The Forge of Black Holes: Stellar Mergers as a Source of Spin

Original: "Formation of rotating supergiants via stellar mergers in dense clusters: Implications for black hole natal spins"

· Ishaan Satish, Kyle Kremer, Fulya Kıroğlu, Daichi Tsuna, Shahed Shayan Aran

arXiv:2607.05495v1 · 2026-07-06 · CC BY · ⏱ 1 min · High Energy Galaxies

In densely populated star clusters, mergers of massive stars before collapse impart rapid spin to newborn black holes, explaining data from gravitational-wave observatories.

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In tight star clusters, an invisible forge is at work: stars collide, merge, and their dance spins future black holes to dizzying speeds. One merger—and the featureless hole gains a spin that can fling it from the cluster. These are the objects that gravitational-wave detectors catch. Perhaps this is the key to the mystery of long gamma-ray bursts.

🎯 In the standard picture, isolated massive stars lose almost all their angular momentum and produce black holes with negligible spin. However, a single close merger in a dense cluster can spin up the future black hole nearly to the limit. Remarkably, even a modest increase in the binary fraction among massive stars boosts the number of such rapidly rotating objects by an order of magnitude.

🎬 Such black holes resemble Gargantua from 'Interstellar': their rapid spin is set at birth.

v_{\rm esc} \approx 66 \left( \frac{M_{\rm cl}}{10^6 M_\odot} \right)^{1/2} \left( \frac{r_h}{4 \text{ pc}} \right)^{-1/2} \text{ km/s}

allows estimating whether the cluster can retain the black hole merger remnant after asymmetric gravitational wave emission

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The Forge of Black Holes: Stellar Mergers as a Source of Spin