Astronomers have discovered the first known stellar-mass black hole hiding inside the massive star cluster Omega Centauri, solving part of a mystery that has puzzled scientists for decades.
Omega Centauri, located about 18,000 light-years from Earth, is the largest globular star cluster in the Milky Way.
It contains around 10 million stars packed closely together.
Scientists have long believed that the cluster should also contain thousands of black holes left behind after massive stars exploded as supernovas. However, despite years of searching, almost none had been found.
Now, researchers have finally detected the first of these missing black holes by combining more than 20 years of observations from NASA’s Hubble Space Telescope with new data from the James Webb Space Telescope.
Their findings have been published in The Astrophysical Journal Letters.
Previous studies suggested that Omega Centauri contains a giant intermediate-mass black hole at its center.
Computer models also predicted that around 10,000 smaller stellar-mass black holes should be scattered throughout the cluster.
Yet traditional search methods, which look for radio waves, X-rays, or changes in a star’s speed caused by an unseen companion, failed to uncover this hidden population.
Instead, the research team used a different technique called astrometry. This method measures the tiny movements of stars across the sky with extraordinary precision.
By carefully tracking the motion of one star over two decades, the scientists found that it was orbiting an invisible object too massive to be anything other than a black hole.
The newly discovered black hole, named oMEGACat BH-2, has a mass about 4.5 times that of the sun. While this clearly places it in the black hole category, it is surprisingly lighter than scientists expected for a black hole formed in Omega Centauri’s metal-poor environment.
The discovery also overturns an earlier conclusion that the unseen object was a neutron star. By combining Hubble observations collected between 2002 and 2023 with Webb’s highly accurate infrared measurements, the researchers were able to calculate the object’s mass much more precisely. At 4.46 times the sun’s mass, it is simply too heavy to be a neutron star.
The black hole also forms one of the most unusual binary systems ever discovered. Its companion star takes about 94 years to complete a single orbit, making it the longest-period black hole binary known. Such a wide orbit suggests the two objects were not born together. Instead, they likely became partners after gravitational interactions inside the crowded star cluster brought them together.
Researchers estimate that this pairing will survive for less than one billion years before close encounters with other stars eventually separate them. Although that sounds like a long time, it is brief compared with Omega Centauri’s estimated age of about 12 billion years.
The discovery provides valuable clues about how black holes form and interact inside dense star clusters. Understanding these systems is also important because similar black hole pairs can eventually collide, producing the powerful gravitational waves that observatories on Earth are now detecting.
The researchers believe this is only the beginning. By continuing to combine the strengths of Hubble and Webb, and with the future launch of NASA’s Nancy Grace Roman Space Telescope, astronomers hope to uncover many more hidden black holes lurking inside globular star clusters, helping to complete the missing picture of these mysterious cosmic objects.
Source: KSR.
