Located approximately 6,000 light-years away from Earth in the core of the Messier 4 star cluster, this black hole is an incredibly dense region of space with a mass equivalent to 800 Suns, causing nearby stars to orbit around it like “bees buzzing around a hive.”
Eduardo Vitral, an astronomer at the Space Telescope Science Institute in Maryland, USA, said, “It’s too small for us to confidently say it’s something other than a standalone black hole. Instead, it could be a star cluster phenomenon that we do not yet understand with our current knowledge of physics.”
Black holes are formed from the collapse of massive stars and grow by “swallowing” gas, dust, stars, and other black holes in their vicinity. Currently, known black holes tend to fall into one of two categories: stellar-mass black holes, with masses ranging from a few to dozens of times that of the Sun, and supermassive black holes, often referred to as “cosmic monsters,” with masses ranging from a few million to 50 billion times that of the Sun.
The globular star cluster Messier 4 harbors a medium-sized black hole at its center (Image: ESA/Hubble & NASA).
Theoretical intermediate-mass black holes, which have masses between 100 and 100,000 times that of the Sun, are the most elusive black holes in the universe. Despite some indications, no intermediate-mass black hole has been conclusively confirmed to exist.
This poses a puzzle for astronomers. If black holes grow from stellar sizes to supermassive sizes by voraciously consuming their surroundings, why don’t we see them in the intermediate stage, when they are still growing? Is this a significant limitation of human understanding of the universe, preventing us from detecting them?
To search for signs of a hidden intermediate-mass black hole, researchers directed the Hubble Space Telescope towards the Messier 4 star cluster. This cluster is a globular cluster consisting of tens of thousands to millions of densely packed stars, many of which are the oldest stars ever formed in our universe.
There are 180 such globular clusters scattered throughout the Milky Way, and due to their high mass density in the central regions, they provide an ideal environment for “young” black holes to continue growing.
Messier 4 is the closest globular cluster to Earth. With the help of the Hubble and Gaia telescopes, researchers collected data over 12 years to accurately determine its stars’ properties and study their motion around the central region. They discovered that the stars are moving around something massive that cannot be directly detected at the cluster’s center.
Vitral, the astronomer, mentioned a small region that is extremely dense, about three times denser than the previously densest known objects in other globular clusters. The small region that the researchers found is even smaller than expected if its tremendous gravitational force is generated by other densely packed dead stars, such as neutron stars and white dwarfs. It could contain around 40 stellar-mass black holes in a space equal to 1/10 of a light-year, causing the stars to orbit around them in an incredibly dense manner.
This leads to their merging and/or ejection in a cosmic “pinball game” among the stars.
To confirm the discovery of a medium-sized black hole and avoid accidentally uncovering new physics phenomena, researchers emphasize the need for continued observations with the Hubble Space Telescope along with the upcoming James Webb Space Telescope.
“Science rarely discovers something new in a flash,” said Timo Prusti, a scientist at the Gaia observatory. “This could be another step towards confirming the existence of intermediate-mass black holes.”