Scientists have discovered a white dwarf star - a faint, dense star that has burned through most of its hydrogen and helium - circling just 600,000 miles or so from a suspected black hole, in what astronomers are describing as the closest such orbit ever known.
The binary system, known as 47 Tucanae X9 and located about 14,800 light years from Earth, has been on scientists' radar for some time.
Initially, astronomers thought that the system consisted of a sun-like star circling around a white dwarf. White dwarf stars have about the same mass as the sun but are compressed into objects about half the diameter of Earth.
But in 2015, observations by radio telescopes suggested that the X9 white dwarf was actually in orbit around a small black hole.
Black holes are objects so dense with matter that not even photons of light can escape their gravitational pull. They are detected by telltale radiation given off by material they consume from nearby stars and gas.
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At the time, the X9 discovery was unusual because black holes typically were not found in globular star clusters, like X9's.
The latest study has shown that the X9 white dwarf is whipping around its companion about every 28 minutes, a speed that means the objects are about 600,000 miles from each other, roughly 2.5 times the distance between Earth and the moon.
The system is believed to be stable, at least for now. Scientists don't know if the probable black hole, which is siphoning off material from the orbiting white dwarf, will eventually cause the star to break apart.
"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," astrophysicist Arash Bahramian, with the University of Alberta in Canada and Michigan State University, said in a press release. "Luckily for this star, we don't think it will follow this path into oblivion, but instead will stay in orbit."
If the white dwarf continues to lose mass, however, it could evolve into an exotic type of planet or completely evaporate, Craig Heinke, an associate professor of physics at the University of Albert, added in a related NASA press statement.
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Scientists also don't know how the cosmic couple came to exist.
One possibility is that the black hole smashed into a red giant star, which caused the star's outer regions to be stripped away. The star's remaining core then became a white dwarf and settled into orbit around the black hole.
Another possibility is that X9's white dwarf is actually orbiting a neutron star rather than a black hole. The neutron star would spin faster as it collects material from the white dwarf. Scientists have discovered a few of these objects, known as transitional millisecond pulsars, but X9's radio and X-ray emissions are a bit different.
The discovery was made using NASA's Chandra X-ray Observatory, NASA's NuSTAR telescope and the Australia Telescope Compact Array, a radio observatory. The research will be published in an upcoming issue of the Monthly Notices of the Royal Astronomical Society.
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