With masses at least 15 times that of the Sun, type-O stars are the heaviest and brightest of all. And because they live only a few million years, they're quite rare. Having analyzed the O-star populations of six nearby star clusters, a team led by Hugues Sana (Amsterdam University) has concluded that over 70% of O stars—far more than had been thought—have binary companions close enough to exchange matter with them. Interaction with companions, large or small, would therefore seem to dominate the evolution of the massive stars, and it should now be taken into account when interpreting observations of distant star-forming galaxies. A smaller companion might become a 'vampire star' (see the artist's impression), sucking away the O star's hydrogen envelope, or it might itself be swallowed up. The team estimates the probabilities of different O-star fates by deducing the distribution of binary-pair orbital parameters from periodic Doppler shifts in spectra taken mostly with the Very Large Telescope array in Chile. For significant binary interaction and reasonable detection probability, a companion's orbital period should be less than about four years. O stars end their lives in core-collapse supernovae that seed the host galaxy with heavy elements. The team's estimate of the fraction of O stars that lose their hydrogen envelopes to vampires resolves an old puzzle; it's in good agreement with the observed frequency of core-collapse supernovae that exhibit no hydrogen lines. (H. Sana et al., Science 337, 444, 2012.)—Bertram Schwarzschild
