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γ Hydrae
	Location of γ Hydrae (circled)
Observation data; Epoch J2000.0 Equinox J2000.0
Constellation	Hydra
Right ascension	13h 18m 55.29719s
Declination	–23° 10′ 17.4514″
Apparent magnitude (V)	+2.993
Characteristics
Spectral type	G8 III
U−B color index	+0.645
B−V color index	+0.920
Astrometry
Radial velocity (Rv)	–5.4 km/s
Proper motion (μ)	RA: +68.99 mas/yr ; Dec.: –41.85 mas/yr
Parallax (π)	24.37 ± 0.15 mas
Distance	133.8 ± 0.8 ly ; (41.0 ± 0.3 pc)
Absolute magnitude (MV)	–0.15
Details
A
Mass	2.94 M☉
Radius	16±3 R☉
Luminosity	115 L☉
Surface gravity (log g)	3.02 cgs
Temperature	5,087 K
Metallicity [Fe/H]	–0.06 dex
Rotational velocity (v sin i)	8 km/s
Age	372 Myr
B
Mass	0.61+0.12; −0.14 M☉
Orbit
Primary	A
Companion	B
Semi-major axis (a)	67.5±0.6 or 159±7 AU
Other designations
	46 Hydrae, BD–22 3554, FK5 495, HD 115659, HIP 64962, HR 5020, NSV 6180, SAO 181543.
Database references
SIMBAD	data

Gamma Hydrae (γ Hya, γ Hydrae) is a binary star in the equatorial constellation of Hydra. It has an apparent visual magnitude of 3.0,^[2] placing it second in brightness among the members of this generally faint constellation. Based upon parallax measurements made during the Hipparcos mission, this star is at a distance of around 133.8 light-years (41.0 parsecs) from Earth.^[1]

Characteristics

The stellar spectrum of the primary matches a stellar classification of G8 III,^[3] with the luminosity class of III indicating it has evolved into a giant star after exhausting the supply of hydrogen at its core. It has nearly three^[3] times the mass of the Sun and 16 times the Sun's radius.^[5] The star is radiating 115^[3] times the Sun's luminosity from its outer atmosphere at an effective temperature of 5,019 K.^[3] This heat gives it the yellow glow of a K-type star.^[10] Despite having reached an advanced stage in its evolution, it is considerably younger than the Sun with an age of around 372 million years. This is because higher mass stars consume their nuclear fuel at a more rapid rate.^[3]

The secondary component component, named Gamma Hydrae B, shares a common proper motion with the primary star. Radial velocity observations confirmed it to be physically bound to Gamma Hydrae A. It has around 60% the mass of the Sun and is located at 1.6" from the primary. The physical separation is either 67.5 or 159 astronomical units.^[8]

In culture

γ Hya appears on the flag of Brazil, symbolising the state of Acre.^[11]

References

^ ^a ^b ^c ^d ^e ^f van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357, S2CID 18759600
^ ^a ^b ^c ^d Gutierrez-Moreno, Adelina; et al. (1966), "A System of photometric standards", Publications of the Department of Astronomy University of Chile, 1, Publicaciones Universidad de Chile, Department de Astronomy: 1–17, Bibcode:1966PDAUC...1....1G
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j Takeda, Yoichi; Sato, Bun'ei; Murata, Daisuke (August 2008), "Stellar Parameters and Elemental Abundances of Late-G Giants", Publications of the Astronomical Society of Japan, 60 (4): 781–802, arXiv:0805.2434, Bibcode:2008PASJ...60..781T, doi:10.1093/pasj/60.4.781
^ Wielen, R.; et al. (1999), "Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions", Veroeffentlichungen des Astronomischen Rechen-Instituts Heidelberg, 35 (35), Astronomisches Rechen-Institut Heidelberg: 1, Bibcode:1999VeARI..35....1W
^ ^a ^b Arroyo-Torres, B.; et al. (June 2014), "VLTI/AMBER observations of cold giant stars: atmospheric structures and fundamental parameters", Astronomy & Astrophysics, 566: 11, arXiv:1404.7384, Bibcode:2014A&A...566A..88A, doi:10.1051/0004-6361/201323264, S2CID 16778588, A88.
^ ^a ^b ^c Cenarro, A. J.; et al. (January 2007), "Medium-resolution Isaac Newton Telescope library of empirical spectra - II. The stellar atmospheric parameters", Monthly Notices of the Royal Astronomical Society, 374 (2): 664–690, arXiv:astro-ph/0611618, Bibcode:2007MNRAS.374..664C, doi:10.1111/j.1365-2966.2006.11196.x, S2CID 119428437
^ Bernacca, P. L.; Perinotto, M. (1970), "A catalogue of stellar rotational velocities", Contributi Osservatorio Astronomico di Padova in Asiago, 239 (1): 1, Bibcode:1970CoAsi.239....1B
^ ^a ^b ^c Ryu, Tsuguru; Sato, Bun’ei; Kuzuhara, Masayuki; Narita, Norio; Takahashi, Yasuhiro H.; Uyama, Taichi; Kudo, Tomoyuki; Kusakabe, Nobuhiko; Hashimoto, Jun; Omiya, Masashi; Harakawa, Hiroki; Abe, Lyu; Ando, Hiroyasu; Brandner, Wolfgang; Brandt, Timothy D. (July 2016). "High-Contrast Imaging of Intermediate-Mass Giants With Long-Term Radial Velocity Trends". The Astrophysical Journal. 825 (2): 127. arXiv:1603.02017. Bibcode:2016ApJ...825..127R. doi:10.3847/0004-637X/825/2/127. ISSN 0004-637X. PMC 7402361. PMID 32753766.
^ "gam Hya". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2012-02-23.
^ "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, archived from the original on 2012-03-18, retrieved 2012-01-16
^ Astronomy of the Brazilian Flag, FOTW Flags Of The World website