Astronomers have spotted a white dwarf star maintaining the closest orbit ever seen around a black hole.
According to Scientific American, the small star is located in the globular cluster 47 Tucanae, about 15,000 light-years from Earth. The star orbits a black hole at a rate of about once every 28 minutes.
Scientists estimate that the star and its companion black hole are only about 2.5 Earth-moon distances apart – closer than any other star has been found orbiting a black hole.
“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,” Arash Bahramian of the University of Alberta in Canada and Michigan State University said. “Luckily for this star, we don’t think it will follow this path into oblivion, but instead will stay in orbit.”
Even if the star remains in orbit, however, it could ultimately lose the remainder of its fuel to 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,” Craig Heinke of the University of Alberta said. “If it keeps losing mass, the white dwarf may completely evaporate.”
The binary system of the star and black hole is called X9, and the black hole within the system appears to be a stellar-mass black hole, which is relatively small. The research team determined the rate of orbit within the system by noting that X9 increases in X-ray emissions one every 28 minutes. A high level of oxygen in X9 suggests that the star is a white dwarf.
The team made observations using NASA’s Chandra X-ray Observatory and Nuclear Spectroscopic Telescope Array (NuSTAR) as well as the Australia Telescope Compact Array.
Astronomers think that the binary system may have formed when a red giant star encountered a small black hole and the two became gravitationally linked as the red giant shrank to its current white dwarf form.
Another possibility is that the white dwarf is orbiting a neutron star, although the X-rays emitted by X9 do not indicate a strong likelihood that a neutron star is present.
“We’re going to watch this binary closely in the future, since we know little about how such an extreme system should behave,” Vlad Tudor of Curtin University and the International Centre for Radio Astronomy Research in Australia said. “We’re also going to keep studying globular clusters in our galaxy to see if more evidence for very tight black-hole binaries can be found.”
The study will be published in the journal Monthly Notices of the Royal Astronomical Society.