Formation of black hole stars via star--black hole collisions

Yanlong Shi, Qingru Hu, Zhenghao Xu, Douglas N. C. Lin, Norman Murray

First listed 2026-08-31 | Last updated 2026-08-27

Abstract

In dense stellar environments such as globular clusters and active galactic nucleus (AGN) disks, stellar-mass black holes (sBHs) may frequently collide with massive stars. We investigate this process using semi-analytic models, three-dimensional hydrodynamical simulations, and one-dimensional stellar evolution calculations, focusing on collisions between sBHs and a $100\,M_\odot$ main-sequence star. We find that gas drag retains the BH within the stellar envelope unless the impact velocity exceeds $\sim2\sqrt{G(M_\star+M_\bullet)/R_\star}$. The post-collision outcome depends primarily on the BH-to-star mass ratio. For $M_\bullet\gtrsim30\,M_\odot$, the retained envelope is either quasi-spherical or disc-like, but remains dynamically unstable because of shock heating. In contrast, for $M_\bullet\lesssim10\,M_\odot$, the collision forms a ``black hole star'' (BH*): a quasi-hydrostatic, extended stellar envelope surrounding the embedded BH. These results agree with our analytic prediction that BH* formation necessarily requires $M_\bullet\lesssim0.2\,M_\star$. Follow-up \texttt{MESA} calculations further show that, for these low-mass BHs, the shock-heated remnant thermally relaxes without triggering runaway expansion. We discuss several astrophysical implications of BH*s, including their evolution, the possibility of gravitational-wave events from BH binaries assembled within a stellar envelope, and repeated star--sBH collisions as a pathway for rapid BH growth in dense stellar systems. This mechanism may contribute to the formation of massive BHs in high-redshift nuclear star clusters and may be relevant to the origin of the ``little red dots'' discovered by JWST.

Short digest

Shi et al. model collisions between stellar-mass black holes and a 100-solar-mass main-sequence star using drag-based orbital integrations, 3D GIZMO hydrodynamics, and follow-up MESA evolution. Gas drag generally traps the intruding black hole unless its impact speed exceeds roughly twice the mutual escape speed, but the remnant is controlled chiefly by mass ratio: black holes below about 10 solar masses produce quasi-hydrostatic, extended black-hole stars, whereas those above about 30 solar masses leave shock-heated, dynamically unstable spherical or disk-like envelopes. The simulations and analytic argument converge on a BH-star formation criterion of black-hole mass below about 0.2 times the stellar mass, and MESA finds that the low-mass cases thermally relax rather than entering runaway expansion. This supplies a concrete collision channel for embedded-envelope black-hole growth in dense clusters and AGN disks, with a possible, still speculative connection to JWST little red dots.

Key figures to inspect

• Figure 2. This is the paper's clearest formation map: it identifies the impact-velocity and impact-parameter regimes in which gas drag retains the stellar-mass black hole, separating geometric encounters from successful embedded-BH outcomes.
• Figure 3. The imposed-inspiral-heating experiment directly supports the thermal-survival claim by tracking injected energy, stellar expansion, core conditions, and nuclear luminosity during and after the shock-heating interval.
• Figure 5. This diagnostic links collision parameters and black-hole mass to the final classification of the remnant as a quasi-spherical BH star, a disk-envelope system, or a bare black hole, making the mass-ratio dependence visually explicit.
• Figure 6. The envelope mass, size, and rotational-support evolution provide the central structural comparison across black-hole masses and distinguish the stable extended BH-star branch from disk-like remnants.
• Figure 7. The final radial density and mixed hydrogen-abundance profiles show the internal structure left by the collision, connecting the hydrodynamic remnant to the stellar-evolution interpretation of a relaxed black-hole star.