Olbers’ Paradox

I had this in a homework a while ago, written by Olbers in 1823:

If the universe is infinite with an infinite number of stars, then every line of sight will terminate at the surface of a star. Therefore, the whole sky should be bright with starlight.


Have a go and think of why this couldn’t be the case. There are multiple reasons for a dark sky, so there isn’t one right answer!


Here are some ideas I had, but comment any others which I’ve missed:

Wilhelm Olbers

Wilhelm Olbers suggested this in 1823, way before we had Hubble’s evidence of an expanding universe. The paradox, however, is based on a static universe, which isn’t actually the case. Our visible universe is not infinite nor static, so there is a limit to how far we can see. Therefore there isn’t an infinite amount of stars in every line of sight, and there hasn’t been much time for far away starlight to reach us.

There’s also the most annoying argument: dust. Astronomers hate it so much because it absorbs and scatters a lot of light, mostly small wavelength light. If dust absorbs a huge amount of incident light to the point that it obscures our own data, how on Earth are we going to see far away faint stars?

I think it’s also interesting that Olbers is implying here that dark = few stars, but many stargazers would argue that in good dark conditions, more stars can be seen. This is just the Sun’s brightness washing out all other stars in the sky during the day, and light pollution washing out most stars at night. I feel like this isn’t the best argument, but it’s an argument against a white night sky nonetheless.


The basis of the paradox can actually be traced back to Kepler, who used the paradox as an argument against an infinite universe with infinite stars. Good job Kepler!

Comment any other ways you can explain why the sky isn’t blindingly bright, I want to hear them!


  1. I will say this: the first time I saw a truly “dark” sky, with no Moon and minimal light pollution, it was astonishing how bright it was. But most of that brightness comes from the Milky Way. The rest of the sky is noticeably less bright. At the very least, this should suggest to Olbers that the stars are not evenly distributed throughout space.

    Liked by 1 person

  2. That is true. There’s a lot of voids, where stars tend not to be. Olbers also didn’t mention dust’s huge absorption of starlight, which is a big problem in the optical part of the electromagnetic spectrum.


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