Comment by caf4676 on 20/07/2022 at 16:01 UTC

7 upvotes, 5 direct replies (showing 5)

View submission: Ask Anything Wednesday - Physics, Astronomy, Earth and Planetary Science

If there are billions of stars in the galaxy, why is the sky dark?

Replies

Comment by OfAaron3 at 20/07/2022 at 16:31 UTC*

24 upvotes, 0 direct replies

Simply put, because they are so far away. If you turned a lightbulb on 100 miles away, would you be able to see it?

Some stars are brighter and/or closer than others, which is why we can see the ones we see. However, with the expansion of the universe, one day everything will be too far away to see.

As Douglas Adams put it, "Space is big. Really big. You just won't believe how vastly hugely mind-bogglingly big it is."

Comment by amaurea at 20/07/2022 at 19:06 UTC

7 upvotes, 0 direct replies

Here's a simple back-of-the-envelope calculation to help illustrate that there just aren't enough stars in the Milky Way to fill the sky.

A star at a typical Milky Way distance of 10,000 parsec[1] with the same radius as the Sun will have an angular radius of about θ = 1e-10 degrees, and hence an angular area of A = πθ² = 5e-20 square degrees. The full sky has about 41000 square degrees in it, so to fill the whole sky with stars one would need of the order of 41000/5e-20 = 1e24 stars in the Milky Way. That's about ten trillion times more stars than the Milky Way actually has.

1: https://en.wikipedia.org/wiki/Parsec

This ignores important details like the shape of the Milky Way, but it gives the right general impression. Put simply, while a star is big, the galaxy is much, much, much bigger, so it ends up being very dilute.

Comment by teo730 at 20/07/2022 at 17:27 UTC

10 upvotes, 1 direct replies

This is Olber's paradox[1]. The wiki covers most of the explanations in a fairly simple way.

1: https://en.wikipedia.org/wiki/Olbers%27_paradox

Comment by ThoughtCenter87 at 20/07/2022 at 21:31 UTC

3 upvotes, 0 direct replies

There's a couple of reasons for this, but the main reasons are: Distance, star sizes, and star type prevalences.

We'll begin with the issue of distance. Now light never stops traveling in a vacuum, unless an object absorbs it. Stars are large enough that planets won't be able to block all of their light, meaning that we should be able to see all the stars in the galaxy. Right?

Well, think about this. Set a light bulb and walk away from it at night. The farther away you move from the light, the smaller the light gets due to distance. If the Earth had no curvature and was a flat plane (this is a hypothetical to make this analogy work), eventually the light would become so small that it would be impossible to see with the naked eye. This is a similar principle with stars in the night sky, many stars are simply far too small and too far away to be visible with the naked eye from Earth.

Red dwarf stars are thought to be the most common types of stars in the universe. It also just so happens that they're the smallest main sequence stars, and all are so small and far away from Earth that none of them are visible to the naked eye. So essentially, most stars that should be visible in the night sky simply aren't because they're too far away for enough of their light to reach Earth.

What you see in the sky then are the less common types of stars. Red dwarves make up approximately 70% of known stars in the observable universe, meaning that you're left with 30% of other stars to be potentially visible in the night. And even then, not all stars that exist in the universe which aren't red dwarves will be visible, as K- and G- type stars must be fairly close to Earth in order to be visible in the night sky. (Side note: There are some K- and G- type stars visible in the night sky, but they're all fairly close to Earth in terms of light years.)

The most common stars you see in the night sky then are the biggest and brightest stars in the observable universe. The largest stars are also the least common to exist in the universe.

So for the most part, what you see in the night sky are the largest, least rarely formed stars in the universe, with speckles of the more fairly common (but still rare in the grand scheme of things) K- and G- type stars.

*The sky is essentially dark because most stars in the universe are simply too small to be visible from Earth and light up the night sky.** Interestingly however, the ground in the best viewing conditions (i.e no pollution to blot out excess light) is lit up by the stars in the night sky. So in the best viewing conditions, the sky isn't dark, and is in fact quite bright. There are images of the night sky blanketed with stars, and it's beautiful - truly a sight to behold. If you can go to a place with zero pollution, I highly recommend traveling there to see the night sky. In the summer the milky way is even visible in certain parts of the world. I suppose another factor to keep in mind then is pollution - cities and other urban areas will have sufficient pollution to block out most other stars that aren't bright enough.

Comment by yARIC009 at 22/07/2022 at 01:45 UTC

2 upvotes, 0 direct replies

I read all these answers but I don’t see any that seem to give the actual answer. The answer is because the photons come off the surface of the star perpendicular to the surface and because the surface of the star is round the photons traveling out are all going at a slightly different angle away from the center of the sphere. Because of this, the photons are all slowly diverging from one another as they travel away from the star. Get far enough away and the gaps between these vectors are huge. Get light years away and you’re only getting hit by a tiny fraction of the total photons emitted by the star that happened to come off the surface of the sphere/star that were pointed directly at you.