The brightest stars don’t live alone: VLT finds most stellar heavyweights come in interacting pairs

26 July 2012 A study using ESO’s Very Large Telescope has shown that most very bright high-mass stars do not live alone. Almost three quarters of them are found to have a close companion star, far more than previously thought….

26 July 2012

A study using ESO’s Very Large Telescope has shown that most very bright high-mass stars do not live alone. Almost three quarters of them are found to have a close companion star, far more than previously thought. Surprisingly most of these pairs are experiencing disruptive interactions, and about one third are even expected to ultimately merge to form a single star. The results are published in the July 27 issue of the journal Science. An international team has used the VLT to study what are known as O-type stars, which have very high temperature, mass and brightness. These stars have short and violent lives and play a key role in the evolution of galaxies. They are also linked to extreme phenomena such as “vampire stars”, where a smaller companion star sucks matter off the surface of its larger neighbour, and gamma-ray bursts. “These stars are absolute behemoths,” says Hugues Sana (University of Amsterdam, Netherlands), the lead author of the study. “They have 15 or more times the mass of our Sun and can be up to a million times brighter. These stars are so hot that they shine with a brilliant blue-white light and have surface temperatures over 30 000 degrees Celsius.” The astronomers studied a sample of 71 O-type single stars and stars in pairs (binaries) in six nearby young star clusters in the Milky Way. Most of the observations in their study were obtained using ESO telescopes, including the VLT. By analysing the light coming from these targets in greater detail than before, the team discovered that 75% of all O-type stars exist inside binary systems, a higher proportion than previously thought, and the first precise determination of this number. More importantly, though, they found that the proportion of these pairs that are close enough to interact (through stellar mergers or transfer of mass by so-called vampire stars) is far higher than anyone had thought, which has profound implications for our understanding of galaxy evolution. O-type stars make up just a fraction of a percent of the stars in the Universe, but the violent phenomena associated with them mean they have a disproportionate effect on their surroundings. The winds and shocks coming from these stars can both trigger and stop star formation, their radiation powers the glow of bright nebulae, their supernovae enrich galaxies with the heavy elements crucial for life, and they are associated with gamma-ray bursts, which are among the most energetic phenomena in the Universe. O-type stars are therefore implicated in many of the mechanisms that drive the evolution of galaxies.