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.
Did you know that some of the stars that look like single stars are actually multiple stars? That is, more than one star circling each other closely so that they appear from Earth like one star. In fact, sometimes there are more than two stars in the group. For example, the star we think of as Castor – one of the two most prominent stars, Castor and Pollux, in the constellation of Gemini the Twins – is actually six stars!
Other constellations Nick tells us about this month are Orion, Leo the Lion, and Crux – also known as the Southern Cross. And he tells us that 14th April is a good date to look for Jupiter because it will be near the crescent Moon in the evening.
For this and much more, listen to the audio, or read the transcript below for more details.
SEE THE SKY CHART
We provide an embedded sky map (below) and an April 2013 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.
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BUY THE BOOK
Our annual book, ‘The 2013 Australasian sky guide’, by Dr Nick Lomb has more information and star maps for months from December 2012 until December 2013 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 April 2013 monthly sky guide audio
This is the guide to the night sky in April. My name is Nick Lomb. I am the Curator of Astronomy at Sydney Observatory. This podcast is available through the Sydney Observatory website – www.sydneyobservatory.com.au – and it is in the Astronomy section. It is always available at the beginning of each month. We’ll start off the podcast by talking about the stars in the night sky, and we’ll consider what planets are visible at the end.
To start off, it’s a good idea to download the star map that’s available through this website, the monthly sky map, and print it out. It would also help to equip yourself with a torch that should have a red colour.
The way to convert an ordinary torch into a red torch is to put some red cellophane at the front.
The idea of the red light is that it does not wreck your adaptation to the night sky, so you can look at a map and a piece of paper as well as looking up into the sky without having to wait for your eyes to adapt to the darkness again.
Also, it’s a good idea to make yourself familiar with the cardinal directions: north, south, east, and west. East is, of course, where the Sun rises. West is where the Sun sets. And of course, in April, it can start to become a little cool, so dress suitably. Go outside, sit yourself down, and listen to this podcast.
We’ll start off our tour of the April stars by facing north. We will see, of course, the familiar sight of the Australian night sky, the constellation of Orion, to our left and to the north-west. Orion is also visible during the Australian summer evenings.
Orion is an easily recognisable constellation, as it is made up of four stars in a rectangle, with three stars in a row in the middle. The three stars in the row in the middle represent Orion’s belt. Just above the belt, there is a line of three faint stars, and the middle one is the Great Nebula of Orion.
These three stars represent the dagger of Orion, or sometimes they’re referred to as the sword of Orion.
Of course, you might find it a little odd that the giant Orion of Greek mythology wears his sword above his belt. The reason is, of course, that Orion was named in the Northern Hemisphere a long time ago, so when we look at Orion, we actually see Orion upside-down.
Now, as I said, the middle, slightly fuzzy star that we can see in Orion’s dagger is the Great Nebula in Orion. This is the nearest large star-forming region to Earth. It is at a distance of about 1,500 light years from us. That is, light has taken 1,500 years to reach us from the Great Nebula in Orion.
The Great Nebula is one of the more interesting objects to look at through a small telescope because, through a telescope we can see a little bit of fuzziness. And inside the fuzzy area, we can see four stars, in a slightly distorted rectangle in the middle.
These four stars are very young stars, referred to as the Trapezium stars, and the fuzziness around them is the gas and dust out of which new stars are currently forming.
When astronomers look at the Nebula with large telescopes and with special telescopes sensitive to infrared radiation, they can see hundreds of new stars being formed inside the huge cloud of gas and dust that forms the Great Nebula in Orion.
Just looking with our own eyes, we can use Orion as a signpost to find other objects in the night sky. Let’s move to the right of Orion, that is towards the east, and a little bit lower down, and we see two bright stars close together.
These two stars are almost due north in the early evening. The two stars are the two brightest stars in the constellation of Gemini, the Twins. The top one is a star called Pollux, and the lower one is a star called Castor.
Pollux is fairly close to us, at a distance of 34 light years. It is a giant star, slightly reddish, with a surface temperature about 4,500 degrees. This makes it a little bit cooler than our own Sun. Our own Sun has a surface temperature around 5,500 degrees. Pollux has a width 10 or 11 times as much as that of our own Sun.
