Delta Aquilae (δ Aquilae, abbreviated Delta Aql or δ Aql) is a binary star system in the equatorial constellation of Aquila. It is visible to the naked eye with an apparent visual magnitude of approximately 3.4.
Location and Distance
Based on parallax measurements from the Hipparcos satellite, Delta Aquilae is located at a distance of about 50.6 light-years (15.5 parsecs) from Earth. It is drifting closer to the Solar System with a radial velocity of −30 km/s, and is predicted to come to within 25.4 light-years (7.8 pc) of the Sun in approximately 335,000 years.
Stellar System
The binary nature of this system was first reported by H. L. Alden at Yale Observatory in 1936. It is an astrometric binary, meaning the two components orbit each other with a period of 3.422 years and an orbital eccentricity of about 0.36. The presence of the secondary component is revealed by its gravitational perturbation of the primary; the individual components have not been directly resolved with a telescope.
Primary Component (δ Aql Aa)
The primary is an aging subgiant star with a stellar classification of F0 IV (sometimes given as F1IV-V(n) or F3IV). The luminosity class IV indicates it is in the process of exhausting the hydrogen supply at its core and evolving into a giant star.
- Mass: ~1.65 times the mass of the Sun
- Radius: ~2.04 times the radius of the Sun
- Luminosity: ~8.5 times the luminosity of the Sun
- Effective temperature: ~6,958–7,016 K
- Color: Yellow-white (F-type star)
- Rotational velocity: ~87 km/s (projected)
Delta Aquilae A is classified as a Delta Scuti variable star, exhibiting small variations in luminosity caused by pulsations in its outer envelope.
Secondary Component (δ Aql Ab)
The secondary is a smaller star with about 67% of the Sun's mass and an estimated 61% of the Sun's radius. It is likely a K-type star.
Naming and Cultural History
- Along with η Aquilae and θ Aquilae, δ Aquilae formed Al Mizān (Arabic: ألميزان, "the Scale-beam"). In this asterism, δ Aquilae was designated Al Mizān I.
- Jim Kaler suggested the name Almizan Occidental for this star, as it is the westernmost of the three.
- Antonín Bečvář's catalogue lists the name Deneb Okab ("tail of the eagle" in Arabic), though this is considered a likely misnomer, as the star lies in the center of the constellation (the eagle's chest), while ε and ζ Aquilae were traditionally known as Deneb al Okab.
- In the Calendarium of Al Achsasi al Mouakket, it was designated Djenubi Menkib al Nesr (Arabic: منكب ألنسر ألخنوبي), Latinized as Australior Humerus Vulturis, meaning "the southern shoulder of the eagle."
- In Chinese astronomy, δ Aquilae is part of the asterism 右旗 (Yòu Qí, "Right Flag") and is individually known as 右旗三 (Yòu Qí sān, "the Third Star of Right Flag").
- This star, together with η, θ, ι, κ, and λ Aquilae, was once part of the now-obsolete constellation Antinous.
Other Designations
Bayer designation: δ Aquilae (Delta Aql)
Flamsteed designation: 30 Aquilae
Henry Draper Catalogue: HD 182640
Hipparcos Catalogue: HIP 95501
Bright Star Catalogue: HR 7377
SAO: SAO 124603
