eli5:with billions of stars emitting photons why is the night sky not bright?
In: 471
Everything about light weirds me out. Like is every star throwing light out in infinite directions? Like how can I walk 1 foot to the left and still see a stream of photons from a star 10 billion light years away?
The “night sky” is basically the “rest of the universe”, so it’s an unimaginable amount of space.
1. **Time** – Light travels at a finite speed and the universe has a finite age, meaning that beyond a certain distance, the starlight hasn’t had *time* to reach us yet.
2. **Space** – Space is really, really, really, really, REALLY, REALLY big. To get an idea of just how big space is, check out [this site](https://joshworth.com/dev/pixelspace/pixelspace_solarsystem.html), which is essentially the solar system (*just the solar system*) to scale. Interstellar space is much *sparser* than even this; intergalactic space sparser still.
3. **”Spreading out” of light** – A star emits from its surface a specific number of photons per unit of time (This is an oversimplification, and light also behaves as a wave, but this is ELI5). That number doesn’t change. (Well, it does, but not for our purposes.) What *does* change is the area of the sphere these photons cover – they have to “spread out” across the universe. The further they spread out, the dimmer the star is – the fewer photons hit your eye per unit of time, and once that number gets below a certain threshold, you just can’t see it. If you were to move the Sun out to where Sirius is now, it would still be a pretty bright star, but it would be less bright than Sirius, perfectly safe to look at. (Also, we’d all freeze, but that’s beside the point.)
4. **Redshift** – Almost all of the stars you can actually see without a powerful telescope are from the Milky Way. Because the universe is expanding, stars from distant galaxies are “redshifted” – their light gets pushed toward the red end of the EM spectrum, and eventually out of the visible-light range entirely. This effect is stronger the further away from Earth you get.
Thanks for the excellent explanation! If everything is moving away, how will the Milky Way eventually merge with the Andromeda Galaxy (as an example)?
Latest Answers