Black Hole Keeps White Dwarf On A Tight Leash

This graphic features an artist’s impression of X9, a star in the closest orbit known around a black hole. This discovery was made using data from NASA’s Chandra X-ray Observatory (shown in the inset where low, medium, and high-energy X-rays are colored red, green, and blue respectively), plus NASA’s NuSTAR telescope and the Australia Telescope Compact Array. Credit: X-ray: NASA/CXC/University of Alberta/A.Bahramian et al.; Illustration: NASA/CXC/M.Weiss

Imagine living on a planet that whipped around its host sun once every 28 minutes. The sun would rise at say, 6 a.m., set about 14 minutes later and then rise again after another 14 minutes. A day would make you dizzy! Astronomers have recently found evidence for a star that whips around a black hole about twice an hour. This may be the tightest orbital dance ever witnessed for a likely black hole and a companion star.

47 Tucanae in the southern constellation of Tucana the Toucan is a dense cluster of more than half a million stars in our galaxy. It’s located about 14,800 light years from Earth. Credit: ESO

This discovery was made with NASA’s Chandra X-ray Observatory, the NuSTAR orbiting observatory and CSIRO’s Australia Telescope Compact Array or ATCA. All three telescopes stared at this close stellar couple, known as X9, inside the starry interior of the globular star cluster 47 Tucanae. Globular clusters are dense balls of mostly old stars residing in the outer region or “halo” of the Milky Way and other galaxies. They’re absolutely stunning when viewed in a telescope; 47 Tucanae is bright enough to see with the naked eye and one of the best.

Combining observations from all three telescopes, the pair likely contains a black hole pulling material from a white dwarf companion star, a small, superdense, Earth-sized remnant that’s left after a sun-like star exhausts its nuclear fuel. Chandra observes objects by the X-rays they give off and discovered that X9’s X-ray brightness changes in the same way every 28 minutes. Astronomers surmise that’s the time it takes the companion star to complete an orbit around the black hole.

The time is so incredibly brief not only because the black hole is massive, but also because the dwarf orbits so close to the hole, only about 600,000 miles away or 2.5 times the distance between the Earth and moon.

A white dwarf with the mass of our sun is almost the same size as the Earth. However, since all that mass is packed into such a small object, white dwarfs are fantastically dense. A teaspoon of its matter would weigh 15 tons. When the sun runs out of nuclear fuel in about 5 billion years, it will expel its outer atmosphere of hydrogen and helium,  revealing its white dwarf “self” or core, a ball of extremely hot carbon and oxygen atoms. Oxygen has been found in the X9 system, a good sign that the black hole’s companion is a dwarf. Credit: NASA (left). Illustration: Bob King

“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,” said first author Arash Bahramian of the University of Alberta in Edmonton, Canada, and Michigan State University in East Lansing. “Luckily for this star, we don’t think it will follow this path into oblivion, but instead will stay in orbit.”

So the star itself won’t fall in, but eventually it will be whittled down to a measly planet’s worth of material or perhaps even evaporate altogether.

How did the black hole get such a close companion? One possibility is that it collided with a red giant star, causing gas from the outer regions of the star to be ejected.  The remaining core of the red giant, captured by the black hole, would evolve into a white dwarf.

Astronomers will continue to study globular clusters in hopes of finding additional black hole binary stars. In the busy environment at the heart of a globular cluster, occasional stellar mergers lead to the formation of black holes. Most get ejected from the cluster from the recoil, but a few like X9 linger for reasons unknown.