The Ara Constellation

Posted on Friday, 27 October, 2017Friday, 27 October, 2017 by Zheluo Cai

The Ara Constellation.

Ara is a southern constellation situated between Scorpius and Triangulum Australe. Ara 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. In ancient Greek mythology, Ara was identified as the altar where the gods first made offerings and formed an alliance before defeating the Titans. The nearby Milky Way represents the smoke rising from the offerings on the altar, 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. Scorpius runs along the length of its northern border, while Norma and Triangulum Australe border it to the west, Apus to the south, and Paco and Telescopium to the east respectively. The official constellation boundaries, as set by Belgian astronomer Eugène Delporte in 1930, are defined by a polygon of twelve segments. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 16^h 36.1^m and 18^h 10.4^m, while the declination coordinates are between −45.49° and −67.69°. Best viewing month is in July at 9.00pm.

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. 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. 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.

Exoplanets have been discovered in seven star systems in the constellation. Mu Arae 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 supergiant Westerlund 1-26, one of the largest stars known. Although Ara lies close to the heart of the Milky Way, two spiral galaxies (NGC 6215 and NGC 6221) are visible near star Eta Arae.

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.