NASA has discovered an unprecedentedly large cluster of black holes in our nearest galactic neighbor, Andromeda.

The 26 black hole candidates were spotted with the Chandra X-ray Observatory, which made more than 150 observations spread over 13 years.

Each of the black holes was formed after a star collapsed in on itself. Now, as they suck in material from other stars that orbit or pass nearby, the material gives off X-rays as it is consumed. These X-rays are what Chandra has spotted.

To filter Andromeda's black holes from other X-ray sources—such as neutron stars or much larger black holes that are farther away than Andromeda—the astrophysicists had to look at attributes like the brightness, variability, and color of their emissions.

These black holes are relatively easy to spot because they have companion stars that provide the material that emits X-rays. Without these companions, there would be no X-rays. "While we are excited to find so many black holes in Andromeda, we think it's just the tip of the iceberg," said Robin Barnard of the Harvard-Smithsonian Centre for Astrophysics, lead author of the study published in The Astrophysical Journal. "Most black holes won't have close companions and will be invisible to us."

The 26 black holes join a previous group of nine found using Chandra data. They are close to Andromeda's "central bulge," the spherical network of huge, old stars that is at the heart of most galaxies.

Andromeda's bulge is larger than the Milky Way's, and the larger number of stars means there is also a corresponding greater number of black holes for us to spot.