Colours of the Sun (1)

Part One: a bit of light science

For my own peace of mind I must begin this post with a warning that might seem blindingly (!) obvious to you but which I ought to spell out nevertheless: please never, ever look at the Sun without adequate (certified) protective filters. Even more crucially, make absolutely sure that your binoculars’/telescope’s field of view doesn’t even stray close to the Sun. The filters I used to generate the images shown below removed, at a minimum, 99.999% of the intensity of the sunlight.

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What colour is the Sun?

Red and orange, as it rises or sets? Yellow, when it’s low in the sky – and in pretty much every child’s painting? What about in the middle of the day when it’s too bright to stare at, and what does it look like to the astronauts on the International Space Station? Just what is the true colour of the Sun?

It depends …

What we perceive is an admixture of the light that the Sun emits (all the various colours, or wavelengths, it generates), the effect of whatever that light travels through and finally our ability to detect it. Our eyes – actually, our eye and brain in combination and assuming the absence of colour blindness – see reds, oranges and yellows when the Sun is nearer the horizon and its light is therefore passing through a lot of Earth’s atmosphere. The shorter wavelengths of light, towards the blue-violet end of the rainbow, have been preferentially spread out by process called Rayleigh Scattering. This describes the scattering, or spreading, of light by particles smaller than the wavelength of the light: the scattering increases as the wavelength of light shortens. Thus, the blue-violet end of the rainbow spectrum is scattered widely, giving us a blue-coloured sky, whilst the redder colours, with longer wavelengths, are scattered less and are therefore more likely to reach us from the Sun’s direction.

If we can reduce the intensity of the sunlight to safe levels when the Sun is high in the sky and thus passing through much less atmosphere, we’d see the Sun as a whitish disk. Our friendly local astronauts, free of our atmosphere altogether, would also tell us that it looks white. This is because the Sun is emitting light across an incredibly wide range of wavelengths. Although its emission is most intense at wavelengths corresponding to green light, the presence of wavelengths to either side add together to give us white (see here and the figure below for a little more information). It turns out that we can get a hint of this even from our Earth-bound vantage point. Light summer clouds appear as near-white in colour due to a process called Mie Scattering which describes the scatter of light from spheres of a size comparable to the wavelength (colour) of the light – microscopic droplets of water in a cloud fit the bill. In this case, there is no significant variation in the strength of the scattering process with the wavelength of light: thus, all colours are affected approximately equally and we therefore get an idea of the actual colour of the Sun, white.

The emission of light by the Sun approximates to something called Black Body Radiation, with a peak intensity in the green part of the visible colours but significant intensity to either side and out towards ultra-violet and x-rays and towards the far infra-red; see here. Note the sharp spikes in intensity at specific wavelengths; these tell us about the chemical make-up of the Sun; we’ll return to them below.

This is my first ever image of the Sun. It was generated from a stack of frames collected by my entry-level astrocam (Altair Astro gpcam2 290c) and my first ‘proper’ telescope bought post-retirement (Skywatcher 150PL on an EQ3/2 mount with retro-fitted drive motors; see here for the full story) and a homemade solar filter (see here). My field of view was limited to 0.27º x 0.15º so I only captured a portion of the Sun’s disk which has an apparent diameter of about ½º. However, it’s enough to illustrate both the colour of the Sun in visible light and the granulation – the result of thousands and thousands of convection currents, like the swirl of cold milk poured slowly into hot coffee. In fact, the Sun’s surface is reminiscent of the shell of a chicken’s egg. There are lots more images in Part 2 of this post.

This is my current setup for imaging the whole disc of the Sun. The telescope is an AA Starwave 80edr fitted with a Baader 2x Barlow lens and an AA533c camera cooled to -10°C; the mount is a Skywatcher HEQ5, controlled from a laptop via ASCOM/EQMOD using Carte du Ciel and SharpCap (URLs for the software are in the appendix to an earlier post, here, together with installation notes).

Having made a start at describing the colours of the Sun according to our eyesight we now need to dig a little deeper. We tend to assume that we’re looking at the surface of the Sun, but it turns out that things are not entirely straightforward in that regard. Remember, the Sun is a huge ball of gas; it’s a pretty good sphere, which one might expect since both the gravitational force pulling it in and the pressure trying to push it out are both acting in all directions uniformly. However, what we think of as the surface of that sphere is in fact the deepest of the three principal outer layers of the Sun: the Photosphere  (It’s called that because it’s where we perceive most of the Sun’s light to come from: hence, ‘sphere of light’.) The photosphere is at about 4000-6000°C depending on altitude. This is what we see if we look at the Sun through a neutral, or white-light, safety filter. It’s also the layer in which we can see sunspots (patches with a slightly lower temperature which, as a result, appear dark) and faculæ (typically, nearby areas of slightly raised temperature which therefore look brighter).

