Binary stars everywhere: Monash University scientists help rewrite cosmic origin story

A new international study, published in Nature Astronomy, reveals that massive stars are about as likely to form in close binary systems in the low-metal environments of the early Universe as they are today,  reshaping our understanding of stellar evolution and the origins of gravitational wave events.

The research, led by a global team of astronomers and using the European Southern Observatory’s Very Large Telescope in Chile, studied 139 O-type stars in the Small Magellanic Cloud, a nearby dwarf galaxy with just one-fifth the metallicity of our Sun. The findings challenge previous observations that low-mass stars are more likely to be found in binaries in metal-poor environments than in our Galaxy.

Professor Ilya Mandel, from the Monash University School of Physics and Astronomy, and the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), co-authored the study, contributing to the statistical analysis testing whether the abundance of binary stars changes with metallicity, the chemical richness of the stars’ environment.

“Our analysis shows there’s no significant change in the fraction of massive stars in close binaries in metal-poor galaxies like the Small Magellanic Cloud,” said Professor Mandel. “That’s exciting, and it tells us that massive binary formation is a fundamental feature of star formation, even in the early Universe.”

The study found that around 70 per cent of the observed O-type stars are in close binaries, and two thirds of them will interact with a companion during their lifetimes, often leading to dramatic phenomena such as supernovae, black holes, or neutron star mergers.

“This is important because binary star interactions are a key pathway to producing exciting and rare outcomes, such as black holes and neutron stars that collide and emit gravitational waves,” Professor Mandel said. “Understanding how common these binaries are in different environments helps us predict how often we should expect to detect gravitational wave events, not just today, but across cosmic history.”

The discovery has far-reaching implications. It strengthens the case that many of the gravitational wave signals detected by LIGO and Virgo come from binary systems born in the early Universe. It also suggests that binary interactions likely played a large role in shaping galaxies and enriching them with heavy elements.

“By studying how stars evolve in environments that mimic the early cosmos, we get a clearer view of how black holes form, how galaxies evolve, and how the Universe became what it is today,” said Professor Mandel.

The research is part of the Binarity at LOw Metallicity (BLOeM) survey, and brings together more than 70 scientists across Europe, the US, Australia, and Israel.

Professor Mandel said the study exemplifies the power of international collaboration and big data analysis in unlocking the secrets of the cosmos.

“We’re in a golden age of discovery,” he said. “And what we’re learning now will echo in our understanding of the Universe for decades to come.”

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)]

Monash University media release: https://www.monash.edu/science/news-events/news/2025/binary-stars-everywhere-monash-university-scientists-help-rewrite-cosmic-origin-story