July 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 Dr Nick Lomb, Sydney Observatory’s Curator of Astronomy.
In the July sky guide, Nick’s tour takes us past the red star Antares, in the constellation of Scorpius, and tells us much about the Southern Cross and the Pointer stars – part of the constellation of Centaurus. Nick reminds us that our map of the sky is related to our vantage point from Earth. The familiar cross in the sky we think of as the Southern Cross is made up of stars that are relatively far from each other, and from different perspectives in the Milky Way, you would not see the ‘star’ pattern we see from Earth. He also points out that many of the stars that appear from Earth to be single stars are actually double stars, circling around each other.
Listen to the July 2012 night sky guide audio, or read the transcript below.
SEE THE SKY CHART
We provide an embedded sky map (below) and a July 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, 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 July 2012 monthly sky guide audio
Nick Lomb: This is a guide to night sky in July. My name is Nick Lomb. I’m Curator of Astronomy at Sydney Observatory and the Powerhouse Museum. You can find this broadcast on the Sydney Observatory website, which is at www.sydneyobservatory.com.au, and look in the astronomy section. If you’d like to become familiar with the night sky, what you need to do is download the map associated with this broadcast. Simply download it in PDF and then print it out and take it outside with you. Of course, being July, being winter, dress warmly because it can be fairly cool at night.
What you also need with you is a torchlight, and ideally one with a red light, since red does not destroy your adaptation to the night sky. With a red light, you can look at the map and look up in the night sky. If you do not have a red torch, put a little bit of red cellophane in front of an ordinary white torch. That will give you a red light to be able to look at both the map and the night sky.
It would also help if you’re familiar with the cardinal directions, that is, north, west, south and east. You need to know where they are with respect to your location. East is, of course, where the Sun rises. West is where the Sun sets. With those, you get a fairly good indication. As well, at 12:00 noon, the Sun is approximately due north.
Let us begin our tour of the night sky in July. We’ll start in the east where we can see the familiar sight of the constellation of Scorpius, the scorpion. This is a welcome sight, visible in the Australian winter. It’s a very obvious constellation. It is one of the few really bright and easy to find constellations, and it is an excellent sign post to the night sky.
Scorpius is a long curving line of bright stars. At this time of the year, it’s in the eastern sky with the claws of the scorpion high up and a little bit towards the north. The actual sting, or tail, of the Scorpion is towards the south. In the middle of the Scorpion, we find the red star Antares which represents the heart of the Scorpion. Antares is a huge star.
Its name Antares means ‘rival of Mars’. The reason for the name is because Antares is a similar reddish colour to the planet Mars. Occasionally, Mars passes Antares close by, and the two can be seen to be very similar reddish objects close together in the night sky, and that is a very impressive sight when that happens.
Antares is a giant star. It is relatively cool with a surface temperature of around 3,000 degrees Celsius. This may sound like a lot. But compared to our own Sun, which has a temperature of around 5,500 degrees Celsius, it is relatively cool. It is that coolness which gives it its red colour. Because it’s such a huge star, even though it’s relatively cool, it still puts out a huge amount of light. It appears like a bright star in the sky even though it is 600 light years from us. Light has taken 600 years to reach us from the star Antares.
The star is so large that, if you replaced our own Sun by Antares…I should add, at this point, that nobody would want that to happen, then the Earth would be engulfed by Antares, as would the planet Mars and many of the asteroids – the rocky objects that circle the Sun between the paths of Mars and Jupiter. Jupiter would be just outside Antares but not very far from it. The solar system would be a very different place, and it would not be a hospitable place for human beings if Antares replaced the Sun.
Antares has a companion star that circles around it. This is a hot star so there is quite a contrast between the two of them. The companion star has a temperature somewhere around 18,000 degrees Celsius and it has a bluish colour. If people look at those two stars together, the ruddy Antares and the blue companion star, they sometimes describe the companion star as green. This seems to be due to some kind of contrast effect in our eyes.
