Delta Aquarii

Delta Aquarii (δ Aquarii, abbreviated Delta Aqr, δ Aqr), officially named Skat /ˈskæt/,[16] is the third-brightest star in the constellation of Aquarius. The apparent visual magnitude is 3.3,[17] which can be seen with the naked eye. The distance to this star is about 113 light-years (35 parsecs) based upon parallax measurements, and it has a close companion.

δ Aquarii
Aquarius constellation map.svg
Red circle.svg
Location of δ Aquarii (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Aquarius
Right ascension 22h 54m 39.0125s[1]
Declination −15° 49′ 14.953″[1]
Apparent magnitude (V) 3.28[2]
Spectral type A3Vp[3]
U−B color index +0.08[4]
B−V color index +0.05[4]
Radial velocity (Rv)+18.0[5] km/s
Proper motion (μ) RA: -38.904[6] mas/yr
Dec.: -33.027[6] mas/yr
Parallax (π)28.7936 ± 0.7289[6] mas
Distance113 ± 3 ly
(34.7 ± 0.9 pc)
Absolute magnitude (MV)−0.17[7]
Mass2.51[8] M
Radius2.4[9] R
Luminosity48[6] L
Surface gravity (log g)3.73[10] cgs
Temperature8,650[11] K
Metallicity [Fe/H]–0.21[12] dex
Rotational velocity (v sin i)81[13] km/s
Age0.3[14] Gyr
Other designations
Skat, Scheat, 76 Aquarii, BD-16 6173, FK5 866, HD 216627, HIP 113136, HR 8709, SAO 165375.[15]
Database references


δ Aquarii (Latinised to Delta Aquarii) is the star's Bayer designation. It also has the Flamsteed designation 76 Aquarii.

It bore the traditional name Skat (also rendered Scheat, Seat, Sheat, etc., which was erroneously applied to Beta Pegasi in late medieval times), from the Arabic الساق al-sāq "shin".[18]

In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[19] to catalogue and standardize proper names for stars. The WGSN approved the name Skat for this star on 21 August 2016, and it is now so included in the List of IAU-approved Star Names.[16]

In Chinese, 羽林軍 (Yǔ Lín Jūn), meaning Palace Guard, refers to an asterism consisting of Delta Aquarii, 29 Aquarii, 35 Aquarii, 41 Aquarii, 47 Aquarii, 49 Aquarii, Lambda Piscis Austrini, HD 212448, Epsilon Piscis Austrini, 21 Piscis Austrini, 20 Piscis Austrini, Upsilon Aquarii, 68 Aquarii, 66 Aquarii, 61 Aquarii, 53 Aquarii, 50 Aquarii, 56 Aquarii, 45 Aquarii, 58 Aquarii, 64 Aquarii, 65 Aquarii, 70 Aquarii, 74 Aquarii, Tau2 Aquarii, Tau1 Aquarii, 77 Aquarii, 88 Aquarii, 89 Aquarii, 86 Aquarii, 101 Aquarii, 100 Aquarii, 99 Aquarii, 98 Aquarii, 97 Aquarii, 94 Aquarii, Psi3Aquarii, Psi2Aquarii, Psi1Aquarii, 87 Aquarii, 85 Aquarii, 83 Aquarii, Chi Aquarii, Omega1 Aquarii and Omega2 Aquarii. Consequently, the Chinese name for Delta Aquarii itself is 羽林軍二十六 (Yǔ Lín Jūn ershíliù, English: the Twenty Sixth Star of Palace Guard).[20]


The spectrum of Delta Aquarii matches a stellar classification of A3 Vp, indicating this is a chemically peculiar A-type main-sequence star that is generating energy through the nuclear fusion of hydrogen at its core. This star has double the Sun's mass and a radius 2.4 times as large.[9] It is radiating 26[9] times the luminosity of the Sun from its outer atmosphere at an effective temperature of around 9,000 K.[21] This heat gives it the characteristic white-hued glow of an A-type star.[22] It has a relatively high rate of rotation, with a projected rotational velocity of 81 km s−1.[13]

Delta Aquarii does not display a strong signal of excess infrared emission that might indicate the presence of circumstellar matter.[23] Delta Aquarii is a probable stream star member of the Ursa Major Moving Group,[24] which has an estimated age of 500 million years.[25]


An analysis of Hipparcos data strongly suggested a close companion object. An orbit was derived with a 483-day period, an eccentricity of 0.12, and an inclination of 41°. When Delta Aquarii was first examined for the companion, it could not be seen. Any possible companion beyond 100 au was constrained to be less than 0.07 M.[14] Infrared interferometric observations did then find a companion, a likely G5 main sequence star around 2 au from the primary.[8]


