Observations

August 2011 night sky guide and podcast

To help you learn about the southern night sky, Sydney Observatory provides an audio guide/podcast, transcript of that audio, and a sky map or chart each month. This month’s audio sky guide is presented by Mel Hulbert, astronomy educator at Sydney Observatory.

HEAR THE AUDIO
You can subscribe with iTunes or upload the audio to your iPod or mp3 player, or listen to it on your computer.

SEE THE SKY MAP
We provide an embedded sky map (below – it can take a moment or two to load) and this August 2011 night sky map (PDF) which shows the stars, constellations and planets visible in the night sky from anywhere in Australia. To view PDF star charts you will need to download and install Adobe Acrobat Reader if it’s not on your computer already.

August 2011 night sky map

BUY THE BOOK
There is more information and detail in our annual book, ‘The 2011 Australian sky guide’ by Dr Nick Lomb. It has information and star maps for months from December 2010 until December 2011 inclusive, plus information about the Sun, twilight, the Moon and tides, and a host of other fascinating astronomical information. You can purchase it ($16.95) at Sydney Observatory and Powerhouse Museum shops or other good bookshops, or online through Powerhouse Publishing.

READ THE TRANSCRIPT (after the jump)

Hello and welcome to the night sky guide for August. My name is Melissa Hulbert and I’m an Astronomy Educator at Sydney Observatory.

Before we start our night sky tour, make sure you download the August sky map from our website www.sydneyobservatory.com.au or, if you have one, use your star map from our book, ‘The Australian sky guide’. There is a lot of astronomical information in this book as well as the monthly star maps.

Armed with your sky map and a small torch with some red cellophane covering it, find a nice dark place away from the glare of the street lights and make sure you know your cardinal directions – north, south, east and west. Remember that the Sun rises in the east, moves through the northern sky during the day and sets in the west; or a small compass will also point you in the right direction. Pick a comfortable spot either on a rug or a deck chair that you can lay back in and make sure you’re wearing something warm during the chilly winter nights. Wait about 5 to 10 minutes and allow your eyes to adapt to the darkness.

Look straight above you. High overhead is Scorpius the Scorpion. The Scorpion is one of the easiest constellations to pick out as it is one of the few that does look like what it’s supposed to represent. It covers about 30 degrees in the sky. Working out degrees in the sky is quite easy. Hold your arm out towards the sky and make a fist. From one side of your fist to the other, this is 10 degrees. Hold your other arm out and spread your fingers out as wide as you comfortably can (so the opposite of a fist), from your little finger to your thumb is 20 degrees. Put your hands side-by-side and you now have 30 degrees, the size the Scorpion covers in the sky. This does work for everyone, as your arm length is proportional to your hand size.

Now, look for the Scorpion’s Heart, Antares, a red supergiant star that is 400 times the diameter of our Sun. Antares means ‘rival of Mars’, and when they are close together in the sky they certainly do look very similar.

If you have a pair of binoculars, then near Antares is a small globular star cluster, M4, which is a group of old stars that lie about 7,000 light years away, making it one of the closest globular clusters to us. Below the sting of the Scorpion are two open star clusters, M7 and M6, which lie about 800 and 2,000 light years away respectively, and are both worth a look. See if you can see the butterfly in M6.

These names I’m giving the clusters are catalog names. ‘M’ stands for Messier and is named after Charles Messier, an 18th century French comet chaser. He made a catalog of 103 fuzzy objects that were not comets so that he didn’t waste his time looking at them. Other astronomers later added a few more objects to the catalog bringing the total to 110.

Follow the scorpion along from its head through its heart, Antares, and to the point where the body meets the tail. The star at this point is Zeta Scorpii and next to it sits NGC 6231, a bright open star cluster containing about 120 stars. This cluster is remarkable for its large population of high-luminosity supergiant stars, which includes two Wolf-Rayet stars. Wolf-Rayet stars are very luminous, very hot stars that have relatively short life times. It is a striking cluster that contains many white and yellow stars as well as many pairs and triplets and is about 6,500 light years away.

Just behind the sting of the scorpion is Sagittarius, which is depicted as a half-man, half-horse aiming an arrow towards Scorpius. Sagittarius was often confused with the other centaur, Centaurus however Sagittarius is different, in that he has a war-like posture with his arrow aimed at the heart of the Scorpion. It is thought that Sagittarius can be traced back to the Mesopotamian archer-god Nergal, who was associated with Irra, the wrathful god of war and fire. However, in our night sky Sagittarius looks more like a teapot than any of these mythical creatures.

