# Chi2 Hydrae

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Binary star system in the constellation Hydra

For other stars with this [Bayer designation](/source/Bayer_designation), see [χ Hydrae](/source/Chi_Hydrae)

Chi2 Hydrae Location of χ2 Hydrae (circled in red) Observation data Epoch J2000.0 Equinox J2000.0 (ICRS) Constellation Hydra Right ascension 11h 05m 57.57000s[1] Declination −27° 17′ 16.2727″[1] Apparent magnitude (V) 5.65 - 5.94[2] (5.85 + 7.57)[3] Characteristics Spectral type B8 III-IVe + B8.5 V[4] U−B color index −0.26[5] B−V color index −0.06[5] Variable type Algol (detached)[2] Astrometry Radial velocity (Rv) +30.6±0.9[6] km/s Proper motion (μ) RA: +34.417[1] mas/yr Dec.: −16.477[1] mas/yr Parallax (π) 4.7616±0.0652 mas[1] Distance 685 ± 9 ly (210 ± 3 pc) Orbit[7] Period (P) 2.2677 d Eccentricity (e) 0.00 Periastron epoch (T) 2439925.545 JD Argument of periastron (ω) (secondary) 0.00° Semi-amplitude (K1) (primary) 123.3 km/s Semi-amplitude (K2) (secondary) 168.9 km/s Details[8] χ Hya A Mass 3.605±0.078 M☉ Radius 4.391±0.039 R☉ Luminosity 344[9] L☉ Surface gravity (log g) 3.65[10] cgs Temperature 11,750±190 K Rotational velocity (v sin i) 112±10[3] km/s Age 158[3] Myr χ Hya B Mass 2.632±0.049 M☉ Radius 2.160±0.030 R☉ Luminosity 66[9] L☉ Surface gravity (log g) 4.23[3] cgs Temperature 11,100±230 K Rotational velocity (v sin i) 60±6[3] km/s Other designations χ Hya, CD−26°8342, HD 96314, HIP 54255, HR 4317, SAO 179522[11] Database references SIMBAD data

**Chi2 Hydrae**, Latinised from χ2 Hydrae, is a [binary star](/source/Binary_star) system in the equatorial [constellation](/source/Constellation) of [Hydra](/source/Hydra_(constellation)). Based upon an annual [parallax shift](/source/Stellar_parallax) of 4.6 [mas](/source/Milliarcsecond) as seen from Earth, it is located roughly 685 [light years](/source/Light_year) from the [Sun](/source/Sun). It is visible to the naked eye with a combined [apparent visual magnitude](/source/Apparent_visual_magnitude) of about 5.7.

A [light curve](/source/Light_curve) for Chi2 Hydrae, plotted from *[TESS](/source/Transiting_Exoplanet_Survey_Satellite)* data[12]

This is a [detached](/source/Detached_binary) [eclipsing binary](/source/Eclipsing_binary) star system with an [orbital period](/source/Orbital_period) of 2.27 days and an essentially circular orbit having a measured [eccentricity](/source/Orbital_eccentricity) of 0.00.[7] The eclipse of the primary by the secondary component reduces the visual magnitude of the system by 0.29, while the eclipse of the secondary diminishes the magnitude by 0.27.[4]

The primary, component A, is a magnitude 5.85 [B-type star](/source/B-type_star) with a [stellar classification](/source/Stellar_classification) of B8 III-IVe,[3] suggesting it may be part way along the path of [evolving](/source/Stellar_evolution) into a [giant star](/source/Giant_star) from a [subgiant](/source/Subgiant_star). It has about 3.6 times the [mass of the Sun](/source/Solar_mass) and 4.4 times the [Sun's radius](/source/Solar_radius),[8] although it may be tidally deformed since its radius is 86%[9] of the [Roche radius](/source/Roche_radius).[13] With an estimated age of 158 million years, it has a [projected rotational velocity](/source/Projected_rotational_velocity) of 112 km/s.[3]

Component B is a magnitude 7.57[3] [B-type main sequence star](/source/B-type_main_sequence_star) with a class of B8.5 V.[4] It has 2.6 times the Sun's mass and 2.16 times the radius of the Sun. The star is filling 60% of its Roche radius.[9]

