Astronomers have pinpointed the likely progenitor of binary black hole mergers – massive stars stripped of their outer layers by powerful stellar companions in a process known as chemically homogeneous evolution. The revelation, announced September 2, 2025, sheds light on the formation of these cosmic collisions, events that ripple through spacetime as gravitational waves.
For years, the origins of binary black holes, systems where two black holes orbit each other and eventually merge, have remained a puzzle. These mergers are detected by observatories like LIGO and Virgo, but understanding how these pairs form is crucial too refining our models of stellar evolution and the universe’s black hole population. The new research suggests that these black holes aren’t typically born already paired,but rather are created through dynamic interactions within dense star clusters,specifically involving stars that have undergone chemically homogeneous evolution.
A team led by Dr. Giacomo Fragione at Northwestern University used detailed computer simulations to demonstrate that stars in close binary systems can mix their interiors so thoroughly that they evolve uniformly. This process, occurring in metal-poor environments, results in stars that are less extended and more easily stripped of their outer layers by their companion.The stripped material doesn’t escape; it falls back onto the remaining core, accelerating its collapse into a black hole. This scenario efficiently produces black holes with the masses and spins observed in gravitational wave events.
“This is a key piece of the puzzle,” explains Fragione. “We’ve shown that chemically homogeneous evolution can naturally explain the observed properties of binary black holes, notably their masses and spins.” The simulations reveal that this process favors the formation of black holes in the 30 to 60 solar mass range, consistent with the majority of mergers detected so far.
the research, published in The Astrophysical Journal Letters, analyzed over 3,000 binary star systems. It indicates that the majority of binary black hole mergers originate from these dynamically formed systems, rather than isolated binary stars.Future gravitational wave observations,particularly with enhanced detectors,will provide further tests of this model and help refine our understanding of black hole formation. The findings have implications for estimating the number of black holes in the universe and predicting the rate of future merger events.