Every year there are at least 500 gamma-ray bursts in the universe. They destroy everything within a few hundred light years of the source. What do these extraordinary explosions mean for intelligent life in the cosmos?by Oliver Morton / January 20, 2000 / Leave a comment
High above the earth’s atmosphere, in one of Nasa’s most expensive spacecraft, sit eight crystal discs of sodium iodide. Now and then they are lit with the faintest of glows. The cause is a sudden surge of gamma rays-electromagnetic radiation like that which makes up visible light, but of shorter wavelength-sleeting through the spacecraft. Within seconds, if all works according to plan, news of the gamma rays’ passage bounces back down to Earth and on to the internet, so that other telescopes can seek out the rays’ source. To astrophysicists, the source of such gamma-ray bursts is one of the most enticing questions around.
For most of the earth’s other inhabitants, gamma-ray bursts are a subject of only passing interest. As far as astronomy goes, the public finds the search for new abodes of life far more interesting than the peculiar emanations of distant stars and galaxies. Following the inconclusive suggestion of fossils in a Martian meteorite and the discovery of ever more, ever stranger, planetary systems around nearby stars, the possibility of life elsewhere is reasserting itself as a central theme in the exploration of space, and gamma-ray astronomy seems remote from these “astrobiological” concerns. But, as we shall see, there is a disturbing link between these two fields.
Gamma-ray bursts were discovered in the 1960s, when satellites looking for nuclear tests started to see them regularly. They were a surprise; unrelated to any objects or events that could be seen in any other wavelengths. Some thought they came from inside our galaxy, the result of disturbances on the surfaces of the dense neutron stars into which some old massive stars collapse. Others thought that they occurred further afield.
The Compton Observatory-the spacecraft which carries those gently glowing crystals-has ended this debate. Its detectors pick up almost one a day, and it is estimated that they miss one third of all gamma-ray bursts because of the way the earth blocks the sky. All told, there are some 500-600 gamma-ray bursts a year. With this much data, it has been possible to look for any patterns that they make on the sky. If they came from old stars in the galaxy it would be reasonable to expect them to cluster in the galaxy’s central plane. They don’t. The sources are scattered evenly across the sky, and it is now accepted that they can be found in distant galaxies.