{{Short description|Binary star in the constellation Hydra}}{{confuse|R Hydrae}} {{Starbox begin | name = Rho Hydrae }} {{Starbox image | image= {{Location mark |image=Hydra_IAU.svg|alt=|float=center|width=350 |label=|position=right |mark=Red circle.svg|mark_width=8|mark_link=Alpha1 |x=822|y=134 }} |caption=Location of ρ Hydrae (circled) }} {{Starbox observe | epoch = [[J2000.0]] | equinox = [[J2000.0]] ([[International Celestial Reference System|ICRS]]) | constell = [[Hydra (constellation)|Hydra]] | ra = {{RA|08|48|25.97057}}<ref name=vanLeeuwen2007/> | dec = {{DEC|+05|50|16.1283}}<ref name=vanLeeuwen2007/> | appmag_v = 4.34<ref name=Mermilliod1986/> }} {{Starbox character | type = [[subgiant]]<ref name=Zorec2012/> | class = A0 Vn<ref name=Zorec2012/> | b-v = −0.04<ref name=Mermilliod1986/> | u-b = −0.04<ref name=Mermilliod1986/> | r-i = | variable = }} {{Starbox astrometry | radial_v = +32.8<ref name=Wilson1953/> | prop_mo_ra = −17.33<ref name=vanLeeuwen2007/> | prop_mo_dec = −29.41<ref name=vanLeeuwen2007/> | parallax = 9.21 | p_error = 0.21 | parallax_footnote = <ref name=vanLeeuwen2007/> | absmag_v = −0.83<ref name=Anderson2012/> }} {{Starbox detail | source = <ref name=Zorec2012/> | mass = {{Val|3.24|0.05}} | radius = 5.04<ref name=tycho-gaia/> | luminosity = 242 | temperature = 9,795 | gravity = | metal_fe = | rotational_velocity = 128 | age_myr = 350<ref name=Gontcharov2012/> }} {{Starbox catalog | names = [[Bayer designation|ρ Hya]], [[Flamsteed designation|13 Hya]], [[Durchmusterung|BD]]+06°2040, [[Henry Draper Catalogue|HD]] 75137, [[Hipparcos Catalogue|HIP]] 43234, [[Bright Star Catalogue|HR]] 3492, [[Smithsonian Astrophysical Observatory Star Catalog|SAO]] 117146<ref name=SIMBAD/> }} {{Starbox reference | Simbad = rho+Hya }} {{starbox end}} '''Rho Hydrae''', [[Bayer designation|equally written]] '''ρ Hydrae''', is a [[binary star]]<ref name=Eggleton2008/> in the equatorial [[constellation]] of [[Hydra (constellation)|Hydra]]. It is visible to the naked eye with an [[apparent visual magnitude]] of 4.34.<ref name=Mermilliod1986/> The distance to this system, based upon an annual [[stellar parallax|parallax shift]] of 9.21 [[milliarcsecond|mas]],<ref name=vanLeeuwen2007/> is about 354 [[light year]]s. At that distance, the visual magnitude is diminished by an interstellar [[Extinction (astronomy)|extinction]] of 0.06 magnitudes, due to intervening dust.<ref name=Gontcharov2012/><ref name=Zorec2012/>
The primary component has an [[stellar classification]] of A0 Vn, suggesting that it is an [[A-type main sequence star]],<ref name=Zorec2012/> but [[stellar evolution]] models indicate it has left the [[main sequence]] and is thus a [[subgiant star]].<ref name=Zorec2012/> It has 5.04 the [[solar radius|radius of the Sun]]<ref name=tycho-gaia/> and 3.2 times the [[solar mass|Sun's mass]]. Rho Hydrae is around 350 million years old<ref name=Gontcharov2012/> and has a high rate of spin, with a [[projected rotational velocity]] of 128 km/s. It radiates 242 times the [[solar luminosity]] from its [[stellar atmosphere|outer atmosphere]] at an [[effective temperature]] of 9,795 K.<ref name=Zorec2012/> The companion is a magnitude 11.9 star at an [[angular separation]] of 12.1 [[arc second]]s along a [[position angle]] of 146°, as of 2000.<ref name=WDSC2014/>
==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|Epsilon Hydrae (ε Hya)]]. This shape in the Greco-Roman tradition, which draws on trading and navigation histories shared with nearby older-recorded astrologies is an [[asterism (astronomy)|asterism]] that represents the head of the water snake.<ref name=Burnham2013/>
