Most very bright stars have companions

Most very bright stars have companions. 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...