A Stellar Blender: Black Holes May Be Pulverizing Stars at the Center of the Milky Way
In the heart of our Galaxy lies Sgr A⋆, a supermassive black hole weighing about four million times the mass of the Sun. Surrounding it is a swarm of stars known as the S-cluster, and potentially, as J. Haas and collaborators argue in their recent paper, a hidden population of stellar-mass black holes that might be drastically reshaping their environment. This “star grinder,” as the authors call it, could be the reason why some types of stars are mysteriously absent near the Galactic centre.
Where Do the Black Holes Come From?
The paper begins by exploring the possible origins of this black hole cluster. Past theoretical and observational work has long suggested the presence of many stellar-mass black holes (those with masses around 10 times that of the Sun) within the central parsec (about 3.26 light-years) of the Milky Way. Haas et al. discuss several formation pathways, including relics from the early formation of Sgr A⋆, dynamical migration from the larger nuclear star cluster, and most importantly, the in situ formation of black holes through repeated starbursts in gaseous accretion discs. These bursts produce many massive stars, which eventually collapse into black holes. Over time, this process could concentrate up to hundreds of thousands of stellar-mass black holes near the center.
The Star Grinder: Collisions in Action
To estimate the number of black holes in this region, the authors model a “grinder” effect, where stars are destroyed by collisions with the surrounding black holes. Using basic physical principles, they derive how often such collisions would occur. It turns out that if the black hole density is around 2 × 10⁸ per cubic parsec, collisions would destroy massive O-type stars (those more than 20 times the mass of the Sun) in about 5 million years, while B-type stars (roughly 10 times the Sun’s mass) could survive for around 55 million years. These estimates match the observed distribution: while many B stars exist in the S-cluster, O stars are notably absent—possibly because they've already been ground down.
Mapping the Invisible: A Density Profile for Black Holes
The paper then uses this idea to map a likely radial density profile for the black hole cluster. By looking at which types of stars are missing or present at different distances from Sgr A⋆, Haas et al. place constraints on how dense the black hole population must be at each location. Their results suggest a bump-like distribution: relatively low densities close to the black hole (due to observational mass limits), a peak density within the S-cluster, and declining numbers beyond 0.04 parsecs where O stars reappear. These predictions are surprisingly consistent with recent observations of so-called “hyper-compact radio sources,” which may be black hole candidates.
Hypervelocity Stars and Their Missing Twins
One especially intriguing piece of evidence comes from hypervelocity stars—B-type stars found in the outer halo of the Milky Way, moving fast enough to escape the Galaxy. These stars are thought to be the result of binary systems disrupted by Sgr A⋆, ejecting one star and capturing the other. If that’s true, the remaining B stars should still be in the S-cluster today. But all known B stars there are under 25 million years old, while the ejected counterparts must be at least 50 million years old. The authors suggest that these older stars may have been destroyed by the grinder, leaving only the younger ones for us to observe.
Beyond Collisions: Grazing Encounters and Dusty Clues
In the final discussion, Haas and collaborators consider the broader consequences of this model. If such a dense black hole cluster really exists, it could also help explain other mysteries, such as the presence of dust-enshrouded objects near the S-cluster. These might be stars partially stripped by grazing encounters with black holes, where only the outer layers are torn away. Over time, multiple such interactions could erode a star entirely. They also speculate that gravitational lensing effects and signals from gravitational waves might one day provide direct evidence of this hidden population.
Conclusion: A Violent Sculptor at Work
Overall, this paper paints a compelling picture of a destructive environment at the Milky Way’s centre. If Haas et al. are correct, then the missing massive stars—and perhaps even other oddities near Sgr A⋆—can be explained by a dense and deadly cloud of stellar-mass black holes, silently reshaping the inner Galaxy one collision at a time.
Source: Haas
