# Tau1 Hydrae

> Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Tau1_Hydrae
> Markdown URL: https://mediated.wiki/source/Tau1_Hydrae.md
> Source: https://en.wikipedia.org/wiki/Tau1_Hydrae
> Source revision: 1344979499
> License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/)

Triple star system in the constellation Hydra

For other stars with this Bayer designation, see [τ Hydrae](/source/Tau_Hydrae).

Tau1 Hydrae Location of τ1 Hydrae (circled in red) Observation data Epoch J2000.0 Equinox J2000.0 (ICRS) Constellation Hydra Right ascension 09h 29m 08.89655s[1] Declination −02° 46′ 08.2649″[1] Apparent magnitude (V) +4.59 (4.60 + 7.15)[2] Characteristics Spectral type F6 V + ? + K0[3] B−V color index +0.411±0.015[2] Astrometry Radial velocity (Rv) +10.85±0.28[4] km/s Absolute magnitude (MV) +3.28[5] τ1 Hydrae A Proper motion (μ) RA: +107.115[6] mas/yr Dec.: −29.652[6] mas/yr Parallax (π) 56.2938±0.5309 mas[6] Distance 57.9 ± 0.5 ly (17.8 ± 0.2 pc) τ1 Hydrae B Proper motion (μ) RA: +138.487[7] mas/yr Dec.: −17.371[7] mas/yr Parallax (π) 55.3675±0.0638 mas[7] Distance 58.91 ± 0.07 ly (18.06 ± 0.02 pc) Orbit[8] Period (P) 2,807±23 d Eccentricity (e) 0.33±0.12 Periastron epoch (T) 2445260 ± 150 JD Semi-amplitude (K1) (primary) 2.98±0.39 km/s Details τ1 Hydrae A Mass 1.20[9] M☉ Radius 1.4[10] R☉ Luminosity (bolometric) 3.369[2] L☉ Surface gravity (log g) 4.12±0.14[9] cgs Temperature 6,473±220[9] K Metallicity [Fe/H] −0.01[2] dex Rotational velocity (v sin i) 30.4±1.5[5] km/s Age 3.61[2] Gyr τ1 Hydrae B Mass 0.86[11] M☉ Radius 0.81[10] R☉ Luminosity 0.435[7] L☉ Temperature 5,197[7] K Other designations τ1 Hya, 31 Hydrae, BD−02°2901, GJ 348, HD 81997, HIP 46509, HR 3759, SAO 136895, WDS J09291-0246[12] Database references SIMBAD τ1 Hya AB τ1 Hya A τ1 Hya B

**Tau1 Hydrae** is a [triple star](/source/Triple_star)[3] system in the equatorial [constellation](/source/Constellation) of [Hydra](/source/Hydra_(constellation)). Based upon the annual [parallax shift](/source/Stellar_parallax) of the two visible components as seen from Earth,[1] they are located about 18 parsecs (59 ly) from the [Sun](/source/Sun). The system has a combined [apparent visual magnitude](/source/Apparent_visual_magnitude) of +4.59,[2] which is bright enough to be visible to the naked eye at night.

The inner pair of stars form a single-lined [spectroscopic binary](/source/Spectroscopic_binary) with an [orbital period](/source/Orbital_period) of about 2,807 days and an [eccentricity](/source/Orbital_eccentricity) of 0.33.[8] The visible member of the pair, component A, is a visual magnitude 4.60[2] [F-type main sequence star](/source/F-type_main_sequence_star) with a [stellar classification](/source/Stellar_classification) of F6 V.[3] During the 1990s, it was thought to be a [Gamma Doradus variable](/source/Gamma_Doradus_variable), but this was later discounted as it shows no short-term [photometric](/source/Photometry_(astronomy)) variability. The star does show some long-term variability, possibly as a result of a [magnetic activity](/source/Stellar_magnetic_field) cycle similar to the [solar cycle](/source/Solar_cycle).[13]

The tertiary member, component B, is a visual magnitude 7.15[2] [K-type star](/source/K-type_star) with a class of K0.[3] It lies at a separation of 1,120 [AU](/source/Astronomical_Unit) from the primary.[14] As of 2012, it was positioned at an [angular separation](/source/Angular_separation) of 67.5 [arc seconds](/source/Arc_second) along a [position angle](/source/Position_angle) of 4°.[15]

This star along with [τ2 Hydrae](/source/Tau2_Hydrae), [ι Hydrae](/source/Iota_Hydrae) and [33 Hydrae](/source/33_Hydrae) (A Hydrae) were traditionally known as Ukdah, Arabic for "knot".[16] The name Ukdah is now officially applied to ι Hydrae.

