Crux

Crux
Constellation
Crux
AbbreviationCru
GenitiveCrucis
Pronunciations/, genitive s/
SymbolismSouthern Cross
Right ascension 12.5h
Declination−60°
QuadrantSQ3
Area68 sq. deg. (88th)
Main stars4
Bayer/Flamsteed
stars
19
Stars with planets2
Stars brighter than 3.00m5
Stars within 10.00 pc (32.62 ly)0
Brightest starAcrux (α Cru) (0.87m)
Messier objects0
Meteor showersCrucids
Bordering
constellations
Centaurus
Musca
Visible at latitudes between +20° and −90°.
Best visible at 21:00 (9 p.m.) during the month of May.

Crux s/ is a constellation centred on four stars in the southern sky in a bright portion of the Milky Way. It is among the most easily distinguished constellations as its hallmark (asterism) stars each have an apparent visual magnitude brighter than +2.8, even though it is the smallest of all 88 modern constellations. Its name is Latin for cross, and it is dominated by a cross-shaped or kite-like asterism that is commonly known as the Southern Cross.

Predominating the constellation is the first-magnitude blue-white star of Acrux (Alpha Crucis), its brightest and most southerly member. There follow four less dominant stars which appear clockwise and in order of lessening magnitude: Mimosa (Beta Crucis), Gacrux (Gamma Crucis), Imai (Delta Crucis) and Ginan (Epsilon Crucis). Many of these brighter stars are members of the Scorpius–Centaurus Association, a large but loose group of hot blue-white stars that appear to share common origins and motion across the southern Milky Way.

Crux contains four Cepheid variables, each visible to the naked eye under optimum conditions. Crux also contains the bright and colourful open cluster known as the Jewel Box (NGC 4755) on its western border. To the southeast figures a large, relatively near dark nebula spanning 7° by 5° known as the Coalsack Nebula, portions of which are mapped in the neighbouring constellations of Centaurus and Musca.

History

The bright stars in Crux were known to the Ancient Greeks, where Ptolemy regarded them as part of the constellation Centaurus.[1][2] They were entirely visible as far north as Britain in the fourth millennium BC. However, the precession of the equinoxes gradually lowered the stars below the European horizon, and they were eventually forgotten by the inhabitants of northern latitudes.[3] By 400 CE, the stars in the constellation we now call Crux never rose above the horizon throughout most of Europe. Dante may have known about the constellation in the 14th century, as he describes an asterism of four bright stars in the southern sky in his Divine Comedy.[4] Others argue that Dante's description was allegorical, and that he almost certainly did not know about the constellation.[5]

Depiction of the Crux by João Faras in May, 1500

The 15th century Venetian navigator Alvise Cadamosto made note of what was probably the Southern Cross on exiting the Gambia River in 1455, calling it the carro dell'ostro ("southern chariot"). However, Cadamosto's accompanying diagram was inaccurate.[6][7] Historians generally credit João Faras[a] for being the first European to depict it correctly. Faras sketched and described the constellation (calling it "Las Guardas") in a letter written on the beaches of Brazil on 1 May 1500 to the Portuguese monarch.[8][9]

Explorer Amerigo Vespucci seems to have observed not only the Southern Cross but also the neighboring Coalsack Nebula on his second voyage in 1501–1502.[10]

Another early modern description clearly describing Crux as a separate constellation is attributed to Andreas Corsali, an Italian navigator who from 1515–1517 sailed to China and the East Indies in an expedition sponsored by King Manuel I. In 1516, Corsali wrote a letter to the monarch describing his observations of the southern sky, which included a rather crude map of the stars around the south celestial pole including the Southern Cross and the two Magellanic Clouds seen in an external orientation, as on a globe.[11][12]

Emery Molyneux and Petrus Plancius have also been cited as the first uranographers (sky mappers) to distinguish Crux as a separate constellation; their representations date from 1592, the former depicting it on his celestial globe and the latter in one of the small celestial maps on his large wall map. Both authors, however, depended on unreliable sources and placed Crux in the wrong position. Crux was first shown in its correct position on the celestial globes of Petrus Plancius and Jodocus Hondius in 1598 and 1600. Its stars were first catalogued separately from Centaurus by Frederick de Houtman in 1603.[13] The constellation was later adopted by Jakob Bartsch in 1624 and Augustin Royer in 1679. Royer is sometimes wrongly cited as initially distinguishing Crux.[2]