eli5:with billions of stars emitting photons why is the night sky not bright?
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Think about a flashlight. It’s emitting a certain number of photons; let’s call it 100 for the sake of easy explanation.
It’s really bright when you put it right on your eye, because all 100 photons enter directly into your eye. If you move it back 5 feet, those 100 photons are spread out across a larger area. That means something like 50 photons are hitting your eye. Move it back to 10 feet. Now, something like 15 photons are hitting your eye. Move it back to 20 feet, and only a single photon is hitting you in the eye.
Stars are massive and bright. They’re also billions of miles away, which means the light is spread out over a HUGE area. That leaves the sky pretty dark from our perspective.
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