Colours of the Sun (2)

 

Part Two: pretty pictures and a little nerdy stuff

For my own peace of mind I must begin this post as I began Part One 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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It’s a fascinating time to be imaging the Sun as we’re near the peak of solar activity in its eleven-year cycle. When I started imaging the Sun a few years ago several days could go by with little or no activity; as 2024 progressed it became a rare day when there weren’t several large and/or complex active regions. The Sun’s disc has a diameter of approximately 109 times that of the Earth so in the images below you’ll realise immediately just how extensive these active regions can be.

Shown above is the neutral/white-light imaging setup I use in order to gather images of the photosphere; I introduced it in Part One. With this I can capture the whole of the Sun’s disc within a 3k x 3k (i.e. 9 megapixel) colour image which gives a decent level of detail. Sunspot active regions and the associated faculæ are easily observed. This is not like taking a snapshot using ones phone or handheld camera unfortunately. The issue is one of atmospheric turbulence: the higher the magnification the more distortions a given exposure may suffer from (I uploaded to my YouTube channel a very short video taken through another of my telescopes in order to illustrate this phenomenon.) To get around this I’ll typically collect up to 2000 frames using the software cited in Part 1 then use a free software package called Autostakkert! in order to select the best – usually the least distorted ten or fifteen percent only – before stacking them to create a single optimised image. This will go into another free package called Registax for sharpening and perhaps basic colour balancing before sending it to an image processing package like AffinityPhoto or Photoshop for final ‘polishing’.

Now, as discussed in Part 1, ‘colour’ is a term that needs a bit of thought. When I use my colour astro-camera the frames that emerge have a green tint because the detector chip inside isn’t the standard RGB of a smartphone or other digital camera but it’s a pattern of RGGB (for reasons I’ll not go into here). Correcting for this is trivial, but in truth one could choose almost any colour for the final image simply by selectively altering the colour balance or colour saturation at the image processing stage. A ‘correct’ balance will yield a white photosphere, but if it’s desired then a yellow-coloured Sun is easily possible. I’ve done both, although I’m increasingly tending to the more neutral/natural white.

When it comes to the remotely accessed setup in Grenada, Spain I mentioned towards the close of Part 1  the whole point of the exercise is to observe at specific wavelengths, using in particular H-alpha light. One is therefore using precision narrow-band filters to select out that one colour associated with the emission from excited hydrogen. A colour astrocam is very inefficient for this – there will be nothing at all recorded in the green or blue channels of course – and it’s far better to use a monochrome camera and then add any desired colour digitally during processing. The rig is shown above; it’s a screenshot from the Zoom-enabled session (- details here; my excellent guide and teacher for the experience was Gary Palmer). The setup we used was the one on the left hand side: a Williams Optics 120 mm refractor, with a ZWO-ASI1600MM astrocam, all on an IOPTRON CEM120 mount.

After a couple of false starts due to the weather, my session took place on 4th July. In order to ‘ground’ the images I used my own backyard setup to capture the Sun’s whole disk in white light using my homemade solar filter. It’s shown below, with the active regions identified using the internationally accepted conventions which one can find online. I find the Space Weather website useful in this regard, and you can find images and associated information for any specified date in their archive (see here). Also useful is the NASA Solar Dynamics Observatory, which includes video sequences for particular dates (see here). I’ll share the rest of the story using the images I later generated from all the data that was gathered on the day. Each of these was generated from original data files containing 2000 exposures which were graded by quality and the best stacked and refined using the packages mentioned above.

My backyard image of the photosphere as it appeared on 4th July, collected and processed as described above.

A couple of composite images overlaid onto my white-light image of the photosphere are shown above. I have marked on my image the active region under observation; superimposed on the side are three narrow-band filter images of that same region. Notice the difference in appearance of the sunspots and the area around them depending on which element we’re utilising: hydrogen, sodium and magnesium. The faculæ (regions of higher temperature) show up well in H-alpha light, sunspots less so; the violence of the Sun’s magnetic field changes in an active region can be seen in the contorted surface patterns. Surface granulation shows up well in sodium light, and the sunspots seem increasingly more clearly revealed as one progresses through Na to Mg. Please note again that the colouration is added digitally – hence the variation: I have been learning how to process the narrow-band monochrome such as these for the first time and therefore trying out various methods. During this period, as an additional complication, I began to migrate from using Photoshop to AffinityPhoto. Unfortunately, there are relatively few online tutorials based on the use of AffinityPhoto although this one was useful; the short books by Dave Eagle were particularly helpful.

