An image of annihilation: data from NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton has revealed a red giant star wandering too close to a supermassive black hole in a galaxy about 250 million light years from Earth. The black hole, located in a galaxy called GSN 069, has a mass about 400,000 times that of the Sun, putting it on the small end of the scale for supermassive black holes.
Once the red giant was captured by the black hole’s gravity, the outer layers of the star containing hydrogen were stripped off and careened toward the black hole, leaving the core of the star—known as a white dwarf—behind.
“In my interpretation of the X-ray data the white dwarf survived, but it did not escape,” said Andrew King of the University of Leicester in the UK, who performed this study. “It is now caught in an elliptical orbit around the black hole, making one trip around about once every nine hours.”
As the white dwarf makes its nearly thrice-daily orbit, the black hole pulls material off at its closest approach (no more than 15 times the radius of the event horizon—the point of no return—away from the black hole). The stellar detritus enters into a disk surrounding the black hole and releases a burst of X-rays that Chandra and XMM-Newton can detect. In addition, King predicts gravitational waves will be emitted by the black hole and white dwarf pair, especially at their nearest point.
What would be the future of the star and its orbit? The combined effect of gravitational waves and a change in the star’s size as it loses mass should cause the orbit to become more circular and grow in size. The rate of mass loss steadily slows down, as does the increase in the white dwarf’s distance from the black hole.
“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,” said 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!”
The Daily Galaxy, Sam Cabot, via University of Leicester