The orange supergiant
Beta Arae, to us its brightest star measured with near-constant
apparent magnitude of 2.85, is marginally brighter than blue-white
Alpha Arae. Seven star systems are known to host planets. Sunlike
Mu Arae hosts four known planets.
Gliese 676 is a (gravity-paired) binary
red dwarf system with four known planets.
In ancient
Greek mythology, Ara was identified as the
altar where the gods first made offerings and formed an alliance before defeating the
Titans.[1] One of the southernmost constellations depicted by
Ptolemy,[3] 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
Bradley Schaefer proposes such Ancients must have been able to see as far south as
Zeta Arae, for a pattern that looked like an altar.[4]
In illustrations, Ara is usually depicted as compact classical
altar with its smoke 'rising' southward.[5] However, depictions often vary. In the early days of printing, a 1482
woodcut of
Gaius Julius Hyginus's classic Poeticon Astronomicon depicts the altar as surrounded by demons.[6]Johann Bayer in 1603 depicted Ara as an altar with burning incense.
Hyginus depicted the same though his featured devils on either side of the flames.
Willem Blaeu, a Dutch
uranographer of the 16th and 17th centuries, drew Ara as an altar for sacrifices, with a burning animal offering unusually whose smoke rises northward, represented by
Alpha Arae.
The Castle of Knowledge by
Robert 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 by
Marcellus Palingenius of 1565, by
Barnabe 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 the
88 modern constellations by area.[11] Its position in the
Southern Celestial Hemisphere means that the whole
constellation is visible to observers south of
22°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 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°.[2]
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, French astronomer
Nicolas-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 to
apparent magnitude 6.5.[d][11]
Just shading Alpha Arae, Beta Arae is the brightest star in the constellation.[16] It is an orange-hued star of spectral type K3Ib-IIa that has been classified as a supergiant or bright giant,[17] that is around 650 light-years from Earth.[18] It is around 8.21 times as massive and 5,636 times as luminous as the Sun.[19] At apparent magnitude 2.85, this difference in brightness between the two is undetectable by the unaided eye.[20] 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.[18] It has been estimated to be between 12.5 and 25 times as massive as the Sun,[21][22] and have around 120,000 times its luminosity.[22]
Alpha Arae is a
blue-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,[23] and has an average of 4.5 times its radius.[24] It is 5,800 times as luminous as the Sun,[23] its energy emitted from its
outer envelope at an
effective temperature of 18,044 K.[24] 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.[23][25]
The third brightest star in Ara at magnitude 3.13 is
Zeta Arae,[26] 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.[27] Were it not dimmer by intervening interstellar dust, it would be significantly brighter at magnitude 2.11.[26]
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.[28]
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, the
outer envelope of the star has expanded to almost 34 times the Sun's radius.[29]
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 the
giant star stage of its
evolution. With 1.12 times the
mass of the Sun, it has an
outer envelope that has expanded to 40 times the Sun's radius.[30] The star is now spinning so slowly that it takes more than eleven years to complete a single rotation.[31]
GX 339-4 (V821 Arae) is a moderately strong variable galactic
low-mass X-ray binary (LMXB) source[32][33] and
black-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.[34]
Exoplanets have been discovered in seven star systems in the constellation.[35]Mu Arae (Cervantes[36]) is a sunlike star that hosts four planets.[37]HD 152079 is a sunlike star with a jupiter-like planet with an orbital period of 2097 ± 930 days.[38]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.
NGC 6193 is an
open 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 the
full 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 by
NGC 6188, a faint
nebula only normally visible in
long-exposure photographs.[1]
NGC 6397 is a
globular 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]
Planetary Nebulae
The
Stingray 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 and
44°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.
^da Silva, L.; Girardi, L.; Pasquini, L.; Setiawan, J.; von der Lühe, O.; de Medeiros, J. R.; Hatzes, A.; Döllinger, M. P.; Weiss, A. (November 2006). "Basic physical parameters of a selected sample of evolved stars". Astronomy and Astrophysics. 458 (2): 609–623.
arXiv:astro-ph/0608160.
Bibcode:
2006A&A...458..609D.
doi:
10.1051/0004-6361:20065105.
S2CID9341088.
^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.
Ridpath, Ian; Tirion, Wil (2001). Stars and Planets Guide. Princeton University Press.
ISBN978-0-691-08913-3.
Ridpath, Ian; Tirion, Wil (2007). Stars and Planets Guide (4th ed.). Princeton University Press.
ISBN978-0-691-13556-4.
Staal, Julius D.W. (1988). The New Patterns in the Sky. McDonald and Woodward Publishing Company.
ISBN978-0-939923-04-5.
Wagman, Morton (2003). Lost Stars: Lost, Missing and Troublesome Stars from the Catalogues of Johannes Bayer, Nicholas Louis de Lacaille, John Flamsteed, and Sundry Others. Blacksburg,
VA: The McDonald & Woodward Publishing Company.
ISBN978-0-939923-78-6.