The universe is a dangerous place. When you look up at the Moon, you can see that it has been scarred over millions of years, by impacts from rocks in space. But there are even greater dangers out there, that could potentially happen at any moment. A supernova is a stellar explosion - where a star literally explodes, releasing huge amounts of energy, and devastatingly changing the space around it. According to Wikipidia, supernovae are rare events, occurring around every 50 years in a galaxy like our own. But, what would happen if a supernova exploded in the vicinity of our planet:
Speculation as to the effects of a nearby supernova on Earth often focuses on large stars, such as Betelgeuse, a red supergiant 427 light-years from Earth which is a Type II supernova candidate. Several prominent stars within a few light centuries from the Sun are candidates for becoming supernovae in as little as a millennium. Though spectacular, these "predictable" supernovae are thought to have little potential to affect Earth. Type Ia supernovae, though, are thought to be potentially the most dangerous if they occur close enough to the Earth. Because Type Ia supernovae arise from dim, common white dwarf stars, it is likely that a supernova that could affect the Earth will occur unpredictably and take place in a star system that is not well studied. One theory suggests that a Type Ia supernova would have to be closer than a thousand parsecs (3300 light years) to affect the Earth.
And from a NASA article on near-Earth supernovae:
Supernovas near Earth are rare today, but during the Pliocene era of Australopithecus supernovas happened more often. Their source was an interstellar cloud called "Sco-Cen" that was slowly gliding by the solar system. Within it, dense knots coalesced to form short-lived massive stars, which exploded like popcorn. Researchers estimate (with considerable uncertainty) that a supernova less than 25 light years away would extinguish much of the life on Earth. The blast needn't incinerate our planet. All it would take is enough cosmic rays to damage the ozone layer and let through lethal doses of ultraviolet (UV) radiation. Our ancestors survived the Pliocene blasts only because the supernovas weren't quite so close.
Death is one of life's inevitabilities, but the fact is that human civilisation has only existed for a blink of an eye, compared to the age of the universe. If you imagine these dangers are not going to happen in your lifetime, you could be right. But then, as time goes on, the chances of a devastating supernova increases, until one day, it will occur. The only solution is to continue the exploration of space and look for other safe havens like Earth. The more we have human populations on other planets, the more likely that the destruction of this planet, will not mean the extinction of our species.Go Top