We Just Found the Closest Black Hole to Our Planet, 18,000 Light Years Away. That’s Not What Makes It Interesting

Its mass is estimated to be 8,200 times that of the Sun, making it a very rare object.

Finding black holes is no easy task. With the exception of more active objects with luminous accretion disks, these objects tend to absorb light rather than emit it. However, their wild gravitational fields often help us detect them in their surroundings.

The closest known. Now, an international team of astronomers has discovered the closest known black hole. The newly discovered hole is “only” 18,000 light years from our solar system.

Intermediate mass. But what really highlights the relevance of this discovery is not the hole’s location, but its mass. The object has a mass 8,200 times greater than our star, the Sun. That means it is an intermediate black hole.

Known black holes are usually divided into two categories: supermassive ones, like the one at the center of our galaxy, with a mass millions of times greater than our sun; and stellar black holes, with a maximum mass of about 100 suns. Between them, there’s a strange void. At least until now.

At a distance of 18,000 light years. The black hole is located about 18,000 light-years away in a region of our galaxy known as Omega Centauri. By comparison, the supermassive black hole at the center of the galaxy is 27,000 light-years away. Omega Centauri is a globular cluster of about 10 million stars visible from the southern hemisphere.

20 years of observation. To find the point where this black hole might be hiding, the team turned to two decades of Hubble observations of the cluster. Two decades, during which they managed to collect 500 images of the cluster and its stars. Curiously, these were mostly images taken to calibrate the space telescope’s instruments.

But thanks to this, the team was able to analyze the movements of the stars in the cluster, focusing on the fast-moving stars. Seven in particular. These stars changed their positions significantly over time. For the team, these movements are the final proof of the existence of a black hole in this region.

What’s more, these stars also allowed the team to estimate that the mass causing these gravitational tugs is 8,200 times that of our Sun. Details of the study were recently published in the journal Nature.

Frozen in time. Astronomers have been wondering for years where these mid-sized black holes are. The problem is that, according to current cosmological models, these black holes should be the remnants of an older universe with smaller galaxies and galactic centers that have not yet accumulated enough matter to be able to host a supermassive black hole.

This discovery and this fact prompted the team to propose a new hypothesis about the origin of the Omega Centauri cluster. According to the hypothesis, these stars should be the remains of an ancient galaxy with its own intermediate black hole at the center. This galaxy would have been absorbed by the Milky Way, and its central black hole would have been isolated, with no more matter to absorb to grow.

In Hatake | Black holes are interesting. And those that form from the collision of two of them are even more so.

Images | ESA/Hubble and NASA, M. Heberle (MPIA)