Alpha Aquarii


Alpha Aquarii, officially named Sadalmelik, is a single star in the constellation of Aquarius. The apparent visual magnitude of 2.94 makes this the second-brightest star in Aquarius. Based upon parallax measurements made during the Hipparcos mission, it is located at a distance of roughly from the Sun.
It forms the primary or 'A' component of a double star designated WDS J22058-0019.

Nomenclature

α Aquarii is the star's Bayer designation. WDS J22058-0019 A is its designation in the Washington Double Star Catalog.
It bore the traditional name Sadalmelik, which derived from an Arabic expression سعد الملك, meaning "Luck of the king". The name Rucbah had also been applied to this star; though it shared that name with Delta Cassiopeiae. It is only one of two stars with ancient proper names to lie within a degree of the celestial equator. The origin of the Arabic name is lost to history. In 2016, the International Astronomical Union organized a Working Group on Star Names to catalogue and standardize proper names for stars. The WGSN approved the name Sadalmelik for Alpha Aquarii on 21 August 2016, and it is now so included in the List of IAU-approved Star Names.
In Chinese, 危宿, meaning Rooftop , refers to an asterism consisting of Alpha Aquarii, Theta Pegasi and Epsilon Pegasi. Consequently, the Chinese name for Alpha Aquarii itself is 危宿一.

Properties

With an age of 53 million years, Alpha Aquarii has evolved into a supergiant with a stellar classification of G2 Ib. It has 5.1 times as much mass as the Sun and has expanded to around 53 times the Sun's radius. It is radiating 2,100 times as much luminosity as the Sun from its outer atmosphere at an effective temperature of 5,383 K. At this heat, the star glows with the yellow hue of a G-type star. Examination of this star with the Chandra X-ray Observatory shows it to be significantly X-ray deficient compared to G-type main sequence stars. This deficit is a common feature of early G-type giant stars.
The visual companion has an apparent visual magnitude of approximately 12.2. It is at an angular separation of 110.4 arcseconds from Alpha Aquarii along a position angle of 40°.