Astronomical Answers! – The bright night sky paradox…

Welcome to the second installment of Astronomical Answers! I can honestly say, you don’t like giving me easy questions to answer- this week I’m going to try explain why the night sky is dark. Or, more precisely, I was asked this- given that there are an almost infinite number of stars in the sky, in different positions and distances, why is our sky mostly black at night and not pretty much all stars with intense brightness?

Now I’d imagine there are several of you reading this rolling your eyes and expecting this to be some easy answered question… that’s what I thought before I started thinking in detail about it. Oh how wrong an assumption could be.

If all lines of sight end in a star, should the sky not look like this all the time? Image credit: Kmarinas86 via the Wikimedia Commons

If all lines of sight end in a star, should the sky not look like this all the time? (If the animation isn’t working, try clicking on the image!)
Image credit: Kmarinas86 via the Wikimedia Commons

So, what is it that makes this such a controversial and hard to answer question? A quick search will reveal the idea of a paradox, first set out by a German physicist- Heinrich Wilhelm Olbers in 1823. The paradox begins by asserting; we see stars all around us, current theory suggests that space is infinite and as such you can (sort of) say that if you look out in any direction far enough, you will eventually see a star. This star is likely to have roughly the same brightness as our own star. So why don’t we see these stars? Surely if there was a star at every point in the night sky, the sky would be as bright as day?

Throughout the 190 years since it was first posed, there have been many attempts to answer the paradox. Here are a few that have stood up to questioning better than others:

The dusty darkness – One explanation that has been proposed is that light is absorbed by the massive amounts of dust that lie between us and those far off stars. The main problem with this suggestion is, the light falling on the dust would eventually heat it up and that too would start glowing- so we can’t really accept that as our explanation!

The next two explanations are the truly controversial explanations- both have arguments for and against:

The infinite versus the finite – A major argument being made about this paradox is the finite nature of a star’s life cycle. When we look at the night sky, we aren’t seeing everything, we’re seeing a snapshot in time. Therefore, there shouldn’t actually be an infinite number of stars in the sky. As stars are born and die, the points of light change but the brightness of the sky stays constant.

Red shifted to the invisible – The principle behind this explanation lies in the expansion of space. It is widely accepted that space, and indeed everything within it, is moving away from us and it’s doing that really quite quickly! When an object is giving off light it causes this light to become ‘red shifted’ according to the Doppler effect. For the benefit of any of you reading that are interested, here’s a quick explanation of Doppler shifting- alternatively, jump past the italic paragraphs!

Let’s imagine an object, a light bulb perhaps, and this light bulb gives out an imaginary amount of light per second- for ease, let’s say one wave of light per second. This light is forming continuously so we can also say that it takes one second for the wave to be completely formed.

So we have a light bulb sat there giving out one wave of light per second. Now, if you could slow down time and watch how the wave of light being emitted from the stationary light bulb forms, you would see a steady chain of waves forming with all the individual waves the same length as each other and steadily moving away from the object. Now if we imagine the light bulb  moving quickly away from us, it’s giving out the same light but it’s doing it over a larger distance in the same amount of time. This means the wave being formed has to be ‘stretched’ over this distance. The effect of this is a longer wave, or wavelength. It’s the wavelength of light that gives it the colour we see, or in the case of extreme Doppler effects, don’t see! The term ‘red shift’ comes from this elongation of the wave, shifting light towards the red end of the spectrum. Anyway, back to Olber and his paradox… 

The Hubble ultra-deep field image was taken in the infrared spectrum to account for this red shift effect Image credit: NASA

The Hubble ultra-deep field image was taken in the infrared spectrum to account for this red shift effect
Image credit: NASA

So, everything is moving away from us but the more important factor in this explanation is how the further an object is away from us, the faster it is moving away from us. Therefore, the further a star is away from us, the more red shifted the light it emits will be. If you follow this through to its inevitable conclusion, you eventually get to a point where the light shifts into the infrared region, and further- but past this point, it is no longer visible to the human eye. However, this doesn’t explain everything. Stars also give out UV and shorter wavelengths, which would be shifted into the visible spectrum, re-creating the problem! Why don’t we see that light?

Out of the popular explanations, I am personally inclined to agree with the red shifting explanation with aspects of the finite lifespan of stars explanation. But do have to wonder whether the sensitivity of our eyes has something to do with what we observe, and whether the proximity of the sun to the point we are observing from, affects what we see? Many of you will know that the further an object is away from you, the dimmer the light appears- perhaps it is just that effect, which would presumably have to be greater than the energy added by having more stars at that distance? Just a few thoughts for you to mull over until the next article!

And if I’ve not managed to convince you, try this fun explanation from the YouTube Channel, Minute Physics by Henry Reich:http://www.richannel.org/why-is-the-nights-sky-dark

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About Mark Nottingham

Mark is a PhD student in the Isotope Geochemistry and Cosmochemistry group at UoM. Primarily working on the RIMSKI and RELAX noble gas instruments.
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3 Responses to Astronomical Answers! – The bright night sky paradox…

  1. If the redshift explanation is correct, then you most likely do see visible light that was initially UV before the redshift process. However, the emission spectrum for our sun (http://legacy.chemgym.net/environmental_chemistry/topic_1a/images/emission_spectrum.gif) shows that a typical stars output of UV is substantially less than the visible emission. Therefore a star whose only visible light arriving at Earth, is EM waves that were initially UV, the star would appear a lot less bright, and so the problem isn’t fully re-created to the extent that there’s still a bright night sky paradox.

    • Mark Nottingham says:

      Admittedly, this isn’t my research field, so my ideas are based on transposed principles- but, the UV-A output is comparable to the red output of a star. I agree that the overall UV output across the spectrum is a much lower intensity than through the visible region (particularly when considering the intensity of visible light is a combination of all individual ROYGBV wavelengths, whilst the sum of the UV region has considerably fewer high intensity components). I believe the explanation that resolves this lower intensity is the presumption that, the further out you go, the greater the number of stars there are contributing to the overall light intensity. Or at least that was my understanding whilst I was reading about this paradox!

      I’ll see if I can come up with a more convincing argument this week. Thanks for the comment!

  2. Victor Badilas says:

    You’re awesome!!

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