Closest star around black hole uncovered

A star has recently been discovered in orbit around a black hole in what might be the closest orbit between such a pair ever seen. New data from NASA’s Chandra X-ray Observatory revealed a white dwarf is racing around the black hole twice every hour.

 
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The white dwarf is being stripped of its gas by the black hole. It could end up as some variety of exotic planet, or may evapourate entirely. The inset was taen with the Chandra X-ray Observatory
Credits: X-ray: NASA/CXC/University of Alberta/A.Bahramian et al.; Illustration: NASA/CXC/M.Weiss

A star has recently been discovered in orbit around a black hole in what might be the closest orbit between such a pair ever seen. New data from NASA’s Chandra X-ray Observatory revealed a white dwarf is racing around the black hole twice every hour.

The binary system, X9, is 14,800 lightyears away in the globular cluster 47 Tucanae, a dense collection of stars. In 2015 radio observations led to the suggestion that the pair was a black hole pulling material from a companion white dwarf.

Every 28 minutes, X9 changes its X-ray brightness following the same pattern. It’s thought the changes track the time taken for the star to circle the black hole, orbiting at a distance only 2.5 times the separation between the Earth and the Moon.

“This white dwarf is so close to the black hole that material is being pulled away from the star and dumped onto a disk of matter around the black hole before falling in,” says Arash Bahramian from the University of Alberta. “Luckily for this star, we don’t think it will follow this path into oblivion, but instead will stay in orbit.”

 


Read more stories about black holes from BBC Sky at Night Magazine:


 

Researchers are uncertain how the white dwarf came to be so close. It could be that a black hole collided with a red giant, stripping the larger star of its outer gas leaving behind a white dwarf. Over time, the star fell in towards the black hole until they were close enough that material began being stripped off the white dwarf.

The find may give a new way to search for the usually difficult to spot black holes.

“For a long time astronomers thought that black holes were rare or totally absent in globular star clusters,” says Jay Strader from Michigan State University. “This discovery is additional evidence that, rather than being one of the worst places to look for black holes, globular clusters might be one of the best.”


 

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