Red giants’ weight loss trick

Artist's impression of the red giant and its dust shell. Credit: Anna Mayall

These old stars don’t need diets, they use stellar winds to lose weight instead.

Red giants are the galaxy’s recyclers, releasing gas and dust via stellar winds every year as they die. This matter will then form the next generation of stars and planets, but no one has understood what’s driving the weight-shedding winds.

A new study from The University of Sydney has suggested a solution for the mass loss problem. By directly imaging the stars, the researchers found a shell of dust, relatively close to the star’s surface, surrounding them.

“This is the material condensing out of the gas being emitted from the star, the first stage in its journey out into the rest of the galaxy,” said PhD student Barnaby Norris, lead author of the study published in Nature. “The really cool thing is that we were also able to measure the size of the dust grains, which gave us a possible answer to the central question – how are these winds propelled?”

The researchers used a technique called aperture-masking interferometry, which allowed them to clearly see both the huge star and its surrounding shell. The dust grains turned out to measure about half a micron across, much bigger than had previously been assumed.

When dust grains reach this size, they are able to reflect light from the star through a process called scattering, and are pushed along by the starlight, much like a small sail. There’s a large amount of these dust grains, so when they are pushed they can drag some gas with them, forming the stellar wind.

This weight loss trick can save these giant stars from a violent death, as stars above a certain mass will die in a supernova. “The mass-loss occurring via these winds can lower the mass of the star below this threshold, so it instead has a peaceful end as a white dwarf,” Norris said.

These winds will also influence the birth of new planets, as the majority of the chemical elements critical to the formation of earth-like planets and life come from the matter shed by dying red giant stars. “That means the Earth and everybody living on it are quite literally made of the stardust we are studying with our new techniques.”

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