- The Southern Cross -
It’s size and the fact that it looks more like a kite than a cross, often makes finding it for the first time quite difficult. It doesn’t help that a few false crosses can also be found up there.
Fortunately there is an easy way to locate the Southern Cross and be certain you have the right one. Each night, as it passes across the sky, the Southern Cross has trailing behind it two bright stars known as the pointers, so named since they seem to point to the top of the Cross (they do in fact just miss but ....... ).
Since no other cross has a similar signpost they are a sure indication you have the right one.
These two pointer stars, alpha Centauri and beta Centauri, are again very bright and consequently almost impossible to miss. Beta Centauri is the 11th brightest star in the sky while alpha Centauri is the 3rd brightest star of all.
Historically the Southern Cross has had an interesting life. Although the stars in the Cross were known to the ancient Greeks, they regarded them as the hind legs of the constellation Centaurus, the centaur, who today surrounds the Cross.
It wasn’t until the year 1516 that it was first described as a cross and not until later that century that it became adopted as a separate constellation by astronomers.
In the same way that Europeans made pictures familiar to them, the aboriginal people of Australia also made their own pictures out of the stars of the southern cross.
To some of the Aboriginal communities of Central Australia the Southern Cross looks not like a cross but the footprint of a giant wedgetailed eagle called Waluwaru. The two pointers are a throwing stick that has been used to hunt the eagle and the dark patch between the two brightest stars of the Cross is the eagle’s nest.
To others the Cross represents the four daughters of Mululu, a group elder. Mululu had made his daughters promise that after his death they would find a way to join him in the sky, which, when the time came, they dutifully did. Now they can be seen as the four brightest stars of the Southern Cross. Their father, Mululu can be found close by, watching and protecting them forever, as the bright star alpha Centauri.
How to find south using the Southern Cross
Finding your direction using celestial bodies is nothing new. For countless centuries this was the only means of doing so. So, how can we use the southern stars to navigate our way around without the aid of modern instruments?
In order to find your direction all you have to do is locate the point in the sky about which all the stars seem to revolve, called the South Celestial Pole (SCP). Once you have found this point, simply drop an imaginary line straight down to the horizon to find true south. Once south has been found all the other cardinal points can be determined.
Of course the hard part is finding the South Celestial Pole in the first place. In the northern hemisphere there is luckily a relatively bright star (Polaris) at almost exactly the north celestial pole, the northern counterpart to the SCP.
Although we too in the southern hemisphere have a star sitting almost exactly at the pole (sigma Octantis) it is unfortunately about the 6,000th brightest star in the entire sky, whereas Polaris comes in at number 48.
So, without the aid of a bright star to guide the way, how do we find the SCP? Fortunately we have the Southern Cross to help.
There are three easy ways we can use the Cross to find the South Celestial Pole.
The first is to simply extend the long axis of the Cross from the bottom star by 4½ times its length. This won't get you to the pole perfectly but it will get you close enough to be useful.
The second is to extend the long axis of the Cross below the Cross for as far as you like. Then, draw another line starting from ½ way between the two pointer stars and at 90 degrees to the line joining them. Where these 2 lines cross is once again roughly the South Celestial Pole.
The third and final easy way is to draw a line between the bright stars Acrux and Achernar (the 9th brightest star in the entire sky). The SCP can be found approximately half way along this line.
Finding the Southern Cross throughout the year
Like all stars (and hence constellations) the stars that make up the Southern Cross rotate throughout the night around the South Celestial Pole. Since this motion is due to the earth spinning around they will complete one full circle in one day. This means the actual location of the Cross in the sky will depend on the time of night you try and observe it.
If you were to look at one particular star at the same time each night however you would also notice that it appears to be in a slightly different position from night to night. This is because it rises approximately 4 minutes earlier each day (for a full explanation on these motions go here).
Since most people tend to only look at the stars of an early evening this means that although it is roughly the same time of night each time, the Cross will appear in a different position in the sky depending on the time of year you look.
Crisp winter nights around April, May, June and July are the best times to look at the Cross of an early evening.
Interesting objects in and around the Southern Cross visible to the unaided eye
The Jewel Box contains about 50 bright members, the brightest of which are supergiants with average luminosities equal to 80,000 times that of the sun. The cluster is only about 7 million years old, about 50 light years across and about 6,440 light years distant.
This is a dark cloud of gas about 60 light years across and 500 light years distant that is blocking our view of the stars in the Milky Way behind it. There is only one star visible to the naked eye within the Coal Sack.
First included in a catalogue by Ptolomy 1,800 years ago it is generally considered to be the best globular cluster in the sky.
Even at a computed distance of about 18,300 light years it still appears as a giant ball of stars through a telescope.
The main ball shape has a diameter of roughly 86 light years across and is estimated to be about 12 billion years old.
