It is a common misconception that the Southern Cross is always visible in the southern night skies. This is in fact not quite so, but those living under it generally find comfort in the notion that should they need its guidance, it will be there.
Since time immemorial, seafarers recognised the four bright stars in the shape of a cross as a dependable signpost. Two of the stars of Crux, Acrux and Gacrux, are commonly used to mark south. Following the line defined by these two stars for approximately 4.5 times the distance between them, leads to a point close to the Southern Celestial Pole. (Don’t use the False Cross to its right which has dimmer stars set further apart.)
The Southern Cross lies in the heavens about one third of the way between the Southern Celestial Pole and the Equator.
This tiny constellation once formed the foot of the Centaur as part of Centaurus, but became a constellation in its own right in the sixteenth century. Of exactly how this came about, there are different versions. One is that in 1501 Amerigo Vespucci named it the Almond, while in 1520 members of Magellan’s crew during their voyage circumnavigating the globe, called it the Southern Cross.
In 1592 Emerie Mollineux also mentions the constellation by this name, which was officially adopted by Bayer when he included it in his star atlas in 1603.
Credit is given to the French astronomer Augustin Royer for defining it as a separate constellation and naming it Crux Australis in 1679.
It was known, however, long before that. Dante, in his great work Purgatorio (1313) mentioned it and gave the symbolic moral virtues of Justice, Prudence, Temperance and Fortitude to the four points.
There are actually five main stars in the Crux, viz Alpha Crucis, Beta Crucis, Gamma Crucis, Delta Crucis and Epsilon Crucis. Three of these are of the brightest stars visible to the naked eye.
Acrux, or Alpha Crucis, the bottom of the cross, is the 14th brightest star in the southern skies, distanced 370 light-years from us. It is actually a binary system (2 stars), both stars roughly of the same size. They are huge B-type stars of between 15 to 20 times the mass of our Sun. This is considered to be one of the very finest binaries in the heavens and they are separated by a span of roughly six times the diameter of our solar system.
Beta Crucis (Mimosa), the brightest star of the group, is a blue-white giant with a mass of about 20 times that of our Sun. It is a pulsating star and lies 490 light-years from Earth.
Gamma Crucis (Gacrux), forms the top of the cross and is orange rather than white which relates to its cooler surface. It is a red giant star of about the same size as Alpha and Beta Crucis and is the 25th brightest star in the sky. It lies out at a distance of about 220 light years.
Delta Crucis, the western arm, is another star like Alpha and Beta, weighing the same as about 15 Suns at a distance of 570 light years.
These are the 4 main stars which form the points of the cross. (The slightly redundant fifth star is Epsilon Crucis.)
Two thousand years ago they were just visible at the horizon in the northern hemisphere and were well known to the Greek astronomers. Over time, however, precession has brought the cross far to the south and it is no longer visible at latitudes north of 26 degrees, except during April and May when it is evident south of latitude 30 degrees north. Then people in places such as Arabia, Egypt, Mexico and South China can still see it.
Its distinction as a feature of the night skies has brought about its use in several countries of the southern hemisphere. Brazil depicted it on postage stamps in 1889. It also appears on the Coat of Arms of the Commonwealth of Australia and in South Africa a most prestigious award was the Honoris Crux Decoration for bravery in dangerous situations. The five brightest stars of Crux (Alpha, Beta, Gamma, Delta and Epsilon Crucis) appear on the flags of Australia, Brazil, New Guinea, Papua and Samoa, while the New Zealand flag shows four, omitting Epsilon.
The Southern Cross contains several Deep Sky Objects of which the best known are The Jewel Box and the Coal Sack. The first, also called the Kappa Crucis galactic cluster, is ranked among the finest objects in the southern Milky Way and was discovered by Nicolas Louis de Lacaille in 1751-1752. It lies at a distance of about 7 500 light years and consists of approximately 100 stars, about fifty of which are a mixture of sparkling supergiants, brilliant reds and blues intermingled with yellows and whites.
Ironically, just to the south of this brilliant pouch of light lies one of the darkest and nearly starless areas of the galaxy, the Coal Sack Nebula. Nebulae are patches of dust and gas, mostly matter left by the death of a star out of which new stars are created.
The Coal Sack is possibly the nebula nearest to us and, just visible to the naked eye, lies east of Alpha Crucis at a distance of 400-500 light years. It is about 60-70 light years in diameter.
The False Southern Cross
When looking for the Southern Cross, it is easy to get confused with the False Cross, Vela (The Sails).
Like Crux, this constellation also used to be part of a much larger constellation, Argo Narvis - the ship of the Jason and the Argonauts - which in 1877 was sub-divided by Gould.
The lesser stars Velorum, in conjunction with the brighter Carinae, form the False Cross which can be mistaken for the Southern Cross (Crux).
Vela lies in a part of the Milky Way known as the Gum Nebula, after the Australian astronomer Colin S. Gum, who drew attention to it in 1952. Near the centre of the gigantic Gum nebula lies the Vela Supernova remnant (SNR), within which is the Vela pulsar, about 1 500 light years away.
To find Vela, look towards the zenith in the late evening sky, and look a little north of a line between the Southern Cross and Canopus.