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Most stellar heavyweights exist in pairs

Using data from ESO's Very Large Telescope, astronomers have revealed that the hottest and brightest stars, which are known as O stars, are often found in close pairs.

india Updated: Jul 27, 2012 19:40 IST

Using data from ESO's Very Large Telescope, astronomers have revealed that the hottest and brightest stars, which are known as O stars, are often found in close pairs.

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 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. 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 30,000C,” said Hugues Sana, from the University of Amsterdam, Netherlands, who is the lead author of the study.

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 per cent of all O-type stars exist inside binary systems, a higher proportion than previously thought, and the first precise determination of this number.

Mergers between stars, which the team estimates will be the ultimate fate of around 20-30 per cent of O-type stars, are violent events. But even the comparatively gentle scenario of vampire stars, which accounts for a further 40-50 per cent 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.

For instance, in the case of vampire stars, the smaller, lower-mass star is rejuvenated as it sucks the fresh hydrogen from its companion. Its mass will increase substantially and it will outlive its companion, surviving much longer than a single star of the same mass would.

The victim star, meanwhile, is stripped of its envelope before it has a chance to become a luminous red super giant.

Instead, its hot, blue core is exposed. As a result, the stellar population of a distant galaxy may appear to be much younger than it really is: both the rejuvenated vampire stars, and the diminished victim stars become hotter, and bluer in colour, mimicking the appearance of younger stars.

Knowing the true proportion of interacting high-mass binary stars is therefore crucial to correctly characterise these faraway galaxies.
The only information astronomers have on distant galaxies is from the light that reaches our telescopes.

Without making assumptions about what is responsible for this light we cannot draw conclusions about the galaxy, such as how massive or how young it is.

According to Sana, this study shows that the frequent assumption that most stars are single can lead to the wrong conclusions.