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.
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.
Basically because space has a lot of space haha theirs just not wnough their to reflect light back so you end up only seeing where its coming from which is the star or planet the light reflected or emitted from
“Space is big. REALLY **BIG**. You just won’t believe how mind-bogglingly huge it is!” …It’s so big that some stars are so far away from us that their light hasn’t had time to reach us in the billions of years the universe has existed, even traveling at the speed of light. And, the universe is expanding. So, those distances are only getting bigger. So, the light from some distant stars will NEVER reach us.
It is, you just can’t see it because some haven’t arrived yet and some have redshifted into radio.
Have you ever been outside on a very clear night in an area without light pollution from a city? It’s bright enough to see pretty well. Most stars we see are incomprehensibly far away though, so the intensity of all that light is still not enough to even come close to the light of the sun (daytime). The fact you see them at all, considering how far they are, is a testament to how bright they are
Redshift. If we could see microwaves, the whole sky would be lit up due to the cosmic microwave background left over from the early universe. However because light has a speed limit and space is expanding, a lot of light either hasn’t had enough time to reach us or has shifted out of the spectrum we can see.
I’ve heard that if we were closer to the center of the Milky Way it would never get dark out because of the amount of stars.
The way I have heard it:
Imagine a cup of lemonade, you drink it, it tastes fine. When you add water, you taste less of the lemonade, adding more and more water makes you taste less and less lemonade.
Imagine that light is the lemonade, and water is the distance between us and the stars.
Because space is big. Bigger than you can possibly imagine. Yes even bigger than whatever you’re imagining now. There are all those stars emitting light but the space between them is so vast and empty that the light is dispersed.
All those stars in the universe emitting light are light a hundred people with flashlights spread across a football field at night. You will be able to see each flashlight in the dark but it will not illuminate the field as if it’s daytime.
Latest Answers