51 Hydrae

This article is about k Hydrae and is not to be confused with κ (kappa) 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]

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]
Distance	170 ± 2 ly (52.1 ± 0.6 pc)
Absolute magnitude (MV)	1.36[2]
Details
Mass	1.31[4] M☉
Radius	13.47+0.48 −1.26[1] R☉
Luminosity	54.9+0.8 −1.5[1] L☉
Surface gravity (log g)	2.25[5] cgs
Temperature	4,255±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
SIMBAD	data

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]