Castor, the second of the two twin stars of Gemini, is a very interesting object. It is the lower of the two stars. At a distance of 51 light years from us, it is a little bit further than Pollux. That means that the two stars are not actually related to each other; they just happen to lie in the same direction.
To the unaided eye, Castor appears like a single star. But if you look at Castor through a telescope, you can see it is made up of two stars. These two stars circle around each other, taking about 460 years to do so.
Of course, you would need a lot of patience to actually see any motion with such a long period. There is, in addition, a third faint, reddish star in the system, but that one is much fainter and not quite as obvious as the other two.
So far, we have mentioned three stars in Castor: the two stars we can see through a telescope and a fainter, reddish star, all circling around each other. However, when astronomers examine each of the three stars in detail, they find that each of them is a double star.
We cannot see these separately. We cannot look through a telescope and see their companion stars.
But when astronomers measure the velocities of the stars using a device called a spectroscope, they find that the objects are sometimes moving towards us and sometimes they are moving away from us. That can only happen if they’re circling and there are two stars circling around each other.
Each of these three stars in Castor is a double star in its own right. So, although looking at Castor with our own eyes we just see one object, it is in fact, six stars.
So we can look at this one pinpoint of light in the sky with our eyes and we’re actually seeing six stars in that direction, which is quite an amazing fact to note.
These two stars, Pollux and Castor, are very prominent stars, and there are legends associated with them. According to a legend from the ancient Greeks, the two stars protected sailors, especially the ship Argo, which travelled to retrieve the Golden Fleece, and the sailors on the Argo were always protected by these two stars, Pollux and Castor.
Let us go towards the right, towards the east, and we reach the star, Regulus. Regulus is a star that lies on the ecliptic, which is the path taken by all the planets and the Sun and the Moon as they move across the sky during the year.
So, Regulus lies on the path taken by the Moon, and that means that it is sometimes covered by the Moon. When the Moon covers a star, astronomers call that an occultation, and Regulus is often occulted by the Moon.
Regulus is the brightest star in the constellation of Leo the Lion. It is relatively close to us at a distance of 77 light years. The name ‘Regulus’ means ‘the little king’.
The rest of the stars of Leo the Lion are quite faint, and it can be quite difficult to make out the constellation. But Regulus is itself easy to find, so Regulus provides an easy way of finding the whole constellation of Leo the Lion.
Regulus is a fairly bright star. It puts out somewhere around 140 times as much light as our own Sun. So, intrinsically it’s bright, but it does not appear so bright in our own sky. It has a mass about three-and-a-half times that of our own Sun.
Regulus has a companion star, so Regulus is a double star. But that companion is quite a long way away from the main star of Regulus, something like 100 times as far away from Regulus as the dwarf planet Pluto is from our own Sun.
Since it is so far away, the companion takes a long time to circle Regulus. It takes 130,000 years. Obviously, nobody has actually seen the companion star to Regulus make a complete circuit.
Now if we go further towards the east, towards the right and almost due east, we find the star Spica. Spica, again, lies in the ecliptic, just like Regulus, so Spica is often occulted or covered by the Moon.
Spica is intrinsically a very bright star which is at a distance of 260 light years from us. Spica appears fairly bright in our sky, but not as bright as it actually is. Close up, it puts out over 2,000 times as much energy as our own Sun.
Spica, as I said, is in the eastern sky. One way of finding it is to look for a group of four stars which form a twisted rectangle in the sky. These are the stars of the constellation of Corvus the Crow. If we extend two of the stars of Corvus directly downwards, we reach Spica. That is the easiest way to be sure we’re looking at the star Spica.
The Sun tends to pass Spica in the Northern Hemisphere’s autumn, or in the Southern Hemisphere’s spring. Because it passes in the Northern Hemisphere autumn, which is at harvest time, the name ‘Spica’ means ‘ear of wheat’. So it is a star that is always associated with harvesting.
If you look at Spica it appears like one star, a single star. Even through a telescope it appears like one star. But through a spectroscope, a device used by astronomers to break light up into its components and a device which allows astronomers to measure the velocity of the stars, astronomers can determine that Spica is made up of two stars circling around each other fairly quickly. They take just four days to complete one circuit of each other.
As the two stars are so close to each other, they’re actually distorted. The shape of the stars is not a nice, round globe that we always imagine stars to be. But their shape is something like that of a football.
