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 Dr Nick Lomb, Sydney Observatory’s ‘astronomer about town’ (he was Curator of Astronomy at Sydney Observatory until the end of 2009). Since then Nick has been pursuing other astronomical interests and projects as well as continuing his valued association with Sydney Observatory.
SEE THE SKY MAP
We provide an embedded sky map (below – it can take a moment or two to load) and this printable July 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.
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)
This is a guide to the night sky in July. My name is Nick Lomb. I was a curator of astronomy for Sydney Observatory in the Powerhouse Museum until the end of 2009. You can find this podcast in the Monthly sky guide section of the Sydney Observatory website.
If you’d like to become familiar with the night sky, what you need to do is download the map associated with this podcast, simple to download in PDF. Print it out, and take it outside with you. Dress warmly, because in July the nights or evenings tend to be fairly cool.
What you also need is some kind of torch light, and preferably one with a red light because red does not destroy your adaptation to the night sky. You can look at the map and look up at the night sky. If you don’t have a red torch, put a little bit of red cellophane in front of an ordinary white torch, and that will give you a red light to be able to look at both the map and the night sky.
You also need to be 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. That gets you a fairly good indication. Around 12 noon, the Sun is roughly due north.
So, let us begin our tour of the night sky in July. Let us start in the east, where we can see the familiar sight of the constellation of Scorpius the Scorpion. This is a very welcome sight, visible in the Australian winter. It’s a very obvious constellation. It’s one of the really few bright and easy to find constellations, so it’s an excellent signpost for the night sky.
It 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 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 reddish 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 has a similar reddish colour to the planet Mars.
Occasionally, when Mars passes close by, as it does on some occasions, they look like two very similar, reddish objects close together in the night sky. That can be very spectacular when it happens.
As I said, Antares is a giant star. It is relatively cool, with a surface temperature around 3,200 degrees Celsius. This may sound like a lot, but compared to our own Sun, which has a temperature around 5,500 degrees Celsius, it is a relatively cool star. It is that coolness which gives it its red colour.
Because it’s so big, so huge, even though it’s relatively cool it still puts out a huge amount of light. So it appears to be a very bright star, even though it is 600 light years from us. Light has taken 600 years to reach us from Antares.
The star is so large if you replaced our own Sun by Antares, and I should say that we wouldn’t want that to happen, then the Earth would be engulfed by it, 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 circling around it. That is a hot star, so there’s quite a contrast between the two. The companion star’s temperature is at 18 thousand degrees Celsius and it has a bluish colour.
If people look at the two stars together, the reddish Antares and the blue companion star, they sometimes describe the companion star as green. That seems to be some kind of contrast effect in our eyes.
This companion star appears small compared to huge Antares. However, it’s actually larger than our own Sun. It’s something like four times wider than our own Sun, and has a mass about 10 times that of our own Sun. It’s 2,000 times as bright as our own Sun.
We do not, as yet, know how long the companion star takes to circle around Antares, but the current estimate that it’s something like 1,000 years.
Now let us move from the eastern part of the sky, from Scorpius the Scorpion, to the north. We’re facing north and looking up. The most obvious star that we can see is a star called Arcturus. This star is part of the constellation of Bootes, which is the Herdsman. Arcturus means the Bear Watcher.
Arcturus is one of the brightest stars in the sky; the fourth brightest star in the sky. It is the brightest star in the Northern Hemisphere as seen from the Northern Hemisphere.
It’s 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 that it moves around the centre of our own galaxy a little bit slower than our own star, the Sun. Of course, we move around, we on Earth, move around 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 an intriguing suggestion that Arcturus does not come from our own galaxy, but comes from a small galaxy that merged with our own galaxy many thousands of millions of years ago.
That would explain why Arcturus has a different motion to other stars in this part of the galaxy as it’s moving a little bit more slowly around the centre.
Let us now move 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’s in the north-west.
The name Regulus means Little King. It’s 77 light years away. It gives off more than 100 times as much light, or at least if we were close to the star we would see it intrinsically give off something like 100 times as much light as our own Sun.
It’s a fairly hot star, 12,000 degrees Celsius, which compares, if you recall, with our own Sun at about 5,500 degrees Celsius. Regulus is a star right on the ecliptic, right in the path of the planets, and the Sun, and the Moon as they move along across the sky.
Consequently, Regulus can be occulted, or covered, by the moon, and that is an event that often happens. Regulus also has a companion star circling around it. It’s a fairly low-mass star, but it’s a large distance away from Regulus. In fact, this companion is 4,000 times as far away from Regulus as the Earth is from the Sun.
This large distance means that it circles around Regulus, the main star of Regulus, very slowly. It’s believed that it would take at least 100,000 years I repeat that, 100,000 years for the companion star of Regulus to circle around it.
I’m afraid none of us are going to be around to see the companion star complete a circuit of Regulus, but people in the distant future will be able to estimate the exact period. For the moment, all we can do is make an estimate that it would take somewhere around 100,000 years.
Interestingly, this companion star this little companion star that’s circling around Regulus is also a double star. The two stars of 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 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’s the best time to view it in the early evening because it’s very high up in the sky, and it is sort of standing vertically due south at this time of the year in the early evening.
If you look at the Southern Cross, you can easily see four stars. There’s also a fifth star, which sadly is becoming lost due to light pollution in our cities. If you’re looking at the Southern Cross from a dark sky in the country, the fifth star is nice and prominent.
But if you look at it from the suburb of a major city, whether it’s in Sydney, or Melbourne, or Adelaide, or Perth, then the fifth star in the Southern Cross is getting hard 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. That is the brightest star of the Southern Cross.
