# HR 3750

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Binary star system in the constellation Hydra

HR 3750 Observation data Epoch J2000 Equinox J2000 Constellation Hydra[1] Right ascension 09h 27m 46.7799s[2][failed verification] Declination −06° 04′ 16.282″[2][failed verification] Apparent magnitude (V) 5.40[citation needed] Characteristics Evolutionary stage subgiant[3] Spectral type G1.5IV-V[4] Apparent magnitude (G) 5.1625[5] Astrometry Radial velocity (Rv) +54.93±0.21[6] km/s Proper motion (μ) RA: −242.6±0.4 mas/yr[5] Dec.: −52.7±0.3 mas/yr[5] Parallax (π) 32.30±0.36 mas[5] Distance 101 ± 1 ly (31.0 ± 0.3 pc) Absolute magnitude (MV) A:2.76; B:4.28[3] Orbit[6] Period (P) 34.53±0.16 years Semi-major axis (a) 0.4089±0.0079″ Eccentricity (e) 0.366±0.022 Inclination (i) 85.4±0.0° Longitude of the node (Ω) 150.42±0.39° Periastron epoch (T) 2,455,083±211 JD Argument of periastron (ω) (primary) 347.5±6.7° Semi-amplitude (K1) (primary) 5.42±0.17 km/s Semi-amplitude (K2) (secondary) 6.30±0.25 km/s Details[6] HD 81809 A Mass 0.87±0.08 M☉ Radius 1.95±0.08 R☉ Luminosity 3.44±0.43 L☉ Luminosity (bolometric) 5.8±0.3[7] L☉ Surface gravity (log g) 3.7981±0.0054 cgs Temperature 5,619±125 K Metallicity [Fe/H] −0.49±0.14 dex Rotation 40.2[7] days Rotational velocity (v sin i) 2.0±1.5 km/s Age 9.75±1.78 Gyr HD 81809 B Mass 0.83±0.01 M☉ Radius 1.12±0.03 R☉ Luminosity 1.31±0.10 L☉ Surface gravity (log g) 4.2776±0.0027 cgs Temperature 5,833±72 K Metallicity [Fe/H] −0.23±0.06 dex Age 11.1±1.5 Gyr Other designations BD−05 2802, GJ 344, HD 81809, HIP 46404, HR 3750, SAO 136872, LTT 3482, 2MASS J09274680-0604164[2] Database references SIMBAD data ARICNS data

**HR 3750** is a [binary star](/source/Binary_star) system in the [equatorial](/source/Celestial_equator) [constellation](/source/Constellation) of [Hydra](/source/Hydra_(constellation)) at a distance of 101 [light years](/source/Light_year). This object is visible to the naked eye as a dim, white star with an [apparent visual magnitude](/source/Apparent_visual_magnitude) of 5.4. It is receding from the Earth with a heliocentric [radial velocity](/source/Radial_velocity) of 57.9±0.2 km/s.[5] This binary is unusual because its eruptions[*[clarification needed](https://en.wikipedia.org/wiki/Wikipedia:Please_clarify)*] do not seem to conform to the [Waldmeier effect](/source/Waldmeier_effect)—i.e. the strongest eruptions of HR 3750 are not the ones characterized by the fast eruption onset.[8] Kinematically, the binary belongs to the [thick disk](/source/Thick_disk) of the [Milky Way](/source/Milky_Way) galaxy—a population of ancient, metal-poor stars.[3]

The star system is a spectroscopic binary with a 32 year, nearly edge-on orbit. The primary, HD 81809 A's visual magnitude is 5.610±0.005 while the secondary, HD 81809 B's visual magnitude is 7.115±0.015. The larger star, HD 81809 A, has been hypothetised to have engulfed a 0.36 [M☉](/source/Solar_mass) [red dwarf](/source/Red_dwarf) star 1–3 billion years ago to explain its unusually older age than expected from theorethical models,[3] but the age discrepancy has been resolved as of 2026[\[update\]](https://en.wikipedia.org/w/index.php?title=HR_3750&action=edit) with newer data, discarding the need of a stellar merger. The secondary component, however, appears to be more metal-enriched than the primary, which is unexpected for stars that presumably [formed](/source/Star_formation) in the same envoirment. This indicates that component B had its metallicity changed after a collision with (metal-enriched) planetesimals.[6]

HD 81809 A[7] has a well defined chromospheric activity cycle with a period of 7.3±1.5 years.[9]

## References

1. **[^](#cite_ref-constellation_1-0)** [Roman, Nancy G.](/source/Nancy_Roman) (1987). ["Identification of a constellation from a position"](https://doi.org/10.1086%2F132034). *[Publications of the Astronomical Society of the Pacific](/source/Publications_of_the_Astronomical_Society_of_the_Pacific)*. **99** (617): 695. [Bibcode](/source/Bibcode_(identifier)):[1987PASP...99..695R](https://ui.adsabs.harvard.edu/abs/1987PASP...99..695R). [doi](/source/Doi_(identifier)):[10.1086/132034](https://doi.org/10.1086%2F132034). [Constellation record for this object](https://vizier.cds.unistra.fr/viz-bin/VizieR-4?source=VI%2F42%2Fout&%2F%2Ftables=VI%2F42%2Fout&-c=HR+3750&-c.eq=J2000&-c.r=++2&-c.u=arcmin&-c.geom=r&-source=&-source=VI%2F42%2Fout&-out=RA1875&-out=DE1875&-out=Cst) at [VizieR](/source/VizieR).

1. ^ [***a***](#cite_ref-SIMBAD_2-0) [***b***](#cite_ref-SIMBAD_2-1) [***c***](#cite_ref-SIMBAD_2-2) ["HD 81809"](http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=HD+81809). *[SIMBAD](/source/SIMBAD)*. [Centre de données astronomiques de Strasbourg](/source/Centre_de_donn%C3%A9es_astronomiques_de_Strasbourg). Retrieved June 21, 2021.

