This recent discovery was made by an international team of seventy astronomers, and includes Newcastle University. They used the European Very Large Telescope in Chile to monitor the velocity of massive stars in the Small Magellanic Cloud. The researchers publish their findings in Nature Astronomy.

For the past twenty years, astronomers have known that many massive stars in the metal-rich Milky Way have a partner. In recent years, it has become clear that the interaction between these partners is important for the evolution of massive stars. However, until now, astronomers were unsure if massive stars in metal-poor galaxies could also be part of a binary system. Now, it turns out that this is indeed the case.

Time machine

"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 massive stars outside of the Milky Way is difficult because the stars are far away and we receive little light from them. The researchers used the FLAMES spectrograph on the Very Large Telescope of the European Southern Observatory in Chile. It is one of the largest telescopes on Earth. FLAMES has 132 optical fibers, each of which can be directed at a different star, thus allowing many stars to be observed simultaneously.

Accelerate and decelerate

Over a period of 3 months, the researchers observed the acceleration and deceleration of 139 massive O-type stars at 9 different times. These stars have masses between 15 and 60 times that of our Sun. They are hot, shine brightly, and end their lives in supernova explosions, which causes the star's core to collapse into a black hole. The results show that over 70% of the observed stars accelerate and decelerate, which is a clear sign for having a nearby partner.

"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, the Netherlands. "Perhaps some of those systems end up as two black holes orbiting each other. It’s an exciting result."

The researchers have planned to observe the same stars 16 more times in the near future. They aim to reconstruct the precise orbits of the binary stars, determine the masses of their components, and study the nature and properties of the companion star.

"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," says co-author Tomer Shenar of Tel Aviv University (Israel).

Co-author Dominic Bowman from Newcastle University concludes "Massive stars in binaries are particularly interesting because they can have drastically different end products to stars born without a partner. These results help constrain how binary stars evolve from birth to death, which is part of the ongoing SYMPHONY project at Newcastle University."

Funding:

The research received funding from UK Research and Innovation (UKRI) in the form of a Frontier Research grant under the UK government’s ERC Horizon Europe funding guarantee (SYMPHONY; PI Bowman; grant number: EP/Y031059/1), and a Royal Society University Research Fellowship (PI Bowman; grant number: URF\R1\231631).

Reference

Sana, H., Shenar, T., Bodensteiner, J. et al. A high fraction of close massive binary stars at low metallicity. Nat Astron (2025). https://doi.org/10.1038/s41550-025-02610-x

Press release adapted with thanks from the Netherlands Research School for Astronomy (NOVA), Amsterdam, the Netherlands: https://www.astronomie.nl/nieuws/en/massive-star-in-metal-poor-environment-often-have-close-partner-4639 

Image: Massive stars in the Small Magellanic Cloud. Of the stars studied, seventy percent (the red diamonds) appear to accelerate and decelerate. This indicates the presence of a partner. (c) ESO/Sana et al.