Mu Hydrae

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μ Hydrae
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Hydra
Right ascension 10h 26m 05.42630s[1]
Declination −14° 19′ 56.2675″[1]
Apparent magnitude (V) 3.83[2]
Spectral type K4 III[3]
U−B color index +1.83[2]
B−V color index +1.47[2]
Variable type Suspected[4]
Radial velocity (Rv) +40.81±0.36[5] km/s
Proper motion (μ) RA: −129.17[1] mas/yr
Dec.: −79.76[1] mas/yr
Parallax (π) 13.93 ± 0.18[1] mas
Distance 234 ± 3 ly
(71.8 ± 0.9 pc)
Radius 45 R
Surface gravity (log g) 1.5 cgs
Temperature 3,999±8 K
Metallicity [Fe/H] −0.12 dex
Rotational velocity (v sin i) 6.0 km/s
Other designations
μ Hya, 42 Hydrae, BD−16° 3052, FK5 389, HD 90432, HIP 51069, HR 4094, SAO 155980.[6]
Database references

μ Hydrae, Latinised as Mu Hydrae, is a solitary[7] star in the equatorial constellation of Hydra. It is visible to the naked eye with an apparent visual magnitude of 3.83.[2] Positioned just 1.8° to the south-southwest is the planetary nebula NGC 3242.[8] Mu Hydrae has an annual parallax shift of 13.93 mas,[1] which yields a distance estimate of 234 light years.

This is an evolved K-type giant star with a stellar classification of K4 III,[3] having used up its core hydrogen and has expanded to around 45 times the radius of the Sun.[5] It is a suspected variable star, with a brightness that varies about 0.03 in magnitude.[4] The relatively cool outer atmosphere has an effective temperature of 3999 K.[5]


  1. ^ a b c d e f van Leeuwen, F. (2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752Freely accessible, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357. 
  2. ^ a b c d Mermilliod, J.-C. (1986), Compilation of Eggen's UBV data, transformed to UBV (unpublished), SIMBAD, Bibcode:1986EgUBV........0M. 
  3. ^ a b Houk, N.; Smith-Moore, M. (1988), University of Michigan Catalogue of two-dimensional spectral types for the HD stars. Declinations -26°.0 to -12°.0, 4, Department of Astronomy, University of Michigan, 
  4. ^ a b Samus, N. N.; Durlevich, O. V.; et al. (2009), General Catalogue of Variable Stars, 1, Bibcode:2009yCat....102025S. 
  5. ^ a b c d Massarotti, Alessandro; et al. (January 2008), "Rotational and radial velocities for a sample of 761 HIPPARCOS giants and the role of binarity", The Astronomical Journal, 135 (1): 209–231, Bibcode:2008AJ....135..209M, doi:10.1088/0004-6256/135/1/209. 
  6. ^ "mu. Hya -- Variable Star", SIMBAD Astronomical Database, Centre de Données astronomiques de Strasbourg, retrieved 2017-01-06. 
  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:0806.2878Freely accessible, Bibcode:2008MNRAS.389..869E, doi:10.1111/j.1365-2966.2008.13596.x. 
  8. ^ O'Meara, Steve (2007), Herschel 400 Observing Guide, Cambridge University Press, p. 107, ISBN 0521858933.