A supernova blast 41,000 years ago started a deadly chain of events that led to the extinction of mammoths and other animals in North America, according to two scientists. ...a key piece of evidence for the supernova is a set of 34,000-year-old mammoth tusks riddled with tiny craters. ...Whatever caused the craters had to have been traveling around 6,214 miles per second, and no other natural phenomenon explains the damage, they said. They think the supernova exploded 250 light-years away from Earth, which would account for the 7,000-year delay before the tusk grain pelting. It would have taken that long for the supernova materials to have showered to Earth. Then, 21,000 years after that event, the researchers believe a comet-like formation from the supernova's debris blew over North America and wreaked havoc.

This means that any supernova that could affect us would have been seen 6750 years ago. Or a different time depending on the distance. Basically, we won't see it coming!

Source: Discovery Channel