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.
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| 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 16th century
French Renaissance poets who called themselves La Pléiade. They, in their turn, were named after an analogous
group of seven 3rd 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.]
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 45th 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.
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| 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.
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| 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.
