An interstellar tale writ in space dust.

Space, it turns out, is not a pristine place. When stars explode, they make an awful mess.

On an earthly scale, think of the diffusing black plumes that blot the sky when Hollywood detonates a Chevrolet. On a stellar scale, hydrogen-gas clouds speckled with dirty carbon grit drift between stars, blocking light, soaking up heat, and emitting characteristic radiation.

But what is that sticky interstellar soot really like?

To find out, Adoif N. Witt, a physicist at the University of Toledo in Ohio, and his colleagues cooked up hydrogenated amorphous carbon, or HAC, exposed it to ultraviolet light, and studied its unique glow. In essence, they find that exposing loose HAC grains to hot hydrogen gas and ultraviolet radiatidon - a striking parallel to known conditions near expanding and exploding stars - sharply perks up their red light radiation.

In fact, the distinctive red glow of certain carbon-rich nebulae, including one called the Red Rectangle, bears a strong resemblance to the photoluminescence of synthesized HACs, Witt says. In deep space, radiation from nearby stars causes dust clouds to give off a reddish hue. The dust grains themselves- each a mineral core with an HAC coating - soak up light of one wavelength, then emit light of another. Yet when conditions around the dust change, so does its photoluminescent efficiency, says Witt.

He reported these findings at a meeting of the Materials Research Society in San Francisco this week.

To simulate conditions near expanding and exploding stars, Witt's team formed HAC films at 300 kelvins in a vacuum, then exposed them to ultraviolet light and high-energy hydrogen atoms and measured the HAC's spectra. In a series of experiments, they found that carbon exposed separately to ultraviolet rays or hydrogen yields a paltry red glow.

However, when carbon meets up with hydrogen in the presence of ultraviolet light, the two bond. This increases the HAC's photoluminescence, brightening the distinctive red radiation.

The upshot, says Witt, is that astronomers can use naturally occurring HACs hovering among the stars as remote sensors, sending back to Earth information about the deep space environment.

"Call them sensors or probes, if you like," Witt says. "What we now have is a laboratory material whose photofuminescence varies in efficiency under measurable conditions. This is very useful. Astronomers can use telescopes to locate dust clouds in space that can tell us by the light they emit what their interstellar environments are like."