The companion star appears small compared to huge Antares. In reality, it is larger than our own Sun. It is something like four times wider than our own Sun, and it has a mass of about 10 times that of our own Sun. It is 2,000 times as bright as the Sun. We do not, as yet, know how long the companion star takes to circle around Antares. The current estimate is that it would take something like 1,000 years.
Now let us move to the eastern part of the sky, from Scorpius the Scorpion, to the north. If we face north and look up, the most obvious star that we can see is a star called Arcturus. This star is part of the constellation Boötes, which is the Herdsman. Arcturus means ‘Bear Watcher’. Arcturus is one of the brightest stars in the sky. It is the fourth brightest star in the sky. It is the brightest star in the Northern Hemisphere as seen from the Northern Hemisphere. It is a slightly orange coloured star, at a distance of 37 light years from us. It is closer than Antares which, as you recall, is 600 light years away. But just like Antares, it appears like a bright star in the sky.
Arcturus is an interesting star for a variety of reasons. One of them is that it moves around the centre of our own galaxy a little bit slower than our star, the Sun. Of course, we on Earth move around the centre of our own galaxy together with the Sun. Arcturus is believed to be a somewhat older star than our own Sun. It comes from an older generation of stars in the galaxy.
There has been a suggestion which may or may not be true, but it’s still an intriguing one, that Arcturus does not come from our galaxy, but comes from a small galaxy that merged with our own many thousands of millions of years ago. This would explain why Arcturus has a different motion to other stars in this part of the galaxy, as it is moving a little bit more slowly around the centre.
Let us move now to face west. The most obvious star that we can see is a star called Regulus. Regulus is towards the west and a little bit towards the right and somewhat towards the north. That is, it is in the north-west. The name Regulus means ‘Little King’. It is 77 light years away. Intrinsically, it gives off something like 100 times as much light as our own Sun. It is a fairly hot star, 12,000 degrees Celsius. This compares, if you recall, with our own Sun, which has a temperature of about 5,500 degrees.
Regulus is a star right on the ecliptic, right on the paths of the planets and the Sun and the Moon as they move along the sky. Consequently, Regulus can be occulted, or covered by the Moon and that is an event that is fairly common. Regulus also has a companion star circling around it. It’s a fairly low mass star which is a long way away from Regulus.
This companion star is roughly 4,000 times as far away from Regulus as the Earth is from the Sun. This large distance means that the companion star circles around the main star of Regulus very slowly. It is believed that it would take at least 100,000 years for the companion star of Regulus to circle around it.
Unfortunately, none of us are going to be around to see that complete circuit be finished, but people in a distant future will be estimating the exact period. For the moment, all we can do is make an estimate that it takes somewhere around 100,000 years. Interestingly, this companion star, the little companion star that circles around Regulus is also a double star. The two stars, with the companion, take around 1,000 years to circle around each other.
Let us now move to the southern part of the sky. If you face south and look up, you can see in the early evening the Southern Cross. This is the best time to look at the Southern Cross, the most famous group of stars in the Southern Hemisphere. It is the best time to view it because it’s very high up in the sky, and it’s sort of standing vertically due south at this time of the year, in the early evening.
If we look at the Southern Cross, we can easily see four stars. There’s also a fifth star, which sadly is becoming lost to light pollution in our cities. If you’re looking at the Southern Cross from a dark sky, from a country spot for example, the fifth star is nice and prominent. However, if you look at this fifth star from the suburb of a major city, whether it’s Sydney or Melbourne or Adelaide or Perth, the fifth star in the Southern Cross is getting harder to see.
The bottom star of the Southern Cross, at least at this time of the year, is a star that we call Acrux, or alternatively, Alpha Crucis. This is the brightest star of the Southern Cross. If you look at Acrux through a small telescope, you can see that it’s actually a double star, or two stars. There is in fact a third star nearby as well. They’re about 320 light years from us. The light that we can see today left Acrux 320 years ago.
Going clockwise, the star on the left of the cross is a star called Beta Crucis, which also has a proper name, Mimosa. This is the second brightest star of the Southern Cross. Mimosa is a fairly hot star at a distance of 350 light years from us. It’s important to note that stars that appear close together in the sky, such as the stars of the Southern Cross, are not necessarily close together in reality.
