Royal Astronomical Society of New Zealand The Evening Sky in December 2007 - what you can see.

Notes by Alan Gilmore, University of Canterbury's Mt John Observatory, www.canterbury.ac.nz

The brightest stars in the sky are in the east and south. Sirius, the brightest of all the stars, is due east at dusk, often twinkling like a diamond. Left of it is the bright constellation of Orion with 'The Pot' at its centre. Further left, in the northeast is the V-shaped pattern of stars making the face of Taurus the Bull with the Pleiades/Matariki/Seven Sisters cluster to its left. Canopus, the second brightest star, is high in the southeast. Low in the south are Crux, the Southern Cross, and the Pointers, Alpha and Beta Centauri.

At the beginning of the month Mars rises in the northeast around 11 pm: orange coloured and as bright as Sirius. By the end of December it is already up at sunset and appears in the early twilight. We pass 88 million km from the 'Red Planet' in mid-month. It will still be tiny in a telescope, needing 110x magnification to make it look as big as the full moon does to the naked eye.

The Milky Way is low in the sky, visible around the horizon from the west, through south into the eastern sky. The broadest part is in Sagittarius low in the west, fading in the twilight. The Milky Way narrows toward Crux in the south and becomes faint in the east below Orion. The Milky Way is our edgewise view of the galaxy, the pancake of billions of stars of which the sun is just one. The thick hub of the galaxy, 30 000 light years away, is in Sagittarius. The nearby outer edge is the faint part below Orion. A scan along the Milky Way with binoculars will show many clusters of stars and a few glowing gas clouds.

The Clouds of Magellan, LMC and SMC, high in the in the southern sky, are two small galaxies about 160 000 light years away. They are easily seen by eye on a dark moonless night. The larger cloud is about 1/20th the mass of the Milky Way galaxy, the smaller cloud 1/30th. That's still many billions of stars in each. The Magellanic Clouds orbit our galaxy once in two billion years.

In some Maori astronomical lore the bright southern Milky Way makes the canoe of Maui with Crux being the canoe's anchor hanging off the side. The last remnant of the Scorpion's tail, in the southwest is the canoe's prow. In this picture the Clouds of Magellan are the sails.

Canopus, high in the southeast, is the second brightest star in the sky. It moves upward during the night as the stars appear to circle clockwise around the south celestial pole, SCP. Canopus is a white star, hotter than the sun, 300 light years away. Seen up close it would be 13 000 times brighter than the sun.

Low in the south are the Pointers, Beta and Alpha Centauri, and Crux the Southern Cross. In some Maori astronomical lore the bright southern Milky Way makes the canoe of Maui with Crux being the canoe's anchor hanging off the side. In this picture the Scorpion's tail can be the canoe's prow and the Clouds of Magellan are the sails.

Sirius is the brightest star both because it is relatively close, nine light years away, and 23 times brighter than the sun. When low in the sky it is shining through a lot of air. Warm and cool layers in the air break the white light into colours making it twinkle.

Left of Sirius is the constellation of Orion. The line of three stars makes Orion's belt in the classical constellation. To southern hemisphere sky watchers they make the bottom of 'The Pot'. The faint line of stars above and right of the three is the Pot's handle. Rigel, directly above the line of three stars, is a bluish supergiant star, 70 000 times brighter than the sun and much hotter. It is 800 light years away. Orange Betelgeuse, below the line of three, is a red-giant star, cooler than the sun. It makes up for its duller surface by its size: hundreds of times the diameter of the sun. It is 15000 times brighter than the sun and 400 light years away.

Left of Orion is a triangular group making the (upside down) face of Taurus the bull. Aldebaran, is the brightest star in the V pattern. The name is Arabic for 'the eye of the bull'. Still further left is the Pleiades/ Matariki cluster, also called the Seven Sisters, impressive in binoculars .

Very low in the north is the Andromeda Galaxy, easily seen in binoculars on a dark night and faintly visible to the eye. It is similar in size and shape to our galaxy but 3 million light years away.

The Geminid meteor shower should be seen in the morning hours of Saturday December 15. The meteors will appear to radiate from the constellation of Gemini, initially in the northeast but moving to the north toward dawn. From dark rural locations at the south end of the country around 27 meteors should be seen in the pre-dawn hour, roughly 3:30-4:30 a.m. The rate rises to around 47 in Northland where the radiant is higher. The shower peaks after sunrise in New Zealand.

