Alpha Aquarii, officially named Sadalmelik,[1] 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 690abbr=offNaNabbr=off 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.
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". The name Rucbah had also been applied to this star; though it shared that name with Delta Cassiopeiae.[2] 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).[1]
In Chinese, Chinese: 危宿 (Chinese: Wēi Xiù), meaning Rooftop (asterism), refers to an asterism consisting of Alpha Aquarii, Theta Pegasi and Epsilon Pegasi.[3] Consequently, the Chinese name for Alpha Aquarii itself is Chinese: 危宿一 (Chinese: Wēi Xiù yī, English: the First Star of Rooftop).[4]
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, which translates to a distance of, or 760 light-years. The 2007 Hipparcos reduction gave a parallax of, translating into a distance of, or 520 light-years. Measurements by the Gaia spacecraft on its second data release (DR2) give a parallax of, giving a much lower distance of (250 ly). The third Gaia data release (Gaia DR3) give a parallax of, translating to a distance of, or 660 light-years.
Alpha Aquarii has 6.3 times as much mass as the Sun and has expanded to around 80 times the Sun's radius. With insufficient mass to explode as a supernova, it will most likely become a massive white dwarf similar to Sirius B.[5] It is radiating 3,900 times as much luminosity as the Sun from its outer atmosphere at an effective temperature of . 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°.