Cosmos: A Spacetime Odyssey2014

John Herschel:
Father, do you believe in ghosts?

William Herschel:
Why, yes, my son.

John Herschel:
You do? I would not have thought so.

William Herschel:
Oh, no, not in the human kind of ghost. No, not at all. But look up, my boy, and see a sky full of them!

John Herschel:
The stars, father? I do not follow.

William Herschel:
Every star is a sun as big, as bright as our own. Just imagine how far away from us you'd have to move the Sun to make it appear as small and faint as a star. The light from the stars travels very fast, faster than anything, but not infinitely fast. It takes time for their light to reach us. For the nearest ones, it takes years. For others, centuries. Some stars are so far away, it takes eons for their light to get to Earth. By the time the light from some stars gets here, they are already dead. For those stars, we see only their ghosts. We see their light, but their bodies perished long, long ago.