Dying stars create cosmic sandblasts

Astronomers have discovered that dying stars are producing violent cosmic sandstorms by blasting streams of grit into space over thousands of years.

An artist’s impression of the sandstorm surrounding the star W Hya. Credit: Anna Mayall

They used a European telescope in Chile to study the atmospheres of red giant stars near the ends of their lives in greater detail than ever before. Their observations revealed powerful winds of gas and dust streaming from the stars at the speed of a rocket. This superwind is 100 million times stronger than the solar wind from our own Sun.

This cosmic storms, thought to last for around 10,000 years, strip away half the material from within the star until only a fading remnant is left. Our Sun will undergo a similar fate in about five billion years time. Until now, the cause of the superwind has been a complete mystery and astronomers assumed they were driven by minute grains of dust that formed in the stars’ atmospheres and absorbed its light which then drove it away from the star.

However computer modelling has shown that the grains cannot be this small because they would become too hot and evaporate. To their surprise, the scientists discovered they must be much larger, almost a micrometer across.

At this size, the grains behave like mirrors and reflect the starlight instead of absorbing it. This means they stay cool and can be blasted out into space intact like a sandstorm.

The dust grains are driven from the stars at 10km per second, or 20,000 mph. Two stars were studied closely, W Hya in the constellation of Hydra the seasnake, and R Dor in Doradus the dolphinfish, which appears as the brightest star in the sky in the infrared.

The discovery was made by an international team led by Barnaby Norris, of the University of Sydney, and including scientists from the Universities of Manchester, Paris-Diderot, Oxford and Macquarie University, New South Wales.

Their work using the Very Large Telescope operated by the European Southern Observatory in Chile is published in Nature this week.

Professor Albert Zijlstra, Director of Jodrell Bank, Manchester, said: “The dust and sand in the superwind will survive the star, and later become part of the clouds in space from which new stars form.

“The sand grains at that time become the building blocks of planets. Our own Earth has formed from star dust. We are now a big step further in understanding this cycle of life and death.”

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