The good ship Argo
As the late summer sky darkens, the stretch of the Milky Way, running from the constellation Norma, through the Southern Cross and on to Canis Major and Orion, swings into view. The central section of this band is the one-time giant constellation Argo Navis, so named by the Greeks after the ship Argo which carried Jason and his Argonauts on their search for the Golden Fleece.
Earlier, the Egyptians saw this constellation as a boat bearing Isis and Osiris, and named it accordingly. From Cairo, in the time of the Old Kingdom (around 3000 BC), this boat rode clear across the southern sky, but today the effects of precession give it the appearance of having run aground on the reefs of the horizon.
During the sixteenth century Argo was shown as an antique galley, the best bet at that time of what a classical Greek galley might have looked like. However, the majority of charts in the seventeenth century abandoned any pretensions to antiquarian precision and became frankly jingoistic. First the galley became transformed into a contemporary fully rigged ship, and then English cartographers decked her out with the colours of an Admiral of the Red.
Such revisionism was very much the mood of the age. Edmund Halley had planted a new constellation, Rubur Carolinii (King Charles’s Oak), in a successfully sycophantic gesture; he was awarded his degree and later, deservedly, became Astronomer Royal.
Nicholas Louis de Lacaille spent four years from 1750 at the Cape of Good Hope making a systematic survey of the southern skies, and it is to this work that we owe the definition of many of the ‘locals’. He took the first steps to subdividing the unwieldy bulk of Argo by setting up Malus, the Mast, and Pyxis Nautica, the Mariner’s Compass, as separate constellations. Later, in the nineteenth century, Benjamin Gould followed suit, dividing the core of the original constellation into the the three parts which we know today: Carina, the Keel, Puppis, the Poop, and Vela, the Sail. Finally, with the formalisation of the boundaries of the constellations by the International Astronomical Union in 1930, Malus, which had never achieved general recognition, was incorporated in Pyxis and so disappeared from the charts.
If the sky in the direction of Scorpius and Sagittarius provides the greatest show off earth (see New Zealand Geographic, Number 6) then that towards Carina is a close rival. In terms of star density, as well as virtuoso displays of nebulosity and rich clusters, this area is a more than adequate compensation for the lost glories of the winter nights. Our line of sight is along the inner or Carina section of the Sagittarius arm which is thick with dust and gas and the site of star formation. For an observer in suburbia, even binoculars will start to reveal the extraordinary richness of this area preceding the Southern Cross as it swings around the South Celestial Pole.
Since only some of the stars appear to rise in the east and set in the west, but others, the circumpolar stars, merely rotate about the South Celestial Pole without ever going below the horizon, terms such as ‘above’ and ‘below’, ‘east’ and ‘west’ are misleading, if not meaningless. Thus, when Crux is
swinging below the pole, the Pointers are west of it, but six months later, at the same time of night, Crux will be passing over the Pole and the Pointers will lie to the east. To avoid this confusion astronomers describe the relative position of stars as ‘preceding’ or ‘following’ and `north’ or ‘south’. Thus the Pointers follow Crux, which in turn follows Carina.
The most obvious features in the vicinity of Argo are the three crosses and the bright star Canopus, alpha Carinae. Though part of this pattern, Crux, the Southern Cross, is a constellation in its own right. Immediately preceding Crux is the ‘Diamond Cross’, which is part of Carina and noticeable for its geometrical perfection and the fact that the southern and preceding point of the diamond is Miaplacidus, beta Carinae, the second brightest star in Carina. The opposite end of the figure is the naked eye ‘star’ theta Carinae, which on examination with even binoculars is seen to be a cluster of stars, IC 2602, notable for the two lanes of bright stars which are its distinguishing feature.
Completing the triad is the False Cross, a rather larger and more distorted cross than that of Crux, but sufficiently striking to be often mistaken for the Southern Cross, which is unequivocally identifiable by the adjacent Pointers, alpha and beta Centauri.
Preceding the entire procession is Canopus, alpha Carinae, the second brightest star. Interestingly, 150 years ago it was outshone by its neighbour beta Carinae. This star is today only visible with binoculars or a small telescope, but in 1843 it appeared nearly as bright as Sirius. Thirty years later it had faded to well below naked eye visibility. Since then it has been very gradually increasing in brightness and is now magnitude 6.2, at the limit of seeing with the unaided eye and brightening slowly.
Through the telescope it is apparent that what we can see is not the star itself but rather an enclosing shroud of gas and dust which is so dense that it entirely obscures the energy source at its core which is heating it to incandescence. Examined in deep infra-red this nebula is the brightest object in the sky outside the solar system, and in visible light the tiny, brilliant core has the shape of a homunculus.
The description by the Argentinian astronomer, Enrique Gaviola, 1944 reads, “Under a power of 1,200 diameters and star images not larger than 1″ a shape resembling a `homunculus’, with its head pointing north-west, legs opposite and arms folded over a fat body, could be clearly seen.”
This was confirmed by the Australian astrophotographer David Malin. Breaking off from photography with the great 3.9-meter diameter AngloAustralian telescope, he examined this nebula by eye: “The warm, red-orange appearance and soft, rounded markings across the elongated egg shape confirm (Gaviola’s) vivid description, enhanced by quivering life-like motions imparted by the Earth’s atmosphere. At its centre a surprisingly faint white star could be seen—the star which has given its name to the (beta Carinae) nebula.”
Alongside the Homunculus nebula lies the striking Keyhole nebula, first drawn by Sir John Herschel when at the Cape of Good Hope in 1838, and which has shown changes since then. Both of these nebulae lie embedded within the huge beta Carinae nebula, which is the brightest and richest such structure in the sky. With a diameter of 2° it is larger than the field of view of most telescopes and strikingly bisected by a dark band which forms a right-angle beside the Keyhole nebula. Nearby, within a small open cluster, is the apparently insignificant star HD 93129A, which was in 1986 the brightest star found in our galaxy. About 100 times as massive as the Sun and with a surface temperature of about 29,000K, it is in real or absolute terms a million times brighter. Only novae and supernovae outshine such stars, of which three are known in the beta Carinae nebula.
Near this great nebula are a number of open clusters, the most magnificent of which, indeed, one of the most magnificent in the entire sky, is NGC 3532. Even a modest telescope will show about 150 stars in it, while the full membership is something better than 400 stars.
If one were condemned to limit one’s observing to only one section of the sky then this is without doubt where I would elect to serve my sentence. The area within five degrees of the Keyhole nebula is rich enough to reward a lifetime’s study.