Venus (not shown) is the brilliant 'Morning Star' rising in the east two hours before the sun. Saturn (not shown) is in the morning sky, the right-hand star of a similar pair of stars in the northeast.

The Evening Sky in December 2007

Chart produced by Guide 8 software; www.projectpluto.com. Labels and text added by Alan Gilmore

The Evening Sky in December 2007 - using the chart

To use the chart, hold it up to the sky. Turn the chart so the direction you are looking is at the bottom of the chart. If you are looking to the south then have 'South horizon' at the lower edge. As the earth turns the sky appears to rotate clockwise around the south celestial pole (SCP on the chart). Stars rise in the east and set in the west, just like the sun. The sky makes a small extra rotation from night to night as we orbit the sun.

Prominent stars are in the eastern and southern sky on December evenings. Sirius, the brightest star, is due east, twinkling like a diamond. Left of it is Orion, with "The Pot" at its centre. Further left is Taurus and the Pleiades/Matariki/Seven Sisters star cluster. The Pointers and Crux, the Southern Cross, are low in the south. The Milky Way is bright along the skyline from southwest to southeast but becomes fainter below Orion and into the north. Venus is the only bright planet in the evening sky, setting in the southwest twilight.

The Large & Small Clouds of Magellan (LMC and SMC) appear as two luminous patches, easily seen by eye in a dark sky. They are two galaxies like the Milky Way but much smaller. Each is made of billions of stars. The Large Cloud contains many clusters of young luminous stars seen as patches of light in binoculars and telescopes. Both clouds are about 160 000 light years away, very close by for galaxies. (1 light year is about 10 000 billion km.)

47 Tucanae, looks like a faint fuzzy star on the edge of the SMC. It is a globular cluster, a ball of millions of stars. A telescope is needed to see a peppering of stars around the edge of the cluster. Though it appears on the edge of the SMC it is one-tenth the distance, 15 000 light years away, and is has no connection to the Small Cloud. Globular clusters are mostly very old, 10 billion years or more; at least twice the age of the sun.

Tarantula nebula is a glowing gas cloud in the LMC. The gas glows in the ultra-violet light from a cluster of very hot stars at centre of the nebula. The cloud is about 800 light years across. It is easily seen in binoculars and can be seen by eye on moonless nights. This nebula is one of the brightest known. If it was as close as the Orion nebula then it would be as bright as the full moon.

Canopus is the second brightest star. It is 14 000 times brighter than the sun and 300 light years away. Sirius, low in the east on spring evenings, is the brightest star in the sky.

Alpha Centauri is the closest naked-eye star, 4.3 light-years away. Alpha Centauri is a binary star: two stars about the same size as the sun orbiting around each other in 80 years. A telescope that magnifies 30x shows the pair. (A very faint and slightly closer star, Proxima Centauri, probably orbits the Alpha pair.)

Omega Centauri is a globular cluster like 47 Tucanae but bigger. Its total mass is six million times the sun's. It is 17000 light years away and 200 light years across. Omega Centauri is the biggest globular cluster in our galaxy. It may have once been the core of a small galaxy that was captured by the Milky Way.

The Coalsack nebula, in Crux, is a cloud of dust and gas about 300 light years away, dimming the more distant stars in the Milky Way. Many similar 'dark nebulae' can be seen, appearing as slots and holes in the Milky Way. These clouds of dust and gas eventually coalesce into clusters of stars.

The Jewel Box also in Crux, is a compact cluster of young luminous stars about 7000 light years away. The cluster formed less than 10 million ago. To the eye it looks like a faint star.

Eta Carina nebula is a glowing gas cloud about 8000 light years away. The golden star in the cloud, visible in binoculars, is Eta [Greek η] Carinae. It is estimated to be to be 60 times heavier than the sun and a million times brighter but is dimmed by dust clouds around it. It is expected to explode as a supernova "soon", that is in the next few thousand years. Many star clusters are found in this part of the Milky Way.

The Southern Sky in December

Chart produced by Guide 8 software; www.projectpluto.com. Labels and text added by Alan Gilmore

The Southern Sky in December

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