Sigma Hydrae

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Sigma Hydrae
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Hydra
Right ascension  08h 38m 45.43747s[1]
Declination +03° 20′ 29.1701″[1]
Apparent magnitude (V) 4.48[2]
Spectral type K2 III[3]
U−B color index +1.23[2]
B−V color index +1.21[2]
Radial velocity (Rv)27.28±0.19[4] km/s
Proper motion (μ) RA: −19.48[1] mas/yr
Dec.: −15.92[1] mas/yr
Parallax (π)8.75 ± 0.25[1] mas
Distance370 ± 10 ly
(114 ± 3 pc)
Absolute magnitude (MV)−0.84[5]
Mass3.07 M
Radius27.6[7] R
Luminosity295 L
Surface gravity (log g)2.21±0.13[8] cgs
Temperature4,491±51 K
Metallicity [Fe/H]0.13±0.06[8] dex
Age430 Myr
Other designations
σ Hya, 5 Hydrae, BD+03°2026, FK5 1224, HD 73471, HIP 42402, HR 3418, SAO 116988.[9]
Database references

Sigma Hydrae (σ Hydrae, abbreviated Sigma Hya, σ Hya), also named Minchir,[10] is a solitary[11] star in the equatorial constellation of Hydra. It is visible to the naked eye with an apparent visual magnitude of 4.48.[2] The estimated distance to this star from the Sun, based upon an annual parallax shift of 8.75 mas,[1] is around 370 light-years. At that distance, the visual magnitude of the star is diminished by an interstellar extinction factor of 0.16,[4] due to intervening dust.


σ Hydrae (Latinised to Sigma Hydrae) is the system's Bayer designation.

It bore the traditional name Minchir, appearing as Minchir es-schudscha' on Bode's large star atlas, Uranographia, which is derived from the Arabic Minkhir al-Shuja‘, "the Nostril of Hydra", for this star.[citation needed] The name is erroneously[citation needed] spelt as Al Minliar al Shuja in the Yale Bright Star Catalogue.[12]. In 2016, the IAU organized a Working Group on Star Names (WGSN)[13] to catalog and standardize proper names for stars. The WGSN approved the name Minchir for this star on 5 September 2017 and it is now so included in the List of IAU-approved Star Names.[10]

This star, along with Delta Hydrae (Lisan al Shudja), Epsilon Hydrae, Zeta Hydrae, Eta Hydrae and Rho Hydrae, were Ulug Beg's Min al Azʽal, "Belonging to the Uninhabited Spot".[14] (According to a 1971 NASA memorandum, Min al Azʽal or Minazal were the title for five stars : Delta Hydrae as Minazal I, Eta Hydrae as Minazal II, Epsilon Hydrae as Minazal III, Rho Hydrae as Minazal IV and Zeta Hydrae as Minazal V.[15])

In Chinese, 柳宿 (Liǔ Sù), meaning Willow (asterism), refers to an asterism consisting of Sigma Hydrae, Delta Hydrae, Eta Hydrae, Rho Hydrae, Epsilon Hydrae, Zeta Hydrae, Omega Hydrae and Theta Hydrae.[16] Consequently, Sigma Hydrae itself is known as 柳宿二 (Liǔ Sù èr, English: the Second Star of Willow).[17]

The people of Groote Eylandt, used the name Unwala ("The Crab") for the star cluster including this star, Delta Hydrae (Lisan al Shudja), Epsilon Hydrae, Zeta Hydrae, Eta Hydrae and Rho Hydrae.[18]


This is an evolved K-type giant star with a stellar classification of K2 III.[3] The measured angular diameter of this star, after correction for limb darkening, is 2.25±0.03 mas.[19] At its estimated distance, this yields a physical size of about 27.6 times the radius of the Sun.[7] It has about three times the mass of the Sun and radiates 295 times the solar luminosity from its outer atmosphere at an effective temperature of 4,491 K. Sigma Hydrae is around 430 million years old.[6]


  1. ^ a b c d e f van Leeuwen, F. (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.
  2. ^ a b c d Mermilliod, J.-C. (1986), Compilation of Eggen's UBV data, transformed to UBV (unpublished), SIMBAD, Bibcode:1986EgUBV........0M.
  3. ^ a b Houk, N.; Swift, C. (1999), "Michigan catalogue of two-dimensional spectral types for the HD Stars", Michigan Spectral Survey, 5, Bibcode:1999MSS...C05....0H.
  4. ^ a b Famaey, B.; et al. (January 2005), "Local kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the concept of superclusters", Astronomy and Astrophysics, 430 (1): 165–186, arXiv:astro-ph/0409579, Bibcode:2005A&A...430..165F, doi:10.1051/0004-6361:20041272
  5. ^ 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.
  6. ^ a b Luck, R. Earle (September 2015), "Abundances in the Local Region. I. G and K Giants", The Astronomical Journal, 150 (3): 23, arXiv:1507.01466, Bibcode:2015AJ....150...88L, doi:10.1088/0004-6256/150/3/88, 88.
  7. ^ a b Lang, Kenneth R. (2006), Astrophysical formulae, Astronomy and astrophysics library, 1 (3rd ed.), Birkhäuser, ISBN 3-540-29692-1. The radius (R*) is given by:
  8. ^ a b Prugniel, Ph.; et al. (July 2011), "The atmospheric parameters and spectral interpolator for the MILES stars", Astronomy & Astrophysics, 531: A165, arXiv:1104.4952, Bibcode:2011A&A...531A.165P, doi:10.1051/0004-6361/201116769.
  9. ^ "* sig Hya". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2017-01-08.
  10. ^ a b "Naming Stars". Retrieved 16 December 2017.
  11. ^ Eggleton, P. P.; Tokovinin, A. A. (September 2008), "A catalogue of multiplicity among bright stellar systems", Monthly Notices of the Royal Astronomical Society, 389 (2): 869–879, arXiv:0806.2878, Bibcode:2008MNRAS.389..869E, doi:10.1111/j.1365-2966.2008.13596.x.
  12. ^ [1]
  13. ^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
  14. ^ Allen, R. H. (1963). Star Names: Their Lore and Meaning (Reprint ed.). New York, NY: Dover Publications Inc. p. 249. ISBN 0-486-21079-0. Retrieved 2010-12-12.
  15. ^ Rhoads, Jack W. (November 15, 1971), Technical Memorandum 33-507-A Reduced Star Catalog Containing 537 Named Stars (PDF), Jet Propulsion Laboratory, California Institute of Technology, retrieved 2017-01-09.
  16. ^ (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.
  17. ^ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 28 日
  18. ^ Selin, Helaine, ed. (1997), Encyclopaedia of the history of science, technology, and medicine in non-western cultures, Dordrecht: Kluwer Academic Publishers, p. 105.
  19. ^ Richichi, A.; et al. (February 2005), "CHARM2: An updated Catalog of High Angular Resolution Measurements", Astronomy and Astrophysics, 431 (2): 773–777, Bibcode:2005A&A...431..773R, doi:10.1051/0004-6361:20042039.