A giant cloud of hot water vapour surrounding a dying star can only be explained by the interaction of ultraviolet starlight breaking down molecules, according to new data from ESA's Herschel space observatory.

Finding water around IRC+10216 in the first place (it was detected in 2001) was a surprise because dying carbon stars weren't thought to create much water. Initially, astronomers thought that the water must be derived from evaporating comets or even planets, but the water ranges in temperature from -200 degrees Celsius to +800 degrees, the higher temperatures being much too hot to have come from icy bodies.

Now, thanks to Herschel's PACS and SPIRE instruments, astronomers have deduced that the culprit must be ultraviolet light. Specifically, the light from surrounding stars can penetrate deep through IRC+10216's clumpy dust cloud to break up molecules such as carbon monoxide and silicon monoxide, releasing oxygen atoms. The oxygen atoms then bind to hydrogen molecules, forming water.

“This is the only mechanism that explains the full range of the water’s temperature,” says Leen Decin, Katholieke Universiteit Leuven, Belgium, the lead author of the paper detailing this work. “This is a good example of how better instruments can change our picture completely.”

Decin's team plan to extend the observations to other carbon stars. “We are very hopeful that Herschel will find the same situations around those stars too,” she says.

The discovery of water and carbon compounds around other stars is a crucial step in the search for life outside our Solar System. Thanks to Herschel, astronomers now know that the secret ingredient for creating water around this star at least, is ultraviolet light from nearby stars.

The results are published in this week's edition of the journal Nature.