## References

1. ^ [***a***](#cite_ref-dr3_1-0) [***b***](#cite_ref-dr3_1-1) [***c***](#cite_ref-dr3_1-2) [***d***](#cite_ref-dr3_1-3) [***e***](#cite_ref-dr3_1-4) Vallenari, A.; et al. (Gaia collaboration) (2023). ["*Gaia* Data Release 3. Summary of the content and survey properties"](https://doi.org/10.1051%2F0004-6361%2F202243940). *Astronomy and Astrophysics*. **674**: A1. [arXiv](/source/ArXiv_(identifier)):[2208.00211](https://arxiv.org/abs/2208.00211). [Bibcode](/source/Bibcode_(identifier)):[2023A&A...674A...1G](https://ui.adsabs.harvard.edu/abs/2023A&A...674A...1G). [doi](/source/Doi_(identifier)):[10.1051/0004-6361/202243940](https://doi.org/10.1051%2F0004-6361%2F202243940). [S2CID](/source/S2CID_(identifier)) [244398875](https://api.semanticscholar.org/CorpusID:244398875). [Gaia DR3 record for this source](https://vizier.cds.unistra.fr/viz-bin/VizieR-4?-source=+I%2F355&-from=nav&-nav=cat%3AI%2F355%26tab%3A%7BI%2F355%2Fgaiadr3%7D%26key%3Asource%3D1355%2Fgaiadr3%26HTTPPRM%3A%26-out.add%3D.%26Source%3D%3D%3D3532312225289255040%26-go+%25%23Sesame%23%25%26) at [VizieR](/source/VizieR).

1. ^ [***a***](#cite_ref-gcvs_2-0) [***b***](#cite_ref-gcvs_2-1) Samus’, N. N.; Kazarovets, E. V.; Durlevich, O. V.; Kireeva, N. N.; Pastukhova, E. N. (January 2017), "General catalogue of variable stars: Version GCVS 5.1", *Astronomy Reports*, **61** (1): 80–88, [Bibcode](/source/Bibcode_(identifier)):[2017ARep...61...80S](https://ui.adsabs.harvard.edu/abs/2017ARep...61...80S), [doi](/source/Doi_(identifier)):[10.1134/S1063772917010085](https://doi.org/10.1134%2FS1063772917010085), [ISSN](/source/ISSN_(identifier)) [1063-7729](https://search.worldcat.org/issn/1063-7729), [S2CID](/source/S2CID_(identifier)) [255195566](https://api.semanticscholar.org/CorpusID:255195566).

1. ^ [***a***](#cite_ref-Torres2010_3-0) [***b***](#cite_ref-Torres2010_3-1) [***c***](#cite_ref-Torres2010_3-2) [***d***](#cite_ref-Torres2010_3-3) [***e***](#cite_ref-Torres2010_3-4) [***f***](#cite_ref-Torres2010_3-5) [***g***](#cite_ref-Torres2010_3-6) [***h***](#cite_ref-Torres2010_3-7) Torres, G.; et al. (February 2010), "Accurate masses and radii of normal stars: modern results and applications", *The Astronomy and Astrophysics Review*, **18** (1–2): 67–126, [arXiv](/source/ArXiv_(identifier)):[0908.2624](https://arxiv.org/abs/0908.2624), [Bibcode](/source/Bibcode_(identifier)):[2010A&ARv..18...67T](https://ui.adsabs.harvard.edu/abs/2010A&ARv..18...67T), [doi](/source/Doi_(identifier)):[10.1007/s00159-009-0025-1](https://doi.org/10.1007%2Fs00159-009-0025-1), [S2CID](/source/S2CID_(identifier)) [14006009](https://api.semanticscholar.org/CorpusID:14006009).

1. ^ [***a***](#cite_ref-Malkov2006_4-0) [***b***](#cite_ref-Malkov2006_4-1) [***c***](#cite_ref-Malkov2006_4-2) Malkov, O. Yu.; et al. (February 2006), "A catalogue of eclipsing variables", *Astronomy and Astrophysics*, **446** (2): 785–789, [Bibcode](/source/Bibcode_(identifier)):[2006A&A...446..785M](https://ui.adsabs.harvard.edu/abs/2006A&A...446..785M), [doi](/source/Doi_(identifier)):[10.1051/0004-6361:20053137](https://doi.org/10.1051%2F0004-6361%3A20053137), [hdl](/source/Hdl_(identifier)):[10995/73280](https://hdl.handle.net/10995%2F73280).

