Delta Aquarii

Delta Aquarii (δ Aquarii, abbreviated Delta Aqr, δ Aqr), officially named Skat, is the third-brightest star in the constellation of Aquarius. The apparent visual magnitude is 3.3, which can be seen with the naked eye. The distance to this star is about 113 ly based upon parallax measurements, and it has a close companion.

Nomenclature
δ Aquarii (Latinised to Delta Aquarii) is the star's Bayer designation. It also has the Flamsteed designation 76 Aquarii.

It bore the traditional name Skat (also rendered Scheat, Seat, Sheat, etc., which was erroneously applied to Beta Pegasi in late medieval times), from the Arabic الساق al-sāq "shin".

In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalogue and standardize proper names for stars. The WGSN approved the name Skat for this star on 21 August 2016, and it is now so included in the List of IAU-approved Star Names. In Chinese, 羽林軍 (Yǔ Lín Jūn), meaning Palace Guard, refers to an asterism consisting of Delta Aquarii, 29 Aquarii, 35 Aquarii, 41 Aquarii, 47 Aquarii, 49 Aquarii, Lambda Piscis Austrini, HD 212448, Epsilon Piscis Austrini, 21 Piscis Austrini, 20 Piscis Austrini, Upsilon Aquarii, 68 Aquarii, 66 Aquarii, 61 Aquarii, 53 Aquarii, 50 Aquarii, 56 Aquarii, 45 Aquarii, 58 Aquarii, 64 Aquarii, 65 Aquarii, 70 Aquarii, 74 Aquarii, Tau2 Aquarii, Tau1 Aquarii, 77 Aquarii, 88 Aquarii, 89 Aquarii, 86 Aquarii, 101 Aquarii, 100 Aquarii, 99 Aquarii, 98 Aquarii, 97 Aquarii, 94 Aquarii, Psi3Aquarii, Psi2Aquarii, Psi1Aquarii, 87 Aquarii, 85 Aquarii, 83 Aquarii, Chi Aquarii, Omega1 Aquarii and Omega2 Aquarii. Consequently, the Chinese name for Delta Aquarii itself is 羽林軍二十六 (Yǔ Lín Jūn ershíliù, the Twenty Sixth Star of Palace Guard).

Properties
The spectrum of Delta Aquarii matches a stellar classification of A3 Vp, indicating this is a chemically peculiar A-type main-sequence star that is generating energy through the nuclear fusion of hydrogen at its core. This star has double the Sun's mass and a radius 2.4 times as large. It is radiating 26 times the luminosity of the Sun from its outer atmosphere at an effective temperature of around 9,000 K. This heat gives it the characteristic white-hued glow of an A-type star. It has a relatively high rate of rotation, with a projected rotational velocity of 81 km s−1.

Delta Aquarii does not display a strong signal of excess infrared emission that might indicate the presence of circumstellar matter. Delta Aquarii is a probable stream star member of the Ursa Major Moving Group, which has an estimated age of 500 million years.

Companion
An analysis of Hipparcos data strongly suggested a close companion object. An orbit was derived with a 483-day period, an eccentricity of 0.12, and an inclination of 41°. When Delta Aquarii was first examined for the companion, it could not be seen. Any possible companion beyond $100 au$ was constrained to be less than. Infrared interferometric observations did then find a companion, a likely G5 main sequence star around $2 au$ from the primary.