With so many stars packed into such a small volume of space the stars are obviously a lot closer to each other than they are in the sun's neighborhood. In fact the star density is about 25,000 times more than in our part of the Milky Way. The average distance between stars in omega Centauri is about one tenth of a light year.
Lying on the opposite side of the Southern Cross to the Pointers is one of the best bright nebulae in the sky, the Eta Carina Nebula.
At a distance of about 7,500 light years it appears as a slightly brighter looking smudge in the middle of the Milky Way. Through a pair of binoculars or a telescope however it shows itself as the wonderful object it really is.
The nebula has a diameter of about 300 light years, which makes it about 20 times bigger than the other famous southern sky nebula, the Great Nebula in Orion.
Located within this star forming region, and probably formed less than a million years ago, is a truly massive star that is spectacular in every way possible, eta Carina (the nebula is named after the star).
With at least 100 suns worth of material it is perhaps the most massive star in the entire Milky Way galaxy. It pumps out over 4 million times the energy the sun does and will end its life in a hypernova explosion.
Being such a massive star means that eta Carina uses its available fuel very quickly and consequently has a very short life span, probably no more than a million years. This means it is already approaching the end of its life.
First catalogued by Edmund Halley in 1677, eta Carina increased in brightness by 6 times before fading back to its original brightness by 1811. It then steady increased again until, in 1843, it reached a brilliance nearly 100 times its original. At that point, despite its distance it outshone all the stars in the sky except for Sirius.
Since then it has faded until it is now only just visible to the unaided eye. It is thought that this fading act is due to the material it ejected from its outer layers in 1843 blocking our view of the star in the middle. Interestingly, in 1999 it once again doubled in brightness.
The outburst of 1843 is perhaps a precursor to the star exploding in the near future. Exactly when it will happen is open for debate. Some astronomers think it won’t happen for about 100,000 years. Others think it could happen any second.
When it does explode it will be easily visible during the daytime and bright enough to read by at night. Apart from the light show the effects of the explosion will probably be felt by the earth, although the atmosphere should hopefully protect us from the worst of it. It might be a different story for any astronauts and satellites however.
Stars of the Southern Cross (and pointers)
Trailing behind the Southern Cross each night are two very bright stars that seem to show the way to our famous constellation.
Out of the two, the star furthest from the cross is the one more worthy of our attention and a star we should all get to know a little bit better.
Known as alpha Centauri, it is the third brightest star in the night sky and at a distance of 4.36 light years (approximately 41,000,000,000,000 km) it is our nearest known neighbor in space beyond the solar system.
Even though it is the closest star to the sun, to put this distance into perspective, if we made the distance from the earth to the sun equal to one metre, alpha Centauri would be situated 274km away.
Apart from being our next door neighbour, alpha Centauri is actually a triple star system composed of three gravitationally bound stars. The two main stars are called alpha Centauri A and alpha Centauri B. The tiniest star in the system is a red dwarf known as alpha Centauri C or Proxima Centauri.
The two brightest stars take about 80 years to orbit about each other at an average distance of about 3.6 billion km, a bit more than the distance from the sun to the planet Uranus.
Visible through telescopes as a magnificent double star, these two stars are remarkably sun like, with alpha Centauri A being a near twin of the sun. Apha Centauri A is calculated to be 1,708,000 km wide, or 1.227 times the size of the Sun, while alpha Centauri B is 1,204,000 km, or 0.865 times the Sun's diameter. They are perhaps the finest example of a binary star system visible in the entire sky.
At 4.22 light years away, Proxima Centauri is nearer to the earth than the other two stars in the system, by a distance roughly 10,000 times the distance from earth to the sun. This technically makes it the closest individual star to the sun. It orbits the other two stars in a huge circle that takes between 500,000 and a million years to complete.
Discovered in 1915 it is one of the least luminous stars ever found and is 13,000 times fainter than the sun. If it replaced the sun it would be only 45 times brighter then the full moon and appear only 1/20th its diameter.
The alpha Centauri system has one of the largest proper motions across the sky. In about the year 6000 this motion will have brought it close enough to the other pointer star, beta Centauri, that they will become a wonderful double star to look at.
As the second pointer star to the Southern Cross, beta Centauri is very prominent in the sky. It helps that it is also the 11th brightest star in the night sky.
Lying at a distance of 525 +/- 43 light years, beta Centauri turns out to be not a single star but in fact 3 stars.
The system actually consists of 2 stars orbiting each other collectively known as beta Centauri A, with a third star called beta Centauri B orbiting those two.
The star beta Centauri B orbits the pair at a minimum distance of 110 AU and takes at least 250 years to make the trip.
The pair beta Centauri A consists of two identical stars that orbit each other with a period of 357 days. Direct observation shows an average separation of 2.6 AU, although the distance between them varies between o.5 AU and 4.7 AU.
