Castor

This is in a very real way a postscript. My previous post, Mizar, marked something of a turning point in my pursuit of astronomy and then astrophotography. I wrote of a childhood hobby, carried out on a pocket-money budget and before personal computers were dreamt of, now reborn in retirement with a modest, but nevertheless much-improved budget. The evening’s observations that had given rise to that post had seen me reach a place of comfort with telescope, astrocam, software … in fact, the whole kit and caboodle – a place of more confidence than happenstance or happy accident. There remains more to learn than is already learnt, that probably goes without saying given my continuing status as a novice but I have at last passed my ‘driving test’. Thus, with a second clear and still night presenting itself in as many weeks – a rare coincidence this past winter – the temptation to set everything up again and try out my new skills was irresistible.

Gemini: a set of stars at all sorts of distances from the Earth – i.e. not bound to one another in any meaningful sense other than that they all reside in our galaxy, The Milky Way. Gemini is an aphorism, not a constellation. Their brightness and apparent proximity suggested particular shapes to our forebears; this is a much-studied form of ‘pattern-recognition’ (see here for a brief article outlining current thinking on the process.)    (Image created from ‘Stellarium’, a free-to-download computer package allowing one to generate bespoke star maps.)

Serendipity provided me with a ready-made target. The vice chair of the amateur astronomy society I joined shortly before the SARS-COV2 virus appeared in the UK (Steve, see here for details) had recently posted a detailed review of the night sky in spring. Within this was a section on the aphorism Gemini. In particular, it mentioned the constituent star Castor, a binary system much like Mizar in some ways. It has two principal components and a more distant third – each of which is itself a binary, the partners all being too faint and too close to observe directly; so six stars in all, just like Mizar. Six stars, five orbits; all taking place about 51 light years away (see diagram below). This was interesting in a generic sort of way, but the key factor for me was that the two brightest stars orbit each other at a relatively small distance. Indeed, from Earth, their angular separation is only about 6 arcsec*. Thus, I had a pretty good test subject for the claim I made in my post on Mizar: that I could probably fully resolve objects only ~5 arcsec apart. The game was on.

Each of the three paired partners orbiting each other, that’s three; Castor A and Castor B orbiting each other, and then finally AB and the two Castor C partners orbiting each other. Again, highly analogous to the Mizar ‘sextuplet’: a beautifully complex dance to gravity’s tune.

Castor was going to be pretty high in the sky at the times I was planning to observe it so, by keeping the tripod and mount relatively low to the ground, extending the legs only in order to level the mount, the small finder-scope was still easily useable without needing to balance on steps! The image was taken using the red-light torch bought for me by my daughter; it’s a light that doesn’t wreck ones night vision. By contrast, right at the top of the image is one of the nearby street lights that can be quite a nuisance, although less so than external security lights and passing car headlamps. Unfortunately, to get a view of both Polaris (for Polar Alignment of the mount) and of the southern sky for a lot of the interesting stuff, the front of my drive is pretty much my only option. On the positive side, it’s led to several lovely (socially distanced) conversations with passing ‘night owls’. The picture on the right is of my laptop display as I located Castor – the A⸱⸱⸱B stars were immediately visible; I’ve circled the pair. I confess to staring at the screen for a minute or two; in part this was the pleasure of seeing two well-resolved stars, but it was also in growing celebration of the fact that I had at last ‘cracked’ polar alignment – there was only minimal drifting of the stars on the screen.

This is the final stacked image showing Castor A and Castor B (lower right); an enlarged version is inset (upper left). The image represents the best 30% of 5000 40 ms sub-exposures. The two stars (or rather, the two pairs of stars – their respective partners being too faint and close to observe directly) are well-resolved: my system’s estimated resolution would seem to be entirely justified. The labelled figure shown below will clarify this further I hope. Castor A and B orbit each other with a period of about 445 years; their partners, Aa⸱⸱⸱Ab and Ba⸱⸱⸱Bb have orbit times in the region of 9¼ and 3 days respectively.

Given my fixation on the relatively bright near-neighbour Castor A and Castor B pair I confess that I didn’t even look for the more distant Castor C. Given that the pair are classified as cool dwarf stars and have luminosities less than 10% of the Sun’s, and that they eclipse each other as they orbit around their common centre of gravity (their barycentre: see my earlier post here) it hardly seemed worth the search. I was wrong. What I should have done was to collect data at longer exposure times, ignoring the fact that the A and B stars would be over-exposed; nevertheless, by increasing the brightness of the stacked image in my old version of Photoshop it was still just about possible to make out the third component: Castor C. This is shown in the inverted (‘negative’) image below. Caster C orbits the AB system every 14,000 years approximately; the pair of stars making up this faint companion orbit each other in a little under 20 hours!

The labelled and annotated figure above summarises the results of my evening’s observational experiment; the images have been inverted ('negative') in order to show Castor C more clearly.

Facts and figures beyond those offered in the stargazing guide referred to at the opening of this post were taken from two online sources: here and here. Both sites contain a wealth of additional information should you wish to learn more. One additional fact with which to close the post: because the orientation of their orbit is not face-on to the Earth, the angular separation of Castor A and B varies with time: in 1970 it was only 2 arcsec and by 2100 it will reach a maximum of 6.5 arcseconds.

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* From one horizon across the arc of the sky to the opposite horizon is 180º (i.e. half a circle), but a degree is too large a unit for many purposes, so we may divide it into sixty arc minutes (arcmin, sometimes simply ′). The Moon and the Sun both appear to be about 30 arcmin across for example, ½º. Even an arcmin is too large on occasion so we may divide that again into sixty arc seconds (arcsec or ′′). (This extract taken from my previous post, here.)