This is a transcript of a podcast of the February 2007 night sky guide. Download and listen to the podcast as you gaze up at the night sky.
Nick Lomb: This is a Guide to the Night Sky in February 2007. My name is Nick Lomb. I’m Curator of Astronomy at Sydney Observatory. If you want to become familiar with the night sky, go outside, settle down, and look up into the night sky while you’re listening to this podcast.
You should also be familiar with the cardinal directions. East is where the sun rises, west is where the sun sets, north is where the sun is at it’s highest, and south is obviously the opposite direction to north. Once you have those directions it becomes much easier to follow the description of the night sky.
In the southern sky, the Southern Cross is in the southeastern sky. It is a very obvious constellation. It’s a compact group of four bright stars close together, with two bright stars, the two pointer stars, below them and pointing to the Southern Cross.
There are similar groups of stars just above the actual Southern Cross but they are much broader and much larger than the Southern Cross itself. The Southern Cross is much more compact and of course has the two pointers pointing up towards it.
There is a fifth star in the cross, the faintest star, a star known as Epsilon Crucis. But that can be difficult to see in cities where the sky is becoming bright due to light pollution: too many unnecessary lights pointed up into the night sky.
Of the two pointer stars, one found furthest from the cross is known as Alpha Centauri and the one above it is Beta Centauri. Alpha Centauri is the closest star system to earth. Looking at it through a small telescope, you can see two stars, very spectacular two stars. They look like a pair of car headlights in the distance. These two stars circle around each other and they take about eighty years to do so. But they’re not actually the closest stars to Earth because there’s a closer star that’s circling around the other two. That star, known as Proxima Centauri, is out of the field of view of a telescope, a small telescope, and is much fainter than the other two. So it makes it somewhat harder to find, but it is a little bit closer than the other two.
Above it, Beta Centauri, it is a very bright star but much further away.
Above them, of course, is the Southern Cross. Extending the main axis of the Southern Cross towards the right, towards the west, and extending it four times, we will reach the South Celestial Pole, the point about which all the stars appear to rotate in the sky. And if you drop a line down from that point down towards the horizon, that is the south point so that it’s a very useful way of being able to find south.
If you extend the main axis of the cross further, we reach the star Achernar and that is a very bright star. And it’s the end of a long line of stars known as Eridanus, the river, and that is one of the longest constellations in the night sky. And Eridanus, apart from Achernar, consists of faint stars, but it reaches up to the northern sky, up to the constellation of Orion. Orion, being of course, a favorite constellation of the southern sky, of the Australian summer sky, and it is a very obvious and easy to pick constellation and it can be used to find other constellations.
So in fact it’s useful to become familiar with bright constellations like the Southern Cross and Orion for they can then be used to find out, to discover, other constellations. Orion can be recognized but it’s… There are four stars in a rectangle: more or less in a rectangle, with three stars in a row in the middle, and they represent the belt of the hunter Orion. The brightest star, the one that is the star at the top of the four stars in the rectangle outlining Orion, is a star called Rigel. And it’s a very hot star with a temperature of about twelve thousand degrees Celsius. And that can be compared with the temperature of the visible surface of our own sun, which is about 5000 degrees. So Rigel is a very hot star. It lies to the distance of about seven hundred and seventy years light years. So light from Rigel has taken seven hundred and seventy years to reach us.
Another bright star, almost as bright, or possibly brighter star in Orion, is the one, which is much lower down closest to the horizon, and that is the star Betelgeuse. That is a star, because of its somewhat reddish colour, is one of the few stars in the night sky that makes it easy to pick a colour.
Most stars just appear like white points of light but Betelgeuse is quite obviously a reddish star. And it has this red, or pale red colour, because it is a cool star. It’s a star much cooler than our own sun. It has a temperature of about three thousand degrees Celsius.
So again, for comparison, the visible surface, our own sun has a temperature of around five thousand degrees. So three thousand makes it fairly cool and that’s obvious by it’s pale red colour.
