Alpha Aquarii, officially named Sadalmelik, is a single yellow supergiant 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 by the Gaia spacecraft, it is located at a distance of roughly 690 ly from the Sun. It is drifting further away from the Sun with a radial velocity of 7.5 km/s. It forms the primary or 'A' component of a double star designated WDS J22058-0019; the secondary or 'B' component is UCAC2 31789179. However, the pair is only an optical binary, with UCAC2 31789179 being a background star.

Nomenclature

α Aquarii (Latinised to Alpha 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 (sa‘d al-malik), meaning "Luck of the king" or “arm/support of God”. 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 (WGSN) to catalogue and standardize proper names for stars. The WGSN approved the name Sadalmelik for Alpha Aquarii (WDS J22058-0019 A) on 21 August 2016, and it is now so included in the List of IAU-approved Star Names (Delta Cassiopeiae was given the name Ruchbah). In Chinese, 危宿 (Wēi Xiù), meaning Rooftop (asterism), 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 (Wēi Xiù yī, ).

Properties

With an age of 53 million years, Alpha Aquarii has evolved into a supergiant with a stellar classification of G2 Ib. It lies within the Cepheid instability strip of the Hertzsprung–Russell diagram, near the red (cooler) edge, but is not classified as a variable star. However, variable cores have been detected in the hydrogen lines, which are originating in a circumstellar envelope. The star has a massive stellar wind that reaches supersonic velocity in the chromosphere. There is some uncertainty about Alpha Aquarii's distance. The original Hipparcos catalog gave a parallax of 4.3 mas, which translates to a distance of 233 parsecs, or 760 light-years. However, the 2007 Hipparcos reduction give a parallax that implies a distance of 161 pc, or 520 light-years. The third Gaia data release (Gaia DR3) give a parallax of 4.94 mas, translating to a distance of 202 pc, or 660 light-years. Alpha Aquarii's angular diameter has been measured at 3.066 mas. At its estimated distance, it translates to a radius of times the radius of the Sun. With insufficient mass to explode as a supernova, it will most likely become a massive white dwarf similar to Sirius B. It is radiating 3,900 times as much luminosity as the Sun from its outer atmosphere at an effective temperature of 5,190 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 (UCAC2 31789179) 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°.