The biggest black holes in the Universe may be built through chains of violent mergers deep inside crowded star clusters.
The largest black holes ever detected through gravitational waves may not have formed directly from collapsing stars, according to new research led by Cardiff University. Instead, scientists say these enormous objects were likely built through repeated black hole collisions inside extremely crowded star clusters.
The international team studied version 4.0 of LIGO–Virgo–KAGRA’s Gravitational-Wave Transient Catalog (GWTC4), which includes 153 highly confident black hole merger detections.
Researchers investigated whether the heaviest black holes in the catalog could be “second-generation” black holes. These objects would form after earlier black holes merged together, then later merged again inside dense star clusters, where stars can be packed up to a million times more tightly than in the region surrounding our Sun.
The findings, published in Nature Astronomy, suggest the most massive black holes seen through gravitational waves belong to a distinct population shaped by repeated mergers rather than ordinary stellar collapse.
Gravitational Waves Reveal How Giant Black Holes Grow
“Gravitational-wave astronomy is now doing more than counting black hole mergers,” explains lead author Dr. Fabio Antonini from Cardiff University’s School of Physics and Astronomy.
“It is starting to reveal how black holes grow, where they grow, and what that tells us about the lives and deaths of massive stars. This is exciting because we can use the information to test our understanding of how stars and clusters evolve in the Universe.”
By studying the gravitational-wave signals, the researchers identified two separate groups of black holes:
- a lower-mass population consistent with ordinary stellar collapse
- a higher-mass population whose spins appear exactly like those expected from hierarchical mergers in dense star clusters
Scientists say the spin patterns of the heavier black holes provide especially strong evidence that they formed through repeated collisions.
“What surprised us most was how clearly the high-mass black holes stand out as a separate population,” recalls co-author Dr. Isobel Romero-Shaw, Ernest Rutherford Fellow at Cardiff University.
“Unlike the lower-mass systems we analyzed, which were generally slowly-spinning, the higher-mass systems are consistent with having more rapid spins, oriented in seemingly random directions. This is the exact signature you would expect if black holes were repeatedly merging in dense star clusters.
“That makes the cluster origin much more compelling than it was with earlier catalogues.”
Evidence Strengthens for the Black Hole “Mass Gap”
The study also offers the strongest evidence so far for a mysterious “mass gap” predicted by astrophysicists. According to this long-standing theory, extremely massive stars should explode violently and destroy themselves before they can collapse into black holes.
As a result, there should be a forbidden range of black hole masses that stars cannot produce directly.
The researchers identified this transition among black holes with masses around 45 times greater than the Sun.
Dr. Antonini said: “In our study, we find evidence for the long-predicted pair-instability mass gap — a range of masses where stars are not expected to leave behind black holes at all. Gravitational-wave detectors have successfully found black holes that appear to sit in or near that gap, which we identify at around 45 solar masses.
“So, the key question now is, are these black holes telling us that our models of stellar evolution are wrong, or are they being made in another way?
“The biggest black holes in the current sample seem to be telling us about cluster dynamics, not just stellar evolution.
“Above about 45 solar masses, the spin distribution changes in a way that is hard to explain with normal stellar binaries alone, but is naturally explained if these black holes have already been through earlier mergers in dense clusters.”
Black Hole Discoveries Could Help Study Stellar Nuclear Reactions
The team also used the transition near the mass gap to investigate a key nuclear reaction involved in helium burning inside massive stars.
Researchers say future gravitational-wave observations may provide valuable insights into nuclear physics because the pair-instability mass limit depends on reactions occurring deep inside stellar cores.
“In the future, gravitational-wave data may help scientists study nuclear physics, because the mass limit set by pair instability depends on the nuclear reactions taking place in the cores of massive stars,” added co-author Dr. Fani Dosopoulou, a research associate at Cardiff University.
Reference: “Gravitational-wave constraints on the pair-instability mass gap and nuclear burning in massive stars” by Fabio Antonini, Isobel M. Romero-Shaw, Thomas Callister, Fani Dosopoulou, Debatri Chattopadhyay, Yonadav Barry Ginat, Mark Gieles and Michela Mapelli, 7 May 2026, Nature Astronomy.
DOI: 10.1038/s41550-026-02847-0
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