'Vampire stars' more common than thought

Posted by Kate Taylor

Most bright, massive stars have a companion star - in many cases with one a 'vampire' star sucking mass from the other, say astronomers using ESO's Very Large Telescope (VLT).

Around three quarters of O-type stars have such a companion, and around a third of these pairs are expected ultimately to merge into a single star.

"These stars are absolute behemoths," says Hugues Sana of the University of Amsterdam.

"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 members of binaries in six nearby young star clusters in the Milky Way. By analysing their light, they found that 75 percent of all O-type stars exist as part of binary systems, a higher proportion than previously thought.

More importantly, though, they found that far more of these pairs than thought are close enough to interact - through stellar mergers or transfer of mass -  with profound implications for our understanding of galaxy evolution.

O-type stars are comparatively rare, but have a powerful effect on their surroundings. The winds and shocks coming from these stars can both trigger and stop star formation, their  supernovae enrich galaxies with the heavy elements crucial for life and they're associated with gamma-ray bursts, which are among the most energetic phenomena in the universe.

"The life of a star is greatly affected if it exists alongside another star," says Selma de Mink of the Space Telescope Science Institute.

"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."

Until now, most astronomers believed that closely-orbiting massive binary stars were rare, something that was only needed to explain exotic phenomena such as X-ray binaries, double pulsars and black hole binaries.

But the new study shows that this isn't the case: these heavyweight double stars are common - and have lives that 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 - meaning it may appear much younger than it actually is.

"The only information astronomers have on distant galaxies is from the light that reaches our telescopes," says Sana.

"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. This study shows that the frequent assumption that most stars are single can lead to the wrong conclusions."