eli5:with billions of stars emitting photons why is the night sky not bright?
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If light traveled instantaneously, then we’d have a problem. You could pick any random direction, draw and infinite imaginary line extending from it and, at some amount of distance, it would hit a star. Could be millions or billions or trillions of lightyears, but it would inevitably hit a star. Any direction. You couldn’t possibly pick a direction that doesn’t point to a star.
So the “night sky” would be a bit like being inside a star. Not cool.
What saves us is that light has a speed, and it isn’t very fast, considering the distances.
So, if you pick any random direction, it is very unlikely to be a direction from which light is currently coming from a star. Most likely, you’ll pick between two visible stars. Of course, even further down, we’re pointing at a star. But that star’s light is either still on its way to us, or more likely is outside of the observable universe and will never reach us.
With billions of billions of billions of stars in the observable universe, it feels like it would be unlikely to point to direction that doesn’t end up in a star. But stars are tiny (considering the distances) and very far away. It’s very unlikely to pick a direction that intersects with one. So the night sky is mostly dark.
A silly analogy would be: imagine that we buried Russia 100m deep in water balloons. I’m only picking Russia because it’s the biggest country. That’s a lot of water balloons, right? Now imagine we dumped them all in the ocean and waited for them to spread around all the oceans. You’d think it would be impossible to be swimming anywhere in the sea and not be surrounded by them. But in fact, you’d be lucky to see one. That’s how big the ocean is.
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