Crux

This article is about the constellation. For other uses, see crux (disambiguation).

"Southern Cross" redirects here. For other uses, see Southern Cross (disambiguation).

Crux
click for larger image
Abbreviation	Cru
Genitive	Crucis
Symbology	Southern Cross
Right ascension	12.5 h
Declination	−60°
Area	68 sq. deg. Ranked 88th
Number of stars (Bayer-Flamsteed)	16
Number of bright stars (magnitude < 3)	4
Number of nearby stars (Distance < 100 ly)	1
Brightest star	Acrux (α Cru) (App. magnitude 0.87)
Nearest Star	Gacrux (γ Cru) (Distance: 88 ly)
Meteor showers	Crucids
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 (IPA: /ˈkrʊks/, Latin: cross), commonly known as the Southern Cross (in contrast to the Northern Cross), is the smallest of the 88 modern constellations, but nevertheless one of the most distinctive. It is surrounded on three sides by the constellation Centaurus while to the south lies the Fly (Musca). Crux was originally thought of by ancient Greeks as part of Centaurus, but was defined as a separate asterism in the 16^th century after Amerigo Vespucci's expedition to South America in 1501. Vespucci mapped the two stars, Alpha Centauri and Beta Centauri as well as the stars of the Crux. Although these stars were known to the ancient Greeks, gradual precession of the equinoxes had lowered them below the European skyline so that they were forgotten. For example at the latitude of Britain in 5000 BC the constellation still was completely visible at springtime midnight.^[1]

Notable features

With the lack of a significant pole star in the southern sky (Sigma Octantis is closest to the pole, but is so faint as to be useless for the purpose), two of the stars of Crux (Alpha and Gamma, Acrux and Gacrux respectively) are commonly used to mark south. Following the line defined by the two stars for approximately 4.5 times the distance between them leads to a point close to the Southern Celestial Pole.

Alternatively, if a line is constructed perpendicularly between Alpha Centauri and Beta Centauri, the point where the above line and this line intersect marks the Southern Celestial Pole. The two stars are often referred to as the "Pointer Stars" or "White Pointers", allowing people to easily find the top of Crux.

Contrary to some people's belief, it is not opposite Ursa Major. In fact, in tropical regions both Crux (low in the South) and Ursa Major (low in the North) can be in the sky from April to June. It is exactly opposite Cassiopeia on the celestial sphere, and therefore cannot be in the sky with the latter at the same time. For locations south of 34° southern latitude Crux is always completely in the sky.

If you use the Southern Cross to find south, be careful to distinguish it from the False Cross. The Southern Cross is somewhat kite-shaped, and it has a fifth star (ε Crucis). The False Cross is diamond-shaped and does not have a fifth star like ε Crucis.

Notable deep sky objects

The Coalsack Nebula is the most prominent dark nebula in the skies, well visible to the naked eye as big dark patch in the southern Milky Way.

Another deep sky object within Crux is the Open Cluster NGC 4755, better known as the Jewel Box or Kappa Crucis Cluster, that 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 spread across an area of about 20 light-years square.

History

The Southern Cross appearing on a number of flags

Due to precession of the equinox the stars comprising Crux were visible from the Mediterranean area in antiquity, so their stars had to be known by Greek astronomers. However, it was not regarded as a constellation of its own, but rather as part of Centaurus.

The invention of Crux as a separate constellation is generally attributed to the French astronomer Augustin Royer in 1679. It was known in that shape well before that, however.

The five brightest stars of Crux (α, β, γ, δ, and ε Crucis) appear on the flags of Australia, Brazil, New Zealand (epsilon omitted), Papua New Guinea, and Samoa, and also the Australian States and Territories of Victoria, the Australian Capital Territory, the Northern Territory, as well as the flag of Magallanes Region of Chile, and several Argentine provincial flags and emblems. The flag of the Mercosur trading zone displays the four brightest stars (epsilon omitted). Crux also appears on the Brazilian coat of arms. A stylized version of Crux appears on the Eureka Flag. The constellation was also used on the dark blue, shield-like patch worn by personnel of the U.S. Army's Americal Division, which was organized in the Southern Hemisplere, on the island of New Caledonia, and also the blue diamond of the U.S. 1st Marine Division, which fought on the Southern Hemisphere islands of Guadalcanal and New Britain.

A stone image of the constellation has also been left at the archaeological site of Machu Picchu, Peru.

Other names for Crux

• In ancient Hindu astrology, the modern Crux is referred to as "trishanku".
• The Māori name for Crux is "Te Punga" - "the anchor". It is thought of as anchor of Tama-rereti's waka (the Milky Way), where the Pointers are its rope.
• In Tonga it is known as Toloa — duck; it is a duck flying over, heading south, and one of his wings (δ) is wounded because Ongo tangata — 2 men — α and Β Centauri threw a stone at it. The Coalsack is known as Humu — triggerfish, because of its shape.^[2]
• Among Tuaregs, the 4 most visible stars of Crux are considered iggaren, i.e. four Maerua crassifolia trees.

Stars

Crux is clearly visible above the aurora australis in this photograph taken from Dunedin, New Zealand. The red giant Gacrux is clearly a different colour to the other three main stars, which are blue-white

Stars with proper names:

• (α^{1,2 Cru) – 0.87 (double 1.3, 1.73) Acrux}
• (β Cru) 1.25 Becrux or Mimosa
• (γ Cru) – double 1.59, 6.42 Gacrux
• (δ Cru) 2.79 Decrux [Delcrux]
• (ε Cru) 3.59 Juxta [Juxta Crucem]

Stars with Bayer designations:

ζ Cru 4.06; η Cru 4.14; θ¹ Cru 4.32; θ² Cru 4.72; ι Cru 4.69; κ Cru 5.89; λ Cru 4.62; μ¹ Cru 4.03; μ² Cru 5.08

Stars with Flamsteed designations:

35 Cru; 39 Cru

Other notable stars:

• HD 108147 7.00 – has a planet

External links, References

1. ^ this image
2. ^ Kik Velt; Stars over Tonga

• Letter of Andrea Corsali 1516-1989: with additional material ("the first description and illustration of the Southern Cross, with speculations about Australia ...") digitised by the National Library of Australia.
• The Deep Photographic Guide to the Constellations: Crux.
• The Cambridge Guide to the Constellations, Michael E. Bakich, Cambridge University Press, 1995, pg. 85.
• Universe: The Definitive Visual Dictionary, Robert Dinwiddie, DK Adult Publishing, (2005), pg. 396.
• Southern Cross Starry Night Photography.

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