30 March 2017 by Brian Jones
Although Crux (the Cross) is the smallest in area of all the constellations, it is also one of the most famous. Often referred to as the Southern Cross, this is something of an iconic group. Crux is depicted on the flags of countries like Australia and New Zealand and images of the constellation are found on many postage stamps as well as on coins, banknotes and innumerable paintings.
The stars forming this tiny group were noted by the Greek astronomer Ptolemy on a list of 48 constellations he drew up during the 2nd century. However, at the time Ptolemy was compiling his list, the stars we now recognise as forming Crux were regarded as part of the neighbouring constellation, Centaurus (the Centaur). Crux was only described as a separate group over a millennium later, this by the Italian navigator Andreas Corsali in 1516, although in 1504 Corsali’s countryman and fellow navigator Amerigo Vespucci had noted a number of important southern stars, including a particular group of six that were probably the four main stars in Crux, together with the nearby Alpha and Beta Centauri.
The first appearances of Crux as an individual constellation were on celestial globes made by the English globe maker Emery Molyneux in 1592 and by the Dutch cartographer Petrus Plancius in 1598. From then on, the constellation gained its own identity as a separate group, appearing on other celestial globes and in various atlases throughout the 17th century. Modern star charts depict it surrounded on three sides by Centaurus.
Locating the Cross
Because Crux lies in the southern sky, it can never be seen by observers located at mid-northern latitudes and only really rises from locations south of Central America and southern India. Visible in its entirety from latitudes south of 25º N, it is completely hidden from view to those situated north of latitude 35º N.
From places on or south of the equator, the constellation is easily spotted, and is visible from locations in the southern hemisphere at practically any time of year. From most of the United States, the Southern Cross never rises above the horizon, so ramblers, walkers and backyard astronomers in America need to be at least as far south as Hawaii, or the southern parts of Florida or Texas (about 26° N) to be able to catch a glimpse of the constellation.
As far as those located in the northern hemisphere’s tropical and subtropical regions are concerned, the month of May is a fairly good time for finding Crux in the mid-evening sky. Observers in the southern hemisphere, however, need only glance at the heavens, where they will see the distinctive shape of Crux hanging well up in the sky. Although its distinctive shape should be readily identifiable, it can be tracked down by following an imaginary line from the bright star Arcturus in Boötes, southwards through Spica in Virgo, and on through the main part of Centaurus. This chart depicts the evening sky as it appears during evenings in late March and throughout April, and shows the main stars needed to guide you to the constellation Crux.
The pattern of stars forming Crux should not be confused with the four stars which straddle the borders of the nearby constellations Carina and Vela. Known collectively as the False Cross, and shown on this chart, these are found a little way to the west of Crux and form a very similar, albeit slightly larger, pattern.
The tiny but noticeable constellation of Crux has long been a source of inspiration to those journeying south. Typical of the sentiments aroused by the sight of the Cross are those expressed in 1817 during a scientific expedition to Brazil by the German biologist Johann Baptist Ritter von Spix and his companion, the German explorer and botanist Carl Friedrich Philipp von Martius, who recorded that ‘…on the 15th of June, in latitude 14° S, we beheld for the first time that glorious constellation of the southern heavens, the Cross, which is to navigators a token of peace… We had long wished for this constellation as a guide to the other hemisphere; we therefore felt inexpressible pleasure when we perceived it in the resplendent firmament.’
The stars of the Cross
Although Crux is the smallest of the constellations, it plays host to numerous objects of interest to the backyard astronomer, all of which are set against the Milky Way, which forms a wonderful backdrop. Three of the four main stars making up the Cross are obviously blue or blue-white in colour while the other, Gamma, has a definite orange/red tint.
The brightest star in Crux is Acrux, a name which seems to have been first coined by the American astronomy writer and cartographer Elijah Hinsdale Burritt in a star atlas compiled by him in the 1830s (click for more about star names). The fact that the stars of Crux were once part of the constellation Centaurus is alluded to by the 16th-century Arabian astronomer Al Tizini, who defined the position of this star as being near the ankle of the right hind foot of the Centaur. Closer examination of Acrux with even a small telescope will show that it is in fact a double star with components of 1.4 and 1.9, shining from a distance of around 320 light years (click for more about magnitudes).
Beta Crucis is a magnitude 1.25 blue star whose light has taken around 280 years to reach us, and is somewhat closer than the blue giant star Delta Crucis which shines at magnitude 2.79 from a distance of just under 350 light years. This means that the light we see coming from Delta set off towards us around the time of the Great Fire of London!
Binoculars will reveal both components of the orange-red double star Gamma Crucis (click for more about star colours). What you will see is a magnitude 1.6 star apparently accompanied by a fainter magnitude 6.7 companion. However, this is merely a line of sight effect and these two stars are not physically related. Gamma itself shines from a distance of 89 light years, roughly a quarter of the distance of the fainter star.
The Jewel Box
Resolvable into individual stars even through binoculars, the splendid open star cluster NGC 4755 lies at a distance of around 6,500 light years. Also known as the Jewel Box, it contains over 100 individual stars and can be spotted with the naked eye as a 4th-magnitude fuzzy star-like object just to the south-east of Beta Crucis, both objects lying within the same field of view of binoculars.
When viewed through binoculars or a small telescope, the Jewel Box can be seen to contain a number of very bright bluish-white stars. However, there is one super giant star within the cluster that has a distinctly red tint and, when viewed through telescopes, lends a beautiful splash of colour to this stunning object. This fine cluster was discovered by the French astronomer Nicolas Louis de Lacaille in around 1752, although it was the English astronomer John Herschel who named it the Jewel Box and said that when viewed through a telescope it was an ‘extremely brilliant and beautiful object’. He also likened it to ‘a superb piece of fancy jewellery’.
The Coal Sack
The Coal Sack dark nebula is visible to the naked eye and, apart from the presence of a few faint foreground stars, takes on the appearance of a dark patch silhouetted against the brighter background of the Milky Way. The Jewel Box cluster appears to be situated close to the northern edge of the Coal Sack, although this association is no more than a line of sight effect, the Coal Sack itself being located only around 500 light years away. The Coal Sack is the most prominent object of its type in the entire sky and, as its name suggests, takes the form of a huge, dark cloud of absorbing dust which blots out the light from the stars beyond – the overall effect being that of a huge and mysterious hole in the surrounding star fields. Happy stargazing!
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