A supernova, or the explosion at the end of a giant star's life, is one of the most spectacular and destructive phenomena in the natural world. Although the risk is small, a nearby supernova could be devastating to life on Earth because of the intense radiation emitted in the form of a gamma ray burst.
What happens when a star exhausts the last of the hydrogen in its core and ends its life depends upon the size of the star. Sufficiently large stars - those much larger than Earth's own Sun - blow apart in a massive explosion called a supernova. According to scientists working with NASA's Chandra X-Ray Observatory, there are two types of supernovas: those which happen when a massive giant star experiences a core collapse (a "Type II" supernova), and those which happen when a smaller star which has already collapsed into a white dwarf experiences a sudden, brief core re-ignition, like a fire restarting from hot embers, and explodes (a "Type Ia" supernova). When supernovas happen, say the NASA scientists, they "produce a brilliant visual outburst that can be as intense as the light of several billion Suns."
That's a potentially serious problem for planets orbiting nearby stars. Supernovae emit intense radiation. At the vast distances of interstellar space, this is still usually harmless, but as the supernova occurs, an extraordinarily intense stream of particles called a gamma ray burst is emitted from each of the star's poles. Ken Parelius says the gamma ray burst contains "more energy than our Sun will (release) in its entire lifetime." A planet caught in the middle of the beam could potentially have its ozone layer stripped away, and its surface would be exposed to deadly radiation. Koji Mukai and Eric Christian, also of NASA, suggest that a supernova which occurred within 30 light-years of Earth's solar system could potentially cause mass extinctions on Earth - including wiping out the human species.
Fortunately, there are no stars that close to Earth which are expected to explode in the near future. However, that limit is a hypothetical one, and there's still considerable uncertainty and concern about what would happen at a somewhat greater distance - especially from a reigniting white dwarf star, since those events are much harder to predict. In recent years, scientists have been poring over rock and soil samples in search of evidence of gamma ray bursts associated with old supernovae - ones that occurred at a safe distance, but still exposed Earth to noticeable levels of gamma radiation. In 2009, for instance, a Japanese research team in Antarctica reported that they had found chemical evidence of gamma ray exposure in 1006 A.D. and 1054 A.D. Chinese astronomers recorded supernovae in both those years; the second of the two created the Crab Nebula.
Those supernovae were far enough away that they did not pose a significant threat - the Crab Nebula, for example, is 6,500 light-years away. However, the gamma ray bursts they emit are a far more worrying problem. Because the bursts are so much more intense than the rest of the explosion, they can harm planets that are much more than just a few dozen light-years away. The bursts are only emitted in direct beams from the two poles, which limits the damage, but the distance at which they are dangerous is far greater. A gamma ray burst is one of the explanations that has been advanced for the Ordovician Extinction of 440 million years ago, when most marine invertebrate species went extinct.
For several years now, for instance, scientists have been carefully examining a Wolf-Rayet star, WR 104, which is about 8,000 light-years away. That's far enough to be harmless - unless, when it explodes, it emits a gamma ray burst in Earth's direction. That burst might be enough to cause mass extinctions and endanger human survival. Right now, according to coverage in Forbes magazine, astronomers are trying to calculate in what direction the star's axis is pointing. If it's north or south pole is not pointing within a few degrees of Earth's location, they say, Earth should be quite safe - from WR 104, anyway.