This light source, along with comparable strength (apparent magnitude) Epsilon, [[Delta Hydrae|δ Hya]] (Lisan al Sudja), [[Zeta Hydrae|ζ Hya]], [[Eta Hydrae|η Hya]], and [[Sigma Hydrae|σ Hya]] (Minchir), were Ulug Beg's ''Min al Azʽal'', "Belonging to the Uninhabited Spot".<ref name=allen1963/> 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: [[Delta Hydrae|δ Hya]] as ''Minazal I'', [[Eta Hydrae|η Hya]] as ''Minazal II'', [[Epsilon Hydrae|ε Hya]] as ''Minazal III'', ρ Hya as ''Minazal IV'' and [[Zeta Hydrae|ζ Hya]] as ''Minazal V'' (exclude [[Sigma Hydrae|σ Hya]]).<ref name=rhoads1971/>
In [[Chinese language|Chinese]], {{lang|zh|柳宿}} ({{lang|zh-Latn|Liǔ Sù}}), meaning ''[[Willow (Chinese constellation)|Willow]]'', refers to an [[asterism (astronomy)|asterism]] consisting of ρ Hydrae, [[Delta Hydrae|δ Hydra]], [[Sigma Hydrae|σ Hydrae]], [[Eta Hydrae|η Hydrae]], [[Epsilon Hydrae|ε Hydrae]], [[Zeta Hydrae|ζ Hydrae]], [[Omega Hydrae|ω Hydrae]] and [[Theta Hydrae|θ Hydrae]]<ref>{{in lang|zh}} ''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}}.</ref> Consequently, ρ Hydrae itself is known as {{lang|zh|柳宿四}} ({{lang|zh-Latn|Liǔ Sù sì}}, {{langx|en|the Fourth Star of Willow}}).<ref>{{in lang|zh}} [http://aeea.nmns.edu.tw/2006/0605/ap060528.html AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 28 日] {{Webarchive|url=https://web.archive.org/web/20110522112415/http://aeea.nmns.edu.tw/2006/0605/ap060528.html |date=2011-05-22 }}</ref>
The people of [[Groote Eylandt]] called ''Unwala'', "The Crab", for the star cluster including this star, [[Delta Hydrae|δ Hya]] (Lisan al Sudja), [[Epsilon Hydrae|ε Hya]] (Ashlesha), [[Eta Hydrae|η Hya]], [[Zeta Hydrae|ζ Hya]] and [[Sigma Hydrae|σ Hya]] (Minchir).<ref name=selin1997/>
==References== <references> <ref name=allen1963>{{citation | last=Allen | first=R. H. | year=1963 | author-link=Richard Hinckley Allen | title=Star Names: Their Lore and Meaning | url=https://archive.org/details/starnamestheirlo00alle/page/249 | access-date=2010-12-12 | edition=Reprint | publisher=Dover Publications Inc | location=New York, NY | isbn=0-486-21079-0 | page=[https://archive.org/details/starnamestheirlo00alle/page/249 249] | url-access=registration }}</ref>
<ref name=rhoads1971>{{citation | url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19720005197_1972005197.pdf | first1=Jack W. | last1=Rhoads | title=Technical Memorandum 33-507-A Reduced Star Catalog Containing 537 Named Stars | publisher=Jet Propulsion Laboratory, California Institute of Technology | date=November 15, 1971 }}</ref>
<ref name=selin1997>{{citation | page=105 | url=https://books.google.com/books?id=raKRY3KQspsC | title=Encyclopaedia of the history of science, technology, and medicine in non-western cultures | editor-first=Helaine | editor-last=Selin | editor-link=Helaine Selin | location=Dordrecht | publisher=Kluwer Academic Publishers | year=1997 |isbn = 9780792340669}}</ref>
<ref name=vanLeeuwen2007>{{citation | title=Validation of the new Hipparcos reduction | last1=van Leeuwen | first1=F. | journal=Astronomy and Astrophysics | volume=474 | issue=2 | pages=653–664 | date=2007 | arxiv=0708.1752 | bibcode=2007A&A...474..653V | doi=10.1051/0004-6361:20078357 | s2cid=18759600 | postscript=. }}</ref>
<ref name=Anderson2012>{{citation | title=XHIP: An extended hipparcos compilation | last1=Anderson | first1=E. | last2=Francis | first2=Ch. | journal=Astronomy Letters | volume=38 | issue=5 | pages=331 | year=2012 | bibcode=2012AstL...38..331A | doi=10.1134/S1063773712050015 | arxiv=1108.4971 | s2cid=119257644 | postscript=. }}</ref>
