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Delta Hydrae
Observation data; Epoch J2000.0 Equinox J2000.0 (ICRS)
Constellation	Hydra
Right ascension	08h 37m 39.36627s
Declination	+05° 42′ 13.6057″
Apparent magnitude (V)	4.146
Characteristics
Spectral type	A1 Vnn
U−B color index	+0.003
B−V color index	+0.008
Astrometry
Radial velocity (Rv)	4.10 km/s
Proper motion (μ)	RA: −70.19 mas/yr ; Dec.: −7.90 mas/yr
Parallax (π)	20.34 ± 0.63 mas
Distance	160 ± 5 ly ; (49 ± 2 pc)
Absolute magnitude (MV)	+0.68
Details
Mass	2.88 M☉
Radius	2.7 R☉
Luminosity	42.7 L☉
Temperature	11,055±376 K
Rotational velocity (v sin i)	285 km/s
Age	244 Myr
Other designations
	δ Hya, 4 Hydrae, BD+06° 2001, FK5 1223, HD 73262, HIP 42313, HR 3410, SAO 116965.
Database references
SIMBAD	data

Delta Hydrae, Latinized from δ Hydrae, is a double star in the equatorial constellation of Hydra. It is visible to the naked eye with an apparent visual magnitude of 4.146.^[2] Based upon an annual parallax shift of 20.34 mas, it is located about 160 light years from the Sun.

This is a double star^[10] with an angular separation of 2.6±0.1 arc second along a position angle of 265.1°±1.0°, as of 2003.^[11] The brighter component is an A-type main sequence star with a stellar classification of A1 Vnn.^[3] It is spinning rapidly with a projected rotational velocity of 285 km/s. This is giving the star an oblate shape with an equatorial bulge that is 20% larger than the polar radius.^[3] It has an estimated 2.88^[6] times the mass of the Sun and 2.7^[7] times the Sun's radius.^[7] The star is about 244 million years old and it radiates 42.7^[8] times the solar luminosity from its outer atmosphere at an effective temperature of 11,055 K.^[6]

The companion has a visual magnitude of 11.15.^[10] X-ray emissions have been detected from this location in space, which may be coming from a companion star.^[11]

Name and etymology

In the catalogue of stars in the Calendarium of Al Achsasi Al Mouakket, this star was designated Lisan al Shudja, which was translated into Latin as Lingua Hydri, meaning the snake's tongue.^[12] This star, along with ε Hya, ζ Hya, η Hya, ρ Hya and σ Hya (Minchir), were Ulugh Beg's Min al Azʽal, "Belonging to the Uninhabited Spot".^[13]

According to the catalogue of stars in the Technical Memorandum 33-507 - A Reduced Star Catalog Containing 537 Named Stars, Min al Azʽal or Minazal were the title for five stars :δ Hya as Minazal I, η Hya as Minazal II, ε Hya as Minazal III, ρ Hya as Minazal IV and ζ Hya as Minazal V (exclude σ Hya)^[14]

In Chinese, 柳宿 (Liǔ Sù), meaning Willow (asterism), refers to an asterism consisting of δ Hydrae, σ Hydrae, η Hydrae, ρ Hydrae, ε Hydrae, ζ Hydrae, ω Hydrae and θ Hydrae^[15] Consequently, δ Hydrae itself is known as 柳宿一 (Liǔ Sù yī, English: the First Star of Willow.)^[16]

The people of Groote Eylandt called Unwala, "The Crab", for the star cluster including this star, ε Hya, ζ Hya, η Hya, ρ Hya and σ Hya (Minchir).^[17]

References

^ ^a ^b ^c ^d ^e 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, S2CID 18759600.
^ ^a ^b ^c ^d Cousins, A. W. J. (1984), "Standardization of Broadband Photometry of Equatorial Standards", South African Astronomical Observatory Circulars, 8: 59, Bibcode:1984SAAOC...8...59C.
^ ^a ^b ^c ^d Belle, G. T. (2012), "Interferometric observations of rapidly rotating stars", The Astronomy and Astrophysics Review, 20 (1): 51, arXiv:1204.2572, Bibcode:2012A&ARv..20...51V, doi:10.1007/s00159-012-0051-2, S2CID 119273474.
^ de Bruijne, J. H. J.; Eilers, A.-C. (October 2012), "Radial velocities for the HIPPARCOS-Gaia Hundred-Thousand-Proper-Motion project", Astronomy & Astrophysics, 546: 14, arXiv:1208.3048, Bibcode:2012A&A...546A..61D, doi:10.1051/0004-6361/201219219, S2CID 59451347, A61.
^ 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.
^ ^a ^b ^c ^d ^e 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.
^ ^a ^b ^c Pasinetti Fracassini, L. E.; et al. (2001), "Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics", Astronomy & Astrophysics, 367 (2) (3rd ed.): 521–24, arXiv:astro-ph/0012289, Bibcode:2001A&A...367..521P, doi:10.1051/0004-6361:20000451, S2CID 425754.
^ ^a ^b McDonald, I.; et al. (2012), "Fundamental Parameters and Infrared Excesses of Hipparcos Stars", Monthly Notices of the Royal Astronomical Society, 427 (1): 343–57, arXiv:1208.2037, Bibcode:2012MNRAS.427..343M, doi:10.1111/j.1365-2966.2012.21873.x, S2CID 118665352.
^ "* del Hya". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2017-01-03.
^ ^a ^b Mason, B. D.; et al. (2014), "The Washington Visual Double Star Catalog", The Astronomical Journal, 122 (6): 3466, Bibcode:2001AJ....122.3466M, doi:10.1086/323920, retrieved 2015-07-22
^ ^a ^b De Rosa, R. J.; et al. (July 2011), "The Volume-limited A-Star (VAST) survey - I. Companions and the unexpected X-ray detection of B6-A7 stars", Monthly Notices of the Royal Astronomical Society, 415 (1): 854–866, arXiv:1103.4363, Bibcode:2011MNRAS.415..854D, doi:10.1111/j.1365-2966.2011.18765.x, S2CID 84181878.
^ Knobel, E. B. (June 1895), "Al Achsasi Al Mouakket, on a catalogue of stars in the Calendarium of Mohammad Al Achsasi Al Mouakket", Monthly Notices of the Royal Astronomical Society, 55: 429, Bibcode:1895MNRAS..55..429K, doi:10.1093/mnras/55.8.429.
^ Allen, R. H. (1963), Star Names: Their Lore and Meaning (Reprint ed.), New York: Dover Publications Inc, p. 249, ISBN 0-486-21079-0, retrieved 2010-12-12.
^ Jack W. Rhoads - Technical Memorandum 33-507-A Reduced Star Catalog Containing 537 Named Stars, Jet Propulsion Laboratory, California Institute of Technology; November 15, 1971
^ (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.
^ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 28 日 Archived 2011-05-22 at the Wayback Machine
^ Helaine Selin, ed., Encyclopaedia of the history of science, technology, and medicine in non-western cultures, Dordrecht, Kluwer Academic Publishers, 1997, p.105.