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107 ings. Our encounter with that cloud may occur in the next 50 thousand years."! Our Sun enters a denser environment each time it crosses the plane of the Galaxy. Some scientists have suggested that these crossings coincide with mass extinctions on Earth, but others find the evidence weak; we crossed the plane 2 million years ago without any sign of major damage to our planet.” Other potential disasters include a nearby supernova that strips our atmosphere’s ozone layer. On the average, one supernova every 50 million years is close enough to engulf the Earth in its expanding ejecta; some researchers once thought that a nearby supernova had killed the dinosaurs. (Sagan put a more positive spin on supernovas, observing that the evolution of life on Earth is driven in part by cosmic rays originating in the deaths of such massive suns.) Even a flare caused by a “star quake” can bathe the Earth in gamma-ray and X-ray radiation. The greatest potential killers are gamma-ray bursters, whose blasts of energy are concentrated into deadly beams. Short duration bursters, which may be generated when neutron stars merge with each other or with black holes, are more common but far less powerful than long-duration bursters, which may be the result of massive stars collapsing into black holes. Bursters can produce a flux of gamma radiation that would sterilize nearby life-bearing planets. The good news is that the radiation from one of these awesome events would merely wipe out the ozone layer and pos- sibly darken the sky, devastating the world’s ecology and food production. The bad news, according to one model of how bursters work, is that the radiation pulse might be followed by a month-long blast of extremely ener- getic cosmic-ray particles, as much as 100 times the dose lethal to humans.” The most brilliant explosion known was a gamma-ray burster 9 billion light-years away—more than halfway across the observable universe. If that event had taken place a few thousand light-years from us, it would have been as bright as the mid-day Sun, and it would have dosed Earth with enough radiation to kill off every living thing.°° It is only a matter of time before a burster occurs nearby, warned Leonard and Bonnell. Within the estimated danger range of 3 thousand light-years, we can expect one every 100 million years, similar to the mean time between the largest mass extinctions in Earth’s geological record. This may explain the preferential survival of deep-water organisms and the sudden rise of new species by increased mutation. Gamma-ray bursters may peri- odically reset the biological clock, forcing the evolution of life to start afresh.” Australian astronomer Ray Norris concluded that gamma-ray bursters could have a disastrous effect on us even at the distance of the Galaxy’s center. He expected such a burster every 200 million years. Yet, we have Black Clouds, Dark Planets, Bursting Stars