Safe from a heavenly doom: gamma-ray bursts not a threat to Earth.

For pessimists, the heavens offer a host of doomsday scenarios--an asteroid crashing into Earth or deadly cosmic rays raining down on the planet. But at least earthlings don't have to worry about gamma-ray bursts, according to new findings. Although these high-energy flashes of light--the most energetic outbursts in the universe--could decimate life in an instant, they're unlikely to occur in the Milky Way, two studies conclude.

In an upcoming Nature, researchers report using the Hubble Space Telescope to analyze the home galaxies of 42 long-duration gamma-ray bursts. These bursts, which last for more than 1 second, are produced within a jet of material blasting out of a collapsing star, or supernova. All but one of the galaxies were small, faint, and misshapen--unlike the spiral-shaped Milky Way--note Andy Fruchter of the Space Telescope Science Institute in Baltimore, Md., and his colleagues.

Another team, led by Krzysztof Stanek of Ohio State University in Columbus, came to a similar conclusion. Stanek and his collaborators examined the home galaxies of four gamma-ray bursts, including one of the same galaxies studied by Fruchter's team, and recently posted their results online (http://xxx.lanl.gov/abs/ astro-ph/0604113).

Fruchter and his colleagues compared the character of the burst galaxies with the homes of 16 core-collapse supernovas, explosions of massive stars that leave behind either a neutron star or a black hole. A small fraction of core-collapse supernovas produce gamma-ray bursts (SN: 5/17/03, p. 317).

The team found that gamma-ray bursts have more-limited locations than the core-collapse supernovas. Those supernovas are equally likely to have originated in a misshapen or a spiral galaxy. Moreover, the gamma-ray bursts emanate from the parts of their galaxies where the highest-mass stars form, while the supernovas are more widely distributed.

The finding supports a model in which a gamma-ray burst is generated only by the death of an extremely high-mass star--weighing as much as 10 or more suns. The explosion of such a star not only produces a powerful jet of material but also packs enough power for the jet to plow through the star's outer layers to create a gamma-ray burst, notes study coauthor Stan Woosley of the University of California, Santa Cruz.

The composition of burst galaxies also jibes with the model, he adds. The researchers found that compared with the Milky Way, these galaxies have a much lower concentration of elements heavier than helium. Massive stars with a low abundance of heavy elements don't blow off as much material as others do, making it more likely that the star will indeed form a black hole and produce a gamma-ray burst, Woosley says.

Astronomer Adrian Melott of the University of Kansas in Lawrence argues that that the findings don't exclude gamma-ray bursts in the Milky Way. Although rich in heavy elements, our galaxy contains some stars, snared from satellite galaxies, that have concentrations of heavy elements as low as those observed in the home galaxies of the gamma-ray bursts, he says.

But the observations show that long-duration gamma-ray bursts must be exceedingly rare in the Milky Way, Fruchter asserts. Any irregular satellite galaxy of the Milky Way, such as the Large Magellanic Cloud, "is likely to have more than its fair share" of bursts, he adds. However, these wouldn't destroy life on Earth.

GROUND ZERO Green crosshairs pinpoint the locations of six long-duration gamma-ray bursts that have long since faded. Of the 42 burst galaxies examined, all but one--the spiral galaxy in the top-middle image--are faint, small, misshapen, and irregular, unlike the Milky Way.