The Young Stars Whipping Around the Milky Way's Black Hole Are Missing Their Partners. A Million-Year Simulation Says the Black Hole Ate Them.

A few dozen stars orbit within 0.04 parsecs, about 0.13 light-years, of Sagittarius A*, the four-million-solar-mass black hole at the center of the Milky Way. They are called the S cluster, they move fast enough that astronomers have watched them complete full orbits, and they should not exist. Most are hot B-type stars only a few million years old, yet the black hole's tidal pull is strong enough to shred any gas cloud long before it could collapse into a star. Astronomers call this the "paradox of youth," and a new study led by Rodrigo P. Silva of the University of Coimbra in Portugal uses a different clue to resolve it: the stars' missing companions.

Massive stars in the galactic disk are usually born in pairs. Surveys put the binary fraction for such stars at about 69%, plus or minus 9%. In the S cluster, the observed fraction is 43%, plus or minus 9%, and until December 2024, when a system designated D9 was confirmed, no binary had ever been found there at all. Silva's team, writing in Astronomy & Astrophysics, asked a simple question: if S stars were born in place with a normal complement of partners, how many binaries should the black hole have destroyed by now?

To find out, the researchers ran simulations tracking 100,000 binary systems for one million years, placing them on orbits that match the observed structure of the S cluster and starting with the 69% pairing rate seen among massive stars generally. Then they let Sagittarius A* do what black holes do. Each time a binary swung close to the black hole, the difference in gravitational pull across the pair, the tidal force, tugged the two stars apart or drove them together. Over the million years, 62% of the binaries survived intact, 18% merged into single, more massive stars, and 20% were disrupted, their partners stripped away. The predicted surviving binary fraction came out to 38%, plus or minus 10%.

That number sits comfortably inside the observed 43%, plus or minus 9%. "This indicates a common formation mechanism throughout this region, with decline toward Sgr A* driven primarily by increasing tidal disruption," the authors wrote. In other words, the deficit of pairs is exactly what a population of ordinary massive binaries would look like after a million years next to a supermassive black hole, and it argues that the S stars formed roughly where they are now, most plausibly from the fragmentation of a dense gas shell compressed around the black hole, rather than migrating inward from farther out. The team notes that an outburst from Sagittarius A* roughly six million years ago fits the timeline for such a shell.

The result also weakens the leading alternative. The Hills mechanism proposes that binaries wandering in from elsewhere are torn apart by the black hole, which captures one star into a tight orbit and hurls the other out of the galaxy as a hypervelocity star. Astronomers have indeed found such runaway stars, but the ones traced back to the galactic center are much older than the S stars, which is hard to square with the two populations being born from the same disrupted binaries. The simulation's match to the observed binary fraction, the authors argue, is more naturally explained by in situ formation.

The paper, published August 11, comes with the usual caveat that the S cluster's binary census is still thin; D9 is one confirmed system among a few dozen stars, and the 43% figure carries a wide error bar. Better measurements of the stars' motions and spectra, the kind of data that instruments such as GRAVITY on the Very Large Telescope Interferometer are now producing, should reveal more pairs and tighten the comparison. For now, the count of missing partners is doing what the stars' ages could not: telling astronomers where they came from.