In the pair of H-alpha images shown above one can readily see prominences reaching out into space. The one on the left is associated with an active region not yet in view from the Earth – i.e. it is being generated around the rim. The Sun rotates once in approximately four weeks at the latitude of these prominences, so this one would have appeared within days. (The Sun is a ball of gas, hence the fluid-like variation of a solar ‘day’ depending on where one is looking between the poles and the equator; see here for more information.) Also apparent in these images are the ‘grass-like’ spicules in the near-surface region of the chromosphere. Each of the two images is itself a composite: one set of 2000 short-duration exposures for the surface and another set with longer exposure times for the spicules and prominences. These two sets were separately processed and then combined digitally to yield the final composite images. The original two layers of the final composite image shown above on the right are reproduced below after stacking the selected subset of 2000 exposures for each of them.

Although filaments – a prominence as seen from above – may also be discerned in the coloured images above, particularly the one on the left, I’ve inserted an image below in which I have accentuated them during processing.

Given the novelty of all this I confess that it took a long while to get to grips with the remotely collected data files I was sent by my guide Dave Palmer. However, in the process I’ve learnt a lot, enjoyed myself and am pleased with what I have to show for all the effort. I hope you like the result.

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) …

The Last Moon

Every year for several years I’ve had an invitation spend the morning with two classes of Year 5 pupils at the Churchill School during their ‘Space’ curriculum topic. Not once has this been less than a delight; it’s always great fun (see here and here to get a flavour). This year’s trip took place in late June – timed such that their preferred target of the Moon was up in the daytime and was no more than about half in sun. The reason for not wanting either a crescent Moon or a full Moon is that we want a chance of seeing a selection of craters, lava basins and mountain ranges but we also want some decent shadows arising from a low Sun in the lunar sky. On that latter point, the ‘best’ shadows are those near the terminator – the line that divides night from day. There are several options for such planning, but the website I tend to use is here.

This is my small, i.e. reasonably transportable, telescope and mount: a Skywatcher 72ED on a Skywatcher EQ3/2 mount (- the image above wasn’t taken during this visit). I’ve retro-fitted motors to the mount in order to track the motion of objects across the sky as the Earth rotates; indeed, this is an enormous help when attempting a ‘mass observing event’ since the target – the Moon – isn’t continually drifting out of the eyepiece’s field of view.

Having learnt the hard way that the variation in height amongst ~60 ten year olds can make viewing tricky I’d purchased a right-angle for the eyepiece which could easily be rotated to suit all-comers. That worked well. Inserting an additional lens (a 2X Barlow for those who wish to know) which, in effect, doubles the magnification was less successful since it has the effect of ‘dulling’ the image. At night this isn’t a problem when viewing the Moon, but on a bright and relatively humid morning in mid-summer when the contrast is already reduced it had the effect of ‘washing out’ the details on the lunar surface. However, once committed to the setup there was no time available to reconfigure everything once observing began. One lives and learns.

This wasn’t taken on the day – it’s an image I prepared earlier ;-) However, it does show the sort of view I had hoped to show everyone – albeit somewhat degraded by the daytime conditions. For anyone wishing to identify features on the Moon when observing by eye, using binoculars or a telescope I’d recommend a phone app called ‘Lunar Map HD’ – although there’s a lot of other online resource also available.

Unfortunately, this is the sort of view we all got: the moon on a bright, humid day in the middle of summer - washed out.

