The greatest show off earth

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This is the season of the Scorpion, the most dra­matic of all the constella­tions. Seen by the early Chinese as part of the Blue Dragon, the happy omen of spring, Scorpius rising lies along the horizon like a huge taniwha writhing across the hills. At the same time Orion sets in the west; no longer the giant hunter but rather a great ocean-going canoe rigged with a mat sail.

As the Scorpion rises it is immediately followed by the archer, Sagittarius, which together with the tail of the Scorpion marks that part of the Milky Way which hides the centre of our galaxy. Between us and this centre lie 26,000 light years of stars, gas and dust which hide the black hole or whatever else may be powering the huge output of energy from the hub. At longer wavelengths than our eyes can detect ­infra-red and radio ­images of this hub have been obtained but their exact interpretation is still open to debate.

Nevertheless, what we can see with the unaided eye (or binoculars or a small telescope) is breath­taking in its variety and splendour. It is for this view that American and Japanese astronomers, professionals included, come south armed with even the modest equipment that an airline passenger may pack. When it comes to super-widescreen displays there is nothing that can compare to the winter sky in southern latitudes.

Unfortunately, most of us live in cities, where it is becoming increasingly difficult to see a clear night sky; a sky unpolluted by wasted urban light. This light, issuing into the sky from such sources as street lights, building and adver­tising flood lights, private security lights, and sports arena super-flood lighting, is thrown back at us from the dust, salt and water particles in the atmosphere, turning the night sky from black to blue/grey and masking all but the bright­est stars.

Light pollution in a major conurbation like Auckland forces us to drive at least thirty minutes north or south of Queen Street before we are able to see clearly the amazing arc of the Milky Way, our close companion galaxies called the Clouds of Magellan. But at this time of year the trip is well worth the effort, even if it means not getting to bed before midnight.

The area of sky covered by the constellations Scorpius, Sagittarius, Scutum and Corona Australis is a celestial treasurehouse containing some of the best and brightest objects of every description, as well as the great clouds of gas and dust which form dark lanes and patches along the Milky Way. For good measure Saturn, that most dramatic of the planets, is on the eastern edge of Sagittarius with its rings still tilted enough to be easily visible in a small telescope. As the planet swings around the Sun on its nearly 30 year­long orbital journey, we will see during the next few years the rings appear­ing progressively narrower until finally, for two or three years, they disappear altogether as they are too thin to see when edge on to US.

Apart from the Milky Way the most obvious feature of this area is the constellation Scorpius which once recognised cannot be forgotten. This is one of the few patterns of bright stars which really does conform to the structure of the object after which it is named. Marking the middle of the scorpion’s body is the red star An­tares, alpha Scorpii, a red giant entering its death throes. Antares is 10-15 times the mass of the Sun, and has expanded until it is 600 million kilometres in diameter, four times that of the Earth’s orbit. Brighten­ing and then dimming every five years as its store of hydrogen is depleted, it is on the edge of final col­lapse. Any time now the radiation pressure of the fusion reactions around its core will drop so low that it can no longer support its hugely expanded atmos­phere, which will start to fall inwards. The closer this material falls towards the centre, the greater is the gravitational attraction and the star will continue to shrink, causing core temperatures and pressure to rise. The result will be a final explosive outburst of fission which will appear as a supernova, a bright, apparently new star. This will shine brilliantly for a few weeks and then slowly fade, leaving behind a high density neutron star sur­rounded by an ever ex­panding shell of hot gas expanding into the sur­rounding space.

Antares is also a double star, the great red primary being orbited by a faint green companion which is normally only visible in moderately large, high definition telescopes. However, when, as on the morning of May 22, the Moon passes in front of Antares, there are a few seconds when the primary is occulted and the secon­dary can be seen shining by itself just clear of the edge of the Moon.

Following along the body of the Scorpion around the curve of its tail, one is led to two great star clusters, M6 (NGC 6405) and M7 (NGC 6474). Like the vast majority of objects, these have no common names and are known by their Messier (M) number or New General Catalogue of Nebulae and Clusters of Stars number. There are just too many objects in the sky to use proper names for each. Con­fusingly, most objects have several designations, for over the years there have been a variety of cata­logues compiled for various purposes. Thus many bright stars not only have a proper name, e.g. Shaula, but also a constel­lation name, lambda Scorpii, and at least two catalogue numbers, HD 158926 and SAO 208954. Astronomers often use the two catalogue designations to refer to a star so that a transcription error in one will not lead to total confusion.

To the eye M6 and M7 appear as intriguing clouds of light. In fact, this whole area running north­wards through Sagittarius into Scutum is tantalising for clearly there is much that is at the limit of vision or beyond: faint ill-defined patches of light brighter than the glow of the Milky Way and bigger than single stars. The only solution is to feed more light through the pupils of our eyes.

Armed with a good quality pair of 7×50 bin­oculars (‘night glasses’) this area is seen to be cluttered with stars forming loops, whirls, patterns of every descrip­tion, and the fainter multi­tude coalescing to form star clouds, those sheets of starlight spangled with individual brighter stars. Amongst all this there are patches of ghostly greenish light, the gaseous nebulae — clouds of interstellar hydrogen fluorescing under the impact of intense ultra­violet radiation from nearby stars. The most prominent of these are the close pair of the Lagoon and the Triffid nebulae, M8 and M20, but a sweep northwards along the Milky Way in the direction of Scutum will reveal others such as M17, the Omega Nebula.

In this area are also a number of small, circular, hazy blobs, the globular clusters. These are spheri­cal balls composed of hundreds of thousands of stars which are bound together by gravity and orbit the centre of the galaxy in random orbits inclined to the plane of the Milky Way in which the vast majority of the stars are located. With binocu­lars a number of globular clusters can be seen but they do not generally reveal their stellar nature until inspected through a 150mm diameter telescope or larger. Composed of very old stars formed from the original material of the galaxy, most of these clusters are very far from us. M4, one of the closest, is about 6500 light years away and M19, at 34,600 light years from us, is further than the centre of the galaxy. The most mag­nificent of all the globular clusters in this area is M22 which is visible to the naked eye.

All along the Milky Way and on each side of it are the open cluster of stars ­groups of twenty to two hundred stars in close association. Each cluster is formed from but now free of its own primordial gas cloud. They provide as­tronomers with much of the essential information about the evolution or life of stars, for all members of each cluster were formed at essentially the same time. However, the rate at which stars change is a function of their mass, the bigger they are the more quickly they evolve towards their ultimate fate.

Some of these clusters are large and look like elaborate pieces of jewel­lery; M7 is often referred to as the butterfly brooch, others are small and compact like a tiny setting of brilliants. Like the globular clusters, the open clusters repay further examination with a tele­scope which will bring their fainter members into view and enrich the pat­tern. M24, the Small Sagittarius Star Cloud, is a particularly bright part of the Milky Way. Just north‑west of this brilliant area is the Scutum Star Cloud.

The background to all these individual objects is the glowing band of the Milky Way, while west of the bright stars of Sagitta­rius the great Sagittarius Star Cloud masks the centre of our galaxy, the visually richest and most fascinating area of the heavens.

To sweep through this area with binoculars or even a modest telescope is to see a skyscape as beauti­ful and intriguing as any land or seascape. How ever often you look at this area of the night sky, how ever many photographs you take of it, how ever much you read about its constituent members, you will never exhaust its attractions.