Hubble goes looking for a rare black hole, finds something even weirder

Today, NASA shared a rather interesting update about the goings-on of the Hubble Space Telescope. Astronomers with the Paris Institute of Astrophysics (IAP) were using the telescope to study globular cluster NGC 6397, where they were looking for an intermediate-mass black hole. Globular systems, as NASA describes them, are "extremely dense stellar systems, which host stars that are closely packed together," with NGC 6397 in particular only about 7,800 light-years away from Earth.

The astronomers who were researching NGC 6397 – Eduardo Vitral and Gary Mamon, both of the IAP – were looking for an intermediate-mass black hole in this globular cluster. As the name suggests, intermediate-mass black holes are a theorized third type of black hole that are sized between supermassive black holes that reside in the centers of galaxies and the stellar-mass black holes that are the products of stellar collapse. As NASA notes in a blog post today, there have only been a few intermediate-mass black hole candidates that have been identified.

So, if Vitral and Mamon were able to find an intermediate-mass black hole in NGC 6397, that would be a huge discovery. What they found there was something else. "We found very strong evidence for an invisible mass in the dense core of the globular cluster, but we were surprised to find that this extra mass is not 'point-like' (that would be expected for a solitary massive black hole) but extended to a few percent of the size of the cluster," Vitral said.

Further analysis of the stars' movements – which has been collected over several years – indicated that this extra mass was actually a collection of smaller, stellar-mass black holes. "We used the theory of stellar evolution to conclude that most of the extra mass we found was in the form of black holes," Mamon said.

So, while Vitral and Mamon didn't find the intermediate-mass black hole they thought was at the center of NGC 6397, they did still make a fascinating discovery, as this is the first time a collection of stellar-mass black holes has been discovered at the center of a dense globular cluster like this. Check out the video embedded above for more on this discovery, as well as some more beautiful images of NGC 6397.