Everyone saw the Moon through my telescope as far as I know, although it took some people less time than it did others to get their eye in just the right position to see it. (One person did come back for a second attempt right at the end, having failed to see much when it was their turn. I’m glad that they did; it would have been such a shame to have said nothing and as a result missed the experience.) After we’d finished there was an hour of Q&A back in the classroom. It’s rewarding to be able to show people a view of the Moon that they might not have had before, but it’s definitely huge fun to attempt to field an apparently never-ending stream of testing questions. Every year I’ve been invited to visit this school I’ve been blown away by the energy and insight behind the questions. Yes, there are some that might be written off as left-field or off-piste – but that would be to miss a key point: these are all young people who’re engaged with the wider subject matter, and many were genuinely well-informed. Enthusiasm is worth a lot.

Nothing trumps break time, and nor should it – particularly at that age – and the time rapidly arrived for me to take my rig apart again and return it to the boot of my car. I was helped by my two wonderful Year 5 host teachers, in the course of which they told me about their new teaching assignments for next year. Apparently neither of them will be teaching Year 5 – so my astronomy/space mornings have in all probability come to an end. That visit may well prove to be the last observing session I assist with at the school; no more shared Moon ...

Over my head

[I confess: this is a hybrid post in which I have broken the golden rule of 'knowing my audience'. As a result, you may well choose to read only part of it. The opening few paragraphs speak of my experience in the planning and presentation of a talk on astrophotography to a non-expert audience. After a couple of tiny footnotes you'll get to two appendices. The first of these was written in response to requests to provide a synopsis of the above talk; it includes a lot of links to various packages etc. So far, so good. The second appendix was added as a note of my parallel struggles trying to get a replacement laptop to run all my astrophotography needs in the way my rapidly-fading older one did; it's distinctly nerdy and is also replete with links.]

House of (more) Treasures

 

In a recent post (here) I waxed lyrical on the subject of my first visit to the Canterbury Cathedral Library and Archives: “[sitting] at a desk surrounded by old wood with light filtering in through handmade glass, and to hear at one point the cathedral’s bells drifting through high ceilings”. I have now embarked on a six-session u3a course in the history of printing in Europe, led by Dr David Shaw, so get to spend even more time there. Such a joy.

The course itself, which is proving to be a delight, has thrown up several nuggets of information to be nestled in the memory, awaiting their time. For instance, did you know that the terms ‘lower case’ and ‘upper case’ derive from the days when a compositor – the person who set each letter of a font in place in order that a page might be printed using a manual printing press – had to select the next letter in a given word: their font cases were arranged such that the more common letters, ‘e’ for example, were close at hand (literally in the lower of the usual arrangement of two font cases) and those less commonly required, capital ‘Z’ perhaps, were in the more distant or upper case. When working with the speed allowed by ‘muscle memory’ this could save a lot of time and effort, rather like touch-typing – a skill I have, regrettably, never properly acquired. It’s no wonder the apprenticeship lasted seven years. This would often be followed by a period as a ‘journeyman’ during which the person would travel to various printing works in order to expand their experience and expertise. The size of a font was also defined at this stage, with 72-point corresponding to one inch (1″) – thus, a 12pt font corresponds to letter/number heights that fit within 1/6th of an inch or a little over 4 mm. This was, evidently, an early example of industrial standardisation; paper sizes were similarly standardised.

However, my principal focus in this post is to mention one of the books that David thoughtfully made available for us to marvel at during our mid-session break: Robert Boyle’s 1660 work on what we would now think of as air pressure and the like. Robert Boyle was a founding member of the Royal Society and made seminal contributions to the physical sciences; indeed, the slightly younger (but perhaps nowadays more famous) Isaac Newton used some of Robert Boyle’s work in order to derive an equation for the speed of sound in air. It is a personal pleasure to be able to turn the pages of this beautiful book; moreover, in a straw poll of the twelve other u3a members with me on this course, I discovered several people had retained a memory of hearing about ‘Boyle’s Law’ from their school days – a testament to his legacy.

Having discovered – whilst drafting the earlier blog post referred to above – the extent of the time and energy required of the Cathedral’s hard-pressed Archive & Library staff to generate and supply images of old documents in their collection, I was delighted to find online a ready-made image of another copy of the this very book. The above title page and example illustration comes courtesy of the Science History Institute and is made available under Creative Commons Public Domain Mark 1.0 Universal.

