Long-duration gamma-ray bursts, the most powerful flashes known of high-energy radiation, arise from the explosions of extremely massive stars. If a burst occurred almost anywhere within the Milky Way � and for certain within a few tens of thousands of light-years � it could threaten life on Earth by destroying the fragile ozone layer, triggering climate change and perhaps drastically altering the path of evolution.

Reporting in the May 10 online issue of Nature, a team of astronomers said they analyzed 42 long-duration GRBs - those lasting more than two seconds � using the Hubble Space Telescope. They discovered that the galaxies in which the bursts originated tended to be small, faint and misshapen. Only one burst occurred in a large spiral galaxy similar to the Milky Way.

In contrast, supernovae - also the result of collapsing massive stars - were found in Milky Way-sized spiral galaxies roughly half of the time.

The results indicate GRBs form only in very specific environments, which tend to be different from those found in the Milky Way.

"Their occurrence in small irregulars implies that only stars that lack heavy chemical elements (elements heavier than hydrogen and helium) tend to produce long-duration GRBs," said lead author Andrew Fruchter of the Space Telescope Science Institute.

This means long bursts occurred more often in the past when galaxies did not have a large supply of heavy elements.

Galaxies build up a stockpile of heavier chemical elements through the ongoing evolution of successive generations of stars. Early generation stars formed before heavier elements were abundant in the universe.

The astronomers also found the locations of GRBs differed from the locations of supernovae - which occur much more frequently. The GRBs were far more concentrated in the brightest regions of their host galaxies, where the most massive stars reside. Supernovae, on the other hand, seem to occur throughout galaxies.

"The discovery that long-duration GRBs lie in the brightest regions of their host galaxies suggests that they come from the most massive stars - perhaps 20 or more times as massive as our Sun," said co-author Andrew Levan of the University of Hertfordshire, in the United Kingdom.

Massive stars abundant in heavy elements are unlikely to trigger GRBs, because they may lose too much material through stellar winds emanating from their surfaces before they collapse and explode. When this happens, the stars retain too little mass to produce a black hole, a necessary condition to trigger a GRB.

The energy from the collapse escapes along a narrow jet, like a stream of water from a hose. The formation of directed jets, which concentrate energy along a narrow beam, would explain why GRBs are so powerful.

If a star loses too much mass, it leaves behind only a neutron star, which cannot trigger a GRB. If the star loses too little mass, however, the jet cannot burn its way through the star. This means extremely high-mass stars that puff away too much material might not be candidates for long bursts. Neither are the stars that give up too little material.

"It's a Goldilocks scenario," Fruchter said. "Only supernovae whose progenitor stars have lost some - but not too much - mass appear to be candidates for the formation of GRBs."

Levan said some researchers have suggested it might be possible to use GRBs to follow the locations of star formation. "This obviously doesn't work in the universe as we see it now," he said, "but when the universe was young, GRBs may well have been more common, and we may yet be able to use them to see the very first stars to form after the Big Bang."

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