The stars in the Southern Cross are a very good example. The stars are dispersed three dimensionally in the sky, they’re at different distances. From our own position, they make up the Southern Cross, they look like a cross. From anywhere else in the universe, or even anywhere else in our own galaxy, the stars will not appear like the Southern Cross. They will take up a completely different shape. It’s just from our own viewpoint, they take up this particular interesting configuration.
Going on, we have looked at Acrux and Beta Crucis or Mimosa. Now at the top of the cross there’s a star called Gamma Crucis. This is a relatively cool star so it has an orange colour. Unfortunately, our eyes are not sensitive to colour in the dark, so we don’t normally pick up the colour with the unaided eye. But if you colour photograph the Southern Cross then it’s quite obvious that Gamma Crucis has an orange colour. Its distance is 88 light years. In other words, it’s fairly close to us. It is the closest star of the five main stars of the Southern Cross.
Then going further clockwise, around the Southern Cross, the star on the right is Delta Crucis. It’s a hot star. It’s 364 light years from us. Below Delta, between Delta and Acrux, you’ll find the faintest star in the Southern Cross, Epsilon. From a city, as we discussed earlier, we may not be able to see Epsilon. Possibly, if you really know where it is, you might just be able to glimpse it. But it can be very hard to find because of light pollution. With a pair of binoculars, however, it is easy to find. Epsilon is, again, an orange coloured star like Gamma Crucis. It has a distance of about 230 light years.
Surrounding the Southern Cross, we find the constellation of Centaurus the Centaur. That constellation surrounds the Southern Cross on three sides, to the east to the left, above to the north, and to the right to the west. Centaurus, or the Centaur, represents a Greek legend of half horse half human creatures called centaurs.
These were very warlike and very quarrelsome creatures. Surprisingly, this particular centaur in the sky, next to the Southern Cross, is not like the rest of them. It represents a centaur known as Chiron who was known for his wisdom and his kindness. He was a teacher, and he taught the Greek heroes of antiquity, Jason and Hercules. He taught them subjects like music, poetry, and mathematics.
As a reward, he was placed by the king of the Greek gods, the gods of Greek mythology, Zeus, among the stars. Originally, the Southern Cross in Greek times was just part of the Centaur and represented the hind legs of the Centaur.
The two main stars of Centaurus are the two Pointer stars. The Pointer stars, which always point to the Southern Cross, are the ones which enable us to find and distinguish the Southern Cross from other nearby stars. You can always recognise the Southern Cross by these two Pointer stars. Out of the two Pointer stars, the one furthest away from the Cross in the sky is Alpha Centauri, also known by its Arabic name ‘Rigel Kentaurus’, which means ‘the Centaur’s foot’.
If you look at Alpha Centauri through a telescope, you can see it’s a double star, two stars really close together in the sky. In fact, they’re among the nicest objects to look at through a telescope. To me, the two stars appear like a pair of distant car headlights. These two stars circle around each other in about 80 years. They were furthest apart in 1995, and since then they’ve been coming closer together. The brighter of the two stars is very similar to our own Sun, while the faintest star is a somewhat orange coloured star. It’s a little bit less massive than our own Sun, a little bit cooler, but it’s a slightly larger star than our Sun.
There is a third star in the system which is known by astronomers as Proxima Centauri. We cannot see Proxima through a small telescope, it’s a long way away out of the field of view. It’s two degrees away from the other two stars. The two degrees represents four times the width of the full Moon, so it’s normally outside the field of view of a telescope. But there’s another reason we cannot see it: it’s very faint. It’s a little dwarf star. It gives off about 1/10,000 as much light as our own Sun.
The three stars of the Alpha Centauri system are at a distance of about 4.33, or 4 1/3 light years from Earth. That is, light left Alpha Centauri 4 1/3 years ago which is not that long ago, that is certainly within our own memory. This makes the three stars of the Alpha Centauri system the closest star system to Earth. But remember that Proxima’s just a little bit closer than the other two. Proxima is in fact the closest star to us after our own Sun. This star was only discovered relatively recently. It was discovered in 1915 by an astronomer called R.T. Innes, Robert Innes. He was an ex Sydney astronomer who moved to South Africa and made the discovery while he was working there.
