'Cloudshine' may reveal secrets of star birth

New Scientist ^ | 11/4/05 | Maggie McKee

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Long, infrared exposures of giant gas and dust clouds reveal structures that are invisible at optical wavelengths (Image: J Foster and A Goodman, CfA)

Astronomers have discovered a new way to probe dark clouds of gas and dust in space, shedding light on the mysterious conditions that nurture star birth.

Stars condense from giant clouds of molecular gas and dust that float through space. But these stellar wombs are difficult to study because they are barely visible at optical wavelengths. And other methods that probe their structure are not very precise – for example, estimating how much dust they contain by how red the stars behind them appear.

Now, two researchers at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US, have imaged the clouds' structure at a resolution 50 times better than previous techniques.

They stumbled on the discovery by taking a four-hour infrared exposure of molecular clouds with a 3.5-metre telescope in Spain. "The image just looks dark and empty in the optical, but in the near-infrared, there's all this colour and structure," says Jonathan Foster, one of the pair.

Foster and colleague Alyssa Goodman have dubbed the infrared emission "cloudshine" and believe it comes from faint, infrared light that the clouds scatter from surrounding stars.

Shape and density

The cloudshine traces the shape and density of the clouds because its colour depends on how much dust it has bounced off. Shorter wavelengths scatter off dust at the edges of the clouds, while longer wavelengths make it to the clouds' central regions before bouncing off dust particles there.

The images may help researchers understand exactly what triggers star birth. "We're interested in knowing what physical processes are important in determining the shape of molecular clouds and how that proceeds to star formation," Foster told New Scientist.

"The only way we can do this is by finding a way to look at the density structure of real clouds and comparing them to simulations" that take into account phenomena such as magnetic fields, he says.

The research was part of the Coordinated Molecular Probe Line Extinction Thermal Emission (COMPLETE) survey, which involves studying three star-forming regions relatively close to Earth at a variety of wavelengths. "We're very interested in seeing where different methods work well and where different methods fail," Foster says.