Astrophile: Ferocious superwind will seal sun's doom

Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse

Object: stellar superwinds
Location: red giant stars; our future
Characteristics: dusty, dense and doom-laden

The sun's dying breath will be no feeble wheeze. Instead, our star will go out in a blustering rage, launching a series of stellar sandstorms into the night. Due in 7 or 8 billion years, humans are unlikely to be around to witness this cosmic gale, but if we are it will be awe-inspiring: far faster than any Earthly hurricane, far denser than the solar wind blowing past Earth today and lasting more than 10,000 years.

"Stars like the sun die in a phase of catastrophic mass loss called a superwind," explains Albert Zijlstra at the University of Manchester, UK. Zijlstra and his colleagues have now caught a glimpse of our sun's doom in the dust around other stars, which seems to reveal what drives these sandstorms.

Superwinds were already known to exist, but it was not clear how they could be powered. One idea was that iron-tainted dust might absorb starlight to be pushed outwards. The problem with that model is that the dust would heat up and vaporise before it could really get moving and drive the wind.

Zijlstra's team used the Very Large Telescope in Chile to look at three red giant stars that are in this late stage of life. By using polarisers and masking off parts of the telescope, they could pick out the polarised light coming from shells of dust around each star.

Ferocious Neptune

The dust was so close to these stars that it would certainly be vaporised if it were absorbing light strongly. So it must be made of transparent crystals. The team suspect it is made of clear silicate minerals, similar to ordinary terrestrial sand grains.

In that case, what pushes it outwards? A clue comes from the size of the grains. At around 300 micrometres across, they are much larger than expected, and these large grains should be good at reflecting starlight. By reflecting instead of absorbing, they can be pushed out without getting overheated.

Then the superwind is a kind of inverted sandstorm. "Instead of the wind picking up the sand, here it is the sand driving the wind," says Zijlstra.

Blowing at 10 kilometres per second, the wind is far faster than any Earthly hurricane or even the ferocious winds of Neptune. In astronomical terms that is no great pace - slower than the solar wind blowing past Earth today - but a superwind is many million times denser than the tenuous solar wind, making it vastly more powerful.

White sapphire

A superwind is not a steady breeze but a series of gusts. These aged stars pulsate roughly once per year. When the sun's time comes, such an intermittent superwind will carry away perhaps a third of its mass over a period of 10,000 years or so.

The wind will form a beautiful planetary nebula, and leave behind a bare stellar core to begin its long afterlife as a white dwarf. If Earth has managed to survive that long, then its bare surface may get sandblasted.

In some cases, the authors suggest, the dust grains may not be silicates but crystals of corundum, the mineral that forms some precious gemstones. That means those stars end their lives in storms of white sapphire.

Journal reference: Nature, DOI: 10.1038/nature10935

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