Not only was it fascinating simply to see and touch this original copy of Robert Boyle’s ground-breaking work, but I learnt from David that there was a (tenuous) personal connection to the story of how this rare book came to be in the Cathedral’s collections. As may be seen in the document reproduced below, the book was a part of the collection of a former Rector of a parish not far from where I live. The Rev. Richard Forster was born only seven years after Robert Boyle’s death; he amassed an impressive library containing many hundred books and much else beside. As one might expect, works on theology were prominent but alongside those were mathematical and scientific books. In his will he left the library to his successors in the role. At some point the task of being Rector of Crundale was combined with that of being Vicar of the nearby parish of Godmersham and the library was transferred to the beautiful vicarage at Godmersham.

Reproduced from a document on David's website (here).

The vicarage was situated just outside the boundary wall of Godmersham Park which, as her fans will know, has a close connection with the writer Jane Austin. It also commanded enviable views across the River Stour and as a result of this proximity and its age, suffered from damp at ground level; its library was accessed via an impressive spiral staircase. My parents-in-law lived in the Godmersham parish; indeed, my father-in-law served as churchwarden for fifty years. Moreover, my wife and I were married in Godmersham church in the late 1970s. The wedding was conducted by the then Vicar/Rector, Canon Graham Brade-Birks – who has been mentioned with affection in a previous post on this blog site, here. I’m pretty sure that he was already eligible for retirement when he conducted our wedding service but he was a man with the clear conviction of his calling, and retirement was postponed for as long as was practicable. However, after he eventually retired, the vicarage was sold (to ‘someone in television’ as I recall) in order to raise funds for the Church of England and there followed an extended interregnum: there was, therefore, no successor to whom the library’s contents could be passed. Thus, although non-stipendiary (unpaid, usually part-time) vicars/rectors were subsequently appointed, Canon Brade-Birks was indeed, in effect, the last of the line. Hoping to ensure the survival of the library’s contents, he left it to the nearby Wye Agricultural College where he had taught the odd course on soil science. (I have also written a post mentioning Wye College, where I was employed for a year after leaving school – see here). At the time, the College was a constituent part of the University of London. It was later subsumed into Imperial College and then closed and sold off; the collection that had been looked after by Canon Brade-Birks was eventually handed into the care of Canterbury Cathedral’s Archive & Library.

As I admitted, this is a tenuous link. For such links I am, however, grateful.

 

 (I acknowledge with thanks the editorial suggestions offered by David Shaw.)

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’.

House of Treasures

 

One morning at the end of June I was sitting at a desk in Canterbury Cathedral’s Archives reading room. This was my first time in the Library/Archives, despite having lived and worked within a few miles of the place since 1985; it had taken the kind invitation of historian and retired academic Dr David Shaw – mentioned in my previous post – to bring about a much overdue visit. In front of me were all three volumes of Isaac Newton’s seminal work on the mathematics that govern so much of the observable universe; a work which, quite literally, redefined the way we understand our the world. Rarely have I touched such valuable documents. However, this is a digression and I shall relegate it to a postscript below*. This post is primarily about glass: specifically, glassmaking in England at the time of the Tudors and Stuarts ...

David has participated in several of my u3a science sessions over recent years, including the extended series I lead on the science, technology and art of glass; he had evidently fully discerned the extent to which I am fascinated with glass as a material. In his role as a volunteer at the Archives and Library he had come across a Royal Proclamation which began life in the reign of Elizabeth I and was then re-issued by her successors, as and when required, at least through to Charles I, to whose reign this particular copy can be dated (1615). The order, printed on two sheets of paper, forbad the use of wood as a fuel in the manufacture of glass. Evidently, so much high quality timber was being used in glassmaking that ship-building was suffering, either directly because of a dearth of supply or by virtue of the price having been driven up; this was, in its turn, seen as a threat to national security. Moreover, it was forbidden to commission from any source glassware made or formed using wood as a fuel; the document goes on to forbid the import of glass or glassware and to even to outlaw the trade in glass drinking vessels. The delegated enforcement of such a proclamation fell to the local authorities across the country, there being no police force at the time remember. Thus, multiple copies of the proclamation would be printed for distribution across England and Wales.

