Astronomers have found a star orbiting a black hole twice in an hour in the 'closest orbit ever'

Astronomers have just spotted a star orbiting a black hole at an estimated incredible speed of nearly 1% of the speed of light. In what is considered to be one of the "closest orbit ever" to be seen around a black hole in our very own Milky Way galaxy, the discovery of the stellar couple, known as a binary, has led researchers to uncover that the star completes two orbits of the black hole in just around an hour. The white dwarf star is also the first one to be discovered at such close proximity to a black hole.

While the existence of the binary known as 47 Tuc X9 was already known, the discovery of the new evidence was made using deep space telescopes belonging to Nasa and The Commonwealth Scientific and Industrial Research Organisation (CSIRO). Raw data from Nasa's Chandra and NuStar and CSIRO's Australia Telescope Compact Array helped astronomers find that a white dwarf star was completing its orbit around the black hole in approximately 28 minutes, covering a distance of "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," said first author Dr Arash Bahramian, from the University of Alberta in Canada and Michigan State University in the United States.

Associate Professor James Miller-Jones, from Curtin University and International Centre for Radio Astronomy Research (ICRAR), said, "We think the star may have been losing gas to the black hole for tens of millions of years and by now has now lost the majority of its mass."

He added, "Over time, we think that the star's orbit will get wider and wider as even more mass is lost, eventually turning into an exotic object similar to the famous diamond planet discovered a few years ago."

"Prior to this discovery, the closest star around any likely black hole was a system known as MAXI J1659-152, which is in an orbit with a 2.4-hour period," said Miller-Jones, ScienceAlert reported. "If the likely black holes in both systems have similar masses, this would imply an orbit three times larger in physical size than the one we found in X9."

In other words the distance between the two objects in X9 is around 1 million kilometres (600,000 miles), which is 2.5 times the distance between the Earth and the Moon.

How did a star get so close to a black hole?

Researchers theorise that the black hole may have collided with a red star and as gas was ejected from the star, a binary may have been formed consisting of a black hole and a white dwarf star. The binary's orbit would have shrunk with the emission of gravitational waves and eventually the black hole would have started pulling material from the white dwarf.

"Finding these rare black holes is important, as 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," Geraint Lewis from the University of Sydney told Marcus Strom at The Sydney Morning Herald.

Fortunately, the stellar dance between the two objects is not expected to come to an end any time soon. In fact, researchers believe that the star and the black hole were even closer together in the past and the white dwarf star's orbit even faster than at present. Over time, however, as the black hole continues to pull apart material from the star, the loss of mass may eventually lead to it falling into the black hole.

"Eventually so much matter may be pulled away from the white dwarf that it ends up only having the mass of a planet," said researcher Craig Heinke. "If it keeps losing mass, the white dwarf may completely evaporate."

The research paper detailing the cosmic dance between the two space objects is titled 'The ultracompact nature of the black hole candidate X-ray binary 47 Tuc X9', and has been featured in the Monthly Notices of the Royal Astronomical Society published by Oxford University Press on 14 March 2017.