<ref name=tycho-gaia>{{citation | title=Fundamental parameters and infrared excesses of Tycho-Gaia stars | display-authors=1 | last1=McDonald | first1=I. | last2=Zijlstra | first2=A. A. | last3=Watson | first3=R. A. | journal=Monthly Notices of the Royal Astronomical Society | volume=471 | pages=770–791 | date=October 2017 | issue=1 | doi=10.1093/mnras/stx1433 | doi-access=free | issn=0035-8711 | arxiv=1706.02208 | bibcode=2017MNRAS.471..770M | postscript=. }} [https://vizier.cds.unistra.fr/viz-bin/VizieR?-source=J/MNRAS/471/770&RAJ2000=132.108211 Rho Hydrae's database entry] at [[VizieR]].</ref>
<ref name=WDSC2014>{{citation | display-authors=1 | last1=Mason | first1=B. D. | last2=Wycoff | first2=G. L. | last3=Hartkopf | first3=W. I. | last4=Douglass | first4=G. G. | last5=Worley | first5=C. E. | title=The Washington Visual Double Star Catalog | year=2014 | bibcode=2001AJ....122.3466M | doi=10.1086/323920 | postscript=. | volume=122 | journal=The Astronomical Journal | issue=6 | pages=3466–3471| doi-access=free }}</ref>
<ref name=Eggleton2008>{{citation | last1=Eggleton | first1=P. P. | last2=Tokovinin | first2=A. A. | title=A catalogue of multiplicity among bright stellar systems | journal=[[Monthly Notices of the Royal Astronomical Society]] | volume=389 | issue=2 | pages=869–879 | date=September 2008 | doi=10.1111/j.1365-2966.2008.13596.x | doi-access=free | bibcode=2008MNRAS.389..869E | arxiv=0806.2878 | s2cid=14878976 | postscript=. }}</ref>
<ref name=Burnham2013>{{citation | title=Burnham's Celestial Handbook | series=An Observer's Guide to the Universe Beyond the Solar System | first1=Robert | last1=Burnham | postscript=. | publisher=Courier Corporation | volume=2 | year=2013 | isbn=978-0486317939 | page=1014 | url=https://books.google.com/books?id=aLbDAgAAQBAJ&pg=PA1014 }}</ref>
<ref name=Gontcharov2012>{{citation | title=Spatial distribution and kinematics of OB stars | last1=Gontcharov | first1=G. A. | journal=Astronomy Letters | volume=38 | issue=11 | pages=694–706 | date=November 2012 | doi=10.1134/S1063773712110035 | bibcode=2012AstL...38..694G | arxiv=1606.09028 | s2cid=119108982 | postscript=. }}</ref>
<ref name=Wilson1953>{{citation | last=Wilson | first=Ralph Elmer | year=1953 | title=General Catalogue of Stellar Radial Velocities | journal=Carnegie Institute Washington D.C. Publication | publisher=Carnegie Institution of Washington | location=Washington | bibcode=1953GCRV..C......0W | postscript=. }}</ref>
<ref name=Zorec2012>{{citation | last1=Zorec | first1=J. | last2=Royer | first2=F. | title=Rotational velocities of A-type stars. IV. Evolution of rotational velocities | journal=Astronomy & Astrophysics | volume=537 | page=A120 |date=January 2012 | doi=10.1051/0004-6361/201117691 | bibcode=2012A&A...537A.120Z | arxiv=1201.2052 | s2cid=55586789 | postscript=. }}</ref>
<ref name=Mermilliod1986>{{citation | last1=Mermilliod | first1=J.-C. | title=Compilation of Eggen's UBV data, transformed to UBV (unpublished) | journal=Catalogue of Eggen's UBV Data | year=1986 | publisher=[[SIMBAD]] | bibcode=1986EgUBV........0M | postscript=. }}</ref>
<ref name=SIMBAD>{{cite simbad | title=rho Hya | access-date=2017-01-08 | postscript=. }}</ref> </references>
{{Stars of Hydra|state=collapsed}}
[[Category:A-type subgiants]] [[Category:Binary stars]] [[Category:Hydra (constellation)]] [[Category:Bayer objects|Hydrae, Rho]] [[Category:Durchmusterung objects]] [[Category:Flamsteed objects|Hydrae, 13]] [[Category:Henry Draper Catalogue objects|075137]] [[Category:Hipparcos objects|043234]] [[Category:Bright Star Catalogue objects|3492]] [[Category:Stars with proper names|Minazal IV]]