This is the ‘Proclamation touching Glasses’ (reference CCA CC/A/P/P/1/PR/48) at the heart for my visit to the Cathedral Archives. Unusually, but rather wonderfully in my opinion, Canterbury Cathedral Archives also holds and manages the city archives of Canterbury: were that not the case I may never have had this rare opportunity. (Sadly, a set of records relating to the county, being the archive of the Diocese of Canterbury, was removed from the collection some ten years ago and is now held by Kent County Council in the County Town of Maidstone.) The images shown above were created for me by the Cathedral Archives and are used with the kind permission of the Chapter of Canterbury.

The Tudor rose and the thistles in this illustrated opening letter ‘I’ tie the origin of the proclamation firmly to its period.

You’ll notice in passing, even without even reading the proclamation, that it takes a lot of words to convey what is a relatively straightforward message: it will come as no surprise to you that the people who drafted such legal documents were, I understand, paid by the word. There was however a useful nugget of information offered (see extracted image above): “… there has been discovered and perfected a way and means to make glass with sea coal [and] pit coal … in as good perfection for beauty and use, as formerly was made by wood”. I have used modern spelling but otherwise kept the contemporary use of English. By pit coal, I suspect we’re mostly talking of open-cast mining; where seams met the coast it was possible to gather lumps of coal on the shoreline, hence sea coal. To the modern mind it seems perfectly reasonable for them to have switched to coal given the shortage of timber, but it’s important to remember that the bulk transportation of fuel would have been far from trivial at the time (i.e. before the canal system, let alone the railways). One consequence of the prohibition against using wood was therefore that glass making necessarily declined in wooded regions of the country, like the Weald of Kent and Sussex, and gravitated towards areas having easy access to coal – perhaps like Sunderland, which developed a well-deserved reputation for glassmaking.

All in all, I had a wonderful few hours in the Archives. The welcome I received was second to none and the ‘atmosphere’ in there came pretty close to defining my dream library environment. To sit at a desk surrounded by old wood with light filtering in through handmade glass, and to hear at one point the cathedral’s bells drifting through high ceilings was bliss.


My warmest thanks go to David Shaw for his thoughtful invitation, to the two Archives reading room staff who registered me and settled me in, to their Digitisation Officer for creating digital images of the proclamation and to the Archives and Library Manager who approved my request to show the images in this post and offered some valuable comments on an earlier draft.

* Postscript: Turning the pages of Isaac Newton’s Philosophiæ Naturalis Principia Mathematica (1687; ‘The Mathematical Principles of Natural Philosophy’, see here and here) was a very special treat for anyone with an interest in science, and particularly for a physicist like me. Isaac Newton laid the groundwork for so much of the physical sciences still taught in schools and colleges: an intellectual giant. The text of the Principia was written in Latin, which I cannot read, but Newton and I share a common second language: that of mathematics. I only had time on this visit to enjoy the first of the three-volume work, but that proved ideal in the sense that I could focus properly on the many pages Newton used to define his terms and establish key axioms and proofs. Thus, I could discern that he used ‘C’ for speed, ‘S’ for distance and ‘T’ for time and would write the classic relationship between them as C = S : T (c.f. the c = s/t form one would see today). Moving from there to acceleration, momentum etc. became possible on this basis. Noticeably, there is much space given to the use geometric proofs. There are other historic scientific documents held within the Archive and now that I have my reader’s photo-ID card I shall be able to book a slot for return visits.

P.p.s. One afternoon, back in 1985/6, together with a couple of colleagues who were similarly bored of the mandatory ‘induction course’ we’d been dispatched to by our employer, I got the chance to look through several historical documents in the library at Herstmonceux Castle – including a beautifully illustrated contemporary account of James Cook’s first expedition on the Endeavour. As a schoolboy I don’t believe I ever committed truancy – I loved learning, so why would I – but I have no regrets about skipping class on that occasion ;-)


Further reading
There are a great many books/articles on the history of glass. For those with an interest in the glassmaking of England during the period of history of most relevance to this blog post I can share a few of the online resources I have consulted:
On early-modern glass
For an extended article on the history of glassmaking in England – including the use of wood and switch to coal see here.
On the scientific analysis of medieval glass (a PhD thesis).
For Historic England’s archaeological guidelines see here and also here and here.

