[jim5cents]

Consider that light is the fastest known moving object in the universe. If there was a star that went supernova and the explosion of said star would destroy the Earth, we would find out about it at the same exact moment that we first saw or heard the supernova. Give the size and mass of stars, as well as their distance from Earth, it is near improbable to predict the day, week, month, or year of a supernova. Consider if Alpha Centari, after the Sun, the next closest star from the Earth exploded, we wouldn't know about it for 4.37 years after the supernova. That is how long it would take the light from that explosion to reach Earth.