Southern hemisphere skies offer dazzling spectacles for night viewing, some of which cannot be seen from the northern hemisphere. Clear skies are usual over much of New Zealand, and it is possible to get fine views of the Milky Way and its neighbouring galaxies, the Magellanic Clouds. The most recognisable constellation (pattern of stars) in the sky is the Southern Cross, along with its associated features, the Jewel Box and Coal Sack.
Many of the sights that are visible from the northern hemisphere can also be seen from New Zealand. The constellations of Orion and Scorpius are prominent at certain times of the year. However, New Zealand is too far south to see Polaris, the pole star, or the Great Bear (Ursa Major).
As the sun sets and darkness descends, a number of different features become visible in the sky: the moon, thousands of stars, sometimes one or more planets, faint hazy patches of light and dusty dark regions. Our view of the sky changes over the course of a single night. Some stars appear to rise in the east and set in the west, while others are visible throughout the evening, but seem to circle clockwise around a common point. It is not the stars that are moving during the night, but the viewer: as the earth spins on its axis, different parts of the sky come into view.
Our view of the sky also changes during the course of a year. As the earth orbits the sun, new regions of the sky become visible from one season to the next.
Northern hemisphere observers consider New Zealanders to have an upside-down view of the sky. People standing in each hemisphere are upside down in respect to each other, and have an inverted view of the same object out in space. For this reason it is difficult for southern hemisphere viewers to pick out the shapes for which many constellations were named.
To people in the southern hemisphere, any suggestion that the moon’s light and dark surface features resemble a ‘man in the moon’, is unconvincing. Theirs is an upside-down view of what is seen in the northern hemisphere. The light areas, known as the southern highlands, appear at the top of the moon to southern hemisphere viewers, while the dark areas, such as the Sea of Rains and Sea of Serenity (now known to be vast lava plains), are concentrated in the middle and lower portions of the moon.
The moon is best viewed around the first and last quarters, when half of its face is illuminated. At these times the sun casts long shadows that accentuate the moon’s mountains and crater walls. In the southern hemisphere, the phases of the moon (changes in the moon’s appearance) appear the reverse of those seen in the northern hemisphere. The new moon is followed by a left-hand crescent moon that grows (waxes) towards full moon, and then decreases (wanes) towards a crescent with the curve facing right.
The planets are large celestial bodies that orbit a star (the sun), and five are visible with the naked eye. We see them as bright points of light slowly moving against a background of stars. The planets are always located in a band of the sky that runs close to the same path the sun appears to travel each year (the ecliptic).
On most nights one or more meteors streak across the sky. Although commonly described as shooting stars, they are actually dust and rock that fall from space and burn as they plummet through earth’s atmosphere. Large particles that survive the fall and hit the ground are known as meteorites.
At times during its annual journey around the sun, the earth passes through particularly dusty regions associated with comets, causing meteor showers. During a meteor shower, it is possible to see dozens of meteors in an hour.
Amalgamations of ice and rock, comets are infrequent visitors to our night skies. Comets originate from the outer fringes of our solar system and pass by earth as they orbit the sun. As a comet approaches the sun, its outer layers of ice and rock are vaporised, forming a gaseous plume which is its tail.
Stars are great spheres of intensely hot gas that are undergoing nuclear reactions (similar to those in hydrogen bombs). Stars form in vast clouds of gas and dust known as nebulae, and have a long lifespan – a few million to tens of billions of years – before they exhaust their supply of fuel. The largest stars are about 120 times the mass of the sun, and are known as supergiants. The smallest, known as red dwarfs, are about one-tenth the sun’s mass. The manner in which stars die depends on their size. The products of many star deaths are also known as nebulae, for they are also great clouds of gas and dust, but in this case ejected from the periphery of an exploding star.
Visible year round from New Zealand, the Southern Cross constellation (Crux) and its associates the Pointers are among the brightest stars in the southern sky. Four bright stars form the ends of an imaginary cross with a long axis and short crossbar. The long axis always points in the direction of the South Celestial Pole, and for this reason it serves as a night-time navigational aid.
Along the eastern edge of the Southern Cross is a dark region called the Coal Sack nebula. It is a star nursery, where young stars are forming from dense clouds of glowing gas and dust compressed under intense gravitational force. Just above the Coal Sack and alongside the second brightest star of the Southern Cross is the Jewel Box, a colourful cluster of about 50 stars that can be seen with a telescope.
