Unique explosion: gamma-ray burst leads astronomers to supernova.

Using scores of telescopes, astronomers worldwide are chasing one of the most intriguing stellar explosions detected in nearly a decade. The supernova--a catastrophic

collapse of a massive star--is one of only a handful of these explosions known to have been heralded by a burst of gamma rays.

The observations confirm that material blasting out from a collapsing star generates a gamma-ray burst. The burst races out into space ahead of the visible, fiery glow from the supernova explosion.

A gamma-ray burst typically lies too far away--billions of light-years--and has an afterglow too bright to permit astronomers to detect the underlying supernova. But the new burst, recorded by NASA's Swift satellite on Feb. 18, resided a relatively close 440 million light-years from Earth. Furthermore, the burst was unusually weak, despite lasting nearly 200 seconds--about 100 times as long as the typical burst.

Within 3 minutes of the burst, dubbed GRB 060218, Swift's visible-light telescope pinpointed the source, in the constellation Aries. Then, the race on was on to find the hidden supernova. On Feb. 21, Alicia Soderberg of the California Institute of Technology in Pasadena and her colleagues succeeded, using the large Gemini South Observatory on Cerro Pachon Mountain in Chile. The supernova is expected to reach its peak brightness around March 5, and amateur astronomers in the Northern Hemisphere with a telescope at least 16 inches across have a good chance of viewing the ongoing eruption.

Watching a supernova unfold so soon after the star erupted--particularly one linked so closely to a gamma-ray burst--is only part of the excitement, says Soderberg. Astronomers calculate that this burst packed only about one-hundredth the energy of more-distant bursts. Its low energy was similar to an even closer burst recorded in 1998. Taken together, the two bursts "imply the existence of a significant population of [faint] gamma-ray bursts that go undetected at larger distances," Soderberg says.

These low-energy events could be 30 times as common as more-powerful bursts, calculates theorist Andrew MacFadyen of the Institute for Advanced Study in Princeton, N.J.

In high-energy bursts, a collapsing star expels jets of material at near-light speeds. Chunks within each jet collide to generate the gamma rays. In contrast, lower-energy bursts may originate from a weaker explosion that drives out lower-speed chunks of material in a more diffuse pattern, Mac-Fadyen suggests. When this material smacks into dust and gas surrounding the star, it generates the lower-energy gamma rays. In either case, the collapsing star becomes a black hole or a magnetar, an extremely dense, rapidly spinning star with an enormous magnetic field.

MacFadyen, who has worked on gamma-ray-burst models for more than a decade, doesn't usually do his work from behind a telescope. This time, however, "I'm personally looking to make friends with someone with a telescope because I really want to see a new black hole or magnetar being formed with my own eyes. This is a rare and special opportunity."