Interestingly, the Sun lies in Sagittarius from mid-December until mid-January, meaning that it lies in this constellation at the time of Summer Solstice – its most distant point south of the equator.

The very centre of our galaxy is found in Sagittarius, along with many great binocular and telescopic objects. Alpha Sagittarii is one of several examples where the star labeled as alpha which represents the brightest star in a constellation, is not actually the brightest star. Epsilon Sagittarii has the honour of being the brightest. Sagittarius is known for its nebulae and clusters, 15 of which Messier cataloged – more than any other constellation.

Near the top of the teapot’s lid is M22, a large globular cluster. It is visible to the naked eye in dark skies and appears as a fuzzy blob in binoculars. It takes a telescope with an aperture of 75mm or greater to reveal some of the outer stars, with some of the brightest appearing to have a reddish hue. Even small telescopes will reveal M22’s elliptical appearance and it lies about 10,000 light years away. M22 is considered to be one of the finest examples of a globular cluster in the sky, third only to Omega Centauri and 47 Tucanae.

Winter months remind us of the wonderful Milky Way sights of Scorpius and Sagittarius. But observers should not forget the 13th zodiac constellation – Ophiuchus which lies beside Scorpius and Sagittarius and is often overlooked with these two constellations overhead.

Ophiuchus is an ancient constellation, representing a snake coiled around a man. However, it is now often associated with Aesculapius, a mythical healer said to have the ability to raise the dead. Certainly in Greek mythology, it is Ophiuchus who raises Orion from the dead after he is bitten by the scorpion. Aesculapius is seen holding a snake, most likely as they were seen as a symbol of power.

Ophiuchus is seen as the 13th sign of the zodiac due to the Earth’s precession – meaning the wobble of Earth on its axis. The Sun, Moon and the planets now pass through this constellation – the Sun from 30 November to the 17 December.

Ophiuchus has many interesting and varied objects for the observer including the second-closest star to our Sun – Barnard’s Star, a red dwarf lying 5.9 light years away.

Lying just to the west of the scorpion is the seventh constellation of the zodiac, Libra, The Scales. To the ancient Greeks Libra was part of Scorpius representing the scorpion’s claws and this association is reflected in the names of the brightest stars in Libra – Zubenelgenubi and Zubeneschamali meaning ‘the southern claw’ and ‘the northern claw’ respectively. However, Latin writers considered Libra distinct from Scorpius, with the scales symbolising the equinoxes, the equal lengths of day and night. Two millennia ago, the Sun moving into Libra marked the September equinox (the point at which the Sun moves south of the celestial equator each year), but due to precession, at around 730BC, this point moved into the adjoining constellation Virgo. During the first century BC, in the reign of Julius Caesar, the Romans separated Scorpius into the two distinct constellations we are familiar with today and associated the scales with Astraeia, the goddess of justice who is also represented by the constellation Virgo.

Time to turn and look towards the south. Look slightly to the west of south. Here you will see the Pointers – Alpha and Beta Centauri. Follow the line of the Pointers down towards the southwest and there is the constellation Crux, better known to us as the Southern Cross. Crux is Latin for cross. The Southern Cross, like the Scorpion, is another constellation that actually does look like what it’s supposed to represent. The Pointers point to the Southern Cross and this is one way to check you have the right cross as there are many groups of stars in the southern sky that look like crosses.

The second brightest star in Crux is a marker for a wonderful binocular and telescope object. To find the 2nd brightest star, whose name is Mimosa, look for the star in Crux closest to the Pointers. Now just nearby – at about 10 o’clock, if you imagine a clock face over Mimosa, is a wonderful open star cluster called the Jewel Box. It looks like a sideways ‘A’ in telescopes and binoculars. In a telescope, wonderful colours can be seen with white stars and a red supergiant. Sometimes even green appears but of course there are no green stars – this is just an illusion. The famous 18th century astronomer John Herschel gave the cluster its name as he likened it to a piece of multi-coloured jewellery. It lies just under 5,000 light years from us.

Move back to the Pointers and look at the constellation of Centaurus, which surrounds Crux on three sides. It depicts a Centaur, a mythical beast – half man, half horse. The constellation was said to represent the scholarly Chiron, the centaur who was tutor to many of the Greek gods and heroes. He was put among the stars after accidentally being killed by a poisoned arrow from Hercules.