1. ^ [***a***](#cite_ref-Fuhrmann2018_3-0) [***b***](#cite_ref-Fuhrmann2018_3-1) [***c***](#cite_ref-Fuhrmann2018_3-2) [***d***](#cite_ref-Fuhrmann2018_3-3) Fuhrmann, Klaus; Chini, Rolf (2018), "Fossil Merger of a Population II Star", *The Astrophysical Journal*, **858** (2): 103, [Bibcode](/source/Bibcode_(identifier)):[2018ApJ...858..103F](https://ui.adsabs.harvard.edu/abs/2018ApJ...858..103F), [doi](/source/Doi_(identifier)):[10.3847/1538-4357/aabaff](https://doi.org/10.3847%2F1538-4357%2Faabaff), [S2CID](/source/S2CID_(identifier)) [125357563](https://api.semanticscholar.org/CorpusID:125357563)

1. **[^](#cite_ref-Keenan1989_4-0)** Keenan, Philip C.; McNeil, Raymond C. (October 1989). "The Perkins Catalog of Revised MK Types for the Cooler Stars". *Astrophysical Journal Supplement*. **71**: 245. [Bibcode](/source/Bibcode_(identifier)):[1989ApJS...71..245K](https://ui.adsabs.harvard.edu/abs/1989ApJS...71..245K). [doi](/source/Doi_(identifier)):[10.1086/191373](https://doi.org/10.1086%2F191373).

1. ^ [***a***](#cite_ref-DR3_5-0) [***b***](#cite_ref-DR3_5-1) [***c***](#cite_ref-DR3_5-2) [***d***](#cite_ref-DR3_5-3) 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%3D5745464991235898752%26-go+%25%23Sesame%23%25%26) at [VizieR](/source/VizieR).

1. ^ [***a***](#cite_ref-Di_Mauro_2026_6-0) [***b***](#cite_ref-Di_Mauro_2026_6-1) [***c***](#cite_ref-Di_Mauro_2026_6-2) [***d***](#cite_ref-Di_Mauro_2026_6-3) Di Mauro, Maria Pia; Pezzotti, Camilla; Moedas, Nuno; Catanzaro, Giovanni; Maxted, Pierre F. L.; Corsaro, Enrico; Reda, Raffaele; Scuflaire, Richard; Bonanno, Alfio Maurizio; Giovannelli, Luca; Beck, Paul G. (2026-03-16). ["On the Contradictory Case of the Binary System HD 81809 Hosting Two Pulsating Solar-like Stars Observed by TESS"](https://doi.org/10.3847%2F1538-4357%2Fae40ad). *The Astrophysical Journal*. **1000** (1): 92. [doi](/source/Doi_(identifier)):[10.3847/1538-4357/ae40ad](https://doi.org/10.3847%2F1538-4357%2Fae40ad). [hdl](/source/Hdl_(identifier)):[2108/456765](https://hdl.handle.net/2108%2F456765). [ISSN](/source/ISSN_(identifier)) [0004-637X](https://search.worldcat.org/issn/0004-637X).

1. ^ [***a***](#cite_ref-Egeland2018_7-0) [***b***](#cite_ref-Egeland2018_7-1) [***c***](#cite_ref-Egeland2018_7-2) Egeland, Ricky (2018), "Deconvolving the HD 81809 Binary: Rotational and Activity Evidence for a Subgiant with a Sun-like Cycle", *The Astrophysical Journal*, **866** (2): 80, [arXiv](/source/ArXiv_(identifier)):[1807.10870](https://arxiv.org/abs/1807.10870), [Bibcode](/source/Bibcode_(identifier)):[2018ApJ...866...80E](https://ui.adsabs.harvard.edu/abs/2018ApJ...866...80E), [doi](/source/Doi_(identifier)):[10.3847/1538-4357/aadf86](https://doi.org/10.3847%2F1538-4357%2Faadf86), [S2CID](/source/S2CID_(identifier)) [119537525](https://api.semanticscholar.org/CorpusID:119537525)

1. **[^](#cite_ref-8)** Garg, Suyog; Karak, Bidya Binay; Egeland, Ricky; Soon, Willie; Baliunas, Sallie (2019), "Waldmeier Effect in Stellar Cycles", *The Astrophysical Journal*, **886** (2): 132, [arXiv](/source/ArXiv_(identifier)):[1909.12148](https://arxiv.org/abs/1909.12148), [Bibcode](/source/Bibcode_(identifier)):[2019ApJ...886..132G](https://ui.adsabs.harvard.edu/abs/2019ApJ...886..132G), [doi](/source/Doi_(identifier)):[10.3847/1538-4357/ab4a17](https://doi.org/10.3847%2F1538-4357%2Fab4a17), [S2CID](/source/S2CID_(identifier)) [202888617](https://api.semanticscholar.org/CorpusID:202888617)

1. **[^](#cite_ref-9)** Orlando, S.; Favata, F.; Micela, G.; Sciortino, S.; Maggio, A.; Schmitt, J. H. M. M.; Robrade, J.; Mittag, M. (2017), "Fifteen years in the high-energy life of the solar-type star HD 81809", *Astronomy & Astrophysics*, **605**: A19, [arXiv](/source/ArXiv_(identifier)):[1707.06437](https://arxiv.org/abs/1707.06437), [doi](/source/Doi_(identifier)):[10.1051/0004-6361/201731301](https://doi.org/10.1051%2F0004-6361%2F201731301), [S2CID](/source/S2CID_(identifier)) [118843788](https://api.semanticscholar.org/CorpusID:118843788)

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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