Astronomers solve the mystery of stellar 'superwind'

Apr 11, 2012
An artist's impression of a star and dust shell. [Image: Anna Mayall]

Astronomers at the University of Manchester believe they have found the answer to the mystery of a powerful "superwind" which causes the death of stars.

Writing in Nature, the team of researchers used new techniques which allowed them to look into the of distant, dying stars.

The team, lead by Barnaby Norris from the University of Sydney in Australia, includes scientists from the Universities of Manchester, Paris-Diderot, Oxford and Macquarie University, New South Wales. They used the Very Large Telescope in Chile, operated by the .

At the resolution used by the scientists, one could, from the UK, distinguish the two headlights on a car in Australia. This extreme resolution made it possible to resolve the red , and to see winds of gas and dust coming off the star.

Stars like the Sun end their lives with a 'superwind', 100 million times stronger than the . This wind occurs over a period of 10,000 years, and removes as much as half the mass of the star. At the end, only a dying and fading remnant of the star will be left. The Sun will begin to throw out these gases in around five billion years.

The cause of this superwind has remained a mystery. Scientists have assumed that they are driven by minute , which form in the atmosphere of the star and absorb its light. The star light pushes the dust grains (silicates) away from the star.

However, models have shown that this mechanism does not work well. The dust grains become too hot, and evaporate before they can be pushed out.

The scientists have now discovered that the grains grow to much larger sizes than had previously been thought. The team found sizes of almost a – as small as dust, but huge for stellar winds.

Grains of this size behave like mirrors, and reflect starlight, rather than absorbing it. This leaves the grains cool, and the star light can push them out without destroying them. This may be the solution to the mystery of the superwind.

The large grains are driven out by the star light at speeds of 10 kilometres per second, or 20 thousand miles per hour – the speed of a rocket. The effect is similar to a sandstorm. Compared to grains of sands, the silicates in the stellar winds are still tiny.

Professor Albert Zijlstra, from The University of Manchester's Jodrell Bank Observatory, said the breakthrough changes our view of these superwinds. For the first time, we begin to understand how the superwinds work, and how stars (including, in the distant future, our Sun) die. 
He added: "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."

Explore further: 'Perfect storm' quenching star formation around a supermassive black hole

More information: "A close halo of large grains around red giant stars: a new piece in the mass-loss puzzle," by Barnaby Norris, Peter Tuthill, Michael Ireland, Sylvestre Lacour, Albert Zijlstra, Foteini Lykou, Thomas Evans, Paul Stewart, and Timothy Bedding, Nature (2012).

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User comments : 5

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Squirrel
5 / 5 (1) Apr 12, 2012
Bad news for interstellar travel--hit one of these at sub light speed and it is bye-bye to your spaceship. Could be why there are so many planets and possibilities for life, including intelligent life, but no visitors (Fermi paradox).
Star_Gazer
not rated yet Apr 12, 2012
Bad news for interstellar travel--hit one of these at sub light speed and it is bye-bye to your spaceship. Could be why there are so many planets and possibilities for life, including intelligent life, but no visitors (Fermi paradox).

Good idea.
Terriva
1 / 5 (1) Apr 14, 2012
Grains of this size behave like mirrors, and reflect starlight, rather than absorbing it.
This implies, all smaller grains should appear black, but it's not true. On the contrary, the thinner dust is, the more pale it usually appears. On this behaviour the streak test of mineralogy is based.
Terriva
not rated yet Apr 14, 2012
In addition, the smaller particle is, the narrower portion of spectrum it does absorb, because the long waves cannot hit it. Which is why we are using red lights in foggy environment, for example.
Grains of this size behave like mirrors, and reflect starlight, rather than absorbing it. This leaves the grains cool, and the star light can push them out without destroying them.

The common argument AGAINST de Fatio/Sage theory is the very similar one: the impacts of tachyons would heat the material objects fast. But it seems, the possibility, they could be reflected was never considered...
Torbjorn_Larsson_OM
not rated yet Apr 15, 2012
Small world! The key paper:

"Models show that grains exceeding a certain critical scattering opacity can drive a wind at high Mg condensation and that, for a star of temperature 2700 K, this critical scattering opacity is only exceeded in a narrow range of dust grain radii around 300 nm8. It should also be noted that a narrow range of grain sizes, of the order of ~500nm radius, is predicted based on a self-regulating feedback mechanism: grain growth effectively halts once the critical size is reached, since the dust is then accelerated outwards and gas densities quickly decrease8."

is authored Susanne Höfner, which is out of my alma mater (Uppsala University).

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