eli5:with billions of stars emitting photons why is the night sky not bright?
In: 471
Because they’re extremely far away and not all of their light hits us. We only capture a fraction of the light that is emitted by each star that’s visible.
the same reason you can comfortably stare at headlights from a mile away but not 20 ft away, light scatters from stars in all directions, not just towards us, they are tiny specks at very great distances only some very tiny portion of their light comes your way at all
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.
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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All of the “tiny angle of light emission hitting us means that we barely get any photons at all” is very true and valid, but there is another phenomenon that helps explaining even more such behavior: Red-shifting.
The mechanics of it are a bit rougher to explain, but it’s essentially that the ever-going expansion of the universe causes light’s wavelengths to lengethen so that over a long enough distance, what is blue ends up looking red, and what is red ends up in the infrared spectrum, which cannot be seen with the naked eye.
This is known as Olber’s Paradox. If the universe is populated with a distribution of stars similar to what we see nearby, then the math works out that every sight line should end at a star and the night sky should be bright. However, because the universe appears to have a finite age and the speed of light is also finite, most sight lines end at the very distant remnants of the soup of primordial fire that was the early universe, which was also very hot and therefore very bright.
So the the real answer is not that brightness is too distant or too sparse. The real answer is redshift. The light from very distant stars and from the early universe has been stretched by the expansion of space into wavelengths far longer than what we can see. You may have heard of it as the cosmic microwave background.
Other answers are missing something — it’s not just that space is big and the stars are far away and the light gets diffused away. Imagine you had an infinitely big, infinitely old, unchanging universe. Like OP asked, in this situation any direction you look, you would end up looking at the surface of a star, some just very far away. (The stars being far away doesn’t reduce the brightness because there are a lot more far away stars than near stars in this picture, so the light adds up.) This obviously isn’t what we see, so at least one of the assumptions is wrong.
* If the universe isn’t infinitely big, there could be a “farthest” star, so you don’t get all that light added in from the very far stars.
* If the universe isn’t infinitely old, it could be that the light from the farthest stars hasn’t reached us yet, so there are still plenty of gaps between the stars we do see.
* If the universe is changing over time, not all stars are visible all the time because some expansion removes them from the visible universe.
The answer is some combination of 2 and 3, both of which are consistent with our current understanding of the Big Bang and the expansion of the universe. The point is that you need to have a picture in mind where we can only see a finite number of stars at any given time, rather than an infinite number of stars. Today we call this [Olbers’ Paradox](https://en.wikipedia.org/wiki/Olbers%27_paradox) but the question has floated around since the 1600s at least.
As many others have said it’s because space is big, but this is actually a very important observation – if space was infinite and timeless, as many believed just over a hundred years ago, it WOULD be bright – every line of sight would end in a star. The observation that it’s dark really supports the idea that the universe isn’t infinitely old (since if it was, light from however far away would be able to get to us) and also supports its expansion (since the light can shift away from visible into the “cosmic background radiation.” So yeah, this is a great question and a very important idea!
Three reasons :
1) Light faints the further it is. It doesn’t mean it disappears , it means it loses energy. Our human eyes are not accustomed to this sort of light, we can only see things with a certain energy because that is the way humans evolved. If we could see in the microwave spectrum, we could see that fainted light and the universe would be all bright.
2) Some stars and galaxies are very far away from us. It takes time for light to travel to us, and a lot of this light hasn’t reached us yet.
3) Space itself is expanding. The further space is, the faster it’s expanding, even to the point where space is expanding faster than the speed of light. So for the stars very, very far away, light will never reach us because it’s too slow.
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