Crux
Crux is a constellation of the southern sky that is centred on four bright stars in a cross-shaped asterism commonly known as the Southern Cross. It lies on the southern end of the Milky Way's visible band. The name Crux is Latin for cross. Though it is the smallest of all 88 modern constellations, Crux is among the most easily distinguished, as each of its four main stars has an apparent visual magnitude brighter than +2.8. It has attained a high level of cultural significance in many Southern Hemisphere states and nations.
Blue-white α Crucis is the most southerly member of the constellation, and at magnitude 0.8, the brightest. The three other stars of the cross appear clockwise and in order of lessening magnitude: β Crucis, γ Crucis, and δ Crucis. ε Crucis also lies within the cross asterism. 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 on its eastern border. Nearby to the southeast is a large 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. 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. By 400 AD, the stars in the constellation now called 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. His description, however, may be allegorical, and the similarity to the constellation a coincidence.Image:Southern Celestial Map of Mestre João Faras.gif|thumb|left|Depiction of the Crux by João Faras in May 1500
Venetian navigator Alvise Cadamosto in the 15th century made note of what was probably the Southern Cross on exiting the Gambia River in 1455, calling it the carro dell'ostro. However, Cadamosto's accompanying diagram was inaccurate. Historians generally credit João Faras for being the first European to depict it correctly. Faras sketched and described the constellation in a letter written on the beaches of Brazil on 1 May 1500 to the Portuguese monarch.
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.
Another early modern description clearly describing Crux as a separate constellation is attributed to Andrea Corsali, an Italian navigator who from 1515 to 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.
Emery Molyneux and Petrus Plancius have also been cited as the first uranographers 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. The constellation was later adopted by Jakob Bartsch in 1624 and Augustin Royer in 1679. Royer is sometimes wrongly cited as initially distinguishing Crux.
Characteristics
Crux is bordered by the constellations Centaurus on the east, north, and west, and Musca to the south. Covering 68 square degrees and 0.165% of the night sky, it is the smallest of the 88 constellations. The three-letter abbreviation for the constellation, as adopted by the International Astronomical Union in 1922, is "Cru". The official constellation boundaries, as set by Belgian astronomer Eugène Delporte in 1930, are defined by a polygon of four segments. In the equatorial coordinate system, the right ascension coordinates of these borders lie between and, while the declination coordinates are between −55.68° and −64.70°. Its totality figures at least part of the year south of the 25th parallel north.In tropical regions, Crux can be seen in the sky from April to June. Crux is exactly opposite to Cassiopeia on the celestial sphere, so it cannot appear in the sky with the latter at the same time. In this era, south of Cape Town, Adelaide, and Buenos Aires, Crux is circumpolar, thus always appears in the sky.
Crux is sometimes confused with the nearby False Cross asterism by stargazers. The False Cross consists of stars in Carina and Vela, is larger and dimmer, does not have a fifth star, and lacks the two prominent nearby "Pointer Stars". Between the two is the even larger and dimmer Diamond Cross.
Visibility
Crux is easily visible from the Southern Hemisphere, south of 35th parallel at practically any time of year as circumpolar. It is also visible near the horizon from tropical latitudes of the Northern Hemisphere for a few hours every night during the northern winter and spring. For instance, it is visible from Cancún or any other place at latitude 25° N or less at around 10 pm at the end of April. There are 5 main stars.Due to precession, Crux will move closer to the South Pole in the next few millennia, up to 67° south declination for the middle of the constellation. However, by the year 14,000, Crux will be visible for most parts of Europe and the continental United States. Its visibility will extend to Northern Europe by 18,000, when it will be less than 30° south declination.
Use in navigation
In the Southern Hemisphere, the Southern Cross is frequently used for navigation in much the same way that Polaris is used in the Northern Hemisphere. Projecting a line from γ to α Crucis about times beyond gives a point close to the Southern Celestial Pole which is also, coincidentally, where it intersects a perpendicular line taken southwards from the east–west axis of Alpha Centauri to Beta Centauri, which are stars at an alike declination to Crux and of a similar width as the cross, but higher magnitude. Argentine gauchos are documented as using Crux for night orientation in the Pampas and Patagonia.Alpha and Beta Centauri are of similar declinations and are often referred as the "Southern Pointers" or just "the Pointers", allowing people to easily identify the Southern Cross, the constellation of Crux. Very few bright stars lie between Crux and the pole itself, although the constellation Musca is fairly easily recognised immediately south of Crux.
Bright stars
Down to apparent magnitude +2.5 are 92 stars that shine the brightest as viewed from the Earth. Three of these stars are in Crux, making it the most densely populated as to those stars.Features
Stars
Lacaille gave 13 stars Bayer designations Alpha through Lambda in 1756 and labelled two stars as Alpha and Theta. In 1879, Benjamin Gould added Mu Crucis as he felt the stars were bright enough to warrant their names.Within the constellation's borders, 49 stars are brighter than or equal to apparent magnitude 6.5. The four main stars that form the asterism are Alpha, Beta, Gamma, and Delta Crucis.
- α Crucis or Acrux is a triple star 321 light-years from Earth. A rich blue in colour, with a visual magnitude 0.8 to the unaided eye, it has two close components of a similar magnitude, 1.3 and 1.8, respectively, plus another much wider component of the fifth magnitude. The two close components are resolved in a small amateur telescope and the wide component is readily visible in a pair of binoculars.
- β Crucis or Mimosa is a blue-hued giant star of magnitude 1.3, and lies 353 light-years from Earth. It is a Beta Cephei-type variable star with a variation of less than 0.1 magnitudes.
- γ Crucis or Gacrux is an optical double star. The primary is a red-hued giant star of magnitude 1.6, 88 light-years from Earth, and is one of the closest red giants to Earth. Its secondary component is magnitude 6.5, 264 light-years from Earth.
- δ Crucis is a magnitude 2.8, blue-white hued star about 345 light-years from Earth. Like Mimosa, it is a Beta Cepheid variable.
- ε Crucis is an orange-hued giant star of magnitude 3.6, 228 light-years from Earth.
- Iota Crucis is a visual double star 125 light-years from Earth. The primary is an orange-hued giant of magnitude 4.6 and the secondary at magnitude 9.5.
- Mu Crucis or Mu1,2 Crucis is a wide, double star where the components are about 370 light-years from Earth. Equally blue-white in colour, the components are magnitude 4.0 and 5.1, and are easily divisible in small amateur telescopes or large binoculars.
Scorpius–Centaurus association