A. Earth is going to be hit by a super-mega-death laser focusing the entire power output of a star into a concentrated beam.
B. A long long time ago, a very dense moon was very far from a supernova. Unfortunately, far is relative, and so is speed in space. Specifically, relative to Earth, the moon is now traveling 5% the speed of light and is on a collision course.
In scenario A, there is no way for us to see this coming, because information also cannot travel faster than light. The light is what kills us.
In scenario B, it is theoretically possible to see this rogue-moon coming, since it is possible for light to reflect off of it and reach us before the moon does.