Both stars have identical masses equal to about 9 times that of the sun. They also have temperatures of 22,500K with luminosities of 15,500 times and radii of about 8 times that of the sun.
One of the twins, perhaps both, is also a variable with a period of less than 4 hours.
The stars also appear to be at the point of shutting down their hydrogen fusion. That means they are reaching the end of their lives and beginning to die. At an estimated age of 30 million years, they will expand to become red giants, quite possibly affecting each other quite dramatically.
Low mass stars like the sun become white dwarfs. High mass stars explode as supernovae. The cut off point is somewhere between 9-12 times the sun’s mass. That means the fate of both stars that make up beta Centauri A is consequently unknown. One, or both, could blow up, or they could both become white dwarfs.
If even only one of them were to explode as a supernova it would shine in our sky with the brightness of the full moon.
Alpha Crucis, or Acrux, is the 13th brightest star in the sky. It is also the most southern first magnitude star (just beating alpha Centauri).
Acrux is also a wonderful binary, with both stars easily seen through even a small telescope. Both are hot stars with temperatures about 28000 and 26000K. From their distance of 321 +/- 20 light years, we find respective luminosities of about 25,000 and 16,000 times that of the sun.
Alpha Crucis 2 is a single star with a mass of about 13 times that of the sun. But alpha Crucis 1 is again a double star.
The components of alpha Crucis 1 are around 14 and 10 times the mass of the sun and orbit each other in only 76 days and at a distance of about 1 AU.
With a minimum separation of 430 AU, alpha Crucis 1 and alpha Crucis 2 orbit each other with a period of at least 1500 years.
The masses of alpha Crucis 2 and the bigger star of alpha Crucis 1 suggest that both stars will someday explode. The smaller star of alpha Crucis 1 may however escape that fate and simply become a massive white dwarf.
Not only is it the second brightest star in the Southern Cross, but beta Crucis also comes in as the 19th brightest star in the sky.
The star is a magnificent blue-white, very hot, giant star lying 353 +/- 22 light years away. With a temperature that soars to 27,600 K, such heat causes the star to radiate most of its light in the ultraviolet.
To the eye, beta Crucis would appear 3000 times brighter than the sun. If all the radiation is taken into account however, the luminosity climbs to about 34000 times that of the sun.
As you would expect from such a magnificent star it has a radius about 8 times and mass about 14 times that of the sun.
Beta Crucis is also a close double star. Its components are too near to each other to resolve separately and, separated by around 8 AU, take almost exactly 5 years to orbit each other.
Though no more than about 10 million years old, the star appears to be already nearing the end of its life. That means eventually it will probably blow itself up as a supernova.
As an added bonus, although not directly related to beta Crucis, appearing to sit beside it and visible through even a small telescope is a crimson red carbon star. Well worth a look!
Gamma Crucis is the 24th brightest star in the sky.
It is unusual among naked eye stars as most of the brighter stars visible appear either white or blue. Gamma Crucis however is a cool red giant star and noticeably appears as an orange colour. If you look at it through even a small telescope you will see this distinctive colour in all its splendour.
And a true giant it is. From its nearby distance of 87.9 +/- 1.6 light years and its temperature of 3400K, its luminosity is calculated to be about 1500 times that of the sun. This leads to a radius of about 113 times the sun’s radius. If gamma Crucis was placed where the sun is it would reach out over halfway to the earth!
With a mass perhaps 3 times that of the sun or less, it may well have given up not just fusing hydrogen in its core, which it must do to become a red giant in the first place, but it may also have gone through its helium fusion stage as well.
This blue-white star shines from a distance of 364 +/- 23 light years and appears as the 127th brightest star in the sky.
From its hot 22550K surface it pours out the light, much of it in the ultraviolet, of 5600 suns. From this it is calculated that it has a radius of about 5 times and a mass of about 8.5 times that of the sun.
It is also estimated that it is only 30 million years old at most.
From its spectrum, the star is classed as a subgiant, which means that it has recently given up fusing hydrogen in its core and is about to expand and become a giant.
Like many similar stars, delta Crucis spins very fast, rotating at least 194 km/sec at its equator. This gives a rotational period of less than 1.3 days.
Also like many massive stars it produces a wind, which is estimated to blow at such a rate as to be about 1000 times that of the sun.
Delta Cru is just under the limit at which stars explode as a supernova. Someday it will end its life as a massive white dwarf.
As the faintest member of the 5 main stars in the constellation, and not part of the classic cross shape, epsilon Crucis is occasionally not considered to be part of the Southern Cross.
This is wrong!
It rightfully deserves to be there with the other stars!
Lying at a distance of 228 +/- 9 light years it has a luminosity of about 145 times that of the sun.
Although it isn't obvious when looking at it with your eyes, even a moderate telescope will show that it is an
orange giant star.
