NOVA Online | Death Star | A Bad Day in the Milky Way

If a burster exploded within, say, 3,000 light-years of Earth, our prospects would not be rosy.

A Bad Day in the Milky Way
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How much ozone would be destroyed? Thorsett estimated that if the burster were located near the center of our galaxy, some 30,000 light-years away, the ozone depletion would be a few percent, comparable to that produced by natural disasters like large volcanic eruptions, very intense solar flares, or even a meteor impact on the scale of the one that exploded over Tunguska, Siberia in 1908.

If the burster were closer, say less than 3,000 light-years away, the gamma-ray flux received in a few tens of seconds could wipe out the entire ozone layer for years to come. At the very least, the drastic increase in solar ultraviolet radiation reaching Earth's surface would cause severe skin cancers. For humans and other animals, slow starvation would likely result, as the harmful ultraviolet flux inhibited plant growth and damaged and altered ecosystems supporting the food chain. As in a nuclear winter, the nitric oxides darkening our skies could also cause acid rains and significant cooling of the Earth's surface. Such pollutants would take decades to settle out of the stratosphere.

In the false-color gamma-ray all-sky map at left, a burster suddenly appears, briefly overwhelms all other celestial gamma rays, then vanishes. The graph at right shows the burst as it was detected by the Compton Gamma Ray Observatory.

But that's not all. In addition to the chemical changes in the atmosphere, the nuclear interactions induced by the high-energy gamma rays would rapidly produce huge quantities of radioactive nucleids, such as carbon-14, which has a half-life of 5,700 years. Of course, winds would distribute this fallout worldwide.

It gets worse

Depending on what the mechanism for producing a gamma-ray burst actually is, a nearby burst could wreak even more havoc. Nir Shaviv and Arnon Dar of the Israel Institute of Technology have explored a particularly devastating model for generating gamma-ray bursts from co-orbiting pairs of neutron stars. All neutron star pairs eventually spiral together, losing energy through gravitational radiation as predicted by Einstein's theory of general relativity.

Shaviv and Dar postulate that as the neutron stars begin their own catastrophic merger, jets of matter would be flung from the system at nearly the speed of light. These atoms and ions would be so energetic that they would absorb visible starlight and re-emit gamma rays, which we would detect as a gamma-ray burst. Impinging on our fair planet shortly after the horrific flash of gamma rays, the energetic particles themselves would join in the destruction, triggering still more deadly atmospheric cascades of nuclear interactions lasting up to a month.

If Shaviv and Dar are correct, a collapsing binary neutron star system anywhere nearby would spell doom for our fair planet.

These authors and others note that known pairs of neutron stars exist in our galaxy, including one within about 1,500 light-years. This knowledge has led to the speculation that in the past the Earth has found itself uncomfortably close to a violent neutron star merger. Some estimates hold that one occurs within about 3,000 light-years of the sun every 100 million years on average. Intriguingly, this timescale is roughly the same as the time between mass extinctions in our planet's geological record.

Learn to love the burst

One shouldn't worry too much, though. For one thing, mass extinctions in the past might have been the result of purely terrestrial phenomena, such as climatic changes produced by plate tectonics and volcanic activity, or of more familiar kinds of cosmic disasters, like the asteroid impact thought to have caused the dinosaur extinction at the end of the Cretaceous Period. For another, even if the aforementioned scenarios turned out to be true, we would still have, statistically speaking, about 50 million years until the next gamma-ray burst of doom.

Which gives us time to get to know bursters better. Our understanding of them constantly changes as new findings are reported. For instance, recent afterglow studies have indicated that a burst's energy is beamed in a particular direction rather than radiating in all directions from the source, substantially reducing the burster's total energy requirement.

This image shows M101, a nearby spiral galaxy, which bears two candidates for possible hypernovae—hypothesized explosions of high-mass stars that release perhaps ten times the energy of conventional supernovae.

Moreover, even after three decades of study, the true nature of gamma-ray bursts remains unknown. Many astrophysicists have taken a shine to a new theory, which for the moment has eclipsed the neutron-star-merger scenario in popularity. Evidence is mounting that at least some bursters are more likely associated with star-forming regions than with binary neutron stars. Theoretical models now in vogue indicate that gamma-ray bursts result from "hypernovae"—the collapse of the cores of extremely short-lived massive stars into black holes.

Another theory actually paints gamma-ray bursts in a positive light. University College Dublin researchers Brian McBreen and Lorraine Hanlon recently estimated the effects of a nearby gamma-ray burst on the preplanetary solar nebula, the cloud of condensing star stuff that formed our solar system some 4.5 billion years ago. They calculated that iron in the nebula would have been the major absorber of the high-energy X-rays and gamma rays from such a burst, causing the nebula's dust to become molten in seconds and then cooling slowly to form millimeter-sized chondrules, round granules of cosmic origin. Chondrules, they note, combined to form meteorites and possibly the rocky terrestrial planets, including Earth.

So, you see, despite the gloom-and-doom scenario I painted above, perhaps we should call them bursts of life.

Dr. Jerry Bonnell is an astrophysicist at Goddard Space Flight Center's Laboratory for High-Energy Astrophysics in Greenbelt, Maryland. Fame and fortune dog his every step as he studies gamma-ray bursts and co-edits the Astronomy Picture of the Day Web site (http://apod.gsfc.nasa.gov).

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