Science

The Autumn Stars

by Duncan Lunan

During the summer months the opportunities for observing the stars are limited by the twilight – especially in Scotland, where twilight lasts all night during June and July.  August is a different matter.  By the end of August the Milky Way is overhead, the brilliant stars of the Summer Triangle, Pegasus and Andromeda dominate the night, and the night air is still (relatively) warm.  The Perseid meteors are an added bonus, although they become more numerous after midnight as the Earth turns into the swarm and this year the Moon has spoiled things.  The same applies to artificial satellites: the Sun angle is just right to make them visible against a dark sky, and sometimes several can be seen in the sky at the same time.

A wide field image of the region of sky in which HD 189733b is located. In this image we can see the asterism of the “Summer Triangle” a giant triangle in the sky composed of the three bright stars Vega (top left), Altair (lower middle) and Deneb (far left). HD 189733b is orbiting a star very close to the centre of the triangle.

Arcturus, the brightest star in Boötes, the Herdsman, is one of the brightest stars in the sky, the very brightest north of the Ecliptic, and was accorded a special role in the astrology of the ancient world.  It’s one of the few stars mentioned in the Bible, and often was regarded not as part of the constellation but as an entity in its own right.  In Britain Arcturus is dominant in the western sky during summer, especially in Scotland, in the twilight of June and July.  It is rivalled only by Vega, the brightest star in the Summer Triangle of Vega, Deneb and Altair, and by Capella, low down on the northern horizon.  The other first magnitude stars in the summer sky, such as Regulus and Spica, stand out more because of their positions with few other bright stars nearby.

For the online journal Concatenation I’ve recently reviewed A History of the Universe in 100 Stars, by Florian Freistetter (trans. Gesche Ipsen, Quercus, 2022).  In his discussion of asterisms – groupings of stars, mostly modern, which don’t make up whole constellations, or which include stars from several – his examples include the Plough in Ursa Major, Orion’s Belt, the ‘Coathanger’ in Sagittarius, and the Pleiades as ‘the Seven Sisters’.  Strangely he doesn’t mention the Winter Triangle of Betelgeuse, Sirius and Procyon  (going back to at least 600 BC), nor the Summer one of Vega, Deneb and Altair, devised in the 1950s and enthusiastically promoted by Patrick Moore.  Instead, rather strangely, Freistetter features the ‘Spring Triangle’ of Arcturus, Spica and Regulus in Leo, of which I’d never heard;  it’s credited to Henry Neely (1879-1963), who was born in Ireland and died in the USA.  I looked it up online and then went out to check, before Leo disappeared into the west.  The Spring Triangle is bigger than its counterparts, and less distinctive, because it’s not equilateral like the Winter Triangle, nor isosceles like the Summer one.  There is an alternative Spring Triangle of Arcturus, Spica and Denebola, at the tail of the Lion, which is equilateral;  but as Denebola is well fainter than the other two, it doesn’t grab the attention.

It’s curious that the sky in British latitudes should seem so balanced through the year, with Vega skirting the northern horizon when Capella is overhead in winter, and with the Summer Triangle climbing towards the zenith as the winter one disappears.  The relationships are coincidental: when Stonehenge was built, and the pole star was Thuban in Draco, while Capella was not circumpolar in Britain and disappeared below the northern horizon for part of the year.  Jim Barker’s star maps are drawn for Europe, a little south of the British view, but if he had drawn them ten degrees further south for the mid-USA, then even the Great Bear  (‘The Big Dipper’) would be partly off the map during the winter.

In the ancient world, no relationships in the sky were coincidental: everything had to be significant in itself and in relation to the Earth below.  If the winter sky was ruled by Orion, the Hunter, then he had to have his counterpart in summer, the Herdsman, and there had to be a mystical identity linking the two.  It couldn’t be chance that both constellations contained a bright red star – Betelgeuse in Orion, Arcturus in Boötes – and in ancient texts the names are sometimes interchanged.  Arcturus was the guardian of the Pole, holding the bears in check: like Orion he has his hunting dogs, Canes Venatici, under the paws of the Great Bear and driving him ahead of the Herdsman throughout the summer.

King Cepheus with Cassiopeia

In previous columns I introduced the stars of Orion, Taurus, Canis Major and Minor, Lepus and Scorpius, in relation to the legends of their origin.  Overhead at midnight, by late September, near Auriga the Charioteer  (who’s also been introduced), we find another remarkable legendary grouping:  Perseus, Andromeda, Cepheus, Cassiopeia;  in some versions of the story, Pegasus the winged horse;  and later in the season, Cetus, the sea-monster, in the south.

