Merging stars could give newborn black holes a faster spin

A collision before collapse

Massive stars in crowded clusters can collide or merge with companions before collapsing into black holes. Those encounters add not just mass but angular momentum—the physical quantity associated with rotation. A modeling study asks how much of that rotational inheritance could survive into a newborn black hole.

In an arXiv preprint, Ishaan Satish and colleagues identify a possible route through blue supergiants: hot, massive stars that remain relatively compact compared with red supergiants. The paper was first submitted to arXiv on 6 July 2026 and revised on 6 November 2026; its arXiv record lists it as accepted by Monthly Notices of the Royal Astronomical Society. Under the researchers’ assumptions, some stellar merger products could produce black holes with substantial spins. These are simulation-based predictions, not observations establishing that this pathway operates in nature.

Connecting clusters to individual stars

The team began with 148 simulations from the Cluster Monte Carlo catalog, which models the evolution of dense star clusters. The catalog spans different cluster sizes, concentrations and chemical compositions, covering conditions relevant to the Milky Way’s globular clusters.

The simulations track direct stellar collisions and mergers caused by unstable mass transfer between companion stars. The researchers followed the resulting stars to identify which eventually became black holes. Encounters between a main-sequence star, which is still burning hydrogen in its core, and an evolved giant accounted for about 85% of these black-hole-forming interactions.

To estimate the resulting spins, the team connected the cluster calculations to stellar-evolution models made with MESA. They added models with one-tenth the Sun’s metallicity—the abundance of elements heavier than helium—and used earlier calculations at half the Sun’s metallicity for comparison. Spin estimates were assigned using nearby models in the stellar-mass grid, rather than by individually simulating every encounter.

Why staying compact matters

When a massive star expands into a red supergiant, internal processes can transfer angular momentum from its core into its extended outer layers. Losing those layers can then leave a slowly rotating black hole.

A merger can change that outcome. In the models, adding enough material to an evolved giant can suppress its later expansion, allowing it to end its life as a blue supergiant. If its collapse fails to produce an explosion that ejects most of the envelope, rotating material can fall back toward the newborn black hole. Some forms a surrounding disk, supplying angular momentum as it accretes, while some escapes in outflows.

For the lower-metallicity models, a companion mass roughly 30% or more of the giant’s mass often favored this compact outcome. The threshold was approximate, not universal; the half-solar-metallicity calculations generally required a ratio closer to 50%.

The predicted black hole spins for the compact progenitors were typically around 0.5–0.8 on the dimensionless spin scale, where zero means no rotation and one is the theoretical maximum. Models of more extended progenitors instead yielded spins below about 0.03 under the adopted collapse assumptions.

Birth conditions change the numbers

Not every stellar encounter supplied enough mass to produce this outcome. Within the baseline catalog and the mass range covered by the stellar models, the researchers identified 496 strong candidates for producing spinning black holes. Thirty of those black holes subsequently participated in mergers with other black holes.

Five additional cluster simulations explored uncertain birth conditions. These included putting all sufficiently massive stars into binaries, concentrating massive stars toward the cluster center, and removing the velocity kicks that black holes may receive when they form. The no-kick cases were deliberately optimistic limits on retention and also used a different collapse prescription.

Across the explored conditions, the authors report that up to nearly half of black holes could have progenitors with a stellar interaction history, with up to roughly one in ten experiencing an interaction they classify as significant—one involving a smaller-to-larger stellar mass ratio of about 0.1 or greater. Those upper-end figures are model-dependent, not universal predictions for clusters. Meeting this significance threshold alone does not guarantee the compact progenitor needed for substantial spin-up.

Spin can make escape easier

After two black holes merge, uneven emission of gravitational waves—ripples in spacetime—can kick the remnant through space. Spin can strengthen that recoil.

Using the estimated spins and 1,000 random orientation draws per selected system, the researchers calculated recoil distributions. For a reference cluster with an escape speed of about 66 kilometers per second, they estimated that more than 95% of the selected post-interaction merger remnants would escape. That result applies to this modeled subset, not all black hole mergers.

Escaping remnants cannot remain available for repeated mergers inside the same cluster. Stellar interactions could therefore create spinning black holes while simultaneously making one route to building larger black holes less efficient.

Important uncertainties remain

The cluster models assume no mass is lost during stellar collisions, and the stellar-evolution grid cannot confidently cover the most massive encounters. Spins and revised recoils were calculated afterward, rather than fed back into the cluster evolution. Collapse, fallback and birth kicks were also not modeled self-consistently together.

The study consequently establishes a plausible mechanism, not its measured cosmic contribution. More direct merger calculations and coupled cluster models are needed to determine how often it matters.