How giant black holes grow in busy star clusters

Researchers have identified two distinct populations of black holes using gravitational-wave data, revealing that while smaller black holes come from collapsing stars, the largest ones are “second-generation” giants formed through repeated collisions in crowded star clusters, solving the mystery of how black holes exist in the “forbidden” mass gap

A major study led by Cardiff University, published in Nature Astronomy, has revealed that the universe’s most massive black holes aren’t just born, they are built.

By analysing the latest gravitational-wave data (GWTC-4), researchers have identified two distinct “species” of black holes, proving that the largest ones are second-generation monsters born from repeated, violent collisions.

Two distinct populations

The team analysed 153 black hole mergers detected by the LIGO–Virgo–KAGRA network. Their findings show that black holes fall into two very different categories based on how they were made:

1. The “slow” population:

   • Lower-mass black holes that spin slowly. These are the direct “corpses” of ordinary massive stars that collapsed at the end of their lives.

2. The “violent” population:

   • High-mass black holes with rapid, randomly oriented spins. These are second-generation objects. They formed when two black holes merged, stayed in the area, and then merged again with a third or fourth partner.

Busy star clusters: The cosmic foundries

This hierarchical growth occurs only in “busy” environments such as globular clusters. In these regions, stars and black holes are packed a million times more tightly than in our solar neighbourhood.

• Gravitational traps:

  • Because the environment is so dense, a black hole formed from a merger doesn’t always get “kicked” out into deep space. Instead, it stays in the cluster’s core, where it is likely to find another partner for a second round of collision.

• Spin signatures:

  • The “random” direction of the spins in these heavy black holes is a dead giveaway; it suggests they were brought together by the chaotic dynamics of a cluster rather than being born as twin stars.

Solving the “forbidden” mass gap

The study provides the strongest evidence yet for the pair-instability mass gap. According to stellar physics, there is a “forbidden” range (starting around 45 times the mass of our Sun) where stars should explode so violently that they leave nothing behind, no black hole at all.

The researchers found that while stars can’t directly collapse into black holes in this mass range, the gravitational-wave detectors are seeing black holes there anyway. The Cardiff team argues these “forbidden” black holes are the result of cluster dynamics: they didn’t come from a single star, but from the merger of two smaller black holes that each sat safely below the 45-solar-mass limit.

A new tool for nuclear physics

This discovery is also helping scientists look inside stars.

The exact mass where the “gap” begins depends on specific nuclear reactions (specifically, helium burning).

By pinpointing where the black hole population shifts from stellar-born to cluster-built, astronomers can now test the laws of nuclear physics using the ripples in spacetime.