There are other, more generic, books sitting on my shelves such as:
‘5000 Years of Glass’ edited by H. Tait (The British Museum Press, 2012; ISBN 978-0-7141-5095-6)
‘A Short History of Glass’ by Cloe Zerwick (Harry N. Abrams Inc., 1990; ISBN 0-87290-121-1)
‘The Glass Bathyscaphe’ by A. Macfarlane and G. Martin (Profile Books, 2003; ISBN 1-86197-394-2).

The Pleiades: of daughters, poets and stars

 

In a recent post I succumbed to the temptation to issue a cri de coeur in respect of my exasperatingly slow progress learning how to capture long-exposure frames, perhaps of 5-10 minutes duration, of faint astronomical objects like galaxies and nebulæ. At the tail end of one of my many nights of trials I turned my telescope towards something easy to identify: M45, the Pleiades, or Seven Sisters, which is a cluster of mostly faint stars. (A cluster is simply a relatively large group of stars held by their mutual gravitational attraction.) By this stage – actually, only about 9 pm on a mid-February evening in 2023 but it felt much later – I simply wanted something, anything, to show for all my efforts.

The image on the left was derived from the best 75 of 103 five-second frames. The second image differs only in that I have labelled the stars that have names and have added their visual magnitudes (see explanation below). The field of view here is a square of almost 2° x 2°, so almost four times the apparent size of the Moon or the Sun. For those wanting a little more technical information, I’ve added a footnote [1].

I had never thought of employing these less-than-dramatic images as the basis for a blog post until using them for one of the short ‘science-lite’ pieces I write for my local u3a’s Facebook group. In its turn, this was mentioned in a comment I made on a FB post (see here) on star clusters within our own galaxy, the Milky Way, posted by my ex-colleague Dr Dirk Froebrich (who runs the Beacon Observatory and its excellent HOYS ‘Citizen Science’ programme). Dirk kindly ran the Pleiades star cluster through the software he’s been using and this added an interesting new level to my simple image. Shortly after, my fellow u3a tutor Dr David Shaw told me about a group of 16^th century French Renaissance poets who called themselves La Pléiade. They, in their turn, were named after an analogous group of seven 3^rd century BCE Alexandrian poets who took their name – the Pleiad – from our star cluster. We end up in Greek mythology with the seven daughters of Pleione (literally, the Pleiades); these Seven Sisters had names you’ll see reflected in the labelled image above: Maia, Electra, Taygete, Alcyone, Celaeno, Sterope and Merope. Their mother joins them, as does another mythological figure claiming Pleione as his mother: Atlas. Taken together, these additional layers of information were enough to coax me to the keyboard. [Since originally posting this in mid-July the u3a's Astronomy Advisor, Martin Willock, has shared a link to an article in 'The Conversation' from December 2020 that I hadn't seen; it offers additional insight into ancient mythology surrounding The Pleiades, including that of Aboriginal Australians. See also this piece, which I add in June 2024.]

Occasionally, if the muse is insistent, I might dare to try my hand at free-form or shape/concrete poetry (see here for my very first attempt) but I make no claims whatsoever regarding my (lack of) poetic ability or understanding. Similarly, although in my youth I read translations of the Iliad, Odyssey and Aeneid – copies of which still sit on a shelf in my study – I claim no depth to my knowledge of Greek mythology. I think it wise to avoid straying too far from the scientific theme at the heart of my blog so, even though I am merely an amateur astronomer and astrophotographer, I will focus on this aspect.