Above the photosphere is the Chromosphere – literally, sphere of colour. Its temperature rises with altitude to over 8000°C and it glows with a red colour derived from the hydrogen plasma that makes it up. We don’t normally see the red because the chromosphere is of much lower density than the photosphere and the intensity of the light generated is therefore swamped by the denser photosphere. It is however possible to see it when the brighter photosphere is blocked out at the peak of a total solar eclipse. The chromosphere is the layer of the Sun’s atmosphere associated with giant prominences that may extend thousands of kilometres into space, and filaments (essentially, prominences seen from above) and spicules. The latter are relatively short spikes of red-glowing hydrogen that can give the Sun a sort of ‘grassy’ look. There’ll be several of my images in Part 2. Further out still we get to the Corona, but I’m not going to dwell on that in this post.

So far, we’ve established that the colour we see depends on the temperatures present in the Sun’s outer layers (Black Body Radiation),what the sunlight has travelled through (e.g. Earth’s atmosphere) and the workings of our eyes (or our cameras). We’ve also had a hint that the chemical make-up of the Sun has an influence (e.g. the red colour of the chromosphere being due to hydrogen). The additional important factor arises from the fact that, whilst the Sun is mostly composed of hydrogen (H) and some helium (He), there are many other elements present in small quantities. We can use them to look at the Sun using particular wavelengths/colours of light – these are the emission lines uniquely associated with each given chemical element. For example, if we put energy into a hydrogen atom – and there’s no shortage of energy in the Sun – it enters what’s called an excited state. The natural next step is for that atom to shed the excess energy, typically by emitting a packet of light (a photon); the beauty of that process is that every element has its own ‘fingerprint’ of emitted photon wavelengths or colours. Thus, a principal colour emitted by hydrogen is red; it’s often labelled as Hα, or H-alpha, and has a wavelength of 656 nm (nanometres, 10^-9 m). If we observe the Sun through a filter that transmits only this wavelength of light then we’ll see a red Sun. Moreover, by picking up that one particular wavelength alone we’re effectively looking specifically at how hydrogen, to the exclusion of all other chemical elements, is behaving within this solar environment.

We can track through the ‘rainbow colours’ by using filters that single out the wavelengths emitted by other elements present on the Sun. Sodium, Na, emits light primarily at 589 nm, which is yellow; magnesium, Mg, emits photons at 517 nm and that’s a green light; calcium, Ca, emits blue light at 393/396 nm. For each colour (or wavelength of light) we are focusing on a single element and thus on a slightly different aspect of the Sun’s behaviour; in effect, we can use these selected elements as an internal solar probe in order to complement observations in ‘white’ light (i.e. all the wavelengths together – our view of the photosphere). There is a caveat: the effect of Rayleigh scattering (see above) is to scatter the shorter wavelengths preferentially and getting good Earth-bound images using the blue light from Ca is therefore hampered. As a consequence I show no Ca-light images in this post.

It is in fact possible, with the right equipment either on Earth or on a satellite, to focus on the light emitted from excited atoms of each of the elements in the periodic table up to iron, Fe. (There is insufficient energy in the Sun to make anything heavier than iron – that requires the sorts of energies associated with a supernova explosion. Remember that the next time you look at gold jewellery; it exists because earlier generations of giant stars have died 😉). There is additional information here, here and here.

For we amateurs, however, the bright emission wavelengths of hydrogen, sodium, magnesium and calcium are accessible. The cost of the kit is, however, a very long way beyond my budget. This is particularly true for those wishing to collect images using the light from hydrogen-alpha emission: thousands of pounds. Hence my own ‘white light’ setup which depends only on reducing all the incoming wavelengths to leave only one part in 100,000; I can image only the photosphere: sunspots, faculæ and granulation. Although I’m mostly content with this state-of-affairs I did treat myself to a relatively inexpensive gift during the Summer: I paid for 90-minutes of guided access to a solar imaging setup in Spain. The details are here. I recommend it to anyone with an interest in this area. 

I’ll share some pretty pictures in Part 2 of this post, along with a bit of nerdy stuff on image processing. Until then, keep looking up (safely) …