Several times a day the mightiest explosions in the universe flash in the skies, unseen by human eyes. Believed to stem from the deaths of massive stars, these enigmatic pulses of radiationknown as gamma-ray burstshave long captivated astronomers. But much about the bursts remains a mystery, including, in most cases, the distances at which they occur. To that end, findings presented yesterday at a meeting of the American Astronomical Society may prove insightful.

As the shifting colors of gamma-ray spectrometers show, bursts start out at high energies and fade to lower ones. Studying 16 of the powerful pulses, Edison Liang of Rice University and colleagues found that the rate at which this cooling off occurs appears to be directly related to the distance of the burst when measured in terms of the number of gamma rays emitted since the beginning of the pulse.

So far the technique only works on gamma-ray bursts that have separable peaks of intensity, however. Applied to "chaotic" bursts (those that have multiple, overlapping peaks) it tends to underestimate the cooling rate. Because most gamma-ray bursts belong in the chaotic category, the team is now working on ways to isolate the colors of overlapping peaks, which should enable them to calculate the true cooling rate.

"It's very labor intensive and tedious," Liang remarks, "but we have high hopes it will work." If so, the distances determined using their technique could shed light not only on the elusive bursts themselves, but also on the formation of the early universe.