The other star of the two Pointers is Beta Centauri. That is the one that appears close to the Cross in the sky and is much further away from our own Sun. It’s 525 light years away, once again, demonstrating that two stars that appear to be close together in the sky can be a very large distance apart in reality.
Beta Centauri is a very hot massive star giving off something like 10,000 times as much light as our own Sun. Through a telescope, you can see that it’s a double star. There is a faint star associated with the main primary star. This faint star is still 400 times as bright as our own Sun.
Finally, before we wrap up this view of the stars in July, I’ll mention another object in the constellation of Centaurus, an object called Omega Centauri that is known affectionately to astronomers as Omega Cen. This is a globular cluster, a huge ball of several million stars. It is believed to contain five to 10 million stars. This ball of stars circles independently around the centre of our own galaxy. It’s the most massive, with 160 or so globular clusters of similar balls of stars, that we know about here in our own galaxy, the Milky Way.
At a distance of 16,000 light years, it is relatively close to us, at least for a globular cluster. It’s only visible from the Southern Hemisphere, and it is a very nice subject to study. It circles around the centre of our own galaxy in the opposite way to stars, which suggests that it has a rather interesting history. It has been suggested that it’s a remnant of a small galaxy that was swallowed a few billion years ago by our own galaxy, the Milky Way. It’s the remnant of a very central part, or the nucleus of a small galaxy, and we can see it as the globular cluster, Omega Centauri.
Omega Centauri is visible to the naked eye from a dark sky, but from the city, it can still be easily seen if you look at it through a pair of binoculars. Back in 1985, when Halley’s Comet was nearing the Earth, before its close approach the following year, it passed right by Omega Cen. The comet looked exactly like Omega Cen, and a lot of people at the time saw the two of them in their binoculars and thought that Halley’s comet had split into two. No, it did not.
Now, let us turn to special events and to what planets are visible during the month of July 2012. First, let us mention that, on the 5th of July, the Earth is furthest from the Sun, that is a position that astronomers call aphelion. That will take place on Thursday, the 5th of July at 2:00PM. With the planets in the early evening, there are three planets visible at least in the beginning of the month. The planet Mercury is low in the western sky, but it disappears at twilight during the second half of the month.
On the 3rd of July, Mercury passes three Moon widths from the star cluster Praesepe. This gives a very nice opportunity to look at both Mercury and the star cluster through a pair of binoculars, and we can expect an interesting sight. On the 20th of July, before Mercury disappears, a very thin crescent Moon is below and to the left, or south of the planet.
The red planet Mars is in the north-west. On the 24th of July, the crescent Moon is below and to the left, or west of the planet. The ringed planet Saturn is high in the northern sky. On the 25th of July, the crescent Moon is to the left, or west of Saturn. While on the next evening, the first quarter Moon is directly above the planet.
Let us now turn to the planets visible in the morning twilight. Venus is in the north-eastern sky. For the first half of the month, it is close to the other bright planet, Jupiter. On the 8th to the 10th of July, Venus passes about two Moon widths from the bright star Aldebaran. On the 16th of July, the crescent Moon is below and to the left, or north of Venus. The other bright planet, Jupiter, is also in the north-east. On the 15th of July, the crescent Moon is above and to the left, or north of the planet.
This completes our survey of the night sky in July 2012. If you’d like to learn more about what’s up in the night sky and know it in advance of the podcast, and know it for the full year in advance, you can purchase ‘The Australasian Sky Guide’. This publication is prepared by myself each year. It’s available from Sydney Observatory and the Powerhouse Museum, and also at good bookshops for the very cheap price of $16.95. You can also order it online, but then there are postage charges as well.
Alternatively, you can listen to the monthly podcasts at www.sydneyobservatory.com.au, in the astronomy section, or you can subscribe to them through iTunes. This completes our guide to the night sky in July 2012.