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Mystery of how stars shed major pounds may be solved

A team of astronomers using ESO’s Very Large Telescope believe they have found the answer to the mystery of the ‘superwind’ that causes the death of stars. At the resolution they used, one could, from the UK, distinguish the two headlights on a car in Australia. This extreme resolution made it possible to resolve the red giant stars, 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 solar wind. This wind occurs over a period of 10,000 years when they become a red giant, 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, a white dwarf. The Sun will begin to throw out these gases in around five billion years.

Scientists previously believed that the cause of this superwind is 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 astronomers have now discovered that the grains grow to 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 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. The effect is similar to a sandstorm. Compared to grains of sands, the silicates in the stellar winds are still tiny. The dust in the superwind later become part of the clouds in space from which new stars form, becoming the building blocks of planets such as our own Earth.

Images: 1. An artist’s impression of the dust surrounding a red giant star. 2. Dust grains forming in the atmosphere of a cool luminous star are accelerated away from the star through absorption and emission or scattering of stellar photons. By subsequently colliding with molecules in the surrounding gas, the grains accelerate the molecules, make them collide with other gas molecules and trigger an outflow of gas, or stellar wind.