Cassiopeia was the mother of Andromeda, and angered Neptune by boasting of her daughter’s beauty.  Neptune sent Cetus to ravage the kingdom (Ethiopia);  and Andromeda’s father Cepheus, whose attitude to women’s lib seems to have been somewhat pragmatic, had her chained to a rock in the monster’s path.  She was saved by Perseus, who happened to be passing with the head of the Gorgon Medusa.   (Neptune’s reaction on finding his monster turned to stone is not on record.)   In some versions of the story Perseus was riding Pegasus at the time, but others say he was using winged sandals, and that Pegasus was born when drops of the Medusa’s blood fell on a mountaintop as Perseus passed overhead.

Perseus with Gorgon’s head, Cassiopeia and Andromeda to right

While it’s not very easy to see them as human figures, unlike Orion, the constellation figures corresponding to the legend are easily learned and contain many interesting stars.  Cassiopeia is one of the easiest constellations to recognise, forming a great letter W in the sky, opposite to the blade of the Plough on the far side of the Pole Star.  When Ursa Major is in the north in autumn and winter, Cassiopeia is high overhead, and vice versa in spring and summer;  they obey the same roles as Capella in Auriga, and Vega in Lyra, marking the seasons and the ring of ‘circumpolar’ constellations which never set at Britain’s latitude.  According to tradition Cassiopeia, ‘the Lady in the Chair’, is punished for her actions by hanging upside-down in her throne for much of the year.  Cepheus, her husband, is represented to her upper right by a diamond of somewhat fainter stars, one of which is very important in the history of astronomy – more on that next time.

Perseus’ shape is less distinctive, but the constellation is easily found on the Milky Way between Cassiopeia and Auriga, which is marked by brilliant Capella  (described here with its near neighbours on January 23rd) .  On the side towards Cassiopeia, right on the Milky Way, there’s the famous ‘Double Cluster’ of newly formed stars, represented on old maps as the Gorgon’s head.  It’s a beautiful sight, through binoculars or a telescope at low power, and you can look at it in safety:  nobody’s been turned to stone by it lately.  Perseus has other sinister objects about his person, however – more about that next time, too.

Great Square of Pegasus, Andromeda, finding M31 from Mirach

Pegasus is drawn in the sky as the forepart of a horse  (Patrick Moore used to be sarcastic about that).  Near midnight at the end of August, the Great Square of Pegasus is high in the east, and Alpheratz, the star at top left, is shared with the double line of stars extending back towards Perseus and representing Andromeda.  At right-angles to it, another line of three faint stars extends upwards, and at the top right of the top one, a faint patch is the most distant object visible to the naked eye:  Messier 31, the Great Nebula in Andromeda, the nearest spiral galaxy to our own and the one with which we’re going to collide in 3.75 billion years time.  There has been a lot of discussion about whether it will be a head-on collision or a graze, first time round, but at the moment a head-on collision seems most likely.  The process will take several million years, accompanied by intense bursts of star formation as the dust and gas clouds in the spiral arms run into one another, but the end will be the formation of a giant elliptical galaxy currently going by the unattractive name of ‘Milkomeda’.

collision of M31 and Milky Way, 3.75 billion years ahead

Although M31 is 2.5 million light-years from us, there’s evidence that the collision process has already started, with hydrogen halos a million light-years out already starting to interact and merge.   At present, to see our neighbour requires a dark sky, and in modern conditions, it often needs averted vision.  (For evolutionary reasons, the rods at the fringes of the eyes are better for distinguishing faint objects in the dark, especially moving ones which might be predators or prey.  The cones in the centres evolved when we ceased to be nocturnal and needed high-resolution colour vision in daylight – but even now, human vision is most acute overall around 8 a.m. and 8 p.m., the times when opportunities to eat and possibility of being eaten were at their height.)  The Andromeda spiral is tilted towards us, so a fairly powerful telescope and a photographic time exposure are needed to trace the dark lanes of dust in the disc.  (For more details, see ‘Stars and Nebulae’ on January 30th.)  Over the next three million years, as M31 travels towards us and grows in the sky, those details will become more obvious and even the colours may become more apparent, with the nucleus tinged yellow and red with older stars and the spiral arms still coloured blue by newer ones.  But by 3.75 billion years from now, all that beautiful structure will be starting to distort as mutual gravitational attraction pulls the two spiral out of shape.  The ultimate view will be bland, with stars of similar ages all over the sky and little else to look at.  But there will still be some variety, as supernovae and colliding neutron stars create fresh clouds of dust and gas, allowing some star formation to continue.  The Hubble Space Telescope and the JWST are finding some new stars in even the oldest globular clusters and elliptical galaxies, so the show will not quite be over.

