Gather round for the epic tale of a star’s dangerous encounter with a supermassive black hole. The star survived this rendezvous, but at a terrible price. It was no longer the same star it once was.
The setting of our story is galaxy GSN 069, located 250 million light-years from our corner of the universe. The black hole in question has a mass of about 400,000 times that of our Sun. Certainly on the small size when it comes to supermassive black holes, but nevertheless a powerful gravitational object. The star is believed to have been a red giant; the phase in a star’s life where it has run out of hydrogen at its core and has begun fusing helium, swelling into a large and puffy object.
This particular red giant had the misfortune to meet a supermassive black hole. The black hole lured the star in and stripped it of its outer layers, leaving its exposed core behind. Now, the star is a white dwarf about 20 percent of the mass of the Sun.
As reported in the Monthly Notices Of The Royal Astronomical Society, the star didn't quite make a lucky escape, but it did survive where many others haven't.
"In my interpretation of the X-ray data the white dwarf survived, but it did not escape," author Andrew King of the University of Leicester, said in a statement. "It is now caught in an elliptical orbit around the black hole, making one trip around about once every nine hours."
Its continuous survival around the black hole does not come for free, though. During its nearly thrice-daily approach to the black hole, the star loses a bit of material. As this is captured by the black hole, a burst of X-rays is released, as you can see above. This emission was detected by NASA's Chandra X-ray Observatory and ESA's XMM-Newton observatories.
This sacrifice the white dwarf makes is not completely in vain. As the star loses material, it becomes lighter and its orbit grows wider, taking it further from the black hole. Currently, the star gets as close as 9 million kilometers (5.6 million miles).
This interaction between the black hole and the white dwarf will not end happily, though.
"It will try hard to get away, but there is no escape. The black hole will eat it more and more slowly, but never stop," explained King. "In principle, this loss of mass would continue until and even after the white dwarf dwindled down to the mass of Jupiter, in about a trillion years. This would be a remarkably slow and convoluted way for the universe to make a planet!"
These observations are remarkable for two reasons. The emission that this interaction produces would be visible to our current instruments for just 2,000 years. So we are either incredibly lucky or there should be a lot of these interactions out there. The other remarkable fact is the ability to estimate the properties of a star 250 million light-years away.
The peculiar interaction will also certainly release gravitational waves and the future space-base observatory LISA might be able to catch them in the act.