Alpha Aquarii

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α Aquarii
Aquarius constellation map.svg
Red circle.svg

Location of α Aquarii (circled)
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Aquarius
Right ascension 22h 05m 47.03593s[1]
Declination −00° 19′ 11.4568″[1]
Apparent magnitude (V) 2.942[2]
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]
Radial velocity (Rv) 7.5[4] km/s
Proper motion (μ) RA: +18.25[1] mas/yr
Dec.: −9.39[1] mas/yr
Parallax (π) 6.23 ± 0.19[1] mas
Distance 520 ± 20 ly
(161 ± 5 pc)
Absolute magnitude (MV) –3.882[5]
Mass 6.5 ± 0.3[3] M
Radius 77 ± 15[6] R
Luminosity 3,000[7] L
Surface gravity (log g) 1.76 ± 0.04[3] cgs
Temperature 5,210 ± 100[3] K
Metallicity [Fe/H] +0.17[5] dex
Rotational velocity (v sin i) 6.7 ± 1.5[7] km/s
Age 53[3] Myr
Other designations
El Melik, Rucbah, Saad el Melik, Sadalmelek, Sadalmelik, Sadlamulk, α Aqr, Alpha Aquarii, Alpha Aqr, 34 Aquarii, 34 Aqr, BD−01 4246, FK5 827, HD 209750, HIP 109074, HR 8414, SAO 145862.[8][9][10][11]
Database references

Alpha Aquarii (α Aquarii, abbreviated Alpha Aqr, α Aqr), also named Sadalmelik,[12] is a single 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 during the Hipparcos mission, this star is located at a distance of roughly 520 light-years (160 parsecs) from the Sun.[1]


α Aquarii (Latinised to Alpha Aquarii) is the star's Bayer designation.

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.[11] 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.[13] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[14] to catalogue and standardize proper names for stars. The WGSN approved the name Sadalmelik for this star on 21 August 2016, and it is now so entered in the IAU Catalog of Star Names (Delta Cassiopeiae was given the name Ruchbah).[12]

In Chinese, 危宿 (Wēi Xiù), meaning Rooftop (asterism), refers to an asterism consisting of Alpha Aquarii, Theta Pegasi and Epsilon Pegasi.[15] Consequently, Alpha Aquarii itself is known as 危宿一 (Wēi Xiù yī, English: the First Star of Rooftop.)[16]


With an age of 53 million years,[3] this star has evolved into a supergiant with a stellar classification of G2 Ib.[3] It has 6.5[3] times as much mass as the Sun and has expanded to around 77[6] times the Sun's radius. It is radiating 3,000[7] times as much luminosity as the Sun from its outer atmosphere at an effective temperature of 5,210 K.[3] At this heat, the star glows with the yellow hue of a G-type star.[17] 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.[7]

Sadalmelik has a visual companion, designated 2MASS J22055176-0017468, with an apparent visual magnitude of approximately 12.2. It is at an angular separation of 110.4 arcseconds from Sadalmelik along a position angle of 40°.[10]


  1. ^ a b c d e f van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, Bibcode:2007A&A...474..653V, arXiv:0708.1752Freely accessible, doi:10.1051/0004-6361:20078357 
  2. ^ a b c d e Cousins, A. W. J. (1984), "Standardization of Broadband Photometry of Equatorial Standards", South African Astronomical Observatory Circulars, 8: 59, Bibcode:1984SAAOC...8...59C 
  3. ^ a b c d e f g h i Lyubimkov, Leonid S.; et al. (February 2010), "Accurate fundamental parameters for A-, F- and G-type Supergiants in the solar neighbourhood", Monthly Notices of the Royal Astronomical Society, 402 (2): 1369–1379, Bibcode:2010MNRAS.402.1369L, arXiv:0911.1335Freely accessible, doi:10.1111/j.1365-2966.2009.15979.x 
  4. ^ Wielen, R.; et al. (1999), "Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions", Veröff. Astron. Rechen-Inst. Heidelb, Astronomisches Rechen-Institut Heidelberg, 35 (35): 1, Bibcode:1999VeARI..35....1W 
  5. ^ a b Soubiran, C.; et al. (2008). "Vertical distribution of Galactic disk stars. IV. AMR and AVR from clump giants". Astronomy and Astrophysics. 480 (1): 91–101. Bibcode:2008A&A...480...91S. arXiv:0712.1370Freely accessible. doi:10.1051/0004-6361:20078788. 
  6. ^ a b Nordgren, Tyler E.; et al. (December 1999), "Stellar Angular Diameters of Late-Type Giants and Supergiants Measured with the Navy Prototype Optical Interferometer", The Astronomical Journal, 118 (6): 3032–3038, Bibcode:1999AJ....118.3032N, doi:10.1086/301114 
  7. ^ a b c d Ayres, Thomas R.; Brown, Alexander; Harper, Graham M. (July 2005), "Chandra Observations of Coronal Emission from the Early G Supergiants α and β Aquarii", The Astrophysical Journal, 627 (1): L53–L56, Bibcode:2005ApJ...627L..53A, doi:10.1086/431977 
  8. ^ HR 8414, database entry, The Bright Star Catalogue, 5th Revised Ed. (Preliminary Version), D. Hoffleit and W. H. Warren, Jr., CDS ID V/50. Accessed on line October 3, 2008.
  9. ^ NAME SADALMELIK -- Star in double system, database entry, SIMBAD. Accessed on line October 3, 2008.
  10. ^ a b Entry 22058-0019, The Washington Double Star Catalog, United States Naval Observatory. Accessed on line November 18, 2008.
  11. ^ a b pp. 51, 148, Star-names and Their Meanings, Richard Hinckley Allen, New York: G. E. Stechert, 1899.
  12. ^ a b "IAU Catalog of Star Names". Retrieved 28 July 2016. 
  13. ^ Sadalmelik, Stars, Jim Kaler. Accessed on line October 3, 2008.
  14. ^ IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016. 
  15. ^ (in Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
  16. ^ (in Chinese) 香港太空館 - 研究資源 - 亮星中英對照表, Hong Kong Space Museum. Accessed on line November 23, 2010.
  17. ^ "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, retrieved 2012-01-16 

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