The Southern Cross, or Crux to give it its official name, is the smallest of all the 88 constellations and an iconic feature of the antipodean sky. Alpha (α) Centauri — the third-brightest nighttime star — and nearby beta (β) Centauri act as convenient pointers to the Southern Cross that all lie within a span of an outstretched hand at arm’s length.
Crux is so small that the field of view of a 7×50 binocular easily encompasses the entire cross- or kite-shaped asterism. Its four stars are, clockwise from top in the image above, Acrux (α Crucis), Mimosa (β Crucis), Gacrux (γ Crucis) and delta (δ) Crucis — the latter not having a proper name. Fourth-magnitude epsilon (ε) Crucis is the fainter fifth star between α and δ that features on the national flags of Australia, Brazil, Papua New Guinea and Samoa.
Where and when to see Crux
The Southern Cross is visible at upper culmination (i.e., above the south celestial pole) from all latitudes south of 27° North and circumpolar (never sets) from latitudes south of 33° South. Put simply, Crux can be seen from anywhere south of the Tropic of Cancer when best placed, and never sets at all as seen from southern Chile, Argentina, the southern tip of South Africa (inc. Cape Town), southern Australia (inc. Adelaide, Melbourne, Hobart, Canberra and Sydney) and the entirety of New Zealand. Wherever the Southern Cross is visible in the world, it is highest in the sky at midnight in early April.
The Southern Cross holds an exalted status as a navigational aid too, which likely explains why it features so prominently on the flags of five nations — the four countries listed above, plus New Zealand. Unlike in the Northern Hemisphere, where conspicuous second-magnitude star Polaris lies less than one degree from the north celestial pole, there is no bright star marking the south celestial pole (SCP). However, an imaginary line drawn from Gacrux through Acrux and extended a further four and a half times the separation of the two stars brings you close to the SCP. Drop a vertical line down from the SCP and you have found due south on the horizon.
Among stories of indigenous Australian star lore, the Southern Cross depicts a possum hiding in a tree. For the Tainui Māori of New Zealand, it represents Te Punga, the anchor of a great sky canoe. For Wairarapa Māori, the Southern Cross is Māhutonga, an opening for storm winds to escape through the Milky Way. The ancient Greeks were aware of the cross/kite-shaped asterism and regarded it as part of the constellation Centaurus. As a navigational guide for Christian European mariners making their first forays into the Southern Hemisphere, the star pattern took on another significance in the 15th and 16th centuries. Crux was formally named in the 17th century.
For such a small constellation, the area is rich in deep-sky objects owing to its position, superimposed on a particularly bright section of the Milky Way — except, that is, for a nearby prominent naked-eye ‘void’ that is commonly referred to as the Coal Sack, or Coalsack Nebula. This is the most prominent dark nebula in the sky, a vast molecular cloud some 600 light-years from Earth dimming the light of the more distant Milky Way stars.
Some 320 light-years from the Sun, Acrux is not only the 14th brightest star in the sky and the most southerly first-magnitude star, it is also a beautiful triple star in small telescopes. Binoculars will reveal the wide fifth-magnitude component, while small telescopes can split the magnitude 1.4 and 2.1 components some 4 arcseconds apart.
Nestled in close to Mimosa is the Jewel Box (NGC 4755), a stunning open star cluster, easily visible in binoculars and small telescopes. It was discovered by Nicolas Louis de Lacaille in 1751-52 and Sir John Herschel’s wonderful description of ‘a casket of variously coloured precious stones’ still rings true today. The Jewel Box is a young cluster about 6400 light-years distant.