Most massive stars in the early universe were likely born as binaries

Most massive stars in the early universe were likely born as binaries
by Erica Marchand
Paris, France (SPX) Oct 21, 2025

A new study from Tel Aviv University's School of Physics and Astronomy suggests that most massive stars formed in the early universe were born in pairs, echoing patterns seen among massive stars in our own Milky Way. The finding provides the first strong evidence that binary star formation was common in the early cosmos-an insight that reshapes understanding of how black holes, supernovae, and heavy elements evolved over cosmic history.

The research, led by Dr. Tomer Shenar of Tel Aviv University alongside Dr. Hugues Sana of KU Leuven and Dr. Julia Bodensteiner of the University of Amsterdam, was published in Nature Astronomy.

Massive stars-those exceeding ten times the Sun's mass-are central engines of galactic evolution. A single such star can radiate more energy than a million Sun-like stars, forge most of the universe's heavy elements, and end its life in a supernova, leaving behind a neutron star or black hole.

In the Milky Way, astronomers have long known that most massive stars form in close binary systems, where companion stars orbit tightly enough to exchange matter or even merge. These interactions drastically alter their lifecycles, influencing everything from their explosive deaths to the birth of compact stellar remnants.

The study set out to determine whether such "binarity" also characterized massive stars in the early universe. Because direct observations of early galaxies are impossible due to distance, the team turned to a nearby analogue: the Small Magellanic Cloud (SMC), a satellite galaxy of the Milky Way with very low metallicity-similar to conditions after the Big Bang.

Using the European Southern Observatory's Very Large Telescope in Chile, the researchers launched the BLOeM survey (Binarity at LOw Metallicity). Over two years, they collected spectra from around 1,000 massive stars in the SMC. By examining periodic shifts in the stars' spectra, they identified companion-induced motions revealing binary systems.

From a detailed analysis of 150 of the most massive stars, the team found that at least 70 percent are part of close binaries-marking the first direct and convincing evidence that such systems were prevalent under early-universe conditions, possibly even more so than in the present day.

"These results change our view of the processes that shaped the universe," said Dr. Shenar. "From the formation of black holes of all scales to the chemical enrichment of galaxies with the elements essential for stars, planets, and life itself, binary stars have played a dominant role from the very beginning."

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