A chill stellar wind…

Thermodynamics may be very good at putting freshers to sleep during Monday morning lectures, but it really is a fundamental piece of modern physics. Bluntly, you don’t argue with thermodynamics. Originally developed to increase the efficiency of steam engines, the laws of how energy is transferred between work and heat underpin the entire physical Universe. Even stars.

This beautifully coloured image, around 2 light years long, is of the Boomerang Nebula. It’s a pre-planetary nebula in the Centaurus constellation. A dying star, casting off it’s outer layers into the night. Taken in polarised light, the colours show how light is reflected off the dust grains that make up those two outflow lobes. The picture also shows the finer details of the dust emanating from the central star as it blossoms into a grandiose planetary nebula.

This, most gelid of nebulae happens to be the coldest naturally occuring object in the known Universe. At a mere 1 Kelvin (-272°C), it’s actually colder than the cosmic microwave background. Barely above absolute zero, the coldest any matter can ever be, it’s even cold enough for helium to exist as a superfluid. The reason? Thermodynamics.

When you release a fire extinguisher, the rapidly expanding gas draws energy out of its surroundings, causing the nozzle of that fire extinguisher to rapidly become extremely cold. Exactly the same thing is happening to the Boomerang Nebula. For reasons no one’s entirely certain about, the gas from this nebula is flowing outwards and expanding at a rate of around 164 kilometres per second. That’s ten times the speed of any other nebula like it (the Egg nebula, for instance). This swift expansion consumes the all the energy in the gas, driving such a frigid stellar wind, and causing the entire nebula to be so exceptionally cold.

Ref: Sahai & Nyman 1997
Image: NASA/Hubble Heritage STScI

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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