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). Ben Harding, a long-time member of a local amateur astronomy society, SEKAS, and a source of considerable useful advice, pointed out to me an excellent way to demonstrate the Sun's colour when it's high in the sky and therefore not going through a lot of atmosphere. At the many public events SEKAS participates in he projects a suitably attenuated image of the Sun from a telescope onto card in order to show everyone that it is indeed white - see image below, on the left. This image is used with his kind permission. It turns out that we can also get a hint of this from our Earth-bound vantage point even without equipment. 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.

Solar projection revealing the Sun's true midday colour. On the right is a graph showing the intensity of light emitted by the Sun across the visible wavelengths. It 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) …

To Capture a Sunspot: solar filters

Occasionally I post on social media images captured using one of my telescopes in conjunction with a high framerate astro-camera. The image posted most recently was of the Sun, specifically of sunspots; I have collected a lot of these images in the past few years. In part this is because we’re nearing a period of maximum solar activity in the Sun’s eleven-year cycle and there’s simply more to see, but daytime astronomy also affords benefits when trying out new bits and pieces – it’s so much easier to learn how to handle new equipment in daylight. My recent foray was a case in point; I had bought a second-hand 80 mm refractor (see endnote [1] for more information and advice) and was keen to test its features after weeks of poor weather, ill-health and general busyness. This is definitely not a post on expensive astronomical equipment however, almost the opposite in fact: my aim is to share with you how I capture images of the Sun safely without spending a lot of additional money. It is a direct response to the questions posed to me by a member of a local amateur astronomy club: “Did you buy a solar filter cap or make one? What solar film did you use?”

The very first vaguely successful image of the Sun I managed to get. It was taken using my first telescope and an entry-level astro-camera and the combination of high magnification and small camera detector size meant that I captured only a small segment of the Sun’s disc. However, it remains in the slideshow of background images on my PC because it gives an impression of size and of the Sun’s neutral colour. Look closely and you’ll also see the ever-present convection cells as the Sun’s near-surface rises and falls. I’ve put a few more details into endnote [2].

This brings up to the matter of solar filters: how to reduce the amount of light entering our telescope to a level that will neither burn our eyeballs nor fry our camera’s detector chip. I should note at the outset that I am not discussing in this post the more dramatic phenomena associated with the Sun’s surface – no prominences or flares etc. of the sort shown in the images here. These require highly specialised (and expensive) equipment which limits the light entering the telescope to a specific wavelength only. The bits and pieces I describe here will allow all wavelengths (i.e. all colours) to pass through, but at very low intensity. Indeed, the solar safety filter material I use removes 99.999% of the light; which means that only one part in 100,000 reaches the telescope and your eye or camera.

Hopefully, the following series of images will explain it all …

Shown above is the setup in my garden I used to capture the image in question, a closer view of the solar filter fitted to the front of my telescope and an inset image of the filter’s rear face. Notice that there is a second solar filter fitted to the smaller finderscope, which is used to help find the ‘target’, shown to the upper right of the central image. The orange-coloured filter holder is a lid from an old food container (I think it was bought full of dry-roasted nuts!) with its central part removed using a hacksaw and the edges smoothed using sandpaper. The diameter of cut-out disk is as close as I could get it to the diameter of the telescope tube. The lid rim’s internal diameter has been reduced slightly using a strip of material cut from some anti-slip matting, fixed in place using double-sided tape. The final filter assembly was a snug fit over the front of the telescope.

The essence of the whole DIY project is to find a tube that has some rigidity (enough to hold its shape when picked up) and has an internal diameter just larger than the outside diameter of the telescope in question. In the example above I have used part of the thick cardboard tube in which one might find a bottle of a certain single malt whisky: it just happened to fit nicely onto the 72 mm refractor I now use for observational astronomy and in my visits to primary schools etc. I buy high-quality solar film in A4 sized sheets since it’s a cost-effective way of fabricating several filter assemblies; it’s available from many stockists but I happen to use this one. You’ll also need some epoxy resin adhesive. (In passing, I note that there are pre-made solar filter assemblies also available to buy; a quick scan online suggests that they retail for about £50 and upward each.)

Having found a suitable tube and cut it to an appropriate length, the key next step is to attach the solar safety film in such a way that it is not creased or scratched. I have found that a thoroughly clean sheet of glass is a great help; I keep an unused glass shelf for all such work but a smooth and flat ceramic tile might serve, as would a kitchen worktop if you can find a section that’ll not be needed for a day. Leaving the protective backing sheet on the foil, place it on the glass surface, foil upwards. Now mix enough epoxy resin to be able to run a thin thread around the end of the tube that’s going to take the solar safety filter; try not to get any epoxy on the tube’s inner surface, although small amounts aren’t critical. Carefully lower the end with the epoxy onto the foil sheet, avoiding any twisting or sliding motion: the foil should have remained flat against its backing sheet and the glass. There’s probably no need unless your tube is exceptionally light, but you could gently lay something like a small hardback book across the top in order to apply even downward pressure if you wish. Now walk away and leave the epoxy to set. Once all that’s done you can cut away the remaining foil with a pair of scissors and store it for another day.

