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Coolest bow tie in the universe

By Stephen Cauchi
February 21 2003

Brr ... the Boomerang nebula, where it is minus 272 degrees.

NASA has released a high-quality image of the coldest place found in the universe, a nebula originally discovered by the Siding Springs Observatory in NSW in 1980.

The Boomerang nebula was named by astronomers at the Anglo-Australian telescope, whose weaker resolution suggested a boomerang shape.

But the magnifying power of the Hubble Space Telescope, which snapped the latest image, clearly shows the nebula looks more like a bow tie.

Five thousand light years from Earth in the constellation of Centaurus, the nebula, a gas cloud formed from a dying star, has a temperature of minus 272 degrees.

It is only one degree warmer than absolute zero, the coldest possible temperature, when atoms cease to vibrate and radiate no heat whatsoever.

What is interesting for astronomers is that the nebula is colder than the microwave radiation which pervades all of space.

This radiation is the remnant of the Big Bang, the explosion which forged the universe in trillion-degree temperatures. More than 11 billion years later, this heat has cooled to minus 270 degrees, but is still detectable.

It appears that the ultra-cold gas is flowing from the nebula's central star so fiercely, at 140 kilometres per second, the background radiation is simply blown away.

"One can say the Boomerang acts as a refrigerator," said astronomer Lars-Ake Nyman, who measured its temperature using the European Southern Observatory radio telescope in Chile. He did this by comparing signals received from carbon monoxide in the nebula with signals from the background radiation.

"Normally the cosmic microwave background photons would excite the gas to at least its own temperature. [But] the gas self-shields itself, and the photons from the microwave background do not penetrate deep into the outflow. The low temperature of the gas in the outflow therefore stays low."

The speed and volume of the gas gives the nebula its shape.

Man has produced yet chillier temperatures. In 1995, American researchers cooled rubidium atoms to less than 170 billionths of a degree above absolute zero.


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