Science

The Southern Sky

by Duncan Lunan

‘Write about what you know’ is advice often given to new writers.  It’s not necessarily good advice, and particularly unhelpful to writers on astronomy and space, let alone science fiction.  ‘Write about what you know about’, is a bit more helpful, but doesn’t always fit the case.  In this case, I’ve never been further south than San Diego in California or Miami in Florida, and even there, there were surprises, aspects of the sky which I didn’t expect.  As I don’t know first-hand what the sky looks like further south than that, there’s a real risk that I’ll commit at least one ‘Howler’ like those that I was pointing out in other people’s work, just recently.

One mistake I can at least try to avoid is describing the sky from a northern hemisphere viewpoint  (the mistake NASA allegedly made at the beginning of The Dish, and actually did make in the rush to reconfigure the Parkes radiotelescope to deal with the Apollo 13 emergency.)  In ‘Co-ordinate Systems’  (ON April 4th 2021)  I described the difficulty I had finding suitable maps when I had a monthly astronomy spot on Falkland Islands TV.  Many of the maps which I obtained had the constellation names printed with north at the top, which made them very difficult to use.  But there is one major source of accurate information which I’ve neglected to mention:  at the end of each month’s ‘Night Sky’ section in the UK magazine Astronomy Now, there’s a map for southern observers, looking north, with the South Celestial Pole at the top of the map  (i.e. behind the user)  and with the Zodiacal constellations holding the Moon and the planets at the bottom.  In the December issue, Auriga is at the bottom, with Capella on the northern horizon, with the triangle of the Kids directly above it, and Taurus and Orion above them – everything the other way up from how we see them, with Rigel at top left of Orion instead of bottom right  (Fig. 1).  The brightest star in the sky, Sirius, rides high because it’s south of the celestial equator.  And many of the stars which are low on the horizon and dimly seen at best, from here, are better seen the further south you go:  Antares in Scorpius and Fomalhaut in Piscis Australis are examples, better seen from Turkey, still better from the southern USA, but coming fully into their own south of the equator.

Fig. 1. Venus and the Pleiades over Australia, Aug 2017 by Yuri Beletsky – Orion at top, Betelgeuse at foot

It’s generally agreed by observers that the southern sky is more spectacular than the northern hemisphere’s.  It’s crossed by the richest regions of the Milky Way and contains the Galaxy’s two satellites, the Magellanic Clouds  (Fig. 2). 

Fig. 2. Venus & Jupiter with Milky Way over Mullinger Swamp, South Australia, John Carter, Feb 2019, Magellanic Clouds in trees at right

Humans have been on Australia for at least 30,000 years – some say 60,000-76,000 – and although claims for 176,000 years failed to stand up, there was settlement of Flores in Indonesia by humans 700,000 years ago or more, proving the ability to make short sea crossings.  There’s growing recognition that some legends of ‘the Dreamtime’ before the coming of white explorers have a basis in fact – for example, stories in northeast Australia of ‘a great white wave’ may relate to a mega tsunami caused by a meteorite impact off New Zealand in the 13th century.  For a long while, it’s been noted that the Aborigine name for the Henbury meteorite craters, formed c.2700 BC, translates as “sun trail fire devil stone”.  

Fig. 3. Sagittarius emu. ‘dark constellation’

In recent years increasing attention has been paid to the star lore of the Aborigines, including ‘dark constellations’ – dark nebulae of dust and gas, silhouetted against the brightest regions of the Milky Way, which Aborigine legends portrayed as the Emu, the totemic emblem of many tribes  (Fig. 3).  Orion is Djulpan, the Celestial Canoe  (Fig. 4);  Binyu, the Ringtail Possum, featured as the head of the Southern Cross, while the Coalsack Nebula behind it was a dungeon into which he hurled his wife’s seducer.  Eta Carinae, nearby, dominated the constellation of Cullowgillouric War, the wife of War, the Crow;  Sirius was the brightest star in Warepil, the Wedgetail Eagle, Achernar the brightest in Yerredetkurrk, the Fairy Owl, and the Magellanic Clouds were Kourtchin, ‘the Brolgas’.  It’s suggested that these constellations may correspond to figures in rock paintings dating back to 40,000 BC, and if so, they may be the oldest known portrayals of the sky.  The South African bushmen and tribes in the Amazon basin have similar ‘dark constellations’:  they and the Quechua of Peru saw the Emu as several dark constellations, including two llamas being attacked by a fox  (Fig. 5). 

But right across the southern Milky Way there is a band of hot blue stars, newly formed when the shockwave of a spiral arm swept through that region a few million years ago.  Some of the bright stars are of great intrinsic brightness – Canopus in Argo is an example – but Alpha Centauri  (‘Rigil Kent’ to modern aerial navigators)  is brilliant because it is the nearest major star to us, and a double star at that.  Alpha Centauri is one of the pointers to the Southern Cross, whose fame derives less from its brilliance than from its dramatic position against the black nebula of dust and gas known as ‘The Coal-Sack’, which Sir John Herschel thought was an actual hole in the Milky Way  (Fig. 6).  Not everyone can see the Southern Cross;  my mother never managed to pick it out, when she spent time prewar in Burma, and admittedly there are ‘false crosses’ elsewhere in the sky  (Fig. 7).  But the Guardians or Pointers, Alpha and Beta Centauri  (Fig. 8), should help to identify it, along with the dark background of the Coal-Sack and the distinctive ‘Jewel Box’ open cluster, as well as the striking Carina Nebula, all nearby  (Fig. 9).

One respect in which the southern sky falls short is that there is no southern pole star, corresponding to Polaris, in the northern hemisphere, in the present day  (Fig. 10), and Thuban in Draco in the time of the Pyramids and Stonehenge, much less to Vega in Lyra, which was the north pole star 13,000 years ago. 

