Massive stars in distant, metal-poor galaxies are just as likely to have close companions as their counterparts in our own Milky Way, according to new research published today in Nature Astronomy.
An international team of seventy astronomers, including researchers at the University of Cambridge, used the European Southern Observatory’s Very Large Telescope (VLT) in Chile to measure the motions of massive stars in the Small Magellanic Cloud (SMC). Their observations reveal that a large fraction of these stars are members of binary systems.
Astronomers have long known that in the Milky Way -- a relatively metal-rich galaxy -- many massive stars exist as binary systems. Over the past two decades it has become clear that these interactions strongly influence how such stars evolve. Until now, however, it was uncertain whether the same was true in galaxies with much lower chemical abundances.
"We used the Small Magellanic Cloud as a time machine," explains Hugues Sana from KU Leuven (Belgium). "The Small Magellanic Cloud has a metallicity environment representative of that of distant galaxies when the Universe was only a few billion years old."
Studying such stars is challenging, as they are both distant and faint. To overcome this, the team used the FLAMES spectrograph on the VLT, which employs 132 fibre optics to observe more than a hundred stars at once.
Over three months, the astronomers tracked 139 O-type stars—hot, luminous objects between 15 and 60 times the mass of the Sun—at nine separate epochs. These stars eventually end their lives in supernova explosions, leaving behind neutron stars or black holes. The observations revealed that more than 70 percent of the stars showed measurable accelerations and decelerations, clear evidence of close 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," says 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."
Avishai Gilkis, Assistant Professor at the University of Cambridge’s Institute of Astronomy, added: “We usually observe low-metallicity massive binaries through their endpoints, as gravitational-wave events or supernovae in distant galaxies. This study provides a glimpse into how they start their lives in environments such as those at cosmic dawn.”
The team plans to continue monitoring the same stars sixteen more times in the near future. These additional observations will allow them to reconstruct precise stellar orbits, determine companion masses, and study the properties of the partner stars in detail.
“Our measurements will give cosmologists and astrophysicists studying the young, metal-poor Universe a firmer foundation for understanding how massive binary stars shape their environments,” said Tomer Shenar (Tel Aviv University, Israel).
Scientific paper
A high fraction of close massive binary stars at low metallicity. By: Hugues Sana, Tomer Shenar, Julia Bodensteiner, et al. In: Nature Astronomy, 2 September 2025. [original | preprint (pdf)]