This decades-old mystery behind our galaxy’s largest star cluster may finally be solved

SURREY, England — Scientists may have finally solved a decades-old mystery behind why the stars in the Omega Centauri, the largest star cluster in the Milky Way, move so unusually fast. Turns out, nearly ten million stars haven’t found a way to suddenly speed up. Rather than being influenced by a single massive black hole as previously thought, a study by a collaboration of European researchers suggests the behavior of the stars is the result of combined gravitational influence of numerous smaller black holes and other stellar remnants found at its center.

Omega Centauri is a massive star cluster that’s long been a topic of heated debate among astronomers over the cause of the unexpectedly high stellar velocities of stars moving toward its center. Until a recent study published in Astronomy and Astrophysics, they did not know whether the movement was because of an “intermediate mass” black hole weighing a hundred thousand times the mass of the Sun or a clump of “stellar mass” black holes and other compact objects working in concert.

Stellar-mass black holes are smaller, about a few times the Sun’s mass, and are made after the death of massive stars. Astronomers were excited to discover an intermediate-mass black hole in a study published earlier in 2024 because they could explain how stellar-mass black holes become supermassive.

Supermassive black holes are usually at the center of large galaxies and weigh millions to billions of times the sun’s mass. However, whether supermassive black holes start as supermassive or begin their lives first as smaller stellar-mass black holes is still unknown. Finding an intermediate-mass black hole could help bridge this gap and solve the cosmic mystery.

However, this was not the case for Omega Centauri, which was found to contain a cluster of stellar-mass black holes in this new research.

“The hunt for elusive intermediate-mass black holes continues. There could still be one at the center of Omega Centauri, but our work suggests that it must be less than about six thousand times the mass of the Sun and live alongside a cluster of stellar mass black holes,” says Justin Read, a researcher at the University of Surrey in England and study coauthor, in a statement. “There is, however, every chance of us finding one soon. More and more pulsar accelerations are coming, allowing us to peer into the centers of dense star clusters and hunt for black holes more precisely than ever before.” 

The researchers examined the velocities of stars at the center of the Omega Centauri and added a new data point. The team also looked at the acceleration of pulsars. Like black holes, pulsars are made from dying stars and weigh up to twice the sun’s mass. They are 20 kilometers across and spin up to 700 times a second. Pulsars release radio waves along their spin axis, similar to a spinning top. The radio beam goes past Earth like a lighthouse, making it possible for astronomers to identify them.

Pulsars act like natural clocks with accuracy akin to Earth’s atomic clocks. Astronomers can measure how pulsars are accelerating by measuring the change in their spin rate and discover crucial new information about the gravitational field at Omega Centauri’s center.

“The formation of pulsars is also an active field of study because a large number of them have recently been detected. Omega Centauri is an ideal environment to study models of their formation, which we have been able to do for the first time in our analysis,” explains Andrés Bañares Hernández, a researcher at the Instituto de Astrofísica de Canarias in Spain and lead study author.

Adding the new acceleration measurements with stellar velocities allowed the scientists to more clearly see whether Omega Centauri had an intermediate-mass black hole or a cluster of smaller ones. The question is — will this latest explanation be agreed upon universally? Only time will tell.

“By studying Omega Centauri—a remnant of a dwarf galaxy—we have been able to refine our methods and take a step forward in understanding whether such black holes exist and what role they might play in the evolution of star clusters and galaxies. This work helps resolve a two-decade-long debate and opens new doors for future exploration,” Hernández adds.