Lurking at the heart of the Milky Way, Sagittarius A* (Sgr A*) is a supermassive black hole four million times the mass of the sun, surrounded by a puzzling collection of young, massive stars whose orbits have long defied explanation. Astronomers have proposed various competing theories to account for these stars, but none has been able to explain all of them together.
Through new analysis published as a preprint on the arXiv server, a team led by Xiaochen Zheng at the Beijing Academy of Science and Technology has developed a unified model that accounts for the full complexity of these stellar orbits in one coherent framework.
Strangely differing orbits
The stars in question fall into three distinct groups, each behaving very differently. Closest to Sgr A* is a tight cluster of cool S-stars following highly elongated, randomly tilted orbits. Farther out sits a disk of heavier, hotter stars rotating together in a clockwise direction. Surrounding both is a more scattered population of stars on poorly constrained orbits with a wide range of inclinations.
All three groups are no older than around 15 million years, but their orbits are so different that astronomers have struggled to explain how they could have arisen from the same starting conditions, or indeed whether they share any common origin at all.
Modeling orbits
In their study, Zheng’s team tackled this through a model in which all three populations formed simultaneously from a single disk of gas around Sgr A*. The key ingredient was an intermediate-mass companion: a dense cluster or black hole around 10,000 times the mass of the sun, orbiting farther out—whose long-range gravitational influence gradually sculpted the orbits of the younger stars over millions of years.
At the same time, the stars’ mutual gravitational interactions redistributed energy and angular momentum among them, randomizing the innermost orbits into the chaotic S-star cluster we observe today, while leaving the outer disk largely intact.
Hidden influence of a companion cluster
Through rigorous testing, the team’s model successfully reproduced the full range of observed stellar behaviors, including the S-stars’ high eccentricities and randomly tilted orbits, the clockwise disk’s relatively orderly structure, and a recently identified gap in the S-stars’ orbital distribution that had previously been unexplained. Crucially, it achieved all of this within the stars’ actual lifetimes, which earlier models had failed to do.
Beyond resolving a longstanding puzzle about our galactic center, the findings suggest that a star cluster called IRS-13E, located about 0.13 parsecs from Sgr A*, may be the intermediate-mass companion responsible for shaping these stellar orbits. Through future high-precision observations of IRS-13E’s motion and internal structure, Zheng’s team hope their model could help finally confirm whether a second massive object shares the innermost neighborhood of our galaxy’s central black hole.
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Publication details
Xiaochen Zheng et al, The complex kinematics of the young stars orbiting the supermassive black hole in the Galactic center can be explained by the presence of an intermediate mass companion of Sgr A*, arXiv (2026). DOI: 10.48550/arxiv.2606.08971
Journal information:
arXiv
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A single origin story for the Milky Way’s most mysterious stars (2026, June 26)
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