Close to the second Pointer, Beta Centauri lies Omega Centauri or NGC 5139, the brightest and largest globular cluster in the sky. In fact, it is so bright it was labeled as a star on early charts by Ptolemy and later recorded by Bayer as Omega Centauri. It was found to be a cluster in 1677 by Edmund Halley of Comet Halley fame, so Omega Centauri carries both a star designation and an object catalogue designation. It is easily seen with the naked eye, shining with the luminosity of a million Suns. Its brilliance and large aperture size is in part due to its relative closeness to us – only about 17,000 light years away, making it one of the closest globulars to us.

Very close to Omega Centauri is NGC 5128, one of the strongest radio sources in the sky and is known to astronomers as Centaurus A. Optically in long exposure photographs it appears as a giant elliptical galaxy, split in half by a dust band. It is thought that Centaurus A is the result of a merger between two galaxies – one elliptical and the other spiral. In good skies, Centaurus A can be seen in binoculars, but a telescope is required to see the dust lanes (of the spiral galaxy) intersecting the bright elliptical halves. Centaurus A lies about 13 million light years from us.

Crux sits within one of the arms of our Milky Way and if you are away from the city lights you will see this arm and notice a dark patch between the brightest and second brightest stars of this constellation. This dark patch is called the Coalsack and is a dark nebula – lots of gas and dust that are blocking out the background stars. In Aboriginal mythology the Coalsack formed the head of the Emu and if you follow the dark dust lanes of the Milky Way towards the Scorpion, you will see the Emu’s body and legs. This time of year is perfect for seeing the Emu stretching across the night sky.

So what else can we look forward to seeing in the sky in August 2011?

This month after sunset look towards the west. Mercury starts the month low in the western sky however by month’s end it will be lost in the twilight glare. Venus is lost in the Sun’s glare for the entire month as it moves from a morning to an evening object.

Saturn is also in the western sky during the early evening and this month is really the last opportunity to see Saturn at its best before it starts to get too low.

The Moon pays a visit to Saturn twice this month. On the 4th it’s a five-day-old waxing crescent and on the 31st it’s a three-day-old waxing crescent.

About mid-month, just before midnight a brilliant star-like object shines in the eastern sky. This is Jupiter, the largest planet in our Solar System. It starts the month rising at about 0:21am EST but by month’s end is rising at 10:26pm. On the 20th and 21st, the waning gibbous Moon will pass above and below Jupiter respectively. If you have a pair of binoculars you can see the Galilean satellites, the four largest moons of Jupiter’s known 63, which appear as tiny points of light. A telescope will show you some of the cloud belts of Jupiter however the moons will still appear as points of light.

This month, Mars is in the eastern pre-dawn sky and provides a few interesting events for all you early-birds. On the 7th, Mars passes by M35, an open star cluster in Gemini. M35 is visible to your eye away from city lights and is best seen with binoculars or using low magnification in a small to medium aperture telescope. On the 26th the 26-day-old waning crescent Moon will be just below and slightly to the east of Mars.

This month I have two wildcards for all you daredevils. The first is the minor planet 4 Vesta. Minor planets are also known as asteroids and are leftover debris from the formation of our Solar System. 4 Vesta is the brightest asteroid and is the only one that can be seen with the unaided eye. At magnitude 5.6 it is just within the limits to see with your unaided eye in dark skies, well away from city lights, and will be in the constellation Capricornus. It is at its best in early August as it reaches opposition which is when a celestial object is opposite the Sun in the sky, on the 5th August. A pair of binoculars will help you find it in the suburbs of large cities.

The second wildcard is Comet 45P/Honda-Mrkos-Pajdusakova. This comet passes very close to Earth, a mere 0.06AU (astronomical units) or approximately 9 million kilometres – rather close! It starts the month at magnitude 13 but should rapidly brighten to about magnitude 8 as it passes through the southern constellations. As it brightens it is certainly within the reach of binoculars and small aperture telescopes. The evening of the 13th provides an interesting encounter as the comet passes along the north-east edge of the Small Magellanic Cloud (SMC) and on the morning of the 15th it crosses the Large Magellanic Cloud (LMC) though you will have a tough time spotting the comet in amongst all the nebulae and stars of the LMC.

Of course this is all based on predictions and comets are notoriously unpredictable which reminds me of that famous quote by renowned comet hunter David Levy: “Comets are like cats. They have tails, and they do precisely want they want.”

With that, I wish you clear skies and see you next month under the stars!

This has been Melissa Hulbert from Sydney Observatory with the August 2011 sky guide podcast.

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