  1. ^ a b van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357, S2CID 18759600
  2. ^ Ducati, J. R. (2002), "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system", CDS/ADC Collection of Electronic Catalogues, 2237, Bibcode:2002yCat.2237....0D
  3. ^ Abt, Helmut A.; Morrell, Nidia I. (July 1995), "The Relation between Rotational Velocities and Spectral Peculiarities among A-Type Stars", Astrophysical Journal Supplement, 99: 135, Bibcode:1995ApJS...99..135A, doi:10.1086/192182.
  4. ^ a b Hoffleit, Dorrit; Jaschek, Carlos (1991), "The Bright star catalogue", New Haven,
  5. ^ Wilson, Ralph Elmer (1953). "General Catalogue of Stellar Radial Velocities". Carnegie Institute Washington D.C. Publication. Washington: Carnegie Institution of Washington. Bibcode:1953GCRV..C......0W.
  6. ^ a b c d Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
  7. ^ Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv:1108.4971, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015, S2CID 119257644.
  8. ^ a b Borgniet, S.; Lagrange, A.-M.; Meunier, N.; Galland, F. (2017), "Extrasolar planets and brown dwarfs around AF-type stars", Astronomy & Astrophysics, 599: A57, arXiv:1608.08257, Bibcode:2017A&A...599A..57B, doi:10.1051/0004-6361/201628805, S2CID 118723455
  9. ^ a b c Malagnini, M. L.; Morossi, C. (November 1990), "Accurate absolute luminosities, effective temperatures, radii, masses and surface gravities for a selected sample of field stars", Astronomy and Astrophysics Supplement Series, 85 (3): 1015–1019, Bibcode:1990A&AS...85.1015M
  10. ^ David, Trevor J.; Hillenbrand, Lynne A. (2015), "The Ages of Early-Type Stars: Strömgren Photometric Methods Calibrated, Validated, Tested, and Applied to Hosts and Prospective Hosts of Directly Imaged Exoplanets", The Astrophysical Journal, 804 (2): 146, arXiv:1501.03154, Bibcode:2015ApJ...804..146D, doi:10.1088/0004-637X/804/2/146, S2CID 33401607
  11. ^ Zorec, J.; Royer, F. (2012), "Rotational velocities of A-type stars. IV. Evolution of rotational velocities", Astronomy & Astrophysics, 537: A120, arXiv:1201.2052, Bibcode:2012A&A...537A.120Z, doi:10.1051/0004-6361/201117691, S2CID 55586789.
  12. ^ Takeda, Yoichi; Kang, Dong-Il; Han, Inwoo; Lee, Byeong-Cheol; Kim, Kang-Min (2009). "Can Sodium Abundances of A-Type Stars be Reliably Determined from Na I 5890/5896 Lines?". Publications of the Astronomical Society of Japan. 61 (5): 1165–1178. arXiv:0907.1329. Bibcode:2009PASJ...61.1165T. doi:10.1093/pasj/61.5.1165. S2CID 15454888.
  13. ^ a b Royer, F.; Zorec, J.; Gómez, A. E. (February 2007), "Rotational velocities of A-type stars. III. Velocity distributions", Astronomy and Astrophysics, 463 (2): 671–682, arXiv:astro-ph/0610785, Bibcode:2007A&A...463..671R, doi:10.1051/0004-6361:20065224, S2CID 18475298
  14. ^ a b Ehrenreich, D.; et al. (November 2010), "Deep infrared imaging of close companions to austral A- and F-type stars", Astronomy and Astrophysics, 523: A73, arXiv:1007.0002, Bibcode:2010A&A...523A..73E, doi:10.1051/0004-6361/201014763, S2CID 54913363
  15. ^ "del Aqr -- Star", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2012-01-30
  16. ^ a b "Naming Stars". Retrieved 18 June 2018.
  17. ^ Celis S., L. (October 1975), "Photoelectric photometry of late-type variable stars", Astronomy and Astrophysics Supplement Series, 22: 9–17, Bibcode:1975A&AS...22....9C
  18. ^ Kunitzsch, Paul; Smart, Tim (2006). A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd rev. ed.). Cambridge, Massachusetts: Sky Pub. ISBN 978-1-931559-44-7.
  19. ^ IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016.
  20. ^ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 7 月 7 日
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  22. ^ "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, archived from the original on 2012-03-10, retrieved 2012-01-16
  23. ^ Su, K. Y. L.; et al. (December 2006), "Debris Disk Evolution around A Stars", The Astrophysical Journal, 653 (1): 675–689, arXiv:astro-ph/0608563, Bibcode:2006ApJ...653..675S, doi:10.1086/508649, S2CID 14116473
  24. ^ King, Jeremy R.; et al. (April 2003), "Stellar Kinematic Groups. II. A Reexamination of the Membership, Activity, and Age of the Ursa Major Group", The Astronomical Journal, 125 (4): 1980–2017, Bibcode:2003AJ....125.1980K, doi:10.1086/368241
  25. ^ Monier, R. (November 2005), "Abundances of a sample of A and F-type dwarf members of the Ursa Major Group", Astronomy and Astrophysics, 442 (2): 563–566, Bibcode:2005A&A...442..563M, doi:10.1051/0004-6361:20053222

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