eli5:with billions of stars emitting photons why is the night sky not bright?
In: 471
There are 3 things going on here and most people are only mentioning one of them.
1) light is made out of massless particles called Photons. Our eyes need at least 5-7 photons in order to detect anything. And then the more photons, the brighter the it looks.
2) The brightness of light, or in other terms the density of the photons, is affected by what’s called the “inverse square law”. That means that the photon density is proportional to 1/ (d^2), where d is the distance. For example if you have 2 light sources of equal brightness, one is 1m away and the other is 2m away, the one that is 2m away will appear 1/4 as bright as the 1m one.
Now stars, are VERY far away. The sun is roughly 0.000016 light years away while Proxima Centauri (the closest star to our sun) is 4.2 ly away. If they were the same brightness, Proxima Centauri would appear 1/68,906,250,000 as bright as the sun.
Because of this, a lot of stars are just so far away that the photon density does not allow 5-7 photons to enter our eyes so we just won’t see it.
3) space is not empty. There is a lot of dust and gas just floating there that blocks light.
Latest Answers