A new study suggests that the young stars orbiting the Milky Way’s central black hole may have a surprisingly ordinary origin. But the black hole’s influence may be what makes their origin look more exotic than it really is. Along with shaping their orbits, it may also destroy their stellar companions. The paper outlining this work was published in Astronomy & Astrophysics on Aug. 11.
Chaotic environment
A cluster of stars known as the S cluster sits very close to the Milky Way’s central supermassive black hole, Sagittarius A* (Sgr A*), at a distance of some 0.04 parsecs (0.13 light-years). The black hole has a mass around 4 million times that of the sun.
These B-type stars are surprisingly young—a few million years old on average. But conventional star formation shouldn’t be possible this close to a black hole, since the black hole’s intense gravity should tear apart any gas cloud before it collapses to form stars. Astronomers call this puzzling situation the “paradox of youth.”
There are two proposed ideas for how the young S stars could have formed. In situ formation proposes that the stars formed where we see them today. It involves a powerful Sgr A* outburst about 6 million years ago. The resulting outflow could have compressed surrounding gas into a shell, which then fragmented to form stars. This theory is called gas-shell fragmentation.
Ex situ formation suggests the stars formed farther away and later moved inward. One possibility is that the stars formed farther out in a separate, well-known disk structure, then migrated inward over time, but this struggles to explain the S stars’ high, nearly random orbital eccentricities and why B stars would migrate while O and Wolf–Rayet stars did not.
Another possibility is the Hills mechanism, in which Sgr A* tears apart an incoming binary, capturing one star and flinging its companion as a high-velocity star. But the high-velocity stars found near the galactic center are much older than S stars, and this mechanism would leave behind almost no binary systems among the S cluster. This is inconsistent with the recent discovery of D9—a binary system—in this cluster.
Why the single life?
There is also a mismatch between the binary fraction among S stars and surrounding young populations, and this may be hiding an important clue. Massive stars in the Galactic field have a binary fraction of about 69 ± 9%, while previous observations suggested a lower fraction near Sgr A*. The discovery of D9 prompted the authors to revisit this question.
The team, led by Rodrigo P. Silva of the University of Coimbra in Portugal, refined the count of binary systems among S stars. The observed binary fraction was 43% ± 9%. To test whether the in situ formation pathway matches this number, they then ran computer simulations tracking 100,000 simulated binary star systems for 1 million years.
They incorporated realistic orbits matching the S cluster’s observed structure. Starting with a standard binary fraction seen in massive stars generally (69%), they let gravity and the black hole’s tidal forces play out in the simulations.
In situ over ex situ
Researchers tracked three possible outcomes for each binary: surviving intact, merging into a single star or being torn apart (disrupted). Here is what they found: 62% survived, 18% merged and 20% were disrupted—producing a predicted final binary fraction of around 38% ± 10% of the resulting S-star population. This aligns with the observed number within their uncertainties.
They also modeled how the predicted binary fraction should change with distance from the black hole. They saw consistency across different distances. The results therefore support the idea that the young stars formed near the Galactic center rather than migrating there from elsewhere.
“This indicates a common formation mechanism throughout this region, with the decline toward Sgr A* driven primarily by increasing tidal disruption,” the team writes in the paper. “Our findings support an in situ formation scenario, such as gas-shell fragmentation.”
The researchers say that improved observations in the future, including more precise measurements of stars’ motions and spectra, could reveal more about how many binary stars are present and their formation history.
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Publication details
Rodrigo P. Silva et al, A solution to the paradox of youth in the Galactic centre, Astronomy & Astrophysics (2026). DOI: 10.1051/0004-6361/202660648
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Why are the stars around our galaxy’s black hole missing their companions? (2026, September 12)
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