I've heard that space is teeming with death rays capable of destroying all life on Earth. Please tell me this isn't true.

—Angus McMahon, Frankfurt, Kentucky

Last January 23, astronomers working in Los Alamos, New Mexico, captured an event on film that was so mind-bendingly creepy it could have been dreamed up by Arthur C. Clarke: the brightest and most violent explosion ever detected in the history of science, a cosmic detonation whose force was estimated to be second only to the Big Bang itself. Scientists identified it as a mammoth gamma ray burst, an eerie phenomenon first discovered in 1967 and believed to be a gasp of radiation released just seconds before a dying star collapses into a black hole. Thanks to high-tech space cameras, astronomers are now capable of observing one or two GRBs every day. But do these celestial flameouts pose a mortal threat to us earthlings? The easy answer is no: By the time the January 23 megaburst traveled nine billion light-years to reach us, it had diffused so thoroughly that all that was left was a harmless fireworks display.Not so, however, should one of these cataclysms occur right here in our own galaxy. "It would quite simply be the worst natural disaster you could ever imagine," says NASA astrophysicist Peter Leonard. "The rays would destroy the ozone layer and cook the planet in a kind of cosmic sterilization bath, leaving the Earth radioactive for millions of years." According to astronomical models, it's only every 200 million years or so that a dying star collapses close enough to Earth —within 3,000 light-years—to deal us a catastrophic blow. (Some maverick scientists have even begun to speculate that the last one may have killed the dinosaurs.) Happily, the next Armageddon-style burst is probably still millions of years away, but if it arrives early, we likely won't be able to deflect the galactic death rays, much less see them coming. "It's difficult to make out the gun," notes Leonard, "when you're staring down its barrel."

I live at sea level, and when I go climbing in Colorado I get nosebleeds. Why?

—Linda Flegel, Vancouver, British Columbia

A variety of factors conspire to make mountains a bane to nasal cavities. Everyone knows that high-altitude air is extremely dry (thin, cold, low-pressure air holds less moisture) but the subtleties of the schnozz are more complicated:"The skin inside your nose isn't like the skin on your arms and legs," says Dr. Murray Grossan, a Los Angeles ear, nose, and throat specialist. "It's paper-thin and loaded with blood vessels." A few hours in the mountains, and the dry, oxygen-lean air will transform it into a chapped and bleeding mess. Humidifiers and saline spray can be a bloody nose's best friend, but if the problem lingers, consider consulting your doctor: You may be suffering from a deviated septum.

Why are polar bears so mean?