Hubble searches for rare black hole, finds something even stranger

Today, NASA shared a rather interesting update on the activities of the Hubble Space Telescope. Astronomers from the Institut d’Astrophysique de Paris (IAP) were using the telescope to study globular cluster NGC 6397, where they were looking for a black hole of intermediate mass. Globular systems, as NASA describes them, are “extremely dense star systems, which host very tightly packed stars”, with NGC 6397 in particular only 7,800 light years from Earth.

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

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

Further analysis of star motions – which has been collected over several years – indicated that this extra mass was actually a collection of smaller black holes and stellar mass. “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, although Vitral and Mamon did not find the intermediate mass black hole that they thought was at the center of NGC 6397, they still made a fascinating discovery, as this is the first time that a collection of stellar mass black holes is discovered in the center of a dense globular cluster like this one. Watch the embedded video above to learn more about this find, as well as other beautiful images of NGC 6397.