How a Black Hole Collision Could Explain the Milky Way’s S-Stars

New research shows that a past collision between the Milky Way’s central black hole and a smaller black hole could explain the dynamics of the S-stars, a group of stars that orbit precariously close to our galaxy’s supermassive black hole.

Journey to the Center of the Milky Way

The center of the Milky Way harbors a supermassive black hole (Sagittarius A* or Sgr A*) with a mass of 4 million Suns. The nearest neighbors of this behemoth are a compact disk of massive young stars and a collection of stars called the S-stars, which inhabit the innermost 48 light-days of our galaxy.

While the stars in the disk are orderly, arranged on orbits with moderate eccentricities and low inclinations, the S-stars orbit Sgr A* every which way, careening around the black hole with a wide range of eccentricities and inclinations. So far, the cause of this unusual distribution of stars is unknown.

When Black Holes Collide

Past research has struggled to explain the S-stars’ orbits. A successful theory of S-star origins must account for the stars’ eccentric orbits (e = 0.61, on average) and high orbital inclinations (i = 79º, on average), and it must produce these characteristics within the 15-million-year lifetime of the stars.

In a recent research article, a team led by Tatsuya Akiba (University of Colorado Boulder) offered a new hypothesis to explain these traits: Sgr A* absorbed a smaller black hole in the not-so-distant past, and the aftermath of the merger created the distribution of stars seen today.

The premise is not far-fetched: at the ripe old age of nearly 14 billion years, the Milky Way has likely gulped down neighboring dwarf galaxies and globular clusters multiple times. When a black hole embedded within one of these meals of stars and gas merges with the Milky Way’s central black hole, the collision causes Sgr A* to recoil — potentially rearranging the stars in its vicinity in the process.

Radical Reorganization

Akiba and collaborators used N-body simulations to explore the outcomes of such a collision. In their simulations, Sgr A* is situated within an axisymmetric disk of stars or gas. A smaller black hole falls toward Sgr A*, spirals inward, and merges with the larger black hole. The asymmetric emission of gravitational waves causes Sgr A* to recoil, warping the surrounding collection of stars and gas into an eccentric disk. For the eccentricity of the disk to match what is seen in the disk of stars surrounding Sgr A* today, the incoming black hole must have a mass of roughly 200,000 solar masses.

After the merger, the stars orbiting farther out torqued the inclinations and eccentricities of the innermost stars up to high values through what’s called the eccentric Kozai–Lidov mechanism. After 2 million years of simulation time, this interaction produced a stellar distribution similar to what is seen today: a disk of moderate eccentricity surrounding “S-stars” with eccentric and highly inclined orbits.

While the properties of the modeled S-stars don’t exactly match the properties of the real deal — the simulated stars have, on average, lower inclinations and eccentricities — the authors noted that this work represents a first foray into their hypothesis. Modeling that explores a broader range of parameter space is needed to fully understand the origins of the stars at the center of the Milky Way.

Citation

“On the Formation of S Stars from a Recent Massive Black Hole Merger in the Galactic Center,” Tatsuya Akiba et al 2025 ApJL 987 L27. doi:10.3847/2041-8213/addc5d