Hubble Finds That Earth Is Safe From One Class Of Gamma-ray Burst

Long-duration gamma-ray bursts are powerful flashes of high-energy radiation that are sometimes seen coming from certain types of supernovae (the explosions of extremely massive stars). If Earth were flashed by a nearby long-duration burst, the devastation could range from destroying the ozone in our atmosphere to triggering climate change and altering life's evolution.

Now astronomers analyzing long-duration bursts — those lasting more than one to two seconds — in several Hubble telescope surveys have concluded that the Milky Way Galaxy is an unlikely place for them to pop off. They find that blasts tend to occur in small irregular galaxies where stars are deficient in the heavier elements. The Milky Way's starry population, by contrast, is rich in elements heavier than hydrogen and helium.

Suspecting that knowledge of their environments might help determine what types of stars produce gamma-ray bursts, the astronomers, led by Andrew Fruchter of the Space Telescope Science Institute in Baltimore, Md., used Hubble to examine the environments of 42 long-duration bursts and 16 supernovae. They found that the small fraction of supernovae that produce the bursts live in a very different environment from the average supernova. Fruchter's results appear in the May 10 online edition of the journal Nature.

Fruchter's team found that most of the long bursts in the sample were detected in small, faint, misshapen, (irregular) galaxies, which are usually deficient in heavier elements. Only one of the bursts was spotted in a spiral galaxy like our Milky Way, suggesting that our galaxy is an unlikely host for long-duration bursts. By contrast, the hosts of supernovae were divided equally between spiral and irregular galaxies, those with greater or smaller concentrations of the heavier elements.

Fruchter's team also found that long bursts are far more concentrated in the brightest regions of their host galaxies where the most massive stars reside. Supernovae, on the other hand, occur throughout their host galaxies.

"The discovery that long-duration gamma-ray bursts lie in the brightest regions of their host galaxies suggests that they come from the most massive stars – 20 or more times as massive as our Sun," Fruchter said. "Their occurrence in small irregulars implies that only stars that lack heavy chemical elements tend to produce long-duration GRBs." This means that long bursts happened 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.

Massive stars abundant in heavy elements are unlikely to trigger bursts because they may lose too much material through stellar "winds" off their surfaces before they collapse and explode. When this happens, the stars don't have enough mass left to produce the proper conditions that would trigger the phenomenon.

Astronomers think that gamma-ray bursts are produced by rotating black holes left over from stellar explosions. The energy from the collapse of a star's core escapes along a narrow jet, like a stream of water from a lawn sprinkler. The jet burns its way through the remnants of the star. The formation of directed jets, which concentrate energy along a narrow beam, would explain why the bursts are so powerful. But if a star loses too much mass, it may only leave behind a neutron star, not a black hole, and thus cannot create the jet. On the other hand, if the star loses too little mass before its collapse, the jet cannot burn its way through the dense outer layers of the star.

This means that extremely high-mass stars that puff away too much material may not be candidates for long bursts. Likewise, neither are 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."

Gamma-ray bursts can be divided into two classes: short bursts, which last between milliseconds and about two seconds, and produce very high-energy radiation, and long bursts, which last between two and tens of seconds, and create less energetic gamma rays. Although long bursts are unlikely to strike in galaxies like our Milky Way, short bursts could still happen. Short bursts are believed to arise from collisions between two compact objects, such as neutron stars. However, even with their higher-energy radiation, short bursts are typically 100 to 1,000 times less powerful overall than long bursts and would pose much less of a threat to life if one were to occur in our galaxy.