As they circle around each other during the four days that they take to do so, we see those footballs from different aspects. We actually see a slight variation of brightness of the star Spica during the four day period due to this change of aspect.
The football shapes that you can see during the four day period that circle around each other, are seen at either end-on or side-on. And this obviously changes the brightness that we can see from a distance and this gives rise to the changes in brightness.
Let us move further in our tour of the April night sky, and let us move to the southern part of the sky. High up in the south-east we can see the constellation of the Southern Cross. Of course, the Southern Cross is a very obvious constellation in the night sky, and a constellation of great significance to Australians.
The Southern Cross is on the Australian flag, it’s on the livery of several airlines, and numerous other firms use the Southern Cross as part of their logo. It is a constellation very much recognisable in Australia.
To be sure when we look at the real Southern Cross up in the night sky, and to make sure that we’re looking at the real Southern Cross and not the False Cross, which is somewhat higher up in the southern sky, it’s important to look for the two stars directly below – the two Pointer stars.
And it is those two Pointer stars which really indicate that we’re looking at the right Southern Cross. As well, we can note that the stars of the real cross are brighter than the stars of the ‘false cross’, and they’re closer together, much more compact than the stars of the ‘false cross’.
The two Pointer stars are known as Alpha and Beta Centauri. The lower one of the two pointers is called Alpha Centauri, and it is the star or star system that’s closest to us. Light left Alpha Centauri four-and-one-third years ago, and it is reaching us today.
If we look at Alpha Centauri through a small telescope, it is a very spectacular double star, two stars close together in the field of view of the telescope. To me, through a telescope, Alpha Centauri appears like a pair of car headlights in the distance with the two lights close together.
There is actually a third star in the system, but that one is out of the field of view of a small telescope, and it’s also too faint to be seen with a small telescope. It’s only recognisable by careful study of actual images of that part of the night sky.
This third star is believed to be circling around the other two, and the current state of its path around the two main stars of Alpha Centauri means that it’s slightly closer to us than the other two stars.
Hence, astronomers have given this star the name, Proxima Centauri, meaning that it’s the close by star of Centauri. Proxima Centauri is the closest star to us, but the closest star system is Alpha Centauri, because Proxima is part of the same system.
Above the Pointers, as we have discussed, we find the bright stars of the Southern Cross. If we take the topmost star of the Southern Cross and the one on the right, and we extend the line through them towards the right, we move towards the west and we reach the bright star, Canopus that is almost overhead, very high up in the southern sky.
Canopus is the second brightest star in the sky after Sirius. It is a star that’s 312 light years from us, and because it appears bright, even at that large distance, we can tell that it is intrinsically a very bright star.
Astronomers have established that it gives off about 10,000 times as much energy as our own Sun. It is a very large star, about 100 times the width of our own Sun.
The name Canopus comes from the name of a pilot of a fleet in ancient Greek times and this fleet was sailing back from Troy after the battle there, at Troy.
According to ancient Greek legends, the name of the pilot was Canopus, and it seems the fleet pulled into the port of Alexandria in Egypt, and Canopus died at that port. At this place, the star Canopus was just visible over the horizon, so it was named after the pilot who died at the city of Alexandria.
Canopus is the brightest star in the constellation of Carina the Keel. And the keel refers to the ship, Argo Navis, the constellation known as the Ship. Argo Navis was once a large constellation in that part of the sky, but it has since been broken up into three: Carina, Vela the Sails, and Puppis, which is the stern of the ship.
The Milky Way passes through the north-eastern part of these three constellations and contains many interesting clusters of stars and nebulae, which are sorts of fuzzy objects in the night sky.
The most famous of these fuzzy objects is called Eta Carinae, or the Eta Carinae nebula. This nebula is a large cloud of gas and dust surrounding a star, also called Eta Carinae. This is one of the largest stars we know about in the night sky. It is at least 350 times the mass of our own Sun.
The star is famous because in the 1830s and ’40s it experienced an outburst and suddenly became very much brighter than it had been previously. It became the second brightest star in the night sky after Sirius, which was then and still is the brightest star in the sky.
We should note that Sirius is only nine light years away. Eta Carinae is 7,500 light years away from us. Hence, intrinsically, Eta Carinae is a much, much brighter star than Sirius. It had to become intrinsically extremely bright in the 1840s to appear in our night sky almost as bright as Sirius.