Fig. 10. Southern hemisphere star trails, from near the Equator

Sigma Octantis is the naked-eye star nearest to the south pole at present, and as its designation implies, it’s not conspicuous.  The situation isn’t permanent:  the 26,000 year cycle of the Precession of the Equinoxes will bring the faint but recognisable Delta Velorum to the south celestial pole in 9250 AD  (Fig. 11).  It is possible to find south by the stars in the southern hemisphere, using the Southern Cross and knowing its distance from the Pole  (Fig. 12), and to use it as a clock, as can be done with the Plough in the northern hemisphere.

Due to precession, when Thuban was the north pole star at the building of Stonehenge I, the declination of the Galactic Pole was equal to the latitude of Stonehenge.  As it passed through the Stonehenge zenith, the plane of the Milky Way coincided with the horizon and the Southern Cross would have been visible, though low on the horizon and not conspicuous.  The same applies to Jerusalem during the Crusades, and it’s perhaps a bit more surprising that apparently none of the chroniclers mentioned it.  Again, I’ve never been to Jerusalem, but as far as I can see from maps and photographs, the sky should have been clear to the south and taking atmospheric refraction into account, the Cross should have been visible standing on the horizon.  But, again, it wouldn’t have been conspicuous.

In classical times, due to precession, stars could be seen from Greece which aren’t visible now, and that helps to date Greek sources such as the Sphere of Aratus as having come from still further south  (see ‘The Ancient Constellations’, ON, 30th May 2021,)  In the region they called ‘the Ocean’, containing water constellations like Eridanus, Cetus, Pisces and Aquarius, in the south they placed the huge constellation of Argo  (Argo Navis), the ship in which Jason and his heroes sailed in search of the Golden Fleece  (Fig. 13).  The constellation was too large for practical purposes in modern astronomy and was split into sections such as Puppis, the Poop, Vela, the Sails and Carina, the Keel (Fig. 14).  The second brightest star in the sky, after Sirius, is Canopus in Puppis – to the south of Sirius, but too far to be visible from here  (Fig. 15).  Similarly although Centaurus could once be seen from Greece, it and the Guardians of the Cross are now too far below the horizon.

Fig. 13. Argo Navis as drawn by Hevelius

When it came to the regions nearer the south celestial pole, never seen and mapped by the classical astronomers, the imaginations of their modern counterparts were sorely lacking.  That region of the sky is occupied by such figures as Norma  (the Set-Square), Fornax  (the Furnace), Antlia  (the Air-Pump), Telescopium  and Microscopium  (guess).  Nevertheless the sky itself is full of interesting objects.  Fomalhaut, for instance, is one of the first stars which the IRAS space telescope found to have a circumstellar disc of dust, similar in scale to the Solar System and apparently forming planets  (Fig. 16). 

What seemed to be a gas giant planet within the inner edge of the disc, was definitely in motion and presumably had swept out the clear zone around it.  But sad to say, the feature has expanded and grown fainter over time, and is obviously just a cloud of dust  (Fig. 17).  Still it presumably was formed by a collision, and it’s out of such collisions that planets grow.

Fig. 18. Jewel Box Cluster, wide-field

The Jewel Box Cluster  (Fig. 18)  is one of the most famous Open Clusters of newly formed stars, perhaps second only to the Pleiades in the northern hemisphere, and strikingly close to the Southern Cross  (Fig. 19). 

Fig. 19. Crux with Mimosa and Jewel Box

At the other end of the evolutionary scale, the eta Carina Nebula, to the right of Crux in Fig. 9, contains the star which is most likely to present us with the spectacle of a supernova in the near future  (though that ‘near future’ could still be hundreds or thousand of years).  The surrounding nebula is huge, with features like the Mystic Mountain (Fig. 20)  and the Keyhole Nebula  (Fig 21)  showing clear signs of star formation.  Yet eta Carina itself is in the last stage of life, blowing off large volumes of dust and gas in successive outbursts  (Fig. 22).  Examples were noted by a succession of astronomers including Edmond Halley, who saw it flare to 4th magnitude, but the 1837 outburst was the brightest yet and Sir John Herschel recorded it as briefly rivalling Alpha Centauri at 1st magnitude.  When the next and possibly last one will be, is anyone’s guess.

Fig. 22. eta Carinae clouds from 1837 outburst and earlier

The latest supernova of recent times was SN 1987-A, in the Large Magellanic Cloud  (Fig. 23). 

Fig. 23. Evolution of SN 1987-A between 1994 and 2016, HST, blast wave passing ring ejected 20,000 years before explosion

The Magellanic Clouds look like small detached fragments of the Milky Way, but there’s a lot more to them than that.  They are in fact satellites of the Milky Way, the nearest dwarf galaxies to ourselves in the Local Group, and they’re joined by a bridge of dust, gas and newly formed stars  (Fig. 24), suggesting either that they have been in collision, or were torn apart in a passage through the outer arms of the Milky Way, prior to which they may have been a dwarf spiral. 

Fig. 24. Magellanic Bridge

That would have been worth seeing, at only 160,000 light-years away, and both may be absorbed by the Milky Way in due course.  But the 1987 supernova could happen again, too – the Tarantula Nebula is the most active star-forming region we know, and it hosts the largest star known, R136-a-1  (Figs. 25 and 26).  Although it’s a blue star, so was SN-1987A, unexpectedly, because it had already blown off its outer layers.

This is one of those articles which could go on forever.   But I’ve used up my quota of illustrations, and there’s a deadline to meet, so this is a good place to stop.  More astronomy next time!