A Tour of a Star Survives Close Call with a Black Hole

Astronomers may have discovered a new kind of survival story: a star that had a brush with a giant black hole and lived to tell the tale through exclamations of X-rays.

Data from NASA's Chandra X-ray Observatory and ESA's XMM-Newton uncovered the account that began with 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 this remarkable tale of cosmic perseverance, the white dwarf survives but does not escape. Instead, the white dwarf is now caught in an elliptical orbit around the black hole, making one trip around every nine hours. As the white dwarf makes its nearly thrice-daily orbit, the black hole pulls material off at its closest approach. 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. The black hole and white dwarf pair should also be giving off gravitational waves, especially at their nearest point.

What would be the future of the star and its orbit? The combined effect of gravitational waves and an increase in the star's size as it loses mass should cause the orbit to become more circular and grow in size. In this case, the rate of mass loss steadily slows down, and the white dwarf slowly spirals away from the black hole. The white dwarf may also lose so much mass over trillions of years that it becomes a planet with about the same mass as Jupiter.

Astronomers are interested in these kinds of encounters because they may help explain how black holes of this size grow over time.