# Rho Hydrae

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Binary star in the constellation Hydra

Not to be confused with [R Hydrae](/source/R_Hydrae).

Rho Hydrae Location of ρ Hydrae (circled) Observation data Epoch J2000.0 Equinox J2000.0 (ICRS) Constellation Hydra Right ascension 08h 48m 25.97057s[1] Declination +05° 50′ 16.1283″[1] Apparent magnitude (V) 4.34[2] Characteristics Evolutionary stage subgiant[3] Spectral type A0 Vn[3] U−B color index −0.04[2] B−V color index −0.04[2] Astrometry Radial velocity (Rv) +32.8[4] km/s Proper motion (μ) RA: −17.33[1] mas/yr Dec.: −29.41[1] mas/yr Parallax (π) 9.21±0.21 mas[1] Distance 354 ± 8 ly (109 ± 2 pc) Absolute magnitude (MV) −0.83[5] Details[3] Mass 3.24±0.05 M☉ Radius 5.04[6] R☉ Luminosity 242 L☉ Temperature 9,795 K Rotational velocity (v sin i) 128 km/s Age 350[7] Myr Other designations ρ Hya, 13 Hya, BD+06°2040, HD 75137, HIP 43234, HR 3492, SAO 117146[8] Database references SIMBAD data

**Rho Hydrae**, [equally written](/source/Bayer_designation) **ρ Hydrae**, is a [binary star](/source/Binary_star)[9] in the equatorial [constellation](/source/Constellation) of [Hydra](/source/Hydra_(constellation)). It is visible to the naked eye with an [apparent visual magnitude](/source/Apparent_visual_magnitude) of 4.34.[2] The distance to this system, based upon an annual [parallax shift](/source/Stellar_parallax) of 9.21 [mas](/source/Milliarcsecond),[1] is about 354 [light years](/source/Light_year). At that distance, the visual magnitude is diminished by an interstellar [extinction](/source/Extinction_(astronomy)) of 0.06 magnitudes, due to intervening dust.[7][3]

The primary component has an [stellar classification](/source/Stellar_classification) of A0 Vn, suggesting that it is an [A-type main sequence star](/source/A-type_main_sequence_star),[3] but [stellar evolution](/source/Stellar_evolution) models indicate it has left the [main sequence](/source/Main_sequence) and is thus a [subgiant star](/source/Subgiant_star).[3] It has 5.04 the [radius of the Sun](/source/Solar_radius)[6] and 3.2 times the [Sun's mass](/source/Solar_mass). Rho Hydrae is around 350 million years old[7] and has a high rate of spin, with a [projected rotational velocity](/source/Projected_rotational_velocity) of 128 km/s. It radiates 242 times the [solar luminosity](/source/Solar_luminosity) from its [outer atmosphere](/source/Stellar_atmosphere) at an [effective temperature](/source/Effective_temperature) of 9,795 K.[3] The companion is a magnitude 11.9 star at an [angular separation](/source/Angular_separation) of 12.1 [arc seconds](/source/Arc_second) along a [position angle](/source/Position_angle) of 146°, as of 2000.[10]

## Name and etymology

This system appears among bright stars in a compact pentagon, resembling a quadrilateral due to the suggestive proximity (close arc distance) to [Epsilon Hydrae (ε Hya)](/source/Epsilon_Hydrae). This shape in the Greco-Roman tradition, which draws on trading and navigation histories shared with nearby older-recorded astrologies is an [asterism](/source/Asterism_(astronomy)) that represents the head of the water snake.[11]

This light source, along with comparable strength (apparent magnitude) Epsilon, [δ Hya](/source/Delta_Hydrae) (Lisan al Sudja), [ζ Hya](/source/Zeta_Hydrae), [η Hya](/source/Eta_Hydrae), and [σ Hya](/source/Sigma_Hydrae) (Minchir), were Ulug Beg's *Min al Azʽal*, "Belonging to the Uninhabited Spot".[12] 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](/source/Delta_Hydrae) as *Minazal I*, [η Hya](/source/Eta_Hydrae) as *Minazal II*, [ε Hya](/source/Epsilon_Hydrae) as *Minazal III*, ρ Hya as *Minazal IV* and [ζ Hya](/source/Zeta_Hydrae) as *Minazal V* (exclude [σ Hya](/source/Sigma_Hydrae)).[13]

In [Chinese](/source/Chinese_language), 柳宿 (*Liǔ Sù*), meaning *[Willow](/source/Willow_(Chinese_constellation))*, refers to an [asterism](/source/Asterism_(astronomy)) consisting of ρ Hydrae, [δ Hydra](/source/Delta_Hydrae), [σ Hydrae](/source/Sigma_Hydrae), [η Hydrae](/source/Eta_Hydrae), [ε Hydrae](/source/Epsilon_Hydrae), [ζ Hydrae](/source/Zeta_Hydrae), [ω Hydrae](/source/Omega_Hydrae) and [θ Hydrae](/source/Theta_Hydrae)[14] Consequently, ρ Hydrae itself is known as 柳宿四 (*Liǔ Sù sì*, English: the Fourth Star of Willow).[15]

