How Can We See Something if the Source of the Light is Gone?

"How can light stay travelling if the source of the light is gone? I read that some stars are dead but we can still see them" - Question posed by Vicky.

As the nights are drawing in, and the weather's getting colder, I'd like you to imagine you're in a snowball fight. You grab a fistful of snow, pack it into a vaguely spherical shape, momentarily poke your head out of your hiding place, take aim, throw and duck back down. You're back in hiding before your snowball hits its target, but you know from the groan that your mate just got a faceful of the cold stuff.

That snowball carried on in its journey across the space between you and your target regardless of the fact that you had disappeared from view. Light is faster - a lot faster - but it still takes time to get to places. Before each lucky photon* of starlight that reaches your eyeball is able to do so it must first be emitted by a star. 

Between being released and being absorbed by your retina, it has a journey of hundreds, perhaps thousands of years to complete. Once being sent on its way, it's independent of its parent star, making its lonely way between star systems: it has no more contact with its sun after leaving it than it has with your eye before hitting it. In the same way that the photon wouldn't cease to exist even if your eye were to wink out of existence** before it got there, neither would it be bothered by the death of its parent so far behind it. It just keeps trundling on regardless!




* A 'photon' is a little packet of light.
** No pun intended. Well, o.k, maybe a little one.

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