## References

1. ^ [***a***](#cite_ref-vanLeeuwen2007_1-0) [***b***](#cite_ref-vanLeeuwen2007_1-1) [***c***](#cite_ref-vanLeeuwen2007_1-2) van Leeuwen, F. (2007), "Validation of the new Hipparcos reduction", *Astronomy and Astrophysics*, **474** (2): 653–664, [arXiv](/source/ArXiv_(identifier)):[0708.1752](https://arxiv.org/abs/0708.1752), [Bibcode](/source/Bibcode_(identifier)):[2007A&A...474..653V](https://ui.adsabs.harvard.edu/abs/2007A&A...474..653V), [doi](/source/Doi_(identifier)):[10.1051/0004-6361:20078357](https://doi.org/10.1051%2F0004-6361%3A20078357), [S2CID](/source/S2CID_(identifier)) [18759600](https://api.semanticscholar.org/CorpusID:18759600).

1. ^ [***a***](#cite_ref-Montesinos2016_2-0) [***b***](#cite_ref-Montesinos2016_2-1) [***c***](#cite_ref-Montesinos2016_2-2) [***d***](#cite_ref-Montesinos2016_2-3) [***e***](#cite_ref-Montesinos2016_2-4) [***f***](#cite_ref-Montesinos2016_2-5) [***g***](#cite_ref-Montesinos2016_2-6) [***h***](#cite_ref-Montesinos2016_2-7) Montesinos, B.; et al. (September 2016), "Incidence of debris discs around FGK stars in the solar neighbourhood", *Astronomy & Astrophysics*, **593**: 31, [arXiv](/source/ArXiv_(identifier)):[1605.05837](https://arxiv.org/abs/1605.05837), [Bibcode](/source/Bibcode_(identifier)):[2016A&A...593A..51M](https://ui.adsabs.harvard.edu/abs/2016A&A...593A..51M), [doi](/source/Doi_(identifier)):[10.1051/0004-6361/201628329](https://doi.org/10.1051%2F0004-6361%2F201628329), [S2CID](/source/S2CID_(identifier)) [55251562](https://api.semanticscholar.org/CorpusID:55251562), A51.

1. ^ [***a***](#cite_ref-Eggleton2008_3-0) [***b***](#cite_ref-Eggleton2008_3-1) [***c***](#cite_ref-Eggleton2008_3-2) [***d***](#cite_ref-Eggleton2008_3-3) Eggleton, P. P.; Tokovinin, A. A. (September 2008), "A catalogue of multiplicity among bright stellar systems", *[Monthly Notices of the Royal Astronomical Society](/source/Monthly_Notices_of_the_Royal_Astronomical_Society)*, **389** (2): 869–879, [arXiv](/source/ArXiv_(identifier)):[0806.2878](https://arxiv.org/abs/0806.2878), [Bibcode](/source/Bibcode_(identifier)):[2008MNRAS.389..869E](https://ui.adsabs.harvard.edu/abs/2008MNRAS.389..869E), [doi](/source/Doi_(identifier)):[10.1111/j.1365-2966.2008.13596.x](https://doi.org/10.1111%2Fj.1365-2966.2008.13596.x), [S2CID](/source/S2CID_(identifier)) [14878976](https://api.semanticscholar.org/CorpusID:14878976).

1. **[^](#cite_ref-deBruijne2012_4-0)** de Bruijne, J. H. J.; Eilers, A.-C. (October 2012), "Radial velocities for the HIPPARCOS-Gaia Hundred-Thousand-Proper-Motion project", *Astronomy & Astrophysics*, **546**: 14, [arXiv](/source/ArXiv_(identifier)):[1208.3048](https://arxiv.org/abs/1208.3048), [Bibcode](/source/Bibcode_(identifier)):[2012A&A...546A..61D](https://ui.adsabs.harvard.edu/abs/2012A&A...546A..61D), [doi](/source/Doi_(identifier)):[10.1051/0004-6361/201219219](https://doi.org/10.1051%2F0004-6361%2F201219219), [S2CID](/source/S2CID_(identifier)) [59451347](https://api.semanticscholar.org/CorpusID:59451347), A61.

