These monster black holes did not form the usual way-their history of violence is written into spacetime ripples

Key Points:

• A new study led by Cardiff University reveals that the most massive black holes detected through gravitational waves are not formed directly from collapsing stars but grow via repeated violent mergers in dense star clusters.
• Analysis of 153 black hole merger events from the LIGO–Virgo–KAGRA catalog identified two distinct populations: lower-mass black holes from ordinary stellar collapse and higher-mass black holes with spin patterns indicative of hierarchical mergers in star clusters.
• The research provides strong evidence for a "mass gap" around 45 solar masses, where very massive stars explode without forming black holes, supporting long-standing theoretical predictions about pair-instability supernovae.
• Observed spin distributions above this mass gap suggest that the heaviest black holes have undergone multiple mergers, highlighting the role of dense cluster dynamics over standard stellar evolution alone.
• The findings also offer potential insights into nuclear reactions during helium burning in massive stars, indicating that future gravitational-wave observations could advance understanding of stellar interiors and nuclear physics.