Small adjustments are probably needed to ensure the filter assembly properly fits the end of the telescope. For reasons rooted only in habit, I tend to do this as a final step despite the fact that it’s probably wisest to get all this out of the way before attaching the safety film. There are all sorts of ways of achieving this, depending on how many millimetres larger the tube’s diameter is than the telescope. It may require only a layer of tape to the inside of the tube (- the end away from the foil of course as one doesn’t want to risk damaging the safety film if anything comes adrift). A layer or two of anti-slip matting fixed using double-sided adhesive tape works well, but it’s easy to find self-adhesive strips of felt or neoprene online and these can also be very useful. Remember, you are aiming for a fit snug enough that nothing’s going to fall off accidentally but not so tight that filter-destroying force is needed in order to slide it onto the telescope tube.

Here’s a selection of the filters made thus far for telescopes of diameters from 50 mm (finderscopes and guidescope) to 150 mm in the case of my Newtonian reflector (- in that case, I used the ring from a cake baking tin that had lost its base to rust); I’ve also made filters for both my grandsons’ telescopes. All this was from two (or three?) sheets of solar safety film. Also shown in the image is a scrap of anti-slip matting and lengths of self-adhesive neoprene and of felt.

Now for some images …

The above are some of the whole-disc images of the Sun I have captured prior to the one shown at the top of this post. The numbers and the sizes of sunspots vary enormously. We’re approaching the peak of the current 11-year solar activity cycle (expected in early 2024) so it’s unlikely you’ll look at the Sun at present without seeing any. Given that one could fit 109 Earths across the diameter of the Sun you’ll not be surprised that the largest sunspot in the image top left is several times the Earth’s diameter. Each sunspot cluster is given a unique identifying number – often preceded by ‘AR’ for active region; you can look this up here. Sunspots themselves are regions associated with the Sun’s magnetic field as it protrudes from the surface. Their dark appearance comes from the fact that they may be 2000°C cooler than their surroundings. Look closely and you’ll also notice brighter regions: those around the darker sunspots are called plages whereas the lighter patches often seen most easily near the edge of the solar disk are faculæ; these are associated with hotter regions in our field of view.

If we take a closer look you’ll see that sunspots have a central dark region, the umbra, and a less dark surrounding area, the penumbra where temperatures are at an intermediate level between the umbra and the surrounding surface.

Even the smallest scraps of solar film left over from making a telescope filter can have their uses. I captured this sequence of shots of a partial solar eclipse using my phone with a piece of safety film covering the phone’s camera lenses. The quality is what you’d expect from a handheld phone in a car park several miles from my house, but it was a fun thing to try. However, see below …

This is a better view. It’s another partial eclipse, this time captured using one of my telescopes and astro-cameras. You may be able to discern the silhouette of some of the Moon’s mountain ranges as it clipped the Sun. We are extraordinarily fortunate in the fact that the Sun and the Moon both appear to us on Earth as discs that are about ½° wide – which is why the Moon can cover the Sun when suitably aligned. (By the way, if you hold your little finger out at arm’s length the fingernail end covers about 1° of the sky so it’ll easily cover the Moon; see here.)

Happy sunspot hunting 😊

1700 words + endnotes

Endnotes
[1] For an overview of telescope types and what to consider and look for when buying try these web sites: here and here. I hasten to add that, like other second hand astronomy items, I bought the telescope mentioned in my opening paragraph from someone I knew to be trustworthy; one has to be careful.

Starting out in astronomy need not be prohibitively expensive – getting into photography, or off-road cycling, or many forms of sport, … might be comparable. However, amateur astronomers often talk in terms of ‘falling down the rabbit hole’: if you get hooked by the hobby you’ll find that there’s a never-ending series of spending opportunities ;-) My advice is to think about what it you most want to do/observe and start your search from there, being aware that as your aspirations evolve you may want to upgrade. The above links are only two of a multitude of places to get advice; read them in order to get an overview, but there’s a huge benefit to be had if you can try things out and talk to experienced people face-to-face. My suggestion is that you join your local amateur astronomy society. I’ve had loads of support from the lovely people here and also here and here. Most societies have websites and/or social media groups and you’ll find members only too keen to answer questions and offer informed advice. See here or here for a list containing many such societies in the UK. (Please note that these lists are not completely up to date, but they’ll get you started.) Once you have some equipment of your own you’ll find another slew of websites and helpful social media groups and online videos dedicated to users of similar kit.

[2] We all know that the Sun is both large and massive, and that it’s hot. In terms of size, the diameter at its equator is about 109 times that of our beautiful Earth; it represents 99.8% of the mass in our entire solar system. Its core temperature, which is where the fusion reactions occur that generate its output, has a temperature of about 15 million °C whereas the Sun’s surface temperature is about 5,500°C. As one rises into the corona (its outer atmosphere) the temperature rises again to about two million °C. (See here and here. Thus, what we perceive from Earth is the ‘cooler’ surface, referred to as the photosphere. In fact, the colour we see is strongly affected by the Earth’s atmosphere and by the limitations of our eyes: light from the blue end of the spectrum is preferentially scattered as the mix of wavelengths from the Sun passes through – this gives us our blue sky and leaves the Sun appearing yellow-orange-red as it nears the horizon but blindingly white when it’s high in the sky (- don’t look!). Our eyes fail to give us its intrinsic colour; if we could look at it through protective glasses from a space station our eyes would perceive the Sun as a white disc.

One of the simple calculations I used to set for students in their foundation year was to use something called Wein’s Law in order to estimate the temperature of the Sun’s photosphere. All that’s needed is the wavelength of light at the peak of the Sun’s emission, which we approximate to the wavelength of green-yellow light. If you’re that way inclined, try it out using the link above. The same formula may be used to estimate the surface temperature of other stars, or indeed the temperature within a furnace – the physics is identical. See also Video 13 in my lockdown series ‘Physics in the House’.