Apart from the main two stars of Orion, Betelgeuse and Rigel, there are the three stars in Orion’s belt. The one on the furthest left, or towards the west is called Mintaka. It’s a very hot star; it’s a giant star that’s 900 years away from the earth. In the middle, we have Alnilam, which is again a hot star at a distance of 1300 light years. The one on the right, or the eastern side is Alnitak, which is once again a hot super giant star at a distance of about 800 light years from the earth.
Below the constellation of Orion, you have the constellation of Gemini, the Twins. This constellation is dominated by two stars, the Twins: Castor and Pollux. Castor is the lower one, a little bit towards the left and towards the west. It’s known as the horseman. It’s a very interesting star. It’s 51 light years away. Looking at it with the unaided eye, it just looks like a bright star, but in fact, it is made up of six separate stars. I repeat, six stars, so it’s a very unusual object.
If you look at it through a small telescope, it is a double star. In fact, it was the first double star binary star that was recognized by astronomers. It was recognized by Sir John Herschel in around the year 1800. And these two stars move around each other roughly every 460 years. So, it’s quite a long time to go around each other.
There is a third faint star in the system, which circles the other two. It takes so long to move around them, that nobody quite knows how long it takes to move around them, but it must take over 10,000 years to do so.
Each of these three stars is, in fact, a double star. Astronomers have discovered this by using a device called a spectroscope, or more precisely a spectrograph. It’s a device which breaks light up into components, and we actually can see that there are two stars for each of those three mains stars. They are composed of two stars.
One of those double stars takes nine days to go around each other, another one takes three days, and for what appears to be the faint third star in the system it is also double and the two stars take one day to go around each other.
So, just to summarize, looking at Castor through a telescope, you see a double star two stars and a third faint third star in the system. And each of those three stars is actually a double a very, very close double star. So, altogether, Castor is made up of six stars.
The second one of the Twins is the second one we can see with the unaided eye is Pollux, known as the Boxer. Pollux is 34 light years away. It is somewhat colder than our own sun, with a temperature around 4, 500 degrees, but it is much larger than our own sun. It is eleven times wider and puts out 35 more times light than our own sun.
If you go towards the right, or towards the East, you come across the planet Saturn. Let’s ignore that for a moment and go just a little bit further and we’ll reach the star Regulus. That is the brightest star in the constellation of Leo, the Lion.
It is a star that just lies on the ecliptic, that is the path of the sun and the planets and the moon across the sky. So, it is a star that is often covered by the moon. The name Regulus means it is “the little king.” The star is 77 light years away. It puts out, roughly 140 times as much light as the sun. It is a fairly hot star, 12,000 degrees, compared to our own sun, which is around 5,000 degrees. So, it is a hot star. It has a width of some three and a half times that of our own son.
It has a companion star which circles around Regulus. Again, it takes a very long time. This time astronomers have deduced is that this fainter companion takes at least 130,000 years to move around Regulus. And this companion star of the Regulus is also a double star, so Regulus is in fact is a triple star.
Going back to Saturn, and that’s visible in the northeast sky, just to the left or West of Regulus. And that is, of course, one of the most spectacular objects to look at in the night sky through small telescopes. Through a small telescope, you can make out the rings of the planet as well as the disk itself.
There’s one other planet visible in the night sky in February, and that is Venus. That’s visible as soon it starts becoming dark. It’s in the western sky.
A couple of dates to note, in connection with Venus. On the 7th of February, it’s worth looking at it with a pair of binoculars, because just near it less than two moon widths away is the planet Uranus. Uranus is too faint to be seen with the unaided eye, so it’s very hard to find in the sky. So, on the 7th of February, Venus will provide a pointer to finding Uranus.
While on the 19th of February, there will be a thin crescent moon just below and to the left of Venus. The crescent moon and Venus always provide a very spectacular sight, so that is something to watch out for.
This has been the Guide to the February Night Sky, and it appears on www.sydneyobservatory.com.