1. ^ [***a***](#cite_ref-Reiners2006_5-0) [***b***](#cite_ref-Reiners2006_5-1) Reiners, Ansgar (January 2006), "Rotation- and temperature-dependence of stellar latitudinal differential rotation", *Astronomy and Astrophysics*, **446** (1): 267–277, [arXiv](/source/ArXiv_(identifier)):[astro-ph/0509399](https://arxiv.org/abs/astro-ph/0509399), [Bibcode](/source/Bibcode_(identifier)):[2006A&A...446..267R](https://ui.adsabs.harvard.edu/abs/2006A&A...446..267R), [doi](/source/Doi_(identifier)):[10.1051/0004-6361:20053911](https://doi.org/10.1051%2F0004-6361%3A20053911), [S2CID](/source/S2CID_(identifier)) [8642707](https://api.semanticscholar.org/CorpusID:8642707)

1. ^ [***a***](#cite_ref-dr2a_6-0) [***b***](#cite_ref-dr2a_6-1) [***c***](#cite_ref-dr2a_6-2) [Brown, A. G. A.](/source/Anthony_Brown_(scientist)); et al. ([Gaia collaboration](/source/Data_Processing_and_Analysis_Consortium)) (August 2018). ["*Gaia* Data Release 2: Summary of the contents and survey properties"](https://doi.org/10.1051%2F0004-6361%2F201833051). *[Astronomy & Astrophysics](/source/Astronomy_%26_Astrophysics)*. **616**. A1. [arXiv](/source/ArXiv_(identifier)):[1804.09365](https://arxiv.org/abs/1804.09365). [Bibcode](/source/Bibcode_(identifier)):[2018A&A...616A...1G](https://ui.adsabs.harvard.edu/abs/2018A&A...616A...1G). [doi](/source/Doi_(identifier)):[10.1051/0004-6361/201833051](https://doi.org/10.1051%2F0004-6361%2F201833051). [Gaia DR2 record for this source](https://vizier.u-strasbg.fr/viz-bin/VizieR-S?Gaia%20DR2%203837697972130323456) at [VizieR](/source/VizieR).

1. ^ [***a***](#cite_ref-dr2b_7-0) [***b***](#cite_ref-dr2b_7-1) [***c***](#cite_ref-dr2b_7-2) [***d***](#cite_ref-dr2b_7-3) [***e***](#cite_ref-dr2b_7-4) [Brown, A. G. A.](/source/Anthony_Brown_(scientist)); et al. ([Gaia collaboration](/source/Data_Processing_and_Analysis_Consortium)) (August 2018). ["*Gaia* Data Release 2: Summary of the contents and survey properties"](https://doi.org/10.1051%2F0004-6361%2F201833051). *[Astronomy & Astrophysics](/source/Astronomy_%26_Astrophysics)*. **616**. A1. [arXiv](/source/ArXiv_(identifier)):[1804.09365](https://arxiv.org/abs/1804.09365). [Bibcode](/source/Bibcode_(identifier)):[2018A&A...616A...1G](https://ui.adsabs.harvard.edu/abs/2018A&A...616A...1G). [doi](/source/Doi_(identifier)):[10.1051/0004-6361/201833051](https://doi.org/10.1051%2F0004-6361%2F201833051). [Gaia DR2 record for this source](https://vizier.u-strasbg.fr/viz-bin/VizieR-S?Gaia%20DR2%203837746380705638656) at [VizieR](/source/VizieR).

1. ^ [***a***](#cite_ref-Pourbaiz2004_8-0) [***b***](#cite_ref-Pourbaiz2004_8-1) Pourbaix, D.; et al. (September 2004), "SB9: The ninth catalogue of spectroscopic binary orbits", *Astronomy and Astrophysics*, **424**: 727–732, [arXiv](/source/ArXiv_(identifier)):[astro-ph/0406573](https://arxiv.org/abs/astro-ph/0406573), [Bibcode](/source/Bibcode_(identifier)):[2004A&A...424..727P](https://ui.adsabs.harvard.edu/abs/2004A&A...424..727P), [doi](/source/Doi_(identifier)):[10.1051/0004-6361:20041213](https://doi.org/10.1051%2F0004-6361%3A20041213), [S2CID](/source/S2CID_(identifier)) [119387088](https://api.semanticscholar.org/CorpusID:119387088).

