Massive stars avoid the single life

Video: How a star couple evolves

For massive stars, connecting with a partner is about more than just romance: it's a matter of life and death. Now it seems that pairing up is the norm among the universe's heftiest, most influential stars.

That in turn suggests that inter-star relationships have a bigger role to play in galactic evolution and dynamics than previously thought – with important implications for explaining stellar behaviour, evolution and history.

Though rare compared with stars like our sun, the heft of massive stars, many times bigger than our sun, means they have a disproportionate effect on the chemical make-up of galaxies.

Very massive stars end their lives in huge explosions, seeding the universe with heavier elements, such as iron and oxygen, forged in their nuclear cores. "Massive stars alone evolve to liberate elements that have undergone advanced stages of nuclear fusion," says Ian Howarth of University College London. "They are of central importance in determining the chemistry of the universe."

Unimportant singleton

Until now, most models used to study galactic evolution have assumed that these influential behemoths are single. Star pairs orbiting close together in a system known as a binary, meanwhile, were thought to be unusual, and the contribution of their interactions has been difficult to determine reliably, says Howarth.

To establish whether these assumptions are accurate, Hugues Sana of the University of Amsterdam and colleagues looked at 71 O-type stars residing in six loosely bound, or "open", star clusters within the Milky Way. Searing hot O-type stars have masses greater than 15 times that of our sun.

The team found that more than 70 per cent of these behemoths dance closely around nearby companions to form binaries – a number far higher than expected.

This suggests that an "astonishing" number of massive stars pair up and go through significant physical changes – not accounted for in previous models – before they die, says Sana. Single stars, meanwhile, are a "relatively unimportant part of the population", says Sana's colleague Chris Evans of The Royal Observatory, Edinburgh, in the UK.

Vampire stars

This new understanding may already be leading to a more accurate interpretation of stellar behaviour and evolution.

In 40 to 50 per cent of the O-type stars in the team's sample, the companion is a "vampire star" that siphons material off its partner. By sucking up the larger star's outer layer of hydrogen, the vampire rejuvenates itself and can live longer than other stars of similar mass. The stripped star, meanwhile, often ends up exploding as an unusually hydrogen-poor supernova.

Previously, astronomers had noticed that around a third of stars that explode as supernovae are observed to have surprisingly little hydrogen but could not explain why. The newly-observed dominance of massive star binaries could explain the lack of hydrogen, especially as the researchers report that the proportion of hydrogen-poor supernovae closely matches the proportion of vampire stars found by this study.

A vampire-victim dynamic is not the fate of all massive binaries, however. The researchers calculate that one-third of O-type stars are in partnerships where the mass exchange prompts the two stars to spiral towards each other and violently merge to form a much larger star.

Journal reference: Science, DOI: 10.1126/science.1223344

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