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Chi² Hydrae
	; A light curve for Chi2 Hydrae, plotted from TESS data
Observation data; Epoch J2000.0 Equinox J2000.0 (ICRS)
Constellation	Hydra
Right ascension	11h 05m 57.57000s
Declination	−27° 17′ 16.2727″
Apparent magnitude (V)	5.65 - 5.94 (5.85 + 7.57)
Characteristics
Spectral type	B8 III-IVe + B8.5 V
U−B color index	−0.26
B−V color index	−0.06
Variable type	Algol (detached)
Astrometry
Radial velocity (Rv)	+30.6±0.9 km/s
Proper motion (μ)	RA: +34.417 mas/yr ; Dec.: −16.477 mas/yr
Parallax (π)	4.7616 ± 0.0652 mas
Distance	685 ± 9 ly ; (210 ± 3 pc)
Orbit
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
χ Hya A
Mass	3.605±0.078 M☉
Radius	4.391±0.039 R☉
Luminosity	344 L☉
Surface gravity (log g)	3.65 cgs
Temperature	11,750±190 K
Rotational velocity (v sin i)	112±10 km/s
Age	158 Myr
χ Hya B
Mass	2.632±0.049 M☉
Radius	2.160±0.030 R☉
Luminosity	66 L☉
Surface gravity (log g)	4.23 cgs
Temperature	11,100±230 K
Rotational velocity (v sin i)	60±6 km/s
Other designations
	χ Hya, CD−26°8342, HD 96314, HIP 54255, HR 4317, SAO 179522
Database references
SIMBAD	data

Chi² Hydrae, Latinised from χ² Hydrae, is a binary star system in the equatorial constellation of Hydra. Based upon an annual parallax shift of 4.6 mas as seen from Earth, it is located roughly 685 light years from the Sun. It is visible to the naked eye with a combined apparent visual magnitude of about 5.7.

This is a detached eclipsing binary star system with an orbital period of 2.27 days and an essentially circular orbit having a measured eccentricity of 0.00.^[8] 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.^[5]

The primary, component A, is a magnitude 5.85 B-type star with a stellar classification of B8 III-IVe,^[4] suggesting it may be part way along the path of evolving into a giant star from a subgiant. It has about 3.6 times the mass of the Sun and 4.4 times the Sun's radius,^[9] although it may be tidally deformed since its radius is 86%^[10] of the Roche radius.^[13] With an estimated age of 158 million years, it has a projected rotational velocity of 112 km/s.^[4]

Component B is a magnitude 7.57^[4] B-type main sequence star with a class of B8.5 V.^[5] 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.^[10]

References

^ MAST: Barbara A. Mikulski Archive for Space Telescopes, Space Telescope Science Institute, retrieved 8 December 2021.
^ ^a ^b ^c ^d ^e Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
^ ^a ^b 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:2017ARep...61...80S, doi:10.1134/S1063772917010085, ISSN 1063-7729, S2CID 255195566.
^ ^a ^b ^c ^d ^e ^f ^g ^h 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:0908.2624, Bibcode:2010A&ARv..18...67T, doi:10.1007/s00159-009-0025-1, S2CID 14006009.
^ ^a ^b ^c Malkov, O. Yu.; et al. (February 2006), "A catalogue of eclipsing variables", Astronomy and Astrophysics, 446 (2): 785–789, Bibcode:2006A&A...446..785M, doi:10.1051/0004-6361:20053137, hdl:10995/73280.
^ ^a ^b Nicolet, B. (1978), "Photoelectric photometric Catalogue of homogeneous measurements in the UBV System", Astronomy and Astrophysics Supplement Series, 34: 1–49, Bibcode:1978A&AS...34....1N.
^ 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:1208.3048, Bibcode:2012A&A...546A..61D, doi:10.1051/0004-6361/201219219, S2CID 59451347, A61.
^ ^a ^b Pourbaix, D.; Tokovinin, A. A.; Batten, A. H.; Fekel, F. C.; Hartkopf, W. I.; et al. (2004), "S_B⁹: The ninth catalogue of spectroscopic binary orbits", Astronomy & Astrophysics, 424 (2): 727–732, arXiv:astro-ph/0406573, Bibcode:2004A&A...424..727P, doi:10.1051/0004-6361:20041213, S2CID 119387088.
^ ^a ^b Brown, Timothy M. (January 2010), "Radii of Rapidly Rotating Stars, with Application to Transiting-Planet Hosts", The Astrophysical Journal, 709 (1): 535–545, arXiv:0912.1639, Bibcode:2010ApJ...709..535B, doi:10.1088/0004-637X/709/1/535, S2CID 119253009.
^ ^a ^b ^c ^d 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:1501.06585, Bibcode:2015AJ....149..131E, doi:10.1088/0004-6256/149/4/131, S2CID 118740259, 131.
^ 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:1997A&A...326.1044J.
^ "chi02 Hya", SIMBAD, Centre de données astronomiques de Strasbourg, retrieved 2017-03-17.
^ 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:1997MNRAS.289..869P, doi:10.1093/mnras/289.4.869.