Star clocked at five million km/hr

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Tight spin with black hole a first, say scientists

University of Alberta scientists have found the universe’s biggest, densest, quickest dance partners – a white dwarf that orbits a black hole every 28 minutes.

It’s the closest anyone has ever seen a star orbit a black hole. It’s also likely the fastest orbit, with the star clipping along fast enough to loop around Earth in about 29 seconds.

The U of A’s Arash Bahramian and Craig Heinke published a paper in Monthly Notices of the Royal Astronomical Society recently on a stellar pair in the star cluster 47 Tucanae, which is about 14,800 light years from Earth.

While researchers have known of this pair for years, they initially thought it was a pair of stars. Radio evidence in 2015 suggested that one of the stars might be one of the most exotic objects in physics: a black hole.

When giant stars run out of fuel, their cores collapse and create a giant explosion called a supernova, explained Gregory Sivakoff, U of A physics professor and one of the study’s co-authors. If the star was originally 25 or more times heavier than the Sun, the core collapses so much that it becomes a black hole.

A black hole is a dead star whose gravity is so strong that not even light can escape, said St. Albert’s Bruce McCurdy, an astronomer at the Telus World of Science in Edmonton, who is not affiliated with this study. White dwarfs are what you get when smaller, Sun-sized stars peter out, and pack the mass of a star into a space the size of the Earth.

Sivakoff and his team studied the stellar pair (dubbed X9) using the Chandra and NuSTAR orbital X-ray telescopes and a land-based radio telescope called the Australia Telescope Compact Array.

The X-ray evidence suggests that one of the objects in the pair had a lot of oxygen in it, which is indicative of a white dwarf, Sivakoff said. The radio waves coming off the pair were so consistent and bright that the team concluded that the pair was actually a black hole eating another star. The only other known possibility, a neutron star/white dwarf combo, wouldn’t produce this pattern of radio waves.

By measuring how often the X-ray brightness of the pair changed over time from one object passing in front of the other, the team determined that the dwarf was orbiting the black hole once every 28 minutes. Using math, they determined that the two were just 2.5 times the distance between the Earth and the Moon apart.

Sivakoff said this is the closest anyone has ever seen a star orbit a black hole, and the quickest they’ve seen one orbit the other. He estimated that the white dwarf was zipping along at about 5 million km/h – that’s fast enough to go around the equator in about 29 seconds.

Researchers know from other binary star systems that this pair would have originally been two stars that were close enough for their atmospheres to touch, Sivakoff said. The bigger star would have blown up without enough force to fling away its neighbour, collapsed into a black hole, and started to suck in matter from the dwarf.

As that happened, the two dance partners would have pulled each other closer together. Matter from the dwarf would have spun towards the hole, forming a disk. As matter got closer to the hole, it would accelerate and heat up enough to produce X-rays we can detect on Earth.

Sivakoff said the team wasn’t sure if the black hole would eventually devour the white dwarf or if the two would continue to dance indefinitely.

While these dance partners are too far from Earth to affect us, McCurdy said studying them could help researchers understand the gigantic black hole at the centre of our galaxy.

“It’s kind of cool understanding what’s going on out there.”

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Kevin Ma

Kevin Ma joined the St. Albert Gazette in 2006. He writes about Sturgeon County, education, the environment, agriculture, science and aboriginal affairs. He also contributes features, photographs and video.