The orange supergiantBeta Arae, to us its brightest star measured with near-constantapparent magnitude of 2.85, is marginally brighter than blue-whiteAlpha Arae. Seven star systems are known to host planets. SunlikeMu Arae hosts four known planets.Gliese 676 is a (gravity-paired) binaryred dwarf system with four known planets.
In ancientGreek mythology, Ara was identified as thealtar where the gods first made offerings and formed an alliance before defeating theTitans.[1] One of the southernmost constellations depicted byPtolemy,[3] it had been recorded byAratus in 270 BC as lying close to the horizon, and theAlmagest portrays stars as far south asGamma Arae. ProfessorBradley Schaefer proposes such Ancients must have been able to see as far south asZeta Arae, for a pattern that looked like an altar.[4]
Johann Elert Bode's illustration of Ara, from hisUranographia (1801)
In illustrations, Ara is usually depicted as compact classicalaltar with its smoke 'rising' southward.[5] However, depictions often vary. In the early days of printing, a 1482woodcut ofGaius Julius Hyginus's classicPoeticon Astronomicon depicts the altar as surrounded by demons.[6]Johann Bayer in 1603 depicted Ara as an altar with burning incense. Indeed, frankincense burners were common throughout the Levant especially in the Yemen, where they are known as Mabkhara. This required live coals or burning embers called Jamra', in order to burn the incense.Willem Blaeu, a Dutchuranographer of the 16th and 17th centuries, drew Ara as an altar for sacrifices, with a burning animal offering unusually whose smoke rises northward, represented byAlpha Arae.
The Castle of Knowledge byRobert Record of 1556 lists the constellation stating that "Under the Scorpions tayle, standeth the Altar.";[7] a decade later a translation of a fairly recent mainly astrological work byMarcellus Palingenius of 1565, byBarnabe Googe states "Here mayst thou both the Altar, and the myghty Cup beholde."[8]
Covering 237.1 square degrees and hence 0.575% of the sky, Ara ranks 63rd of the88 modern constellations by area.[11] Its position in theSouthern Celestial Hemisphere means that the wholeconstellation is visible to observers south of22°N.[11][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".[12] The official constellation boundaries, as set by Belgian astronomerEugène Delporte in 1930,[c] are defined by a polygon of twelve segments. In theequatorial coordinate system, theright ascension coordinates of these borders lie between16h 36.1m and18h 10.4m, while thedeclination coordinates are between −45.49° and −67.69°.[2]
Bayer gave eight starsBayer 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, French astronomerNicolas-Louis de Lacaille recharted the stars of Ara from Alpha through to Sigma, including three pairs of stars next to each other as Epsilon, Kappa and Nu.[14]
Ara contains part of the Milky Way to the south of Scorpius and thus has rich star fields.[1] Within the constellation's borders, there are 71 stars brighter than or equal toapparent magnitude 6.5.[d][11]
Beta Arae, apparent magnitude 2.85, is the brightest star in the constellation, about 0.1 mag brighter thanAlpha Arae although the difference in brightness between the two is undetectable by the unaided eye.[16] Beta is an orange-hued star ofspectral type K3Ib-IIa that has been classified as asupergiant orbright giant,[17] and lies around 650 light-years from Earth.[18] It is over 8 times as massive and 5,636 times as luminous as the Sun.[19] Close to Beta Arae isGamma Arae, a blue-hued supergiant of spectral type B1Ib. Of apparent magnitude 3.3, it is 1110 ± 60 light-years from Earth.[18] It has been estimated to be between 12.5 and 25 times as massive as the Sun,[20][21] and have around 120,000 times its luminosity.[21]
Alpha Arae is ablue-white main sequence star of magnitude 2.95, that is 270 ± 20 light-years from Earth.[18] This star is around 9.6 times as massive as the Sun,[22] and has an average of 4.5 times its radius.[23] It is 5,800 times as luminous as the Sun,[22] its energy emitted from itsouter envelope at aneffective temperature of 18,044 K.[23] ABe 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 polarstellar wind with a terminal velocity of approximately 1,000 km/s.[22][24]
The third brightest star in Ara at magnitude 3.13 isZeta Arae,[25] an orange giant of spectral type K3III that is located 490 ± 10 light-years from Earth.[18] 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.[26] Were it not dimmer by intervening interstellar dust, it would be significantly brighter at magnitude 2.11.[25]
Delta Arae is a blue-white main sequence star of spectral type B8Vn and magnitude 3.6, 198 ± 4 light-years from Earth.[18] It is around 3.56 times as massive as the Sun.[27]
Epsilon1 Arae is an orange giant of apparent magnitude 4.1, 360 ± 10 light-years distant from Earth.[18] It is around 74% more massive than the Sun. At an age of about 1.7 billion years, theouter envelope of the star has expanded to almost 34 times the Sun's radius.[28]
Eta Arae is an orange giant of apparent magnitude 3.76, located 299 ± 5 light-years distant from Earth.[18] Estimated to be around five billion years old, it has reached thegiant star stage of itsevolution. With 1.12 times themass of the Sun, it has anouter envelope that has expanded to 40 times the Sun's radius.[29] The star is now spinning so slowly that it takes more than eleven years to complete a single rotation.[30]
GX 339-4 (V821 Arae) is a moderately strong variable galacticlow-mass X-ray binary (LMXB) source[31][32] andblack-hole candidate that flares from time to time. From spectroscopic measurements, the mass of the black-hole was found to be at least of 5.8 solar masses.[33]
Exoplanets have been discovered in seven star systems in the constellation.[34]Mu Arae (Cervantes[35]) is a sunlike star that hosts four planets.[36]HD 152079 is a sunlike star with a jupiter-like planet with an orbital period of 2097 ± 930 days.[37]HD 154672 is an ageing sunlike star with aHot 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.
