Why do we have a night sky?


Just saw a video of the hubble telescope zooming into a small patch of seemingly empty night sky revealing millions of stars and galaxies invisible to the naked eye.

Makes me wonder, if space is mostly empty and there’s possibly infinite number of stars and galaxies in any given part of the night sky, shouldn’t the night be as bright as the day?

In: 0

The universe is expanding. A consequence of this is that light from distant objects “shifts” into wavelengths that we can’t see.

What you’re looking for is [Olber’s paradox](https://en.wikipedia.org/wiki/Olbers%27s_paradox).

Stars are really far, so far that even stray atoms and space dust scattered in mostly empty space dim their light eventually. All the stars that you see with naked eye as single stars are closer than 10000 lightyears or so.

Light becomes exponentially more dim the further away you are, following the ‘inverse square law’

ELI5: your voice gets quieter the further away you stand from me. Stars get dimmer the further away you stand from them! If you had really really big ears you could hear me louder. Telescopes have really really big eyes and they can see stars from further away!

Lets start from Olberg’s paradox.

If the universe is infinite, static and eternal we should see a star at every point in the sky. Yet there are definitely some dark spots.

The resolution is that the universe isn’t eternal nor static and the infinite part is well we don’t know but the true size here is irrelevant.

Since we know the universe started from a small dense point (according to general relativity exactly one singular point) it’s not eternal which because light requires time to get to places puts a limit to the furthest most point from which light can reach you. So the visible or observable universe has a finite volume. And so it contains a finite amount of matter so things that can emmit light. So not every point on the sky has to have a star on it.

Ok so whats up with using a telescope to see stars and other structures that otherwise aren’t visible.

There are two things, luminosity and wavelength.

Luminosity is quite straightforward. If you have a radiating surface like a star the amount of radiation (like absolute amount of photons) is lets say some constant value at the surface of the star. This radiation leaves and so if you are further away like a distance r the same amount of radiation gets spread out over a larger spherical surface. The surface of a sphere is 4pi×r² so a unit surface area gets less radiation the further you are as the amount of radiation spreading out is constant but the area over which it spreads out is increasing. Which means the further you are the dimmer the object. Now a telescope can focus on some part of the sky for a long period of time to make an image. Over a longer period of time the telescope collects more light and so an image can be made. Its almost like moving cloaser to the object. Its just that you get fewer photos per unit time but capture the image over a longer period. Thats how you can make good photos at night just let your camera take the image for longer to collect more light. And lets not forget that telescopes have large areas for light collection. That could be a giant lense but its usually a giant mirror.

But dimness isn’t the only reason why you might not see stars so far away. Since the universe isn’t static but it expands the space between distant points grow significantly. This stretching also stretches the wavelength of light, so it redshifts it. (If you are wondering how is energy conserved here, it isn’t.) So objects super far away that emmit in the visible range get redshifted to infrared or below. What ends up as visible light is UV or heigher energy light. Now for Hubble it’s mostly sensitive to visible and UV. So it can only really see distant objects that emmited heigh energy light like star forming reasons. JWST for example is more sensitive to infrared so it can see visible light emmited from distant galaxies.

To sum up. The availability matterial and space for stars we may hope to observe is finite. Due to the expansion of the universe originally visible light gets shifted to infrared so we cant see that. Very distant objects just don’t give you enough radiation per unit time for your eyes to make an image of them.

Olbers Paradox. This is in fact one of the strongest pieces of evidence for the expansion of the universe. This observation — that in an infinite or near-infinite universe, your eye should see the light from a star in every direction you look, but you don’t — means that it must be expanding, so that some of that light never reaches us. That results in the dark sky we see, with only some stars in the field of view. Still a lot though!