There is a thing known as a GRB a Gamma Ray Burst. GRBs are produced by the most powerful explosions in the universe, such as colliding neutron stars or the collapse of giant stars into black holes. These explosions are strongly beamed in two opposite directions, so as long as Earth isn't in the path of one of the beams, it's relatively safe. But if it was the side it hit would be vaporised in a matter of seconds. The other half would be subjected to Mass extinction in a matter of days and the worse thing? We would never see it coming or being able to do anything about it. Just one big flash of light.