NGC 6193 is anopen cluster containing approximately 30 stars with an overall magnitude of 5.0 and a size of 0.25 square degrees, about half the size of thefull Moon. It is approximately 4200 light-years from Earth. It has one bright member, a double star with a blue-white hued primary of magnitude 5.6 and a secondary of magnitude 6.9. NGC 6193 is surrounded byNGC 6188, a faintnebula only normally visible inlong-exposure photographs.[1]
NGC 6397 is aglobular cluster with an overall magnitude of 6.0; it is visible to the naked eye under exceptionally dark skies and is normally visible in binoculars. It is a fairly close globular cluster, at a distance of 10,500 light-years.[1]
TheStingray Nebula (Hen 3–1357), the youngest known planetary nebula as of 2010, formed in Ara; the light from its formation was first observable around 1987.
NGC 6326. A planetary nebula that might have a binary system at its center.
^Although parts of the constellation technically rise above the horizon to observers between the 22°N and44°N, stars within a few degrees of the horizon are to all intents and purposes unobservable.[11]
^Delporte had proposed standardising the constellation boundaries to the International Astronomical Union, who had agreed and gave him the lead role[13]
^Objects of magnitude 6.5 are among the faintest visible to the unaided eye in suburban-rural transition night skies.[15]
^The Castle of Knowledge containing the Explication of the Sphere both Celestiall and Materiall, etc., 1st edition, 1556; Robert Record, London; R. Wolfe (printer) STC 20796
^The zodiake of life written by the godly and zealous poet Marcellus Palingenius Stellatus, wherein are conteyned twelve bookes, new edition, 1565 (1 vol.); Barnabe Googe, London; Henry Denham (printer), for Rafe Newberye dwelling in Fleete streate, Anno. 1565. Aprilis. 18; STC 1915
^Harney, Bill Yidumduma; Cairns, Hugh C. (2004) [2003].Dark Sparklers (Revised ed.). Merimbula, New South Wales: Hugh C. Cairns. p. 201.ISBN978-0-9750908-0-0.
^Jofré, E.; Petrucci, R.; Saffe, C.; Saker, L.; de la Villarmois, E. Artur; Chavero, C.; Gómez, M.; Mauas, P. J. D. (2015). "Stellar parameters and chemical abundances of 223 evolved stars with and without planets".Astronomy & Astrophysics.574: A50.arXiv:1410.6422.Bibcode:2015A&A...574A..50J.doi:10.1051/0004-6361/201424474.S2CID53666931.
^Bagnall, Philip M. (2012).The Star Atlas Companion : What You Need to Know about the Constellations. New York:Springer. pp. 50–54.ISBN9781461408307.OCLC794225463.
^Pepe, F.; Correia, A. C. M.; Mayor, M.; Tamuz, O.; Benz, W.; Bertaux, J. -L.; Bouchy, F.; Couetdic, J.; Laskar, J.; Lovis, C.; Naef, D.; Queloz, D.; Santos, N. C.; Sivan, J. -P.; Sosnowska, D.; Udry, S. (2006). "The HARPS search for southern extra-solar planets. IX. μ Ara, a system with four planets".Astronomy and Astrophysics.462 (2):769–776.arXiv:astro-ph/0608396.Bibcode:2007A&A...462..769P.doi:10.1051/0004-6361:20066194.S2CID119071803.
^Fok, Thomas K. T.; Nakashima, Jun-ichi; Yung, Bosco H. K.; Hsia, Chih-Hao; Deguchi, Shuji (2012). "Maser Observations of Westerlund 1 and Comprehensive Considerations on Maser Properties of Red Supergiants Associated with Massive Clusters".The Astrophysical Journal.760 (1): 65.arXiv:1209.6427.Bibcode:2012ApJ...760...65F.doi:10.1088/0004-637X/760/1/65.S2CID53393926.
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