{{Short description|Star in the constellation Hydra}} {{about-distinguish2|k Hydrae|[[Kappa Hydrae|κ (kappa) Hydrae]]}} {{Starbox begin}} {{Starbox image | image = {{Location mark |image=Libra constellation map.svg|alt=|float=center|width=280 |label=|position=right |mark=Red circle.svg|mark_width=10|mark_link=51 Hydrae |x=840|y=785 }} | caption = Location of 51 Hydrae (circled in red) }} {{Starbox observe | epoch = J2000 | constell = [[Hydra (constellation)|Hydra]] | ra = {{RA|14|23|05.77393}}<ref name=GaiaDR2/> | dec = {{DEC|−27|45|14.4609}}<ref name=GaiaDR2/> | appmag_v = 4.78<ref name=Anderson2012/> }} {{Starbox character | type = giant | class = K4 III<ref name=Keenan1989/> | b-v = {{Val|1.300|0.040}}<ref name=Anderson2012/> | u-b = | variable = }} {{Starbox astrometry | radial_v = {{Val|19.7|0.9}}<ref name=Anderson2012/> | prop_mo_ra = −202.476<ref name=GaiaDR2/> | prop_mo_dec = −117.186<ref name=GaiaDR2/> | parallax = 19.2042 | p_error = 0.2252 | parallax_footnote = <ref name=GaiaDR2/> | absmag_v = 1.36<ref name=Anderson2012/> }} {{Starbox detail | source = | mass = 1.31<ref name=Luck2015/> | radius = {{Val|13.47|0.48|1.26}}<ref name=GaiaDR2/> | luminosity = {{Val|54.9|0.8|1.5}}<ref name=GaiaDR2/> | temperature = {{Val|4255|31|fmt=commas}}<ref name=Luck2015/> | metal_fe = 0.08<ref name=McWilliam1990/> | gravity = 2.25<ref name=McWilliam1990/> | rotational_velocity = | rotation = | age_gyr = }} {{Starbox catalog | names = {{odlist | B=k Hya | F=51 Hya | CD=-27°9803 | HD=125932 | HIP=70306 | HR=5381 | SAO=182483 | NSV=6648 }}<ref name=SIMBAD/> }} {{Starbox reference | Simbad=k+Hya }} {{Starbox end}}
'''51 Hydrae''' is a single<ref name=Eggleton2008/> [[star]] in the [[celestial equator|equatorial]] [[constellation]] of [[Hydra (constellation)|Hydra]],<ref name=SIMBAD/> located 170 [[light year]]s away from the Sun.<ref name=GaiaDR2/> It has the [[Bayer designation]] '''k Hydrae'''; ''51 Hydrae'' is the [[Flamsteed designation]].<ref name=SIMBAD/> This object is visible to the naked eye as a faint, orange-hued star with an [[apparent visual magnitude]] of 4.78. It is moving further from the Earth with a heliocentric [[radial velocity]] of +20 km/s.<ref name=Anderson2012/> Eggen (1971) listed it as a member of the [[Eta Cephei|η Cephei]] group of [[old-disk star]]s.<ref name=Eggen1971/>
This is an [[stellar evolution|evolved]] [[giant star]] with a [[stellar classification]] of K4 III,<ref name=Keenan1989/> which indicates it has exhausted the supply of hydrogen at its [[stellar core|core]] and expanded off the [[main sequence]]. It has 1.31<ref name=Luck2015/> times the [[mass of the Sun]] but has swollen to 13.5<ref name=GaiaDR2/> times the [[Sun's radius]]. The star is radiating 55<ref name=GaiaDR2/> times the [[luminosity of the Sun]] from its enlarged [[photosphere]] at an [[effective temperature]] of 4,255 K.<ref name=Luck2015/>
==References== <references> <ref name=GaiaDR2>{{cite DR2|6269720523431409536}}</ref>
<ref name=SIMBAD>{{cite simbad | title=k Hya | access-date=May 3, 2019 }}</ref>
<ref name=Anderson2012>{{citation | title=XHIP: An extended hipparcos compilation | last1=Anderson | first1=E. | last2=Francis | first2=Ch. | journal=Astronomy Letters | postscript=. | volume=38 | issue=5 | pages=331 | year=2012 | bibcode=2012AstL...38..331A | arxiv=1108.4971 | doi=10.1134/S1063773712050015 | s2cid=119257644 }}</ref>
<ref name=Luck2015>{{citation | title=Abundances in the Local Region. I. G and K Giants | last1=Luck | first1=R. Earle | journal=Astronomical Journal | volume=150 | issue=3 | at=88 | year=2015 | arxiv=1507.01466 | bibcode=2015AJ....150...88L | doi=10.1088/0004-6256/150/3/88 | s2cid=118505114 }}</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 | postscript=. | bibcode=2008MNRAS.389..869E | arxiv=0806.2878 | s2cid=14878976 }}</ref>
<ref name=McWilliam1990>{{citation | last1=McWilliam | first1=Andrew | postscript=. | title=High-resolution spectroscopic survey of 671 GK giants. I - Stellar atmosphere parameters and abundances | journal=Astrophysical Journal Supplement Series | volume=74 | pages=1075–1128 | date=December 1990 | doi=10.1086/191527 | bibcode=1990ApJS...74.1075M | doi-access=free }}</ref>
<ref name=Keenan1989>{{citation | last1=Keenan | first1=Philip C. | last2=McNeil | first2=Raymond C. | title=The Perkins Catalog of Revised MK Types for the Cooler Stars | journal=The Astrophysical Journal Supplement Series | volume=71 | pages=245 | year=1989 | bibcode=1989ApJS...71..245K | postscript=. | doi=10.1086/191373}}</ref>
<ref name=Eggen1971>{{citation | title=The ζ Herculis, σ Puppis, ∈ Indi, and η Cephei Groups of Old Disk Population Stars | last1=Eggen | first1=O. J. | postscript=. | journal=Publications of the Astronomical Society of the Pacific | volume=83 | issue=493 | page=251 | date=June 1971 | doi=10.1086/129119 | bibcode=1971PASP...83..251E | doi-access=free }}</ref> </references>
{{Stars of Hydra}}
[[Category:K-type giants]] [[Category:Hydra (constellation)]] [[Category:Bayer objects|Hydrae, k]] [[Category:Durchmusterung objects|CD-27 09803]] [[Category:Flamsteed objects|Hydrae, 51]] [[Category:Henry Draper Catalogue objects|125932]] [[Category:Hipparcos objects|070306]] [[Category:Bright Star Catalogue objects|5381]]