1. ^ [***a***](#cite_ref-David2015_9-0) [***b***](#cite_ref-David2015_9-1) [***c***](#cite_ref-David2015_9-2) David, Trevor J.; Hillenbrand, Lynne A. (2015), "The Ages of Early-Type Stars: Strömgren Photometric Methods Calibrated, Validated, Tested, and Applied to Hosts and Prospective Hosts of Directly Imaged Exoplanets", *The Astrophysical Journal*, **804** (2): 146, [arXiv](/source/ArXiv_(identifier)):[1501.03154](https://arxiv.org/abs/1501.03154), [Bibcode](/source/Bibcode_(identifier)):[2015ApJ...804..146D](https://ui.adsabs.harvard.edu/abs/2015ApJ...804..146D), [doi](/source/Doi_(identifier)):[10.1088/0004-637X/804/2/146](https://doi.org/10.1088%2F0004-637X%2F804%2F2%2F146), [S2CID](/source/S2CID_(identifier)) [33401607](https://api.semanticscholar.org/CorpusID:33401607).

1. ^ [***a***](#cite_ref-PasinettiFracassini2001_10-0) [***b***](#cite_ref-PasinettiFracassini2001_10-1) Pasinetti Fracassini, L. E.; et al. (February 2001), "Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS)", *Astronomy and Astrophysics*, **367** (3rd ed.): 521–524, [arXiv](/source/ArXiv_(identifier)):[astro-ph/0012289](https://arxiv.org/abs/astro-ph/0012289), [Bibcode](/source/Bibcode_(identifier)):[2001A&A...367..521P](https://ui.adsabs.harvard.edu/abs/2001A&A...367..521P), [doi](/source/Doi_(identifier)):[10.1051/0004-6361:20000451](https://doi.org/10.1051%2F0004-6361%3A20000451), [S2CID](/source/S2CID_(identifier)) [425754](https://api.semanticscholar.org/CorpusID:425754).

1. **[^](#cite_ref-Tokovinin2014_11-0)** Tokovinin, Andrei (2014), "From Binaries to Multiples. II. Hierarchical Multiplicity of F and G Dwarfs", *The Astronomical Journal*, **147** (4): 14, [arXiv](/source/ArXiv_(identifier)):[1401.6827](https://arxiv.org/abs/1401.6827), [Bibcode](/source/Bibcode_(identifier)):[2014AJ....147...87T](https://ui.adsabs.harvard.edu/abs/2014AJ....147...87T), [doi](/source/Doi_(identifier)):[10.1088/0004-6256/147/4/87](https://doi.org/10.1088%2F0004-6256%2F147%2F4%2F87), [S2CID](/source/S2CID_(identifier)) [56066740](https://api.semanticscholar.org/CorpusID:56066740), 87.

1. **[^](#cite_ref-SIMBAD_12-0)** ["tau01 Hya"](http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=tau01+Hya). *[SIMBAD](/source/SIMBAD)*. [Centre de données astronomiques de Strasbourg](/source/Centre_de_donn%C3%A9es_astronomiques_de_Strasbourg). Retrieved 2017-03-22.{{[cite web](https://en.wikipedia.org/wiki/Template:Cite_web)}}: CS1 maint: postscript ([link](https://en.wikipedia.org/wiki/Category:CS1_maint:_postscript))

1. **[^](#cite_ref-Henry1999_13-0)** Henry, G. W.; et al. (March 1999), "tau1 Hydrae: NOT A gamma DORADUS VARIABLE", *Information Bulletin on Variable Stars*, **4688** (4688): 1, [Bibcode](/source/Bibcode_(identifier)):[1999IBVS.4688....1H](https://ui.adsabs.harvard.edu/abs/1999IBVS.4688....1H).

1. **[^](#cite_ref-Allen2012_14-0)** Allen, Peter R.; et al. (August 2012), "Low-mass Tertiary Companions to Spectroscopic Binaries. I. Common Proper Motion Survey for Wide Companions Using 2MASS", *The Astronomical Journal*, **144** (2): 12, [arXiv](/source/ArXiv_(identifier)):[1206.4289](https://arxiv.org/abs/1206.4289), [Bibcode](/source/Bibcode_(identifier)):[2012AJ....144...62A](https://ui.adsabs.harvard.edu/abs/2012AJ....144...62A), [doi](/source/Doi_(identifier)):[10.1088/0004-6256/144/2/62](https://doi.org/10.1088%2F0004-6256%2F144%2F2%2F62), [S2CID](/source/S2CID_(identifier)) [51051184](https://api.semanticscholar.org/CorpusID:51051184), 62.

