Gamma Hydrae
| Observation data Epoch J2000.0 Equinox J2000.0 | |
|---|---|
| Constellation | Hydra |
| Right ascension | 13h 18m 55.29719s[1] |
| Declination | −23° 10′ 17.4514″[1] |
| Apparent magnitude (V) | +2.993[2] |
| Characteristics | |
| Evolutionary stage | horizontal branch[3] |
| Spectral type | G8 III[4] |
| U−B color index | +0.645[2] |
| B−V color index | +0.920[2] |
| Astrometry | |
| Radial velocity (Rv) | −5.4[5] km/s |
| Proper motion (μ) | RA: +68.99[1] mas/yr Dec.: −41.85[1] mas/yr |
| Parallax (π) | 24.37±0.15 mas[1] |
| Distance | 133.8 ± 0.8 ly (41.0 ± 0.3 pc) |
| Absolute magnitude (MV) | −0.15[4] |
| Orbit[6] | |
| Primary | A |
| Name | B |
| Semi-major axis (a) | 67.5±0.6 or 159±7 AU |
| Details[7] | |
| A | |
| Mass | 2.90±0.29 M☉ |
| Radius | 12.47 ± 0.63[a] R☉ |
| Luminosity | 96.8+9.1 −8.3 L☉ |
| Surface gravity (log g) | 2.94±0.05 cgs |
| Temperature | 5,127±30 K |
| Metallicity [Fe/H] | +0.02±0.02 dex |
| Rotational velocity (v sin i) | 8[8] km/s |
| Age | 372[4] Myr |
| B | |
| Mass | 0.61+0.12 −0.14[6] M☉ |
| Other designations | |
| Naga, γ Hya, 46 Hya, NSV 6180, BD−22 3554, FK5 495, HD 115659, HIP 64962, HR 5020, SAO 181543[9] | |
| Database references | |
| SIMBAD | data |
Gamma Hydrae, officially named Naga,[10] 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]
Nomenclature
Gamma Hydrae (Latinized from γ Hydrae, abbreviated γ Hya) is the star's Bayer designation.
In Southeast Asian cultures, particularly in Bali (Indonesia), the constellation Hydra is seen as a nāga (Sanskrit नाग, serpent or dragon). In Japanese, naga coincidentally means "long", which fits the image of the serpent in the sky. The IAU Working Group on Star Names approved the name Naga for Gamma Hydrae A (which marks the snake's tail) on 22 February 2026.[11]
In Chinese astronomy, γ Hydrae forms with π Hydrae the asterism Ping (平), representing a judge.[12] The Chinese name for γ Hydrae itself is Ping yī (平一), the first star of Ping.[13] R. H. Allen's 1899 book Star Names instead claimed the Chinese name Ping Sing, translated as "a Tranquil Star", for ι Hydrae (Ukdah).[14]
Characteristics
The stellar spectrum of the primary matches a stellar classification of G8 III,[4] 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 times the mass of the Sun and 12.5 times the Sun's radius.[a] The star is radiating 97 times the Sun's luminosity from its outer atmosphere at an effective temperature of 5,127 K.[7] This heat gives it the yellow glow of a K-type star.[15] 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.[4]
The secondary 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.[6]
In culture
γ Hya appears on the flag of Brazil, symbolising the state of Acre.[16]
Notes
- ^ a b Calculated, using the Stefan-Boltzmann law and the star's effective temperature and luminosity, with respect to the solar nominal effective temperature of 5,772 K:
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
- ^ Stock, Stephan; Reffert, Sabine; Quirrenbach, Andreas (2018). "Precise radial velocities of giant stars. X. Bayesian stellar parameters and evolutionary stages for 372 giant stars from the Lick planet search". Astronomy and Astrophysics. 616: A33. arXiv:1805.04094. Bibcode:2018A&A...616A..33S. doi:10.1051/0004-6361/201833111.
- ^ a b c d e 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 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
- ^ a b Rosas-Portilla, F.; Schröder, K.-P.; Jack, D. (2022-04-26), "On the physical nature of the Wilson-Bappu effect: revising the gravity and temperature dependence", Monthly Notices of the Royal Astronomical Society, 513 (1): 906–924, arXiv:2203.16593, doi:10.1093/mnras/stac929, ISSN 0035-8711
- ^ 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
- ^ "gam Hya". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2012-02-23.
- ^ "IAU Catalog of Star Names". Retrieved 22 February 2026.
- ^ "Naga (नाग)". All Skies Encyclopaedia. IAU Working Group on Star Names. Retrieved 22 February 2026.
- ^ Ridpath, Ian. "Star Tales – Hydra". Retrieved 22 February 2026.
- ^ "AEEA 天文教育資訊網" [Activities of Exhibition and Education in Astronomy] (in Chinese). Archived from the original on 21 May 2011.
- ^ Allen, R. H. (1899), Star Names: Their Lore and Meaning, p. 249
- ^ "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