Let’s start with an explanation of the other designation I gave in my opening paragraph for this cluster of stars: the Pleiades and the Seven Sisters are covered, but what of M45? Charles Messier was a French astronomer with a special interest in the discovery of new comets. There were, however, objects that might look a little like a comet in a small telescope but which could be ruled out on closer study since, unlike a comet, they moved exactly as a star would in the night sky. One of Messier’s important contributions to comet discovery was the compilation of a list of these non-cometary objects. His catalogue, the final version of which was published in 1781 and which contained 103 of such objects, enabled fellow comet-hunters to avoid ‘wasting their time’. Messier-45, or M45 – the Pleiades – is simply the 45^th entry in his catalogue of things not to bother with if one is trying to spot a comet. Ironically, his ‘avoid’ list has become a ‘to do’ list for many amateur astronomers. This is perhaps especially so for those who prefer astrophotography to visual observation and wish to produce their own images of visually fascinating objects beyond our solar system like nebulæ, supernova remnants and galaxies. In my own case, although I do aspire to capturing a few such images, there’d be no point in saddling myself with such a demanding checklist at my age ;-)

Look them up in books, apps or online and you’ll be told that the Pleiades can be seen with the naked eye within the constellation of Taurus (see diagram below, a screenshot from a piece of free software called Stellarium). Actually, that’s only really true in the absence of significant light pollution since it's a relatively faint cluster of stars. Even under good observing conditions one can usually only pick it out in the corner of one’s eye. This is because, whilst our central vision is great for detail and colour it’s not so good on faint objects – for these we’re better off letting the light fall on the off-centre parts of the retina where the receptors don’t permit a sharply resolved image but are better suited to low light levels. The standard advice is to ‘look away a little’ whilst remaining aware of what’s in the periphery of your vision.

Stellarium is not the only useful navigation aid for the night sky, but it’s certainly a very good one. This view is approximately 60° across, so a small fraction of the 360° horizon; it is the view towards the West as it would have been in mid-February 2023 at about 21:30. The Pleiades is indicated by my arrows. By the way, in Japan the cluster is referred to by a different name: Subaru, which mean ‘unite’. Furthermore, the Pleiades (along with Orion) get a mention in two of the books of the Old Testament - Job, which probably dates to the fifth or sixth centuries BC, and Amos, which refers to events at around 750 BC.

Despite its common name of the Seven Sisters, there are about 1000 stars in the cluster although most are too faint for all but the most powerful telescopes to pick out. Even the brightest stars in the cluster are still relatively faint. The brightness of celestial objects is measured on a scale of apparent magnitudes: a counter-intuitive scale on first sight that has the faintest objects assigned the highest numbers. Each step of 1 on the apparent magnitude scale corresponds to a change in perceived brightness by a factor of 2½ (see footnote [2] for more detail). Thus, bright Venus has a magnitude of -4.7 at the time of writing but much fainter Polaris, the Pole Star, is +2 and this actually means that Venus appears to us more than one hundred times as bright as Polaris. Our eyes can, on a clear night and without too much light pollution, see stars ‘down to’ a magnitude of about 5 or 6; at my age and with the excessive street lighting near my house it’s definitely more 5 than 6 – probably even m = 4! So it's no surprise that the Pleiades are difficult to pick out without a bit of technology since even the brightest of them reaches only 2.8 in magnitude (Alcyone). However, the variation in magnitudes within this faint cluster gave me a means to check out the detection limits of my telescope and astrocamera. It’s apparent that, even with a total of only 6¼ minutes of light-gathering, I can detect stars with a magnitude approaching 10; I’m content with that.

We come now to the clever bit, and something totally new to me until a couple of weeks ago when I read the post by Dr Dirk Froebrich I mentioned above. Dirk and his team are using data from the European Space Agency’s Gaia telescope to identify clusters of young stars within the Milky Way. The Gaia project is progressing with its aim to measure the distance and motion of 2000 million stars (please see footnote [3] for details). Astronomers like those in Dirk’s team can use its database to check which stars within a telescope’s field of view are actually within a cluster – i.e. grouped by virtue of their mutual gravitational attraction – or simply happen to be in the line of sight. To be part of a cluster our stars ought to be at similar distance from us and they ought to be moving together within the Milky Way, albeit with some small additional ‘random’ motion as the stars orbit the centre of their cluster. What Dirk kindly shared is an application of this methodology to the distances and proper motions of the stars in the part of the night sky that includes the Pleiades; the results are shown below.

