An artist's impression of white dwarf star X9 (left) in orbit around a black hole. Credit:NASA The white dwarf star X9 orbits what is very likely a black hole every 28 minutes at an astonishing 12 million kilometres an hour. That's 1 per cent the speed of light. "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 [that matter] falls in," said lead author of the study, Arash Bahramian, from the University of Alberta in Canada. Astronomers made the discovery using the Australia Telescope Compact Array, operated by CSIRO near Narrabri in NSW, backed up with data from two of NASA's space telescopes, Chandra X-ray Observatory and NuStar.

The globular cluster Tucanae 47. The binary system studied is near the cluster's core. Credit:NASA/ESA/HUBBLE Until the ATCA data came through in 2015, astronomers thought the binary system was what is known as a "cataclysmic variable", where a white dwarf draws mass from a nearby sun-like star. Vlad Tudor, a PhD student at Curtin University who worked on the study, said: "We detected strong radio jets from the system in 2015. These were not compatible with a cataclysmic variable system." Vlad Tudor. Credit:Curtin/ICRAR "We think the star might have been losing gas to the black hole for tens of millions of years and has now lost the majority of its mass," said co-author of the study, Associate Professor James Miller-Jones from Curtin University and the International Centre for Radio Astronomy Research.

Professor Geraint Lewis from the University of Sydney, who was not part of the study, said: "Finding these rare black holes is import antas they are not only the end points of massive stars, produced in supernova explosions, they also continue to play a role in the evolution of other stars after their deaths." So will the star collapse into the black hole? Associate Professor Miller Jones said that while most of the gas from the star will be stripped off, this decline in mass will actually see the white dwarf move further from the black hole, to conserve angular momentum of the system. He said there were various formation scenarios for the unusual binary system. The stellar dance between the two objects is taking place inside a globular cluster 47 Tucanae, a group of about a million stars orbiting the galactic centre about 15,000 light years from Earth. Globular clusters have much higher stellar density than other galactic space, so a collision between a star and the black hole is the most likely process.

"The nearest star to our solar system is Proxima Centauri about four light years away. If you were in the centre of this globular cluster there would be 100,000 stars within this distance," Mr Tudor said. A collision between a red giant star and the black hole could have led to the current system, with mass lost through the emission of gravitational waves. Mr Tudor said current gravitational wave detectors operated by LIGO could not pick up emissions from this system, but space-based detectors expected to be launched in the 2030s could detect them. Associate Professor Miller-Jones said the next step will be to determine the mass of the objects. He estimates the white dwarf will only have a mass a few per cent that of the sun and have a planet-sized diameter.