Delta Aquilae (δ Aquilae, δ Aql) is a binary star system in the equatorial constellation of Aquila. It has an apparent visual magnitude of 3.4 and, based upon parallax measurements, is located about 50.6 light-years (15.5 parsecs) from Earth.
Delta Aquilae is an astrometric binary where the two components orbit each other with a period of 3.422 years and an eccentricity of about 0.36. This is a type of binary star system where the presence of the secondary component is revealed by its gravitational perturbation of the primary. The individual components have not been resolved with a telescope.
The primary component, Delta Aquilae A, is a subgiant star with a stellar classification of F0 IV, where the luminosity class of IV indicates it is in the process of exhausting the supply of hydrogen at its core and evolving into a giant star. The mass of the star is 65% greater than the Sun and it has expanded to more than double the Sun's radius. It is radiating around 7–8 times the luminosity of the Sun from its outer atmosphere at an effective temperature of 7,016 K, giving it the yellow-white hue of an F-type star. Delta Aquilae A is a Delta Scuti variable that exhibits variations in luminosity caused by pulsations in its outer envelope. It is spinning rapidly with a projected rotational velocity of about 87 km s−1. This is a lower bound on the azimuthal velocity along the star's equator.
This star, along with η Aql and θ Aql (Tseen Foo) were Al Mizān (ألميزان), the Scale-beam. According to the catalogue of stars in the Technical Memorandum 33-507 - A Reduced Star Catalog Containing 537 Named Stars, Al Mizān were the title for three stars: δ Aql as Al Mizān I, η Aql as l Mizān II and θ Aql as Al Mizān III. Being the westernmost star of the asterism, Jim Kaler has suggested the name Almizan Occidental. On the other hand, Antonín Bečvář includes, with no further explanation, Deneb Okab in his Skalnate Pleso Atlas of the Heavens, meaning the tail of eagle in Arabic; however, the star is situated in the centre of the constellation, which is usually identified with the chest, while the stars ε Aql and ζ Aql have been collectively known as Deneb al Okab by Arabian medieval astronomers, which might suggest that Bečvář's assumption was a misnomer.
In the catalogue of stars in the Calendarium of Al Achsasi al Mouakket, this star was designated Djenubi Menkib al Nesr (منكب ألنسر ألخنوبي - mankib al-nasr al-janúbii), which was translated into Latin as Australior Humerus Vulturis, meaning the southern shoulder of the eagle.
In Chinese, 右旗 (Yòu Qí), meaning Right Flag, refers to an asterism consisting of δ Aquilae, μ Aquilae, σ Aquilae, ν Aquilae, ι Aquilae, 42 Aquilae, HD 184701, κ Aquilae and 56 Aquilae. Consequently, δ Aquilae itself is known as 右旗三 (Yòu Qí sān, English: the Third Star of Right Flag.)
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