The left hand plot shows the distances of the stars one observes in the part of the sky occupied by the Pleiades – some of which may be a part of the cluster and others may simply be in our line-of-sight. Notice the red-coloured spike indicating those stars at approximately the same distance from Earth, a median distance of 444 ± 1 light years. Sitting at the same distance is however not sufficient to qualify a star as being a member of a cluster as it also needs to have a similar proper motion to other members; i.e. it needs to be moving through space as part of the cluster. The plot to the right gives us that information. It shows, for all those stars in the red-coloured spike, their proper motion in right ascension (the celestial analogue of longitude) and declination (the analogue of latitude). Only those stars with similar values, again represented in red, can be considered to be members of the Pleiades cluster. The Gaia data yields a little more in respect of the Pleiades – indeed, all the stars it has measured – in that one may plot the colour of its constituent stars against its absolute magnitude and generate a Hertzsprung–Russell diagram, which gives an indication of the stars’ ages, but I have relegated this to yet another footnote, [4].

Apart from the fun I’ve had in writing this, I’ve learnt once again that there’s always more to see than the obvious: I’ve known of the Pleiades since my first forays into stargazing as a teenager, now I treasure them as a sight even more. It’s also caused me to consider afresh the facts I took for granted when writing about some of the binary star systems I’ve imaged in the past (e.g. here and here). Thanks for reading J

~1750 words + footnotes

Footnotes:

[1]  The 3k x3k pixel image is defined by my Altair Astro 533c astrocamera (4 fps, sensor at -5° C, gain=400, dark level=192; no calibration frames) on a Skywatcher 72ED refractor fitted with a 0.8x reducer/flattener; the telescope was mounted on a Skywatcher HEQ5 GoTo mount controlled from my elderly laptop by Carte du Ciel via ASCOM. The mount was polar aligned (i.e. to the celestial pole, which is close to Polaris, the Pole Star) using a plate-solving routine included in SharpCap – this is what facilitated exposures as long as 5s. Image capture was handled using SharpCap, with the best frames, i.e. a total integration time of only 6¼ minutes, stacked initially using DeepSkyStacker – my first trial of this software – and then using PIPP and Autostakkert3. The image was processed using my old version of Photoshop.

[2]  2.512 to be a little more precise, or to get it entirely correct it’s ⁵√100 – the fifth root of 100 – which means that a star of apparent magnitude 1, m1, will appear one hundred times as bright as a star of m6. For a full description of both absolute magnitudes (M) and the apparent magnitude (m) scale I’ve mostly used in this post see here.

[3]  Distances are determined using measurements of parallax, i.e. the apparent change of position of an object against its background as the observer changes position. (Try holding your thumb at arm’s length and use it to hide a distant object like a TV aerial or street sign whilst one eye is closed; now switch to the other eye and notice the apparent shift in the object’s position relative to its background: that’s parallax.) In the case of Gaia’s measurements of stars, the analogue to switching between eyes is to conduct a pair of measurements six months apart – so on opposite sides of the Earth’s orbit around the Sun. What comes out of these measurements is a parallax angle, and thence corresponding Earth-star distances – calculated using trigonometry on the basis of the diameter of the Earth’s orbit being the base of an isosceles triangle – are usually quoted in Parsecs (pc), which is approximately the same as 3.26 light years (ly). For a fuller explanation, see here or here. Determining the motion of our candidate stars requires that one follows their positions relative to the cosmic background over time; astronomers refer to a star’s ‘proper motion’.

[4]  The colour of a star is a pretty good proxy for its surface temperature (- think about metal being heated, glowing increasingly bright red as the temperature rises until appearing ‘white hot’) and its magnitude may be used instead of its luminosity. So, by plotting a graph of colour against magnitude we’re in effect plotting temperature against luminosity and that can tell astronomers/astrophysicists a lot about the age distribution of stars in a cluster. (There is additional information here and here.) The plot Dirk obtained indicates that the stars which constitute the Pleiades formed no more than 200 Myr ago; the online resources I looked at before drafting this post (e.g. here and here) suggest a figures of 100 Myr ranging up to 150 Myr depending on the stellar evolution model adopted – so that’s a reasonable match.