Hubble Goes Hunting For A Black Hole Missing Link In This Mesmerizing Star Cluster

Astronomers believe they have detected some of the best evidence yet of a rare intermediate-sized black hole lurking at the heart of a globular star cluster. The discovery of the mid-sized black hole located 6,000 light-years from Earth was made using NASA's Hubble Space Telescope.

Typically, black holes come in two sizes, small or extremely large. NASA estimates that our galaxy has 100 million small black holes which were created by exploded stars. Supermassive black holes weighing billions of times that of our own Sun are also found throughout the centers of galaxies. What is harder to find, however, are those black holes that fall in the middle of the other two predominant types.

In order to find these seemingly clandestine mid-sized black holes, astronomers use a variety of techniques. To date, Hubble has been utilized to detect two of the best candidates, 3XXM J215022 40055106 and HLX-1, in dense star clusters on the outskirts of other galaxies. NASA's Chandra X-ray observatory has also been used to find many other possible candidates.


While most of the other possible candidates have been located a great distance away from Earth, a team of astronomers has been able to use Hubble to find good evidence of one closer by in the globular star cluster Messier 4 (M4). The candidate mid-sized black hole has a mass of around 800 solar masses. While the black hole is not able to be seen, astronomers are able to estimate its size by studying the motion of stars that are caught in its gravitational field.

"We have good confidence that we have a very tiny region with a lot of concentrated mass. It's about three times smaller than the densest dark mass that we had found before in other globular clusters," explained Eduardo Vitral of the Space Telescope Science Institute in Baltimore, Maryland. "The region is more compact than what we can reproduce with numerical simulations when we take into account a collection of black holes, neutron stars, and white dwarfs segregated at the cluster's center. They are not able to form such a compact concentration of mass."

The team does caution against getting too excited just yet, however. He says that while the team is unable to completely affirm an intermediate-sized black hole is the central point of gravity, it could also be a stellar mechanism they do not know about with current physics.

Vitral remarked about the possibility of finding a new intermediate-sized black hole closer to home, "You can't do this kind of science without Hubble."