Centre of Milky Way home to 10,000 black holes, study suggests

At the centre of our galaxy lies a supermassive black hole, about 4 million times the mass of our sun. But it's not alone.

Findings support decades-long theory of existence of numerous black holes near galactic centre

The 3.6-metre European Southern Observatory telescope at La Silla is shadowed by the galactic bulge of the Milky Way. At the centre lies a supermassive black hole, Sagittarius A*, and new research has found dozens of surrounding black holes suggesting upwards of 10,000 more could be present. (ESO/S. Brunier)

At the centre of our galaxy lies a supermassive black hole, about four million times the mass of our sun. New research has found that it's not alone.

For some time, the theory has been that the supermassive black holes at the centre of most galaxies, including our own Milky Way, are accompanied by smaller black holes.

In a study published in Nature on Wednesday, a team of astronomers discovered 12 of these outliers, within three light-years of our supermassive black hole, called Sagittarius A*. The findings suggest that there are upwards of 10,000 surrounding the galactic centre.

But since black holes are black, with nothing — including light — able to escape their mighty grasp, how does one exactly go about finding them?

You can't see an inactive black hole, one that isn't consuming anything. But you can see evidence of a black hole's meal.

Search is on

As black holes — the remnants of dead stars — gobble up matter, a particular type of X-ray is burped back out into the cosmos. These types of black holes are referred to as black hole binaries.

In an effort to spot some of these black hole binaries, the team analyzed data that had already been collected by the Chandra X-Ray Observatory in orbit around Earth.

A Chandra image of the galactic centre, is visible with the blue circles indicating the newly discovered black holes. Surrounding red dots are white dwarf stars.

"They're devilishly hard to find," Chuck Hailey, lead author of the paper and astrophysicist with Columbia University told CBC News.

"But every now and then you'll find one of these black holes catch a star, and matter will come out of this star in a disk around the black hole … so we can detect the tip of the iceberg."

They found a dozen of these black hole binaries, and because it's rare for a black hole to grab a star, the scientists can infer that many others are out there that didn't grab a star and are thus invisible.

The astronomers were also careful not to be fooled by the many other intriguing X-ray-emitting objects at the galactic centre. Black holes consuming a dying star emit low-energy X-rays compared to other stars or objects, so they were able to isolate them omitting other, high-energy X-rays.

Black holes are one of the most fascinating objects in our universe. They form after the spectacular death of a massive star — about 10 times that of our own sun — a supernova explosion that can outshine the star's host galaxy.

This artist's rendering shows Sagittarius A* surrounded by a cloud of dust and gas, within which are 12 small black holes. The takeout shows the 12 black holes scientists spotted are actually black hole binaries, with the gas of a nearby star being transferred into the disk around the black hole as it moves toward the centre. (Columbia University)

As gravity takes over and eventually wins, it forms a singularity, a single point in space where gravity is so strong that no object — not even light — can escape its grasp once it crosses its boundary.

Supermassive black holes are believed to form at the same time a galaxy forms, though astronomers aren't certain of the exact process. It's believed that these types of black holes lie at the centre of most galaxies.

We really think we basically understand how things are happening at the centre of the galaxy.- Chuck Hailey, Columbia University

Stars form as gas and dust succumb to gravity and fusion is ignited. And it turns out the galactic centre is surrounded by a halo of gas and dust, a perfect breeding ground for massive stars needed to create black holes. These black holes eventually are believed to congregate around the centre.

"Some of these black holes were born billions of years ago," Hailey said. "Some of them were formed comparatively recently. So they kind of just collect there, it's sort of like a junkyard … they can't escape the pull from the supermassive black hole so they just kind of sit there."

Gravitational waves

What Hailey finds particularly exciting is that the new findings could also aid in more discoveries and the study of gravitational waves, first seen in 2015. Among other ways, these waves of gravity can be created by merging black holes.

Our galaxy is considered fairly normal, which means that gravitational wave theorists can make predictions on how many gravitational wave events might happen in other galaxies. And, Hailey said, perhaps they will find one at the centre of a distant galaxy.

Overall, he's excited by the finding, particularly that it supports the decades-long prediction of these black holes. And he hopes that other astronomers will use publicly available data to further seek out these somewhat elusive mysteries of the universe.

"For me, you can kind of breathe a sigh of relief, that we really think we basically understand how things are happening at the centre of the galaxy."


Nicole Mortillaro

Senior Reporter, Science

Nicole has an avid interest in all things science. As an amateur astronomer, Nicole can be found looking up at the night sky appreciating the marvels of our universe. She is the editor of the Journal of the Royal Astronomical Society of Canada and the author of several books.