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Star in the constellation Aquarius From Wikipedia, the free encyclopedia
Alpha Aquarii, officially named Sadalmelik (/ˌsædəlˈmɛlɪk/),[15] is a single yellow supergiant star in the constellation of Aquarius. The apparent visual magnitude of 2.94[2] 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 light-years (210 parsecs) from the Sun.[5] It is drifting further away from the Sun with a radial velocity of 7.5 km/s.[4]
Observation data Epoch J2000.0 Equinox J2000.0 (ICRS) | |
---|---|
Constellation | Aquarius |
Right ascension | 22h 05m 47.03555s[1] |
Declination | −00° 19′ 11.4634″[1] |
Apparent magnitude (V) | 2.942[2] |
Characteristics | |
Evolutionary stage | Yellow supergiant |
Spectral type | G2 Ib[3] |
U−B color index | +0.699[2] |
B−V color index | +0.971[2] |
R−I color index | +0.49[2] |
Astrometry | |
Radial velocity (Rv) | 7.5[4] km/s |
Proper motion (μ) | RA: +18.59 mas/yr[1] Dec.: −10.45 mas/yr[1] |
Parallax (π) | 4.9451 ± 0.4296 mas[1] |
Distance | 690.8+37.8 −56.1 ly (211.9+11.6 −17.2 pc)[5] |
Absolute magnitude (MV) | −3.882[6] |
Details | |
Mass | 6.31[7] M☉ |
Radius | 70[8] R☉ |
Luminosity | 3,917[7] L☉ |
Surface gravity (log g) | 1.4[7] cgs |
Temperature | 5,190±50[9] K |
Metallicity [Fe/H] | +0.17[6] dex |
Rotational velocity (v sin i) | 6.7±1.5[10] km/s |
Age | 53[3] Myr |
Other designations | |
Database references | |
SIMBAD | data |
It forms the primary or 'A' component of a double star designated WDS J22058-0019; the secondary or 'B' component is UCAC2 31789179.[16] However, the pair is only an optical binary, with UCAC2 31789179 being a background star.[16]
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.[14] 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.[17] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[18] 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).[15]
In Chinese, 危宿 (Wēi Xiù), meaning Rooftop (asterism), refers to an asterism consisting of Alpha Aquarii, Theta Pegasi and Epsilon Pegasi.[19] Consequently, the Chinese name for Alpha Aquarii itself is 危宿一 (Wēi Xiù yī, English: the First Star of Rooftop).[20]
With an age of 53 million years,[3] Alpha Aquarii has evolved into a supergiant with a stellar classification of G2 Ib.[3] 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.[9] The star has a massive stellar wind that reaches supersonic velocity in the chromosphere.[21]
There is some uncertainty about Alpha Aquarii's distance. The original Hipparcos catalog gave a parallax of 4.3±0.83 mas, which translates to a distance of 233±45 parsecs, or 760 light-years.[22] However, the 2007 Hipparcos reduction give a parallax that implies a distance of 161±5 pc, or 520 light-years.[23] The third Gaia data release (Gaia DR3) give a parallax of 4.94±0.43 mas, translating to a distance of 202±17 pc, or 660 light-years.[1]
Alpha Aquarii's angular diameter has been measured at 3.066±0.036 mas.[24] At its estimated distance,[5] it translates to a radius of 70 times the radius of the Sun.[8] With insufficient mass to explode as a supernova, it will most likely become a massive white dwarf similar to Sirius B.[25] It is radiating 3,900 times as much luminosity as the Sun from its outer atmosphere[7] at an effective temperature of 5,190 K.[9] At this heat, the star glows with the yellow hue of a G-type star.[26] 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.[10]
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°.[13]
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