30 July 2012
A new study using the European Southern Observatory’s (ESO) Very Large Telescope (VLT) has shown that most bright high-mass stars, which drive the evolution of galaxies, do not live alone. Almost three quarters of these stars are found to have a close companion star, far more than previously thought. Surprisingly, most of these pairs are also experiencing disruptive interactions, such as mass transfer from one star to the other, and about one third are even expected to ultimately merge to form a single star.
These are the findings of research conducted by Dr Hugues Sana and Prof Alex de Koter from the University of Amsterdam, together with Dr Selma de Mink from Space Telescope Science Insititute in Baltimore (US). The results were published in the scientific journal Science on 27 July 2012.
The universe is a diverse place, and many stars are quite unlike the Sun. An international team has used the VLT to study what are known as O-type stars, which have 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 neighbor — and gamma-ray bursts.
'These stars are absolute behemoth's,' said Hugues Sana from the University of Amsterdam in the Netherlands. '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 30000° 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 percent 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.
'The life of a star is greatly affected if it exists alongside another star,' said Selma de Mink from the Space Telescope Science Institute in Baltimore, Maryland. 'If two stars orbit very close to each other, they may eventually merge. But even if they don’t, one star will often pull matter off the surface of its neighbour.'
Mergers between stars, which the team estimates will be the ultimate fate of around 20 to 30 percent of O-type stars, are violent events. But even the comparatively gentle scenario of vampire stars, which accounts for a further 40 to 50 percent of cases, has profound effects on how these stars evolve.
Until now, astronomers mostly considered that closely orbiting massive binary stars were the exception, something that was only needed to explain exotic phenomena such as X-ray binaries, double pulsars, and black hole binaries. The new study shows that to properly interpret the universe, this simplification cannot be made. These heavyweight double stars are not just common, their lives are fundamentally different from those of single stars.