William Herschel referred to the light of dead stars that we see from Earth as the "ghosts" of those stars. Could this be the reason our companions are called "Ghosts?" Are they the leftover light of stars that the Traveler has visited? The dead light of the Traveler itself? Something else altogether? I believe Destiny is full of these subtle references, and that's why I keep playing. Bungie is still creating a deep meaningful universe even if the story we play is somewhat lacking.