51 Hydrae

51 Hydrae is a single[7] star in the equatorial constellation of Hydra,[6] located 170 light years away from the Sun.[1] It has the Bayer designation k Hydrae; 51 Hydrae is the Flamsteed designation.[6] 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.[2] Eggen (1971) listed it as a member of the η Cephei group of old-disk stars.[8]

This article is about k Hydrae. It is not to be confused with κ (kappa) Hydrae.
51 Hydrae
Observation data
Epoch J2000      Equinox J2000
Constellation Hydra
Right ascension 14h 23m 05.77393s[1]
Declination −27° 45′ 14.4609″[1]
Apparent magnitude (V) 4.78[2]
Characteristics
Evolutionary stage giant
Spectral type K4 III[3]
B−V color index 1.300±0.040[2]
Astrometry
Radial velocity (Rv)19.7±0.9[2] km/s
Proper motion (μ) RA: −202.476[1] mas/yr
Dec.: −117.186[1] mas/yr
Parallax (π)19.2042 ± 0.2252 mas[1]
Distance170 ± 2 ly
(52.1 ± 0.6 pc)
Absolute magnitude (MV)1.36[2]
Details
Mass1.31[4] M☉
Radius13.47+0.48
−1.26[1] R☉
Luminosity54.9+0.8
−1.5[1] L☉
Surface gravity (log g)2.25[5] cgs
Temperature4,255±31[4] K
Metallicity [Fe/H]0.08[5] dex
Other designations
k Hya, 51 Hya, NSV 6648, CD−27°9803, HD 125932, HIP 70306, HR 5381, SAO 182483[6]
Database references
SIMBADdata

This is an evolved giant star with a stellar classification of K4 III,[3] which indicates it has exhausted the supply of hydrogen at its core and expanded off the main sequence. It has 1.31[4] times the mass of the Sun but has swollen to 13.5[1] times the Sun's radius. The star is radiating 55[1] times the luminosity of the Sun from its enlarged photosphere at an effective temperature of 4,255 K.[4]

References
  1. Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
  2. 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.
  3. Keenan, Philip C.; McNeil, Raymond C. (1989), "The Perkins Catalog of Revised MK Types for the Cooler Stars", The Astrophysical Journal Supplement Series, 71: 245, Bibcode:1989ApJS...71..245K, doi:10.1086/191373.
  4. Luck, R. Earle (2015), "Abundances in the Local Region. I. G and K Giants", Astronomical Journal, 150 (3), 88, arXiv:1507.01466, Bibcode:2015AJ....150...88L, doi:10.1088/0004-6256/150/3/88, S2CID 118505114
  5. McWilliam, Andrew (December 1990), "High-resolution spectroscopic survey of 671 GK giants. I - Stellar atmosphere parameters and abundances", Astrophysical Journal Supplement Series, 74: 1075–1128, Bibcode:1990ApJS...74.1075M, doi:10.1086/191527.
  6. "k Hya". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved May 3, 2019.
  7. 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, S2CID 14878976.
  8. Eggen, O. J. (June 1971), "The ζ Herculis, σ Puppis, ∈ Indi, and η Cephei Groups of Old Disk Population Stars", Publications of the Astronomical Society of the Pacific, 83 (493): 251, Bibcode:1971PASP...83..251E, doi:10.1086/129119.
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