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.
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…
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