1. ^ [***a***](#cite_ref-Nicolet1978_5-0) [***b***](#cite_ref-Nicolet1978_5-1) Nicolet, B. (1978), "Photoelectric photometric Catalogue of homogeneous measurements in the UBV System", *Astronomy and Astrophysics Supplement Series*, **34**: 1–49, [Bibcode](/source/Bibcode_(identifier)):[1978A&AS...34....1N](https://ui.adsabs.harvard.edu/abs/1978A&AS...34....1N).

1. **[^](#cite_ref-deBruijne2012_6-0)** 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-Pourbaix2004_7-0) [***b***](#cite_ref-Pourbaix2004_7-1) Pourbaix, D.; Tokovinin, A. A.; Batten, A. H.; Fekel, F. C.; Hartkopf, W. I.; et al. (2004), "SB9: The ninth catalogue of spectroscopic binary orbits", *Astronomy & Astrophysics*, **424** (2): 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-Brown2010_8-0) [***b***](#cite_ref-Brown2010_8-1) Brown, Timothy M. (January 2010), "Radii of Rapidly Rotating Stars, with Application to Transiting-Planet Hosts", *The Astrophysical Journal*, **709** (1): 535–545, [arXiv](/source/ArXiv_(identifier)):[0912.1639](https://arxiv.org/abs/0912.1639), [Bibcode](/source/Bibcode_(identifier)):[2010ApJ...709..535B](https://ui.adsabs.harvard.edu/abs/2010ApJ...709..535B), [doi](/source/Doi_(identifier)):[10.1088/0004-637X/709/1/535](https://doi.org/10.1088%2F0004-637X%2F709%2F1%2F535), [S2CID](/source/S2CID_(identifier)) [119253009](https://api.semanticscholar.org/CorpusID:119253009).

1. ^ [***a***](#cite_ref-Eker2015_9-0) [***b***](#cite_ref-Eker2015_9-1) [***c***](#cite_ref-Eker2015_9-2) [***d***](#cite_ref-Eker2015_9-3) Eker, Z.; et al. (April 2015), "Main-Sequence Effective Temperatures from a Revised Mass-Luminosity Relation Based on Accurate Properties", *The Astronomical Journal*, **149** (4): 16, [arXiv](/source/ArXiv_(identifier)):[1501.06585](https://arxiv.org/abs/1501.06585), [Bibcode](/source/Bibcode_(identifier)):[2015AJ....149..131E](https://ui.adsabs.harvard.edu/abs/2015AJ....149..131E), [doi](/source/Doi_(identifier)):[10.1088/0004-6256/149/4/131](https://doi.org/10.1088%2F0004-6256%2F149%2F4%2F131), [S2CID](/source/S2CID_(identifier)) [118740259](https://api.semanticscholar.org/CorpusID:118740259), 131.

1. **[^](#cite_ref-Jordi1997_10-0)** Jordi, C.; Ribas, I.; Torra, J.; Gimenez, A. (October 1997), "Photometric versus empirical surface gravities of eclipsing binaries.", *Astronomy and Astrophysics*, **326**: 1044–1054, [Bibcode](/source/Bibcode_(identifier)):[1997A&A...326.1044J](https://ui.adsabs.harvard.edu/abs/1997A&A...326.1044J).

1. **[^](#cite_ref-SIMBAD_11-0)** ["chi02 Hya"](http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=chi02+Hya), *[SIMBAD](/source/SIMBAD)*, [Centre de données astronomiques de Strasbourg](/source/Centre_de_donn%C3%A9es_astronomiques_de_Strasbourg), retrieved 2017-03-17.

1. **[^](#cite_ref-MAST_12-0)** [*MAST: Barbara A. Mikulski Archive for Space Telescopes*](https://mast.stsci.edu/portal/Mashup/Clients/Mast/Portal.html), Space Telescope Science Institute, retrieved 8 December 2021.

1. **[^](#cite_ref-Pols1997_13-0)** Pols, Onno R.; et al. (August 1997), "Further critical tests of stellar evolution by means of double-lined eclipsing binaries", *Monthly Notices of the Royal Astronomical Society*, **289** (4): 869–881, [Bibcode](/source/Bibcode_(identifier)):[1997MNRAS.289..869P](https://ui.adsabs.harvard.edu/abs/1997MNRAS.289..869P), [doi](/source/Doi_(identifier)):[10.1093/mnras/289.4.869](https://doi.org/10.1093%2Fmnras%2F289.4.869).

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

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Adapted from the Wikipedia article [Chi2 Hydrae](https://en.wikipedia.org/wiki/Chi2_Hydrae) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Chi2_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.