Why are the stars around our galaxy's black hole missing their companions?

A new study suggests that the young stars orbiting the Milky Way's central black hole may have a surprisingly ordinary origin. The black hole's influence may be what makes their origin look more exotic than it really is. The paper was published in Astronomy & Astrophysics on Aug. 11. The S cluster of stars is close to the black hole and is about 0.04 parsecs away. These B-type stars are young, averaging a few million years old. The black hole's intense gravity is supposed to tear apart gas clouds before they form stars, but this situation is called the "paradox of youth." Two theories are proposed: in situ formation suggests the stars formed where they are now, with an outburst from the black hole compressing gas into a shell that fragmented to form stars. Ex situ formation suggests the stars formed farther away and later moved inward. Another possibility is the Hills mechanism, in which the black hole tears apart a binary, capturing one star and flinging its companion as a high-velocity star. However, the high-velocity stars near the center are older than the S stars, and this mechanism would leave almost no binary systems among the S cluster. The discovery of D9—a binary system—prompted the authors to revisit this question. The team refined the count of binary systems among S stars, finding a binary fraction of 43% ± 9%. They ran computer simulations tracking 100,000 simulated binary star systems for 1 million years, incorporating realistic orbits matching the S cluster's observed structure. They found that 62% of binaries 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. The results support the idea that the young stars formed near the Galactic center rather than migrating there from elsewhere. 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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