August 2012 night sky guide podcast, transcript and sky chart
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 guide is presented by Melissa Hulbert, Sydney Observatory Astronomy Educator.
Highlights Melissa points out for this month include: M22 (a not-very-romantic name for a large globular cluster) and three wonderful Milky Way zodiac constellations: Scorpius (the heart of which is Antares, a red supergiant star), Sagittarius and Ophiuchus – yes, there is a 13th zodiac constellation not many know about!
Melissa also tells us about the two centaurs in the sky: Sagittarius and Centaurus (part of which is the two Pointer stars that help us to identify Crux – also known as the Southern Cross).
Listen to the August 2012 night sky guide audio, or read the transcript below.
SEE THE SKY CHART
We provide an embedded sky map (below) and an August 2012 night sky chart (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.
BUY THE BOOK
Our annual book, ‘The 2012 Australasian sky guide’, by Dr Nick Lomb has more information and star maps for months from December 2011 until December 2012 inclusive (next year’s, likely to be available from November, will have months from December 2012 to December 2012), 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 (additional packing/postage costs apply).
READ THE TRANSCRIPT (after the jump)
Transcript of the August 2012 monthly sky guide audio
Hello and welcome to the night sky 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 at www.sydneyobservatory.com.au. It can be found under the Astronomy tab.
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 – that’s 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. Wait about 5-10 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 catalogue names. ‘M’ stands for Messier and is named after Charles Messier, an 18th century French comet chaser. He made a catalogue 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 catalogue 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 catalogued – 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 south-west 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, 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 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 the Indigenous Dreamtime 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 sky.
So what else can we look forward to seeing in the sky in August 2012?
This month after sunset look towards the north-west. Mars and Saturn are in the constellation Virgo and have some interesting configurations with the bright star Spica. On the 14th the three objects will form a straight line with Mars in the middle, Saturn on the right and Spica on the left. On the 22nd, Mars and the 5-day old crescent Moon are beside each other and will be above Saturn and Spica (which also appear to be side-by-side).
In the early morning sky a brilliant star-like object shines in the east. This is Jupiter, the largest planet in our Solar System. It starts the month rising at about 2:49am Eastern Standard Time (EST) close to the orange-red star Aldebaran, the eye of the bull Taurus, but by month’s end is rising at 1:10am. On the 12th the 24-day old waning crescent Moon is very close to Jupiter and for those of you in Darwin, the Moon will occult Jupiter and its moons with the planet disappearing behind the limb of the Moon at 4:07am and reappearing at 4:41am Central Standard Time (CST).
If you have a pair of binoculars you can see the Galilean satellites, the four largest moons of Jupiter’s known 65, 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.
On the 3rd, the four Galilean satellites provide some interesting events for observers. The first two are visible from eastern states and begin with Europa disappearing behind Jupiter at 4:10am EST and the second is the reappearance of Ganymede 17 minutes later. The final is a dawn event for Western Australian observers and occurs at 5:25am Western Standard Time (WST) when Callisto remains the only moon visible near the planet. This event ends at 6:29am WST when Io moves out from in front of the planet.
This month, Venus and Mercury are the highlights for all you early-birds. On the 13th, the 25-day old waning crescent Moon is between Venus and Jupiter, with the trio forming a straight line. On the 14th Venus will be very close to the limb of the Moon.
Mercury is also in the eastern pre-dawn sky this month, however it will be lower on the horizon and hard to see in the twilight glow.
Don’t forget – in three months time on 14th November, a solar eclipse will occur. More details about this can be found on our website, in the ‘2012 Australasian Sky Guide’ and in upcoming podcasts. If you miss it this time, you will have to wait until 2028 to see another from mainland Australia.
If you have enjoyed this podcast and think you might want to regularly check out what’s in the night sky, why not purchase a copy of Sydney Observatory’s book the ‘Australasian Sky Guide’. It not only contains detailed monthly sky guides, but is jam-packed with astronomical information including rise and set times for the Sun, Moon and planets, tides, and a detailed look at our Solar System and upcoming astronomical events. Only $16.95 from Sydney Observatory and Powerhouse Museum shops or you can purchase it online (additional costs apply).
You can also subscribe for free to our Sydney Observatory monthly sky guide podcasts through iTunes.
I leave you now with this quote from Irish poet, novelist, dramatist and critic, Oscar Wilde: “We are all in the gutter, but some of us are looking at the stars”.
Wishing you clear skies, and see you next month under the stars!
This has been Melissa Hulbert from Sydney Observatory with the August monthly sky guide podcast.