Star spotted in 30-minute ‘orbital dance’ with black hole

14 Mar 2017

Illustration of a star found in the closest orbit known around a black hole, named 47 Tucanae. Image: NASA/CXC/University of Alberta/A.Bahramian et al (x-ray); NASA/CXC/M Weiss (illustration)

The closest star to a black hole ever spotted orbits it about twice every hour, at a distance of around 2.5 times that of Earth to the moon.

Space is a vast expanse, dotted with planets, rocks, ice, gas, mysterious dark matter and, in a small number areas, the potential for life.

There are also stars, many of which are visible to the naked eye, and black holes, the true bad guys of a significant portion of science-fiction space travel stories.

Eating up all around them, black holes are the spoilsports of space but, thanks to a new discovery, we’re finding stars up close and personal with the hungry beasts. The latest discovery of a star within a mere 30-minute orbit of a black hole, however, faces an uncertain future.

This star – part of a binary system including a white dwarf – points to what could well be the tightest orbital dance ever witnessed for a likely black hole and a companion star, according to researchers.

Discovered long ago, it was only in the past couple of years that astronomers realised the white dwarf – almost out of nuclear fuel – was getting ripped apart by a nearby black hole.

The duo are in the globular cluster of 47 Tucanae, a dense cluster of stars in our galaxy about 14,800 light years away from Earth.

“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 Arash Bahramian, lead author of the study with the University of Alberta and Michigan State University (MSU).

“Luckily for this [other] star, we don’t think it will follow this path into oblivion, but instead will stay in orbit.”

It’s not entirely clear how this star, and its white dwarf neighbour, got so close to the black hole – though one suggestion is an explosive coming together of the latter and a red giant star.

The gas from the outer regions of the star could then have subsequently been ejected from the binary. The remaining core of the red giant would form into a white dwarf, which becomes a binary companion to the black hole.

The orbit of the binary would then have shrunk as gravitational waves were emitted, until the black hole started pulling material from the white dwarf.

“For a long time, astronomers thought that black holes were rare or totally absent in globular star clusters,” said Jay Strader, MSU astronomer and co-author of the paper.

“This discovery is additional evidence that, rather than being one of the worst places to look for black holes, globular clusters might be one of the best.”

Gordon Hunt was a journalist with Silicon Republic