The ABC is a useful way of remembering the defining stars of the southern hemisphere:
A is for Alpha Centauri, B is Beta Centauri, and C is the cross.
The Centaurus constellation lies to the east of the Southern Cross. Its two brightest stars, Alpha and Beta Centauri, are commonly known as the Pointers, because an imaginary line between the two stars points towards the cross.
Alpha Centauri appears as the third brightest star in the night sky, after Sirius and Canopus. It shines with a yellow light, and is not a single star, but a triple star system. Two stars orbit around each other every 80 years and both are visible with a good telescope. The third star lies far beyond them, and is so small and faint that it was only discovered in 1915. This is Proxima Centauri, the closest star to our sun at only 4.22 light years away. It appears to orbit the other two stars every 500,000 years.
Beta Centauri, the 11th brightest star, shines with a blue-white light. It is the nearest of the pointers to the Southern Cross, and is 526 light years from earth. It is a double star system, consisting of two giant stars about 15 times bigger than the sun.
In summer Orion becomes prominent in the north-western night sky. Along with the Southern Cross, it is one of the easiest constellations to recognise. People in the southern hemisphere have an upside-down view of what the ancient Greeks recognised as a giant with a sword and belt: New Zealanders see a saucepan. Three bright stars form the base of the pot, and three faint stars its handle. The base of the pot lies along an imaginary line known as the celestial equator: the region above the pot faces south; north is below.
The intense blue star on the handle side of the pot is Rigel. It is the brightest star in Orion, and the seventh brightest in the sky. Classed as a blue supergiant by astronomers, it shines with a light equivalent to 40,000 suns.
Below the pot and diagonally opposite Rigel is Betelgeuse, a distinctive orange-red star. It is the second brightest in Orion and 10th brightest in the sky. A red supergiant, it has a diameter 500 times greater than the sun’s.
Unlike most stars, which burn hydrogen, giants and supergiants have exhausted their hydrogen supply and are starting to burn heavier elements such as helium and carbon. Once these fuels are used up, the giants collapse into their core and trigger a massive explosion known as a supernova.
Winter skies are dominated by Scorpius, a long, S-shaped constellation located in the widest and brightest part of the Milky Way. It is best viewed from the southern hemisphere, where it lies overhead in late winter evenings. It needs little imagination to make out the shape of a scorpion. A prominent orange-red star, Antares, represents the heart of the scorpion. One of the largest stars, it is a red supergiant. Just west of Antares, a line of three stars represents the head and claws, and on the other side of Antares, a line curving downwards is the scorpion’s tail.
The Pleiades are an important star cluster also known to stargazers as the Seven Sisters or M 45, and Matariki to Māori. They are a group of young stars that still dwell in the gas and dust of the nebula from which they formed. In New Zealand their first appearance in the wintry dawn sky around late May or early June heralds the Māori New Year.
The broad band of faint white light stretching in an arc from the north-east to south-east horizon is known as the Milky Way, or Te Ikaroa to Māori. The light comes from clusters of millions of stars so distant from earth they cannot be seen individually. The view is spectacular on dark nights away from densely populated areas – city lights brighten the sky to such an extent that only some stars are visible. In New Zealand the Milky Way is best seen during winter, when the bright central region is directly overhead.
The Milky Way is a spiral galaxy. Our solar system belongs to it, along with 250 billion other stars, and vast clouds of gas and dust. Galaxies are large assemblages of stars bound together by gravity. We have an interior view of the galaxy, looking edge-on towards its centre. If we were to look down on the Milky Way it would appear as a spinning Catherine wheel. Side on, it is disc-shaped with a central bulge. It is this bulge, the widest region of the galaxy, located around the constellations of Scorpius and Sagittarius, that is such a feature of winter skies in the southern hemisphere.
Named after the 16th-century Portuguese circumnavigator Fernão de Magalhães (Ferdinand Magellan), and described by him in 1519, are two hazy patches of light near the South Celestial Pole. Although far from impressive when viewed with the naked eye, the Large and Small Magellanic Clouds are significant features, for they are actually satellite galaxies of the Milky Way. Like the Milky Way they contain an assemblage of millions of stars, gas and dust. At 190,000 light years away, the Small Magellanic Cloud is one of the most distant objects visible with the naked eye.
Just west of the Small Magellanic Cloud is 47 Tucanae, one of the brightest globular clusters in the Milky Way. It appears as a single fuzzy star, but viewed with a good telescope a spherical collection of thousands of stars is apparent.
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