Hubble Space Telescope finds elusive black hole in star cluster: NPR

Astronomers have used the Hubble Space Telescope to find evidence of an elusive type of black hole, one that is about 8,000 times the size of our sun.

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

It’s far more massive than a garden-variety black hole, the kind created 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 center of galaxies and can hold hundreds of thousands to millions of suns.

Scientists have long been on the lookout for medium-sized black holes like this new one, because finding them could shed light on a host of ways black holes can form and why some grow into giant monsters.

Despite many efforts over the years, however, scientists have had no luck finding strong examples of black holes in the so-called intermediate size range, which would include any black hole that is between 100 and 100,000 times more massive. greater than the mass of the sun.

“So people have been wondering, is it hard to find them 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 several 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, as their gravity pulls in everything, including light. But researchers can watch to see if a black hole’s gravity is affecting nearby objects, including stars.

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

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

Because the telescope had made high-quality observations that went back more than two decades, Häberle and his colleagues were able to precisely measure the motion of the 1.4 million stars in the cluster.

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

Ultimately, the researchers 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 must exit the star cluster and disappear forever.

The fact that they remain stuck and centered, says Häberle, “means that there has to be something gravitationally pulling on them so that they don’t escape. And the only object that could be that massive is an intermediate-mass black hole with a minimum mass of at least 8,000 solar masses.

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

He notes that there was an earlier 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 strong” because of 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 being heated as it falls into the black hole.

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

She notes that the only similar observation was the 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 the size of our sun.

“So I think it’s a really big deal. And it’s a much smaller black hole,” she says.

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

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

Another possible way to grow medium-sized black holes, recently suggested by astronomers, is that many stars could collide into a dense cluster like Omega Centauri and become a single very massive star. Later, that massive star will collapse into a medium-sized black hole.

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

The newly discovered black hole “is really going to tell us important information about how those supermassive black holes formed and grew in the first place,” Greene says.

Such supermassive black holes seem to have appeared surprisingly soon after the beginning of the universe, just a few hundred million years after the Big Bang.

That’s according to new observations made with the James Webb Space Telescope, which left astronomers wondering how a black hole could get so big 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 is some tantalizing evidence now that black holes grew earlier than their galaxies.”

Medium-sized black holes that exist today may be relics left over from the early black hole creation process, Greene says, and may provide clues as to how it happened.

“Ultimately to get a complete picture, we need more than just one,” she says, “but this is really opening the door.”