Ara (Latin: "The Altar") is a southern constellation situated between Scorpius and Triangulum Australe. Ara (Greek: Βωμός) was one of the 48 Greek constellations described by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations defined by the International Astronomical Union.
The orange supergiant Beta Arae is the brightest star in the constellation, with an apparent magnitude of 2.85—marginally brighter than the blue-white Alpha Arae. Seven star systems are known to host planets. The sunlike star Mu Arae hosts four known planets, while Gliese 676 is a binary red dwarf system with four known planets. The Milky Way crosses the northwestern part of Ara.
|Pronunciation||/ˈɛərə/, genitive /ˈɛəriː/[a]|
|Right ascension||16h 34m 16.9497s– 18h 10m 41.3407s|
|Area||237 sq. deg. (63rd)|
|Stars with planets||7|
|Stars brighter than 3.00m||2|
|Stars within 10.00 pc (32.62 ly)||3|
|Brightest star||β Ara (2.84m)|
|Visible at latitudes between +25° and −90°.|
Best visible at 21:00 (9 p.m.) during the month of July.
In ancient Greek mythology, Ara was identified as the altar where the gods first made offerings and formed an alliance before defeating the Titans. One of the southernmost constellations depicted by Ptolemy, it had been recorded by Aratus in 270 BC as lying close to the horizon, and the Almagest portrays stars as far south as Gamma Arae. Professor of astronomy Bradley Schaefer has proposed that ancient observers must have been able to see as far south as Zeta Arae to define a pattern that looked like an altar.
In illustrations, Ara is usually depicted as an altar with its smoke 'rising' southward. However, depictions of Ara often vary in their details. In the early days of printing, a 1482 woodcut of Gaius Julius Hyginus's classic Poeticon Astronomicon depicts the altar as surrounded by demons. Johann Bayer in 1603 depicted Ara as an altar with burning incense; the flames rise southward as in most atlases. Hyginus also depicted Ara as an altar with burning incense, though his Ara featured devils on either side of the flames. However, Willem Blaeu, a Dutch uranographer active in the 16th and 17th centuries, drew Ara as an altar designed for sacrifice, with a burning animal offering. Unlike most depictions, the smoke from Blaeu's Ara rises northward, represented by Alpha Arae.
In Chinese astronomy, the stars of the constellation Ara lie within The Azure Dragon of the East (東方青龍, Dōng Fāng Qīng Lóng). Five stars of Ara formed Guī (龜), a tortoise, while another three formed Chǔ (杵), a pestle.
Covering 237.1 square degrees and hence 0.575% of the sky, Ara ranks 63rd of the 88 modern constellations by area. Its position in the Southern Celestial Hemisphere means that the whole constellation is visible to observers south of 22°N.[b] Scorpius runs along the length of its northern border, while Norma and Triangulum Australe border it to the west, Apus to the south, and Pavo and Telescopium to the east respectively. The three-letter abbreviation for the constellation, as adopted by the International Astronomical Union, is Ara. The official constellation boundaries, as set by Belgian astronomer Eugène Delporte in 1930,[c] are defined by a polygon of twelve segments. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 16h 36.1m and 18h 10.4m, while the declination coordinates are between −45.49° and −67.69°.
Bayer gave eight stars Bayer designations, labelling them Alpha through to Theta, though he had never seen the constellation directly as it never rises above the horizon in Germany. After charting the southern constellations, Lacaille recharted the stars of Ara from Alpha though to Sigma, including three pairs of stars next to each other as Epsilon, Kappa and Nu.
Ara contains part of the Milky Way to the south of Scorpius and thus has rich star fields. Within the constellation's borders, there are 71 stars brighter than or equal to apparent magnitude 6.5.[d]
Just shading Alpha Arae, Beta Arae is the brightest star in the constellation. It is an orange-hued star of spectral type K3Ib-IIa that has been classified as a supergiant or bright giant, that is around 650 light-years from Earth. It is around 8.21 times as massive and 5,636 times as luminous as the Sun. At apparent magnitude 2.85, this difference in brightness between the two is undetectable by the unaided eye. Close to Beta Arae is Gamma Arae, a blue-hued supergiant of spectral type B1Ib. Of apparent magnitude 3.3, it is 1110 ± 60 light-years from Earth. It has been estimated to be between 12.5 and 25 times as massive as the Sun, and have around 120,000 times its luminosity.
Alpha Arae is a blue-white main sequence star of magnitude 2.95, that is 270 ± 20 light-years from Earth. This star is around 9.6 times as massive as the Sun, and has an average of 4.5 times its radius. It is 5,800 times as luminous as the Sun, its energy emitted from its outer envelope at an effective temperature of 18,044 K. A Be star, Alpha Arae is surrounded by a dense equatorial disk of material in Keplerian (rather than uniform) rotation. The star is losing mass by a polar stellar wind with a terminal velocity of approximately 1,000 km/s.
At magnitude 3.13 is Zeta Arae, an orange giant of spectral type K3III that is located 490 ± 10 light-years from Earth. Around 7–8 times as massive as the Sun, it has swollen to a diameter around 114 times that of the Sun and is 3800 times as luminous.
Eta Arae is an orange giant of apparent magnitude 3.76.
