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Constellations: Ara 'the Altar'

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The shield of the Science, Mathematics and Engineering faculty of the h2g2 University.Constellations: Overview | Andromeda | Antlia | Apus | Aquarius | Aquila | Ara | Aries | Auriga | Boötes | Caelum
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Telescopium | Triangulum | Triangulum Australe | Tucana | Ursa Major | Ursa Minor | Vela | Virgo | Volans | Vulpecula
Name:Ara (Latin: 'altar')
Genitive:Arae
Short form:Ara
Area:237 sq deg
Co-ordinates:Right Ascension 17h, Declination −53°
Origin:Ancient

Ara 'the Altar' is a Southern Hemisphere constellation found to the south of Scorpius 'the Scorpion' and north of Apus 'the Bird of Paradise'. An arm of our own Milky Way runs through its centre. Ara's shape looks roughly like a lop-sided letter H - a convenient description, as it appears much the same whether it is being regarded by observers from the Northern Hemisphere or upside down from the Southern.

Mythology and History

The origins of Ara are shrouded in the mists of antiquity. To the early Euphratean astronomers, Ara signified an altar or mound probably associated with the temple mound of the Tower of Babel. However, there is some evidence that their Ara was probably the constellation that we now recognise as Libra 'the Scales'. The Greeks also recognised Scorpius, which sits between the two, as a double figure: that of the scorpion's body with arched tail and, separately, the claws to its south, adjacent to what is now Ara. The lack of Arabic names for the stars of Ara suggest that the constellation was unformed before the introduction of Greek astronomy and that somewhere in the various Greek translations, the Euphratean altar moved first to the claws of Scorpius and later to its present position.

Ara is one of the 48 ancient constellations listed by Ptolemy in his Almagest. In classical times Ara was associated with the nearby constellations of Centaurus 'the Centaur' and Lupus 'the Wolf'. In fact, it abutted with Lupus until French astronomer Abbé Nicholas de la Caille, in the 18th Century, for reasons best known to himself, unceremoniously inserted his new constellation of Norma 'the Carpenter's Square' between them. From Greek mythology it became the altar on which the centaur Chiron was to sacrifice Lupus as an offering to the gods.

To the Romans it became Ara Centauri, the altar of Centaurus and also Thymele, the altar of Dionysus. Later the Roman poet Marcus Manilius associated it with the clash of the Roman gods with the Titans, and Ara became the altar where Zeus and his fellow gods took their vows.

An alternative interpretation commonly accepted down to the 18th Century, is that of a censer or a turibulum. This is illustrated in later classical star catalogues as a three-legged dish on which perfumes or small offerings were burned in the temple at the funeral ceremonies of the dead. These illustrations show this constellation as a censer, with the smoke from the offering rising to mingle with the stars of the Milky Way.

Principal Stars

The principal stars of Ara are not particularly bright. Here is one of those anomalies in star classification where the constellation's brightest star is beta Arae of 2.81 magnitude which is located at the centre bar of the H configuration. It is closely accompanied by gamma Arae at 3.28 magnitude a double star. Alpha Arae at magnitude 2.90 is only just less bright than Beta and is a spectroscopic binary star-system.

Epsilon1 and epsilon2 are a line of sight pairing of two similar magnitude stars. Epsilon1 is an orange giant, much further distant than epsilon2 which is itself a triple star system. Mu Arae is a yellow-orange star at a distance of 50 light-years. It is fairly unremarkable except it has been found to have at least four planets in orbit around it.

Star Table

StarDesignationBrightness (m)Distance (light-years)Remarks
β Arabeta Arae+2.81604Orange/Single
α Araalpha Arae+2.90243Double
ζ Arazeta Arae+3.12574Orange giant
γ Aragamma Arae+3.281144Binary
δ Aradelta Arae+3.59187Binary
η Araeta Arae+3.77313Binary
ε1 Araepsilon1 Arae+4.00305Single
ε2 Araepsilon2 Arae+5.2586Multiple
μ Aramu Arae+5.1250Multiple planetary system (4)

Star Clusters and Nebulae

Because of its proximity to the Milky Way, Ara contains a number of star clusters. The most prominent are NGC6397, a bright globular cluster, close to beta Arae, which is visible to the naked eye and is the closest cluster of this type to our solar system. It is 8,200 light years' distant and is best seen through either binoculars or a small telescope, and appears like a misty ball of light. On the other side of the H asterism, NGC6193 and NGC6208 are both open star clusters and over 30 closely related stars can be resolved with binoculars in each.

The Stingray Nebula (Hen-1357) is the remains of a dying star illuminated by the core white dwarf and resembles the classic manta ray in outline. It is the youngest observed planetary nebula, first seen around 1975 but is faint enough to require specialist equipment to see effectively.

Star Clusters and Nebulae Table

CatalogueTypeBrightness (m)Distance (light-years)Remarks
NGC6397Globular cluster+6.57,200 lyClosest globular cluster
NGC6193Open cluster+6.04,300 ly30+ stars
NGC6208Open cluster+8.0939 ly30+ stars
HEN-1357Planetary nebula+10.7518,000 lyStingray nebula

Extrasolar Planets in Ara

Several stars in Ara are known to be orbited by planetary bodies. Mu Arae has four confirmed planets. The innermost planet has a mass approximately the same as our planet Uranus and was the first so-called 'Hot Jupiter' to be found in 2004. It orbits closer to its star than Mercury orbits our own Sun and completes each orbit in nine Earth days. The other three are all gas giants on wide, circular orbits.

Gliese 674 is a red dwarf star at a distance of 15 light years. It has one confirmed planet, Gliese 674 b, which is a gas giant orbiting very close to its parent star every four and a half days.

The star HD 154857 is similar to our own sun and has one confirmed and another possible planet in an eccentric orbit around it. The confirmed planet, HD 154857 b, is another gas giant with an orbital period of 410 days. The second planet, as yet unconfirmed is thought to be much further out with an orbital period of around four and a half years.

HD 154672 b was discovered orbiting a yellow sub-giant star in September 2008. At around five times the mass of Jupiter, it is classed as a 'superjovian' and with a distance of only 0.6AU from its star, it's likely very hot, even though its year is 164 Earth days.

HD 156411 b was announced in the bumper crop of 19 October, 2009. It's a gas giant completing an orbit around its star in 842 days.

GJ 676A b was also announced in the bumper crop with HD 156411 b. GJ 676A b is a superjovian four times Jupiter's mass, orbiting a red giant binary star, GJ 676A, in about a thousand days.

HD 152079 b is a superjovian world orbiting a yellow dwarf star just fractionally more massive than our own Sun. It takes 2,097 days to complete one orbit.


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