The people of [Groote Eylandt](/source/Groote_Eylandt) called *Unwala*, "The Crab", for the star cluster including this star, [δ Hya](/source/Delta_Hydrae) (Lisan al Sudja), [ε Hya](/source/Epsilon_Hydrae) (Ashlesha), [η Hya](/source/Eta_Hydrae), [ζ Hya](/source/Zeta_Hydrae) and [σ Hya](/source/Sigma_Hydrae) (Minchir).[16]

## References

1. ^ [***a***](#cite_ref-vanLeeuwen2007_1-0) [***b***](#cite_ref-vanLeeuwen2007_1-1) [***c***](#cite_ref-vanLeeuwen2007_1-2) [***d***](#cite_ref-vanLeeuwen2007_1-3) [***e***](#cite_ref-vanLeeuwen2007_1-4) [***f***](#cite_ref-vanLeeuwen2007_1-5) van Leeuwen, F. (2007), "Validation of the new Hipparcos reduction", *Astronomy and Astrophysics*, **474** (2): 653–664, [arXiv](/source/ArXiv_(identifier)):[0708.1752](https://arxiv.org/abs/0708.1752), [Bibcode](/source/Bibcode_(identifier)):[2007A&A...474..653V](https://ui.adsabs.harvard.edu/abs/2007A&A...474..653V), [doi](/source/Doi_(identifier)):[10.1051/0004-6361:20078357](https://doi.org/10.1051%2F0004-6361%3A20078357), [S2CID](/source/S2CID_(identifier)) [18759600](https://api.semanticscholar.org/CorpusID:18759600).

1. ^ [***a***](#cite_ref-Mermilliod1986_2-0) [***b***](#cite_ref-Mermilliod1986_2-1) [***c***](#cite_ref-Mermilliod1986_2-2) [***d***](#cite_ref-Mermilliod1986_2-3) Mermilliod, J.-C. (1986), "Compilation of Eggen's UBV data, transformed to UBV (unpublished)", *Catalogue of Eggen's UBV Data*, [SIMBAD](/source/SIMBAD), [Bibcode](/source/Bibcode_(identifier)):[1986EgUBV........0M](https://ui.adsabs.harvard.edu/abs/1986EgUBV........0M).

1. ^ [***a***](#cite_ref-Zorec2012_3-0) [***b***](#cite_ref-Zorec2012_3-1) [***c***](#cite_ref-Zorec2012_3-2) [***d***](#cite_ref-Zorec2012_3-3) [***e***](#cite_ref-Zorec2012_3-4) [***f***](#cite_ref-Zorec2012_3-5) [***g***](#cite_ref-Zorec2012_3-6) Zorec, J.; Royer, F. (January 2012), "Rotational velocities of A-type stars. IV. Evolution of rotational velocities", *Astronomy & Astrophysics*, **537**: A120, [arXiv](/source/ArXiv_(identifier)):[1201.2052](https://arxiv.org/abs/1201.2052), [Bibcode](/source/Bibcode_(identifier)):[2012A&A...537A.120Z](https://ui.adsabs.harvard.edu/abs/2012A&A...537A.120Z), [doi](/source/Doi_(identifier)):[10.1051/0004-6361/201117691](https://doi.org/10.1051%2F0004-6361%2F201117691), [S2CID](/source/S2CID_(identifier)) [55586789](https://api.semanticscholar.org/CorpusID:55586789).

1. **[^](#cite_ref-Wilson1953_4-0)** Wilson, Ralph Elmer (1953), "General Catalogue of Stellar Radial Velocities", *Carnegie Institute Washington D.C. Publication*, Washington: Carnegie Institution of Washington, [Bibcode](/source/Bibcode_(identifier)):[1953GCRV..C......0W](https://ui.adsabs.harvard.edu/abs/1953GCRV..C......0W).

1. **[^](#cite_ref-Anderson2012_5-0)** Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", *Astronomy Letters*, **38** (5): 331, [arXiv](/source/ArXiv_(identifier)):[1108.4971](https://arxiv.org/abs/1108.4971), [Bibcode](/source/Bibcode_(identifier)):[2012AstL...38..331A](https://ui.adsabs.harvard.edu/abs/2012AstL...38..331A), [doi](/source/Doi_(identifier)):[10.1134/S1063773712050015](https://doi.org/10.1134%2FS1063773712050015), [S2CID](/source/S2CID_(identifier)) [119257644](https://api.semanticscholar.org/CorpusID:119257644).

