As November draws on, we see less and less of the core of the Milky Way, and progressively lose sight of the Scorpion as it plunges headlong below the horizon. Over in the east Orion and his dreadful opponent Taurus, the long-horned bull, are emerging from the pre-dawn twilight, preceded by the Pleiades, the Seven Sisters.
Overhead Cetus and Phoenix, the mythical monsters which herald the approach of summer, culminate (lie on the meridian) at midnight by the middle of October, and also mark the very depths of space.
Our galaxy is, in effect, a disc of stars with the Sun and its planets imbedded in it about two thirds of the distance from the centre to the rim. As we are orbiting a star within the thickness of the disc, its member stars appear to us as a band encircling the earth — the Milky Way. The whole system — some 100 billion stars — rotates around the centre of the galaxy, with the Galactic Poles marking the axis of rotation.
Thus, when you look up towards the South Galactic Pole, which lies between Cetus and Phoenix, you are looking down into the depths of space. Looking not merely into the space between the planets of the Solar system, nor just into the gaps between the stars of our galaxy but out to the limit of light, out to the edge of the detectable universe. Every star that you can see is a part of this galaxy (on the scale of the cosmos a “next door neighbour”), but in the gaps between them glimmer the far galaxies.
Only a few of the very brightest can be seen with amateur telescopes, but deep space photographs and CCD images with the great four- and five-metre telescopes show these galaxies arranged in an apparently never ending succession of sheets and clusters right up to the limits of detection, out to distances so great that the light is either too faint to register or red-shifted to radio wavelengths.
Between us and the void lies Cetus, not a whale but the sea monster sent to consume Andromeda, who was set up as a propitiatory sacrifice after her mother Cassiopeia had impiously boasted that her daughter was more beautiful than Nereids. With bulging eyes, fangs like a sabretooth tiger and nostrils like Ngauruhoe, Cetus was seen as having a great curved tail, marked by the star Diphda, and fearsomely taloned forelimbs.
Poised on the bank of the river Eridanus he is about to launch himself across and onto the succulent Andromeda. But the meal is fated never to be consumed, for Perseus, a sort of celestial Tom Mix, is at hand with the Gorgon’s head, the sight of which will turn the monster to stone. Then, as in all the best fiction, our hero will release the damsel from her bonds and they will live happily and fecundly ever after.
During the past few years we have had our own Kiwi version of this played out by Willie the sea elephant, who haunted the Whangamata estuary in amorous pursuit of a dairy herd (New Zealand Geographic, Jan-Mar 1989). The part of Perseus was taken by the dairy farmer who came to their rescue, not with the Gorgon’s head but an electric fence — curiously foreshadowed by the Linum Boreum and Linum Austrinum, now abandoned constellations marking the cords or threads which ran between Pisces and Cetus. Perhaps they should be reinstated as Saepes, the fence.
In the body of Cetus is the star Mira, omicron Ceti, famous as the first star noticed as being variable in brightness. Originally, it was believed that the stars were examples of perfection, unchanging in either position or brightness. Planets, comets and shooting stars were correctly held to be non-stellar, and were located below the celestial sphere upon which the stars were placed. The very occasional supernova was no doubt troublesome, but as such stars fade in a few months and dim below the threshold of visibility, the problem conveniently brushed itself under the carpet.
However, the variable stars are not so obliging, and long period variables of the Mira type fade and brighten again with periods between 150 and 600 days. They thus presented a problem which would not go away, and were yet another indication that the stuff of the universe was inherently the same throughout, and that earth was not the only sphere that was subject to change and decay — an opinion which got Galileo a life sentence of house arrest.
With the advent of modern astronomy and the development of nuclear physics the Mira “problem” became an opportunity: a complex unknown which is changing yields far more information than one which is unchanging. Moreover, these changes allow us to make observational tests of the hypotheses we put up to explain the object. The variable stars are still of great interest to astronomers, who now have good reason to believe that variability marks particularly important stages in the very early and latter part of the life cycle of stars.
Indeed, so important is our understanding of variable stars to our attempts to unravel the story of the universe that their study is one of the major branches of astronomy. It is in the collection of data from variable stars (principally, estimates of the brightness as it changes with time) that the amateur astronomer plays such an important part. There are far more variable stars than the world’s rather small collection of professional astronomers can keep tabs on, and given their special training and the capabilities of their equipment the exercise would be an underutilisation of resources. However, there are a host of experienced and reliable amateur astronomers who are well equipped to monitor these stars and so provide their professional brethren with the necessary data.
The Mira variables are stars approaching the end of their lives as they exhaust the store of hydrogen available for nuclear fusion.
At maximum brightness during the latter part of September, Mira is now fading, and by mid-November should be about magnitude five or six that is, getting towards the limit of visibility and not apparent to the naked eye against the sky glow of city lights.
South of Cetus, beyond the zenith, lies Phoenix, the Egyptian bird which immolates itself in the altar fire every 1460 years, fanning the blaze with its wings as it is incinerated and then reborn. Visible on the southern horizon, this is one of the pre-European constellations and marks the beginning of winter and hence the drawing on of the northern hemisphere’s new year. For us it more appropriately announces the arrival of the fiery furnace of summer.
Apart from the second magnitude star alpha Phoenicis, Ankaa, this constellation is composed of a rather unremarkable scatter of third and fourth magnitude stars, and its sixteenth century depictions show no discernible relationship between the figure and the constituent stars.
South from Phoenix on a line through the South Celestial Pole is Crux, Southern Cross, now inverted and its head, marked by gamma Crucis, tracing out the northern limit of the circumpolar stars as seen from Auckland. These are the stars which never appear to set below the horizon but merely fade at dawn and reappear after sunset. Just which stars are circumpolar depends upon the latitude of the observer; at the equator only the Pole Star is circumpolar, whereas at the poles all stars appear to wheel around on paths parallel to the horizon.
The November sky chart shows a strip of sky 60° wide running from the southern horizon up through the zenith and down to the northern horizon. This is how this strip of sky will appear at about 8pm NZST in the first week of December. If you hold the chart with the zenith mark overhead and you are aligned north/south you will see why half of the chart is printed upside down.
In addition to Cetus and Phoenix a number of the more prominent star groups are shown together with a number of non-stellar objects. The biggest of these are the two clouds of Magellan which are in fact small galaxies so close that, at rather less than 200,000 light years away, they are almost part of our own galaxy.
Just above the northern horizon, below the great square of Pegasus, is the famous Andromeda galaxy. M31. Perhaps the most familiar of all galaxies, its glory is only revealed in photographs; even with quite large telescopes the faint detail structure escapes the eye. Nevertheless with a pair of 7×50 binoculars the view from the cliffs of it setting over the Tasman is to be treasured.