  The presence of a nearby companion alters the evolution of massive stars in
binary systems, leading to phenomena such as stellar mergers, X-ray binaries
and gamma-ray bursts. Unambiguous constraints on the fraction of massive stars
affected by binary interaction were lacking. We simultaneously measured all
relevant binary characteristics in a sample of Galactic massive O stars and
quantified the frequency and nature of binary interactions. Over seventy per
cent of all massive stars will exchange mass with a companion, leading to a
binary merger in one third of the cases. These numbers greatly exceed previous
estimates and imply that binary interaction dominates the evolution of massive
stars, with implications for populations of massive stars and their supernovae.