If you look at Acrux with 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 see today left Acrux 320 years ago.
Going clockwise, the star on the left of the cross is a star called Beta Crucis, or we can call it by its proper name, Mimosa. That is the second-brightest star in the Southern Cross.
It’s a fairly hot star at a distance of 350 light years from us. It’s important to note that the stars that appear close together in the sky, like the stars of the Southern Cross, are not necessarily close together in the sky in reality.
The stars in the Southern Cross are a very good example. The stars are dispersed three-dimensionally in the sky. They are at different distances. From our position, they make up the Southern Cross. They look like a cross.
Anywhere else in the universe, or even anywhere else in our own galaxy, the stars would not appear like the Southern Cross. They would take up a completely different shape. It is just from our viewpoint that they take up this particularly interesting shape.
Going on, we have looked at Acrux and Beta Crucis or Mimosa. At the top of the Cross, there is a star called Gamma Crucis. That 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 our unaided eyes. But with photographs, if you look at colour photographs of 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. It’s the closest star out 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. This is a hot star 364 light years away from us.
Below Delta, between Delta and Acrux, we find the faintest star of the Southern Cross, Epsilon. From a city, as discussed earlier, you 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. But with a pair of binoculars, it is easy to find.
Epsilon is again an orange-coloured star, like Gamma Crucis. It’s at 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. It’s 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 quarrelsome creatures. Surprisingly, this particular centaur in the sky next to the Southern Cross is not a warlike creature.
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 the 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, and 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 is a double star, two stars really close together in the sky. In fact, they are 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 fainter star is a somewhat orange 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 own Sun.
There’s a third star in the system, which is known as Proxima Centauri. We cannot see Proxima through a small telescope because it’s a long way away, out of the field of view. It’s two degrees away from the other two stars, and two degrees represents four times the width of the full Moon, so it’s normally outside the field of view for a telescope.
But there’s another reason why we cannot see it; it’s very faint. It’s a little dwarf star. It gives off about 1/10,000th as much light as our own Sun.
The three stars of the Alpha Centauri system are at a distance of 4.33 light years from Earth, that is, light left Alpha Centauri 4.33 years ago, which is not that long ago. That is certainly within our memory.
This makes Alpha Centauri, the three stars of the Alpha Centauri system, the closest star system to Earth. But remember that Proxima is just a little bit closer than the other two. Proxima is, in fact, the closest star to Earth apart from 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 working there.
The other star of the two Pointers is Beta Centauri. That is the one that appears closer to the cross in the sky and that is much further away. It is 525 light years away, once again demonstrating that two stars that appear close together in the sky can be a very large distance apart in reality. Beta Centauri is 525 light years away from us.
It 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 just see that it is a double star. You can see a faint star associated with the main primary star, though 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 5 10 million stars.
This ball of stars circles independently around the centre of our own galaxy. It’s the most massive of the 160 or so globular clusters, 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’s relatively close to us for a globular cluster. It’s only visible from the Southern Hemisphere, and it is a very nice object to study. It circles around the centre of our 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. This is the remnant, the very central part or the nucleus of the small galaxy and we see it as the globular cluster, Omega Centauri.
It’s visible to the naked eye from a dark sky, but in the city it can still be easily picked up with a pair of binoculars.
Back in 1985 when Halley’s Comet was nearing the Earth, before its close approach in 1986, it passed right by Omega Cen. This comet looked almost exactly like Omega Cen, and a lot of people at that time saw the two of them in their binoculars and thought that Halley’s Comet had split into two.
Now let us turn to the special events and the visibility of planets during this month, July 2011.
This month is interesting, we have two new Moons on the 1st and the 31st. New Moons are of course when the Moon is not visible. And there was once many many years ago a column by another astronomer about the sky for that particular month. And editors had put up as the heading for the column: ‘Two new Moons to light up our skies this month’ which I thought was rather amusing and showed that the sub-editor who made up the heading did not really know much about astronomy.
There are two planets visible in the evening sky. They are the planets Mercury and Saturn. The planet Mercury is visible low in the north-west sky after it becomes dark, soon after sunset. Mercury is always worthwhile looking at. There are many people who have never seen Mercury. It’s a hard-to-see planet as it never strays far from the Sun. So it’s never high up either in the evenings when it’s visible, it’s not high up above the horizon, or it’s not high up above the horizon in the mornings when it’s visible in the eastern sky. But this month in July 2011, it’s visible in the evening sky, low in the north-west after dark.
On 3rd July, the crescent Moon is above and to the left or west of the planet. It’s an event worth looking out for.
The ringed planet Saturn is high in the northern sky. Again, of course, after it becomes dark, after sunset.
On the 7th of the month the crescent Moon has moved near to Saturn and will be to the left or west of the planet. While on the next evening, on the 8th July, the Moon which at that stage will be at first quarter, and it will be directly above the planet.
In the mornings there are three planets visible. To see them you need to look before dawn, before it starts becoming light in the mornings. The bright planet Venus is low in the north-eastern sky. But in the middle of July it will disappear into the twilight and it will not be visible again for over a month.
The planet Mars is in the north-eastern sky. On the 28th July the crescent Moon is below and to the right or east of the planet.
The giant planet Jupiter is in the northern sky. On 24th July the crescent Moon is below and to the left of the planet.
That completes the guide to the planets and to the stars for July 2011. You heard this podcast on the Sydney Observatory blog, www.sydneyobservatory.com.au.