1. **[^](#cite_ref-WDSC2014_15-0)** Mason, B. D.; et al. (2014), "The Washington Visual Double Star Catalog", *The Astronomical Journal*, **122** (6): 3466–3471, [Bibcode](/source/Bibcode_(identifier)):[2001AJ....122.3466M](https://ui.adsabs.harvard.edu/abs/2001AJ....122.3466M), [doi](/source/Doi_(identifier)):[10.1086/323920](https://doi.org/10.1086%2F323920).

1. **[^](#cite_ref-allen1899_16-0)** [Allen, R. H.](/source/Richard_Hinckley_Allen) (1963). [*Star Names: Their Lore and Meaning*](https://archive.org/details/starnamestheirlo00alle/page/250) (Reprint ed.). New York, NY: Dover Publications Inc. p. [250](https://archive.org/details/starnamestheirlo00alle/page/250). [ISBN](/source/ISBN_(identifier)) [0-486-21079-0](https://en.wikipedia.org/wiki/Special:BookSources/0-486-21079-0). Retrieved 2010-12-12. {{[cite book](https://en.wikipedia.org/wiki/Template:Cite_book)}}: ISBN / Date incompatibility ([help](https://en.wikipedia.org/wiki/Help:CS1_errors#invalid_isbn_date))

v t e Constellation of Hydra List of stars in Hydra Hydra in Chinese astronomy Stars Bayer α (Alphard) β γ (Naga) δ ε (Ashlesha) ζ η θ ι (Ukdah) κ λ μ ν ξ ο π ρ σ (Minchir) τ1 τ2 υ1 (Zhang) υ2 φ1 φ2 φ3 χ1 χ2 ψ ω b1 b3 C F Flamsteed 1 2 3 6 (a) 9 10 12 (D) 14 15 17 19 20 21 23 24 25 26 27 (P) 28 29 33 (A) 34 37 44 47 48 50 51 (k) 52 (l) 54 (m) 55 56 57 58 (Solitaire) 59 60 10 Crt 17 Crt 20 Crt 2 Sex Variable R U V W TW EX HS KU LQ V361 V419 V421 V478 V484 HR 3378 3538 3749 (G) 3750 3858 (I) 3862 3919 3923 (Felis) 4162 4328 4339 (β Ant) 4445 5265 HD 72659 74156 82943 86226 86264 90156 (γ Ant) Other 2MASS 1114−2618 ESO 439-26 Gliese 328 Gliese 357 Gliese 433 Gliese 453 GJ 3634 HAT-P-30 HE 1256−2738 HE 1327−2326 LHS 2065 LHS 3003 LTT 3780 SDSS J090745.0+024507 WASP-25 WASP-36 WASP-84 WASP-166 (Filetdor) WISE 0855−0714 WISEA J1141−3326 Exoplanets Gliese 357 d HD 72659 b HD 74156 b c HD 82943 b c HD 86226 b HD 86264 b TW Hydrae b (unconfirmed) WASP-193b Star clusters Messier 48 Messier 68 NGC 5694 Price-Whelan 1 Nebulae Abell 33 Abell 35 NGC 3242 Southern Owl Nebula Galaxies Messier 83 NGC 2617 2642 2697 2708 2758 2835 2848 2865 2890 2935 2936 2937 2960 2962 2986 2992 3030 3052 3054 3081 3109 3124 3200 3285 3285B 3290 3305 3307 3308 3309 3311 3312 3313 3314 3315 3316 3336 3369 3383 3393 3402 3463 3464 3585 3621 3673 3717 3904 3923 3936 4980 4993 5042 5061 5078 5085 5101 5135 5260 5264 5495 5626 Other 3C 196.1 AM 1316−241 Arp 7 CTQ 327 ESO 510-G13 GAMA202627 GLEAM J0917−0012 Hydra A IC 535 MRC 1138−262 Peekaboo Galaxy PKS 1151−348 RX J0911.4+0551 SDSS J0841+0101 TN J0924−2201 Tololo 1247−232 ZwCl 1693 BCG Galaxy clusters Abell 754 Abell 3411 Abell 3412 Bullet Group Hickson 40 Astronomical events GW170817 Category

---
Adapted from the Wikipedia article [Tau1 Hydrae](https://en.wikipedia.org/wiki/Tau1_Hydrae) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Tau1_Hydrae?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.