December 9, 2021

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A nearby star makes an invisible black hole in a very small star cluster 'visible'

A nearby star makes an invisible black hole in a very small star cluster ‘visible’

Astronomers may be able to discover more hidden black holes in the same way, which could shed light on how these mysterious objects formed and evolved.

We now know that there are countless black holes in the universe. Finding them, however, is another matter. This is because you can’t see black holes. They are strange things that nothing can escape – not even particles, not even light. However, researchers have now used a very effective method to discover a black hole hiding in a star cluster outside our Milky Way. This is thanks to the movement of a nearby star.

NGC 1850
Researchers tracked the black hole using the Very Large Telescope. The object was hidden in NGC 1850, a group of thousands of stars about 160,000 light-years away in the Large Magellanic Cloud, a small neighboring galaxy to the Milky Way. NGC 1850 is only about 100 million years old – the twinkling of an eye by astronomical standards. So the fact that a black hole was found in this star cluster is very special. Because it’s the first time a black hole has been discovered in such a young star cluster.

This image shows NGC1850, a group of thousands of stars in the Large Magellanic Cloud, a small neighboring galaxy of the Milky Way about 160,000 light-years away. The red filaments surrounding the cluster, made up of huge clouds of hydrogen gas, are believed to be the remnants of supernova explosions. Photo: ESO, NASA/ESA/M. Romaniello

How did researchers find the black hole? The researchers saw a star blazing, whose strange motion revealed the existence of the black hole. The black hole exerts a gravitational effect on a five-solar-mass star orbiting it.

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Sherlock Holmes
“Just as Sherlock Holmes was tracking down a criminal gang through its slips, we looked at every star in the cluster through a magnifying glass,” said Sarah Saracino, the study’s lead researcher. We then tried to find evidence of black holes without seeing them directly. This means that a nearby star made the virtually invisible black hole in the very small star cluster “visible,” as it were. It is the first time that this detection method has been used to detect a black hole outside our Milky Way.

small black hole
In the meantime, we also know more about this newly discovered black hole. For example, its mass is about 11 times the mass of our Sun, which makes it a small black hole. By the way, this is not the first time that such “small” black holes have been found. Previously, samples have been discovered in other galaxies by observing X-rays emitted by black holes as they gobble up matter. They can also be detected by gravitational waves emitted by black holes when they collide with each other or with neutron stars. But this is very rare. Most black holes in some stellar masses do not betray their existence by emitting X-rays or gravitational waves. “The vast majority can only be discovered through dynamics,” researcher Stefan Dritzler explains. “When they have an ordinary star as their companion, they affect its movement in a subtle but detectable way, so we can track them down with advanced tools.”

The new detection method
The researchers hope to use the new detection method to detect many hidden black holes in the Milky Way and in nearby galaxies. “The result presented here is for only one of the wanted criminals,” Saracino said. “But if you find one, you’re on the right track to discovering more, and hiding in other star clusters.”

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learn more
And the more black holes we discover, the more we learn about how these mysterious objects form and evolve. In this case, more young black holes can be compared to the larger, more mature black holes in older star clusters. In this way, more ideas can be gained about how these objects can “grow” by feeding on stars or merging with other black holes. In addition, mapping the demographics of black holes in star clusters would improve our understanding of the origin of the sources of gravitational waves. “Any discovery we make will be important to our understanding of star clusters and their black holes,” said study researcher Mark Gillis.

With access still under construction very large telescope, which will begin work later this decade, will allow astronomers to discover even more hidden black holes. “ELT is going to revolutionize this region,” says Sarachino. “This will allow us to observe significantly faint stars in the same field of view, as well as search for black holes in globular clusters at much greater distances.” And who knows what scientists will reveal about these strange things.