Exoplanets have been discovered in seven star systems in the constellation. Mu Arae (Cervantes) is a sunlike star that hosts four planets. HD 152079 is a sunlike star with a planet. HD 154672 is an ageing sunlike star with a Hot Jupiter. HD 154857 is a sunlike star with one confirmed and one suspected planet. HD 156411 is a star hotter and larger than the sun with a gas giant planet in orbit. Gliese 674 is a nearby red dwarf star with a planet. Gliese 676 is a binary star system composed of two red dwarves with four planets.
The northwest corner of Ara is crossed by the galactic plane of the Milky Way and contains several open clusters (notably NGC 6200) and diffuse nebulae (including the bright cluster/nebula pair NGC 6188 and NGC 6193). The brightest of the globular clusters, sixth magnitude NGC 6397, lies at a distance of just 6,500 light-years (6.1×1016 km), making it one of the closest globular clusters to the Solar System.
Ara also contains Westerlund 1, a super star cluster that contains the red hypergiant Westerlund 1-26, which is possibly the largest star known with an estimate varying between 1,530 R☉ and 2,550 R☉.
ESO 280-SC06 (also known as ESO 280-6) is a globular cluster 69,800 light years from the Sun in the constellation of Ara. The cluster received this designation when it was catalogued by the European Southern Observatory (ESO) in Chile. Imagery and Hertzsprung-Russell diagrams show a sparsely populated globular cluster with a compact core.HD 152079
HD 152079 is a star in the constellation Ara and has one planet.HD 154857
HD 154857 is a yellow main-sequence star approximately 209 light years away in the constellation of Ara.HD 165493
HD 165493 is double star in the southern constellation of Ara. As of 2011, the two components have an angular separation of 4″ along a position angle of 257°.NGC 6167
NGC 6167 is an open cluster in the constellation of Norma, viewed from Earth it has an apparent magnitude of 6.7.NGC 6193
NGC 6193 is open cluster containing 27 stars in the constellation Ara, visible to the unaided eye. NGC 6193 lies at the center of the Ara OB1 association, which extends over a square degree. The cluster is associated with (and provides the energizing radiation for) neighboring regions of the nebulosity NGC 6188.NGC 6200
NGC 6200 is an open cluster in the constellation Ara, lying close to the galactic equator. It contains one β Cephei variable.NGC 6204
NGC 6204 is an open cluster in the constellation Ara, lying close to the galactic equator. It is 3,540 ly (1,085 pc) distant from Earth. The cluster was discovered on 13 May 1826 by British astronomer James Dunlop.NGC 6208
NGC 6208 is an open cluster in the southern constellation of Ara. With an age of 1.17 Gigayears, it is one of the oldest known open clusters.NGC 6221
NGC 6221 (also known as PGC 59175) is a barred spiral galaxy located in the constellation Ara. It is designated as SB(s)bc in the galaxy morphological classification scheme and was discovered by British astronomer John Herschel on 3 May 1835. NGC 6221 is located at about 69 million light years from earth.NGC 6250
NGC 6250 is a young open cluster of stars in the southern constellation of Ara.NGC 6263
NGC 6263 is an elliptical galaxy in the constellation Hercules. It was discovered by Albert Marth on June 28, 1864.NGC 6300
NGC 6300 is a barred Seyfert spiral galaxy located in the constellation Ara. It is designated as SB(rs)b in the galaxy morphological classification scheme and was discovered by the Scottish astronomer James Dunlop on 30 June 1826. NGC 6300 is located at about 51 million light years away from earth. It is suspected that a massive black hole (300,000 times the mass of Sun) may be at its center, pulling all the nearby objects into it. In turn, it emits large amounts of X-rays.NGC 6326
NGC 6326 is a complex and irregularly structured planetary nebula located in the constellation Ara. It is designated as PN in the galaxy morphological classification scheme and was discovered by the Scottish astronomer James Dunlop on 26 August 1826. NGC 6326 is the result of the ejected material from the central binary star, which is nearing the end of its life. The blue and red color is due to the amount of radiation the star releases, thus causing the gasses to glow. NGC 6326 is located at about 11,000 light years away from Earth.NGC 6328
NGC 6328 is an intermediate spiral galaxy located in the constellation Ara. It is designated as SAB(s)ab in the galaxy morphological classification scheme and was discovered by the British astronomer John Herschel on 2 May 1835. NGC 6328 is located at about 199 million light years away from earth.NGC 6352
NGC 6352 (also known as Caldwell 81) is a globular cluster in the southern constellation Ara. A telescope with a 15 cm (5.9 in) aperture is required to resolve the stars within this loose cluster.S Arae
S Arae (S Ara) is an A-type star in the constellation of Ara. It has an apparent visual magnitude which varies between 9.92 and 11.24.Water Lily Nebula
The Water Lily Nebula, in the southern constellation of Ara, is a pre-planetary nebula also known as IRAS 16594-4656, in the process of developing to a planetary nebula. It is one of the pre-planetary nebulae containing polycyclic aromatic hydrocarbons, organic hydrocarbons otherwise constituting the basis for life.XTE J1650-500
XTE J1650-500 is a binary system containing a stellar-mass black hole candidate and 2000–2001 transient binary X-ray source located in the constellation Ara.
In 2008, it was claimed that this black hole had a mass of 3.8±0.5 solar masses, which would have been the smallest found for any black hole; smaller than GRO 1655-40, the then known smallest of 6.3 MSun. However, this claim was subsequently retracted; the more likely mass is 5–10 solar masses.
The binary period of the black hole and its companion is 0.32 days.
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