Meanwhile, we can enjoy the variety that we have.  As Arthur C. Clarke said, trillions of years from now when only red dwarf stars remain active, their long-lived civilisations may envy us because we knew the Universe when it was young.  As the autumn nights wear on, the harbingers of winter become visible:  Taurus is on the eastern horizon, and its major features are the Open Clusters of the Pleiades and the Hyades – bright new stars, formed within the last million years  (100-125,000 years, for the Pleiades)  and still gravitationally bound together, but with a great future ahead of them.  Make the most of it!

All the bright stars above are also visible at different times of night in winter, spring and summer, and have been covered in those articles, so I’ve tried to avoid too much repetition here.  The larger constellations which occupy the southern sky in autumn contain few bright stars, and are not well seen from Scotland, so I don’t write about them much.  Hercules, for example, now fairly high in the southwest, is a sprawling constellation centred on four stars making the figure of ‘the Keystone’, as in the topmost stone of an arch.  To visualise it as a human shape, it’s best seen upside-down, from the southern hemisphere viewpoint.  On its right-hand side, Messier 13 is one of the best known of the globular clusters, on the side nearest Corona Borealis, the Northern Crown, which lies between Hercules and Boötes.  Corona is an attractive little constellation, but its brightest star, Alphecca, is often the only one visible.  Hercules himself is much better seen from the south of England – those few degrees of latitude make a big difference.  Below him, Ophiucus the Serpent-Bearer is marked by the two relatively bright stars Rasalhague and Rasalgethi, the latter at the bottom of Hercules.  But it has to be said, it’s rare in Troon to make out any more of those constellations, between the glow of Prestwick Airport and the haze where the Firth of Clyde meets the Atlantic.  Lower still, Antares, the brightest star in Scorpius, is best seen over a sea horizon or from a hillside, with nothing like its brilliance as I’ve seen it from California or Turkey.  The four summer months I spent in Turkey in 2005 were a great opportunity for me to explore Sagittarius, Scorpius, Libra and Capricornus, all of which are well-nigh invisible from here.

There is one small constellation, not particularly bright, which deserves a mention.  Delphinus, the Dolphin, above and to the left of Aquila, the Eagle, was translated to the stars as a reward for saving the life of Arion, the wonderful musician whose Lyre contains Vega, the brightest star of summer and autumn.  I have known people with restricted vision who love Delphinus just because it’s small enough for them to see its shape in full.  I have a soft spot for it because at school, I proved to be better than most with Latin poetry.  The late Chris Boyce of The Herald used to invite me to lunch along with Lesley Duncan, Features Editor and now the compiler of the paper’s poetry feature, who was a year or two ahead of me at Marr College.  It fascinated Chris that when we reminisced about our schooldays, we’d end up quoting passages from Ovid that Mr. Nicol, the Principal Teacher of Latin, had made us memorise.  Quod mare non novit, quae nescit Ariona tellus…

Summer Triangle constellation figures with Ophiucus at top right and Delphinus below

When I became involved in the folk scene, I was fascinated to learn that Arion had made it into British Folk tradition, with his instrument evolving from the lyre to the fiddle  (as happened with the harp, in both Scotland and England).  Arion’s name survives as Glasgerion in the Scottish ballad, and Jack Orion in England.  The late folklorist A.L. Lloyd restored Jack Orion to the folk tradition by setting it to the tune of Donald, Whaur’s Your Troosers?, so successfully that most people don’t recognise it, and Bert Jansch devoted almost a whole side of his LP Jack Orion to an extended version of it.  And there’s a Connemara proverb, ‘Never trust Orangemen or fiddlers’, whose accompanying folk tale begins, ‘Jake O’ Reilly was as fine a fiddler as you could find anywhere…’  I almost fell off my chair when I heard the singer-songwriter Buff Wilson launch into that, one night at the Irvine Folk Song Club, having learned it from an old Irish farm-hand.  But if you go outside on a clear autumn night, and find the Summer Triangle overhead, you’ll see Arion’s Lyre made prominent by Vega, and the dolphin who saved his life in Ovid’s poem marked out by the small, perfect constellation to the upper left of Altair.

Delphinus stars, by Robin Scagell

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