By Dr David Whitehouse
BBC News Online science editor
Astronomers have identified the coldest place ever detected in space. It is the gas blown away from a star in the latter stages of its life-cycle.
The Boomerang Nebula
The so-called Boomerang Nebula, one of the youngest of its kind, has been observed in minute detail by the Hubble Space Telescope (HST).
It seems that the central dying star has been expelling gas at a huge rate. As that gas has expanded it has cooled to such a degree that it is now even colder than the cosmic background radiation that bathes all of space.
The young planetary nebula imaged by the HST is in the constellation of Centaurus, 5,000 light-years from Earth. Planetary nebulae form around bright, central stars when they expel gas in the last stages of their lives.
In 1995, astronomers revealed that the Boomerang Nebula is the coldest place in the Universe found so far outside a terrestrial laboratory. With a temperature of -272 Celsius, it is only one degree warmer than absolute zero.
Even the -270 C background radiation from the Big Bang that permeates the cosmos is warmer than this nebula. The Boomerang Nebula is the only object found so far that has a temperature lower than the background radiation.
The recent Hubble image shows faint arcs and ghostly filaments embedded within the diffuse gas of the nebula's lobes. It appears quite different from other observed planetary nebulae. Researchers speculate that the object is so young that it may not have had time to develop more familiar structures.
The remarkable coolness of the gas clouds may be the result of an unusual central star. The clouds appear to have been sculpted by a fierce 500,000 kilometre-per-hour wind blowing ultra-cold gas away from the dying central star.
The dying star has been losing as much as one-thousandth of a solar mass of material each year for perhaps a millennium. This is 10-100 times more than mass-loss seen in other similar objects.
It is the rapid expansion, and subsequent cooling, of the gas cloud that has enabled it to become the coldest known region in the Universe.