Andromeda is 2.5 million light years away. if you live in a dark sky area, it is the furthest thing that can be seen with the naked eye. think about this, it is the closest galaxy to our own, not including our satellite galaxy's the greater and lesser magellanic clouds, that can be seen from the southern hemisphere. the photon that hit your eye have been traveling for 2.5 million years directly to your retina. and they are not pulsed, so that means it is a SOLID stream of photons stretching from Andromeda to your eye.