For decades, astronomers have known that many of the massive stars in our Milky Way are not alone. They often live as pairs, bound together in what scientists call binary systems. These stellar duos orbit one another, shaping each other’s fates in ways that single stars cannot. Yet, until recently, a major question lingered: do such cosmic partnerships exist in galaxies that are far poorer in heavy elements than our own?
An international team of 70 astronomers has now provided an answer—and it is a resounding yes. By peering into one of our nearest galactic neighbors, the Small Magellanic Cloud, they have shown that massive stars in these ancient, metal-poor environments also live with close companions. Their work, published in Nature Astronomy, sheds light not only on the lives of stars, but also on the earliest days of the universe.
Why Metal Matters
When astronomers talk about “metals,” they mean all elements heavier than hydrogen and helium. Our Sun, for example, is considered metal-rich because it contains carbon, oxygen, iron, and many other heavier elements forged in earlier generations of stars.
The Small Magellanic Cloud, by contrast, is metal-poor. It resembles the conditions of galaxies that existed billions of years ago, when the cosmos was still young and metals had only begun to form in the hearts of stars. Studying it is like stepping into a time machine, offering a glimpse of how stars might have lived in the early universe.
Until now, scientists weren’t sure whether massive stars in such environments would still form close partnerships. Without metals, the rules of star formation and stellar winds change, and this could have altered the likelihood of stars pairing up. The new discovery shows that even under these ancient conditions, cosmic companionship endures.
A Telescope That Sees the Invisible
The breakthrough was made possible by the European Southern Observatory’s Very Large Telescope (VLT), one of the most powerful observatories on Earth. Perched in the Chilean desert, the VLT can collect precious light from distant galaxies and split it into detailed spectra, revealing subtle changes invisible to the human eye.
The astronomers used a special instrument called FLAMES, which can observe over a hundred stars at once. This was crucial, because studying massive stars outside our Milky Way is no easy feat—they are faint, far away, and constantly battling the cosmic noise of space.
Over three months, the researchers tracked 139 O-type stars in the Small Magellanic Cloud. These are true giants: each one at least 15 times the mass of our Sun, blazing with heat and brightness, and destined to end in spectacular supernova explosions.
The Dance of Acceleration and Deceleration
What the team looked for was subtle: tiny shifts in the velocity of these stars. If a star seemed to accelerate and then decelerate, this suggested that an unseen companion was tugging it around in orbit.
The results were astonishing. More than 70% of the massive stars showed signs of such orbital motion, evidence that they had partners. This percentage is remarkably similar to what astronomers see in the Milky Way, meaning that the rules of stellar companionship apply even in metal-poor galaxies.
It is as if the universe, no matter how young or chemically simple, insists that its biggest stars rarely shine alone.
Echoes of the Early Universe
The implications stretch far beyond the Small Magellanic Cloud. If massive stars in metal-poor galaxies tend to live with partners, then it is likely the very first stars in the universe—the so-called Population III stars—also did. These first giants, born only a few hundred million years after the Big Bang, seeded the cosmos with the metals that would eventually form planets, life, and us.
Some of those early pairs may have collapsed into two black holes orbiting each other. Billions of years later, as they spiraled closer, their collisions would send ripples through space-time—gravitational waves—that we can now detect with instruments like LIGO and Virgo.
In other words, the distant whispers of the universe’s first stars may still be echoing to us today.
Looking Ahead: Mapping the Orbits of Giants
The current study is only the beginning. The team plans to observe the same stars many more times to reconstruct their orbits, measure their masses, and study their companions in detail. Each new observation will add another piece to the puzzle of how massive stars live, die, and shape the cosmos.
As Tomer Shenar of Tel Aviv University explains, this knowledge will give cosmologists a firmer foundation when modeling the early universe. By understanding the lives of massive stars in metal-poor environments, scientists can better simulate the galaxies that existed when the cosmos was still in its infancy.
Science as a Time Machine
What makes this discovery so breathtaking is that it connects us to an era billions of years before humanity existed. By studying the faint light of stars in a neighboring galaxy, astronomers are effectively peering into the conditions of the young universe. It is a reminder that science, at its best, is not just about data points and measurements—it is about time travel, imagination, and wonder.
The universe is ancient and vast, but discoveries like this remind us that we can still trace its story, one star at a time. Somewhere, in the flickering light of the Small Magellanic Cloud, we are catching glimpses of a cosmic dance that began near the dawn of time—and continues, brilliantly, today.
More information: H. Sana et al, A high fraction of close massive binary stars at low metallicity, Nature Astronomy (2025). DOI: 10.1038/s41550-025-02610-x. Preprint (pdf): www.astronomie.nl/upload/files … m-Obinaries-2025.pdf
