The interpretation of observations of gravitational waves, transients, and distant galaxies in the high-redshift Universe heavily relies on our understanding of massive stars at low metallicity. At high metallicity, a majority of massive stars have at least one close stellar companion, which dramatically impacts their evolution, feedback, and final fate. At low metallicity, constraints on the multiplicity of massive stars over the separation range leading to binary interaction have been crucially missing. Here we show that the presence of massive stars in close binaries is ubiquitous, even at low metallicity. The Small Magellanic Cloud is a neighbouring low-metallicity dwarf galaxy with a metal content of about one fifth solar, hence similar to metallicity environments during and before the peak of star formation in the Universe. Using Very Large Telescope spectroscopy from the Binarity at LOw Metallicity large program, we obtained multi-epoch radial velocity measurements of a representative sample of 139 massive O-type stars across the Small Magellanic Cloud. We find that 45% of them show radial velocity variations which demonstrate that they are members of close binary systems, predominantly with orbital periods of less than one year. After correcting for observational biases, we find that at least 0.70^+0.11^-0.06 of the O stars in our sample are in close binaries, and that at least 68^+7^-8% of all O stars will interact with a companion star during their lifetime. We found no evidence supporting a statistically significant trend of the multiplicity properties with metallicity. Our results indicate that multiplicity and binary interactions govern the evolution of massive stars and determine their cosmic feedback and explosive fates, which has strong implications for our understanding of the high redshift Universe.

Cone search capability for table J/other/NatAs/9.1337/stars (List of studied stars)