1. ^ [***a***](#cite_ref-tycho-gaia_6-0) [***b***](#cite_ref-tycho-gaia_6-1) McDonald, I.; et al. (October 2017), "Fundamental parameters and infrared excesses of Tycho-Gaia stars", *Monthly Notices of the Royal Astronomical Society*, **471** (1): 770–791, [arXiv](/source/ArXiv_(identifier)):[1706.02208](https://arxiv.org/abs/1706.02208), [Bibcode](/source/Bibcode_(identifier)):[2017MNRAS.471..770M](https://ui.adsabs.harvard.edu/abs/2017MNRAS.471..770M), [doi](/source/Doi_(identifier)):[10.1093/mnras/stx1433](https://doi.org/10.1093%2Fmnras%2Fstx1433), [ISSN](/source/ISSN_(identifier)) [0035-8711](https://search.worldcat.org/issn/0035-8711). [Rho Hydrae's database entry](https://vizier.cds.unistra.fr/viz-bin/VizieR?-source=J/MNRAS/471/770&RAJ2000=132.108211) at [VizieR](/source/VizieR).

1. ^ [***a***](#cite_ref-Gontcharov2012_7-0) [***b***](#cite_ref-Gontcharov2012_7-1) [***c***](#cite_ref-Gontcharov2012_7-2) Gontcharov, G. A. (November 2012), "Spatial distribution and kinematics of OB stars", *Astronomy Letters*, **38** (11): 694–706, [arXiv](/source/ArXiv_(identifier)):[1606.09028](https://arxiv.org/abs/1606.09028), [Bibcode](/source/Bibcode_(identifier)):[2012AstL...38..694G](https://ui.adsabs.harvard.edu/abs/2012AstL...38..694G), [doi](/source/Doi_(identifier)):[10.1134/S1063773712110035](https://doi.org/10.1134%2FS1063773712110035), [S2CID](/source/S2CID_(identifier)) [119108982](https://api.semanticscholar.org/CorpusID:119108982).

1. **[^](#cite_ref-SIMBAD_8-0)** ["rho Hya"](http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=rho+Hya). *[SIMBAD](/source/SIMBAD)*. [Centre de données astronomiques de Strasbourg](/source/Centre_de_donn%C3%A9es_astronomiques_de_Strasbourg). Retrieved 2017-01-08.{{[cite web](https://en.wikipedia.org/wiki/Template:Cite_web)}}: CS1 maint: postscript ([link](https://en.wikipedia.org/wiki/Category:CS1_maint:_postscript))

1. **[^](#cite_ref-Eggleton2008_9-0)** Eggleton, P. P.; Tokovinin, A. A. (September 2008), "A catalogue of multiplicity among bright stellar systems", *[Monthly Notices of the Royal Astronomical Society](/source/Monthly_Notices_of_the_Royal_Astronomical_Society)*, **389** (2): 869–879, [arXiv](/source/ArXiv_(identifier)):[0806.2878](https://arxiv.org/abs/0806.2878), [Bibcode](/source/Bibcode_(identifier)):[2008MNRAS.389..869E](https://ui.adsabs.harvard.edu/abs/2008MNRAS.389..869E), [doi](/source/Doi_(identifier)):[10.1111/j.1365-2966.2008.13596.x](https://doi.org/10.1111%2Fj.1365-2966.2008.13596.x), [S2CID](/source/S2CID_(identifier)) [14878976](https://api.semanticscholar.org/CorpusID:14878976).

1. **[^](#cite_ref-WDSC2014_10-0)** Mason, B. D.; et al. (2014), "The Washington Visual Double Star Catalog", *The Astronomical Journal*, **122** (6): 3466–3471, [Bibcode](/source/Bibcode_(identifier)):[2001AJ....122.3466M](https://ui.adsabs.harvard.edu/abs/2001AJ....122.3466M), [doi](/source/Doi_(identifier)):[10.1086/323920](https://doi.org/10.1086%2F323920).

1. **[^](#cite_ref-Burnham2013_11-0)** Burnham, Robert (2013), [*Burnham's Celestial Handbook*](https://books.google.com/books?id=aLbDAgAAQBAJ&pg=PA1014), An Observer's Guide to the Universe Beyond the Solar System, vol. 2, Courier Corporation, p. 1014, [ISBN](/source/ISBN_(identifier)) [978-0486317939](https://en.wikipedia.org/wiki/Special:BookSources/978-0486317939).

