An international group of scientists led by Dr. Barnaby Norris from the University of Sydney in Australia using new techniques which allowed them to look into the atmospheres of dying stars has found an answer to the long-standing mystery of superwind that plays a role in the death of stars.
Structure of a red giant star (Sten Odenwald)
Stars like the Sun end their lives with a superwind, 100 million times stronger than the solar wind. 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 dust grains, 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 new study, published in the journal Nature, suggests that the grains grow to much larger sizes than had previously been thought. The group found sizes of almost a micrometer – 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 starlight can push them out without destroying them. This may be the solution to the mystery of the superwind.
“The breakthrough changes our view of these superwinds,” said Prof. Albert Zijlstra of the University of Manchester’s Jodrell Bank Observatory, a co-author of the study. “For the first time, we begin to understand how the superwinds work, and how stars (including, in the distant future, our Sun) die.”
The large grains are driven out by the starlight at speeds of 10 kilometers per second (20,000 miles per hour) – the speed of a rocket. This effect is similar to a sandstorm. Compared to grains of sands, the silicates in the stellar winds are still tiny.
“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,” Prof. Zijlstra concluded. “The sand grains at that time become the building blocks of planets. Our own Earth has formed from stardust. We are now a big step further in understanding this cycle of life and death.”
