Night Watch

Updated 1st June 2009

Sagittarius star cloud. Image by Werner Kutsche
(more images by Werner Kutsche)

The winter Milky Way shines across the sky from east to west in a blaze of starlight. Roaming through the Milky Way with binoculars is an amazing experience. It's like canoeing down a river of golden stars. When we look towards Sagittarius, halfway up the eastern sky, we are looking towards the centre of our galaxy where just about all the stars are old stars and hence cooler. The star colours here are therefore predominantly reddish yellow, whilst the stars in the outer edges of our galaxy are blue and white, advertising their relative youth and energetic spirit.

For centuries, humans have looked up at the night sky and played games of "join-the-dots" with the stars. Today, these patterns form the 88 constellations that cover the entire sky.

The largest constellation is Hydra the water snake, which slithers across 90 degrees of the sky. To get a sense of how much sky that is, consider that the full Moon spans only half a degree. Whilst the head of Hydra is setting in the west, its tail stars are still overhead!

The smallest constellation is one of the most famous. Its correct name is Crux, the cross, although it's better known as the Southern Cross. At this time of the year, it's high in the southern sky. Its stars are shown on the Australian and NZ flags.

Crux is small, but it's packed with celestial wonders. For example, there's the Jewel Box Cluster (NGC 4755). Seen through a telescope, the stars of the Jewel Box shine diamond-white and sapphire-blue. A lone red star lies between Earth and the cluster, so it looks like a ruby against the other gems. The whole cluster is set against the glittering band of the Milky Way. Of course, you'll need a dark sky to see the Milky Way. The light polluted skies of Adelaide hide it fairly well.

Omega Centauri. Image by Tony Virgo
(more images by Tony Virgo)

High in the sky, above the Southern Cross and the Pointers, look for a small fuzzy star. The Omega Centauri globular star cluster (NGC 5139) is a vast "city" of stars, perhaps a million stars packed into a fairly small, spherical volume of space. It's about 17,000 light-years away, in the constellation Centaurus. It stands highest above the southern horizon around 10 p.m. To really appreciate the cluster, though, you need some help. Binoculars will show you more of its stars. But there's nothing like a telescope to reveal Omega Centauri for what it really is; a stellar city swarming with a million stars.

High in the eastern sky is the constellation of Scorpius, the scorpion. Throughout the ages, almost all civilizations have endowed this particular group of stars with the body of a scorpion.

Over in the northwest, orange Arcturus keeps watch. The star became a famous object in 1933, when its starlight was used to trigger a switch that turned on the floodlights at the World Expo in Chicago. It is a large star, roughly 25 times the diameter of our Sun. It is some 115 times brighter than the Sun, but at its distance of almost 40 light years, the heat we receive from it is equal to the heat from a single candle at a distance of approximately 9 kilometres. Not quite enough to keep the winter chills away!

High up, almost overhead, the blue-white star, Spica is easy to see. Spica actually consists of more than one star, but only one is bright enough to see without a telescope. Astronomers use two different measurements to rate the brightness of a star. Both measurements are called magnitudes, and the lower the number, the brighter the star.

The first measurement, called apparent magnitude, rates how bright an object appears in the sky; Spica rates a "one". But apparent magnitude doesn't tell you if the object is intrinsically bright or just close by. So astronomers also rate stars and galaxies by absolute magnitude. This number tells us how bright different objects would appear if they were all lined up at the same distance. On that scale, Spica gets a minus three. The Sun would only rate a five, which means that Spica is about 1600 times brighter.

Saturn. Image by Paul Haese
(more images by Paul Haese)

If you observe very soon after sunset, you can still get a glimpse of the ringed planet Saturn as it begins to set in the west with Leo, the Lion. Saturn's rings lie on the plane of the equator. Because of the tilt of the equator to the orbital plane, the rings change orientation as seen from Earth. During 2009 the ring system will be edge-on and we will not see much of it. At other times (next in 2016), the rings reach their maximum inclination and display their full splendour to us.

The Moon is Full on June 8th, at Last Quarter on the 16th, New on the 23rd, and at First Quarter on June 29th.