eli5:with billions of stars emitting photons why is the night sky not bright?
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The short answer? Space is absolutely massive, and most of that light misses us.
Imagine dropping a large rock in a swimming pool. The ripples from that rock will spread from the spot where it hit the water out in all directions. If you’re standing 10 feet away, you’ll feel those ripples strongly, because you are close and a large portion of those ripples hit you.
But instead, let’s say you are standing at the edge of a fairly large lake, and someone drops a large rock in the middle of the lake. You’re a few hundred feet away, and only a very small portion of the ripple hits you. It’s so small you barely notice.
Now let’s say you’re in the middle of the ocean. Rocks are constantly dropping elsewhere in the ocean, but the closest ones only drop a few hundred feet away from you. The difference between a rock dropping into the water ten feet away from you and a rock dropping into the water a mile away from you is massive. You only really feel the more closer rocks within a few hundred feet, but those ripples are still tiny.
That’s essentially what it is. We notice the light from the sun being so bright because it is orders of magnitude closer to us than any other star. Other stars are just as or even more bright, but because they are so far away, only a very tiny amount of that light actually hits us.
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