gravity pulls everything in, including light. But researchers can see if a black hole’s gravity affects nearby objects, including stars.

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

“This is actually for technical reasons, to calibrate the instruments,” says Häberle.

Because the telescope had been making high-quality observations for 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 whose motions we have measured is much, much larger than any previous attempt,” he says, adding that the stars “are all moving in random directions and like a swarm of insects.”

Ultimately, the researchers were able to pick out seven stars in the center that were moving much faster than the others. These stars were actually moving so fast that they should simply shoot out of the cluster and be gone forever.

The fact that they are stuck and concentrated in the center, says Häberle, “means that there must be something pulling on them gravitationally, so that they don’t escape. And the only object that can be that massive is a medium-mass black hole with a minimum mass of at least 8,000 solar masses.”

It is unlikely that the black hole is more massive than 50,000 times the mass of the sun, he says. If that were the case, scientists would expect to see many more stars affected by the black hole’s gravity.

He notes that it was claimed as early as 2008 that a candidate for an intermediate-mass black hole had been found in this cluster, but this was disputed.

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

In addition, future observations are already planned with the James Webb Space Telescope. This powerful telescope will be able to look for signs of gas being heated as it falls into the black hole.

“This is really exciting, right? This is only the second black hole where you see individual stars rushing around the black hole,” said Jenny Greene, an astrophysicist at Princeton University.

She notes that the only comparable observation comes from a Nobel Prize-winning study that saw stars flying around the black hole at the center of our Milky Way galaxy. This supermassive black hole 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 much less mass,” she says.

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

One possibility is that small black holes merge to form a larger hole. Evidence for this comes from the detection of gravitational waves from two colliding black holes, an event that produced a black hole about 150 times more massive than the sun.

Another possible way to grow intermediate-mass black holes, recently proposed by astronomers, is for many stars in a dense cluster like Omega Centauri to collide and become a single very massive star. Later, that large star would collapse into an intermediate-mass black hole.

If scientists better understand where medium-sized black holes are and how they grow, they can better understand what role they might play in the evolution of even more massive black holes at the hearts of galaxies.

The newly discovered black hole “will give us really important information about how these big black holes first formed and grew,” Greene said.

Such supermassive black holes appear to have formed surprisingly soon after the universe began, just a few hundred million years after the Big Bang.

This is evident from new observations made with the James Webb Space Telescope. Astronomers wonder how a black hole can become so large so quickly.

Before those 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 some tantalizing evidence that black holes grew before their galaxies did.”

According to Greene, medium-sized black holes as we know them today could be remnants of the early formation process of the black hole, and could provide clues as to how it formed.

“To ultimately get a full picture, we need more than one person,” she says, “but this really opens the door.”

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