Massive stars in low metal galaxies frequently form binaries

Massive stars in low metal galaxies frequently form binaries
by Clarence Oxford
Amsterdam, Netherlands (SPX) Sep 03, 2025

Astronomers have confirmed that massive stars in galaxies with low metal content often exist in binary systems, much like their counterparts in the Milky Way. An international team of seventy researchers from Belgium, the Netherlands, and Israel used the European Very Large Telescope (VLT) in Chile to monitor massive stars in the Small Magellanic Cloud. Their findings appear in Nature Astronomy.

For two decades, it has been known that many massive stars in the Milky Way are part of binary systems. More recently, astronomers realized that such interactions play a key role in the stars' evolution. Until now, however, it was unclear whether this was also true for galaxies poor in heavy elements. The new study shows that it is.

"We used the Small Magellanic Cloud as a time machine," explained Hugues Sana of KU Leuven in Belgium. "Its metallicity is similar to that of distant galaxies when the Universe was only a few billion years old."

Observing these stars beyond the Milky Way is challenging due to their faint light and great distance. The team relied on the FLAMES spectrograph at the VLT, which can target up to 132 stars simultaneously using fiber optics, making it ideal for such surveys.

Over three months, the astronomers studied 139 O-type stars, each 15 to 60 times the mass of the Sun. These luminous, short-lived stars end as supernovae and collapse into black holes. More than 70 percent of the stars showed telltale acceleration and deceleration in their velocities, indicating the gravitational tug of nearby companions.

"The fact that massive stars in the Small Magellanic Cloud have a partner suggests that the first stars in the universe, which we suspect were also massive, had partners too," said co-author Julia Bodensteiner of the University of Amsterdam. "Perhaps some of those systems end up as two black holes orbiting each other. It's an exciting thought."

The team plans sixteen additional observation rounds to map the stars' orbits, measure their masses, and characterize their companions. "Using our measurements, cosmologists and astrophysicists studying the young, metal-poor universe will then be able to rely on our knowledge of massive binary stars with greater confidence," concluded Tomer Shenar of Tel Aviv University.

Research Report:A high fraction of close massive binary stars at low metallicity.