1. **[^](#cite_ref-allen1963_12-0)** [Allen, R. H.](/source/Richard_Hinckley_Allen) (1963), [*Star Names: Their Lore and Meaning*](https://archive.org/details/starnamestheirlo00alle/page/249) (Reprint ed.), New York, NY: Dover Publications Inc, p. [249](https://archive.org/details/starnamestheirlo00alle/page/249), [ISBN](/source/ISBN_(identifier)) [0-486-21079-0](https://en.wikipedia.org/wiki/Special:BookSources/0-486-21079-0), retrieved 2010-12-12 {{[citation](https://en.wikipedia.org/wiki/Template:Citation)}}: ISBN / Date incompatibility ([help](https://en.wikipedia.org/wiki/Help:CS1_errors#invalid_isbn_date))

1. **[^](#cite_ref-rhoads1971_13-0)** Rhoads, Jack W. (November 15, 1971), [*Technical Memorandum 33-507-A Reduced Star Catalog Containing 537 Named Stars*](https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19720005197_1972005197.pdf) (PDF), Jet Propulsion Laboratory, California Institute of Technology

1. **[^](#cite_ref-14)** (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](/source/ISBN_(identifier)) [978-986-7332-25-7](https://en.wikipedia.org/wiki/Special:BookSources/978-986-7332-25-7).

1. **[^](#cite_ref-15)** (in Chinese) [AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 28 日](http://aeea.nmns.edu.tw/2006/0605/ap060528.html) [Archived](https://web.archive.org/web/20110522112415/http://aeea.nmns.edu.tw/2006/0605/ap060528.html) 2011-05-22 at the [Wayback Machine](/source/Wayback_Machine)

1. **[^](#cite_ref-selin1997_16-0)** [Selin, Helaine](/source/Helaine_Selin), ed. (1997), [*Encyclopaedia of the history of science, technology, and medicine in non-western cultures*](https://books.google.com/books?id=raKRY3KQspsC), Dordrecht: Kluwer Academic Publishers, p. 105, [ISBN](/source/ISBN_(identifier)) [9780792340669](https://en.wikipedia.org/wiki/Special:BookSources/9780792340669)

v t e Constellation of Hydra List of stars in Hydra Hydra in Chinese astronomy Stars Bayer α (Alphard) β γ (Naga) δ ε (Ashlesha) ζ η θ ι (Ukdah) κ λ μ ν ξ ο π ρ σ (Minchir) τ1 τ2 υ1 (Zhang) υ2 φ1 φ2 φ3 χ1 χ2 ψ ω b1 b3 C F Flamsteed 1 2 3 6 (a) 9 10 12 (D) 14 15 17 19 20 21 23 24 25 26 27 (P) 28 29 33 (A) 34 37 44 47 48 50 51 (k) 52 (l) 54 (m) 55 56 57 58 (Solitaire) 59 60 10 Crt 17 Crt 20 Crt 2 Sex Variable R U V W TW EX HS KU LQ V361 V419 V421 V478 V484 HR 3378 3538 3749 (G) 3750 3858 (I) 3862 3919 3923 (Felis) 4162 4328 4339 (β Ant) 4445 5265 HD 72659 74156 82943 86226 86264 90156 (γ Ant) Other 2MASS 1114−2618 ESO 439-26 Gliese 328 Gliese 357 Gliese 433 Gliese 453 GJ 3634 HAT-P-30 HE 1256−2738 HE 1327−2326 LHS 2065 LHS 3003 LTT 3780 SDSS J090745.0+024507 WASP-25 WASP-36 WASP-84 WASP-166 (Filetdor) WISE 0855−0714 WISEA J1141−3326 Exoplanets Gliese 357 d HD 72659 b HD 74156 b c HD 82943 b c HD 86226 b HD 86264 b TW Hydrae b (unconfirmed) WASP-193b Star clusters Messier 48 Messier 68 NGC 5694 Price-Whelan 1 Nebulae Abell 33 Abell 35 NGC 3242 Southern Owl Nebula Galaxies Messier 83 NGC 2617 2642 2697 2708 2758 2835 2848 2865 2890 2935 2936 2937 2960 2962 2986 2992 3030 3052 3054 3081 3109 3124 3200 3285 3285B 3290 3305 3307 3308 3309 3311 3312 3313 3314 3315 3316 3336 3369 3383 3393 3402 3463 3464 3585 3621 3673 3717 3904 3923 3936 4980 4993 5042 5061 5078 5085 5101 5135 5260 5264 5495 5626 Other 3C 196.1 AM 1316−241 Arp 7 CTQ 327 ESO 510-G13 GAMA202627 GLEAM J0917−0012 Hydra A IC 535 MRC 1138−262 Peekaboo Galaxy PKS 1151−348 RX J0911.4+0551 SDSS J0841+0101 TN J0924−2201 Tololo 1247−232 ZwCl 1693 BCG Galaxy clusters Abell 754 Abell 3411 Abell 3412 Bullet Group Hickson 40 Astronomical events GW170817 Category

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Adapted from the Wikipedia article [Rho Hydrae](https://en.wikipedia.org/wiki/Rho_Hydrae) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Rho_Hydrae?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.