During the outburst in the 1830s and ’40s, Eta Carinae threw out a lot of dust, and that dust has formed a cocoon around the star and hides it from our view. That is why the star has become much fainter than it was in the 1830s and ’40s.
Intrinsically, the star is still bright, but it’s inside the dust cloud which hides it from our view. Infrared radiation is still transmitted through the dust though. The dust creates its own infrared radiation, and Eta Carinae is the brightest source of infrared radiation in the night sky.
There’s a very famous and spectacular image by the Hubble Space Telescope of this cocoon of dust surrounding the star, and it looks like, on the image, a brain, like two halves of a brain. That is, of course, the two hemispheres of the dust cocoon surrounding the star.
In recent times, astronomers have determined that Eta Carinae is not just a single star, but is two massive stars circling around each other. One of them is just over 100 times as massive as our own Sun, and the other one is probably a little less than 100 times as massive as our own Sun. These two stars circle around each other roughly every five years.
As these stars are so massive, they’re believed to be near the end of their life cycles. Eta Carinae is the best candidate we have for a star about to go supernova, that is, to explode at the end of its lifetime.
As light takes 7,500 years to reach us from Eta Carinae, it’s quite possible that one of the stars of Eta Carinae has already exploded and created a supernova. But that is something we will not know until the light actually reaches us.
Certainly it is a prime candidate to be a supernova sometime between now and the next 10,000 or 20,000 years. If it does become a supernova, it will become very, very spectacular and not only become the brightest object in the night sky, rivaling, and possibly becoming even brighter than the Moon, but it will be visible even during daylight for months on end.
Now let us turn to the special events and the position of the planets for this month of April 2013.
This month, summer time ends on the morning of 7th April; it will end at 3am. So that means that we can sleep in for an extra hour but also has the very nice benefit that we can start watching the sky in the evenings at an earlier time.
Another major event for the month is that there will be a partial eclipse of the Moon. It is a particularly short and shallow eclipse with only 2% – 1/50th of the Moon’s width immersed into the shadow of the Earth. But still it will be worth watching. It will only be visible from Australia but not, unfortunately, for those people in New Zealand. It will occur in the early morning of Friday 26th April, that is, the morning after Anzac Day. The Moon will start moving into the shadow of the Earth 5.52am Australian Eastern Standard Time and leaves the shadow at 6.23am. As I said, it’s a very shallow eclipse; only a very tiny little bite will appear missing from the Moon but it will certainly be worth watching, and looking out for that tiny bite.
With regard to the planets in the evenings: initially there is only one planet visible, the bright planet Jupiter, but later on it is joined by the ringed planet, Saturn.
So, Jupiter is in the north-west sky, to the right or north of the bright star Aldebaran, which is the brightest star in the constellation of Taurus the Bull.
On 14th April the crescent Moon is below and to the left or west of Jupiter, while on the next evening it is above and to the right or north of Jupiter.
The ringed planet Saturn appears low in the eastern sky during the 2nd week of the month.
On Anzac Day, on 25th April, the gibbous Moon is above and to the left or north of the planet. Of course, that will give us a good opportunity to identify the position of Saturn, having the gibbous Moon nearby.
On the next evening, that is, on the evening of 26th April, the Moon will be full, and it will be below and to the right or south of Saturn.
Those of you who rise early will be able to see in the morning twilight the planet Mercury, low in the eastern sky. On 8th April, the crescent Moon is above and to the left or north of Mercury.
On the next evening, on 9th April, the crescent Moon is below but still to the left or north of the planet.
The planet Saturn which has become visible in the evening sky, is still visible in the morning sky, low in the western sky, in the constellation of Libra.
That completes our guide to the planets visible during the month of April 2013. This podcast is available each month on the Observatory website – www.sydneyobservatory.com.au – and you can find it in the Astronomy section of the website.
Alternatively, you can subscribe to the podcast through iTunes.
If you would like more information about the night sky, and have it available all year round in advance, you can purchase the ‘Australasian sky guide’ that I prepare each year.
This publication is available through Sydney Observatory and Powerhouse Museum shops, or at good bookshops throughout the country for the very low price of $16.95.
It can also be ordered online though the Powerhouse Museum or Sydney Observatory websites, but obviously then there is a small mailing cost to be added to the price as well.
And this is the end of the podcast for April 2013.