# V361 Hydrae

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Star in the constellation Hydra

V361 Hydrae Observation data Epoch J2000.0 Equinox J2000.0 Constellation Hydra Right ascension 14h 05m 33.018s[1] Declination −27° 01′ 34.01″[1] Apparent magnitude (V) 15.3[2] Characteristics Evolutionary stage horizontal branch[3] Spectral type sdB+[4] Variable type V361 Hya[5] Astrometry Proper motion (μ) RA: −7.8[2] mas/yr Dec.: −9.4[2] mas/yr Parallax (π) −0.7258±0.3350 mas[6] Distance ~7,200[7] ly (2,231[7] pc) Details Mass 0.47[7] M☉ Radius 0.101[7] R☉ Luminosity 13.3[7] L☉ Surface gravity (log g) 6.10[8] cgs Temperature 34,700[8] K Other designations EC 14026−2647, UCAC4 315-074628 Database references SIMBAD data

**V361 Hydrae** is a [hot subdwarf](/source/Hot_subdwarf) [variable star](/source/Variable_star), the prototype of a class of pulsating variables. It is very approximately 2,200 [parsecs](/source/Parsec) away in the [constellation](/source/Constellation) of [Hydra](/source/Hydra_(constellation)). The star is classified as an extreme [horizontal branch](/source/Horizontal_branch) star; it is small, with only half the [mass](/source/Solar_mass) and a tenth the diameter of the [Sun](/source/Sun), but is 13 times as [luminous as the Sun](/source/Solar_luminosity).

## Discovery

This star was first noted in the [Edinburgh-Cape Blue Object Survey](/source/Edinburgh-Cape_Blue_Object_Survey), a search for blue objects in the [southern celestial hemisphere](/source/Southern_celestial_hemisphere) published in 1997, and it was catalogued as EC 14026−2647. Together with several similar objects from the survey, it was found to pulsate with a period of just a few minutes. These stars were initially referred to as EC 14026 stars and tentatively grouped with the [PV Telescopii variables](/source/PV_Telescopii_variable). In 2000, this star was given the [variable star designation](/source/Variable_star_designation) V361 Hydrae[9] and in 2008 a new class of variable stars was formally created, the V361 Hydrae variables.[10]

V361 Hydrae itself varies in brightness by about a tenth of a [magnitude](/source/Magnitude_(astronomy))[5] with a period of 144 seconds.[4]

## Formation

Subdwarf B stars such as V361 Hydrae are thought to be the result of the ejection of the hydrogen envelope of a [red giant](/source/Red_giant) star at or just before the onset of [helium](/source/Helium) fusion. The ejection left only a tiny amount of hydrogen on the surface—less than 1/1000 of the total stellar mass. The future for the star is to eventually cool down to make a low-mass white dwarf. Most stars retain more of their hydrogen after the first red giant phase, and eventually become [asymptotic giant branch](/source/Asymptotic_giant_branch) stars. The reason that some stars lose so much mass is not well known.[11] At the [tip of the red-giant branch](/source/Tip_of_the_red-giant_branch), the red giant precursors of the subdwarf stars reach their maximum radius, on the order of 0.7 AU. After this point, the hydrogen envelope is lost and helium fusion begins—this is known as the [helium flash](/source/Helium_flash).[12]

## Possible companion

The [spectrum](/source/Stellar_spectrum) of V361 Hydrae is unusual and difficult to interpret. While it is clearly a hot hydrogen-deficient [subdwarf](/source/Subdwarf), the spectrum contains a full series of hydrogen [absorption lines](/source/Absorption_line) that are typical of a class F or G [main sequence star](/source/Main_sequence_star) and these are presumed to originate from an otherwise-unseen companion.[4][8]

## References

1. ^ [***a***](#cite_ref-dr3_1-0) [***b***](#cite_ref-dr3_1-1) 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%3D6270542648889732096%26-go+%25%23Sesame%23%25%26) at [VizieR](/source/VizieR).

1. ^ [***a***](#cite_ref-ucac4_2-0) [***b***](#cite_ref-ucac4_2-1) [***c***](#cite_ref-ucac4_2-2) Zacharias, N. (2012). *The fourth US Naval Observatory CCD Astrograph Catalog (UCAC4)*. VizieR On-line Data Catalog. Vol. 1322. [Bibcode](/source/Bibcode_(identifier)):[2012yCat.1322....0Z](https://ui.adsabs.harvard.edu/abs/2012yCat.1322....0Z).

1. **[^](#cite_ref-guyot2025_3-0)** Guyot, N.; Van Grootel, V.; Charpinet, S.; Farnir, M.; Dupret, M.-A.; Brassard, P. (2025). "The theoretical pulsation spectra of hot B subdwarfs: Static and evolutionary STELUM models". *Astronomy and Astrophysics*. **696**. [arXiv](/source/ArXiv_(identifier)):[2503.16046](https://arxiv.org/abs/2503.16046). [Bibcode](/source/Bibcode_(identifier)):[2025A&A...696A..13G](https://ui.adsabs.harvard.edu/abs/2025A&A...696A..13G). [doi](/source/Doi_(identifier)):[10.1051/0004-6361/202452423](https://doi.org/10.1051%2F0004-6361%2F202452423).

1. ^ [***a***](#cite_ref-kilkenny1997_4-0) [***b***](#cite_ref-kilkenny1997_4-1) [***c***](#cite_ref-kilkenny1997_4-2) Kilkenny, D.; O'Donoghue, D.; Koen, C.; Stobie, R. S.; Chen, A. (1997-06-01). ["The Edinburgh-Cape Blue Object Survey - II. Zone 1 - the North Galactic CAP"](https://doi.org/10.1093%2Fmnras%2F287.4.867). *Monthly Notices of the Royal Astronomical Society*. **287** (4): 867–893. [Bibcode](/source/Bibcode_(identifier)):[1997MNRAS.287..867K](https://ui.adsabs.harvard.edu/abs/1997MNRAS.287..867K). [doi](/source/Doi_(identifier)):[10.1093/mnras/287.4.867](https://doi.org/10.1093%2Fmnras%2F287.4.867). [ISSN](/source/ISSN_(identifier)) [0035-8711](https://search.worldcat.org/issn/0035-8711).

1. ^ [***a***](#cite_ref-gcvs_5-0) [***b***](#cite_ref-gcvs_5-1) Samus, N. N.; et al. (2017). "General Catalogue of Variable Stars". *Astronomy Reports*. GCVS 5.1. **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).

1. **[^](#cite_ref-dr2_6-0)** [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%206270542648889732096) at [VizieR](/source/VizieR).

1. ^ [***a***](#cite_ref-tic_7-0) [***b***](#cite_ref-tic_7-1) [***c***](#cite_ref-tic_7-2) [***d***](#cite_ref-tic_7-3) [***e***](#cite_ref-tic_7-4) Stassun, Keivan G.; et al. (2019). ["The Revised TESS Input Catalog and Candidate Target List"](https://doi.org/10.3847%2F1538-3881%2Fab3467). *The Astronomical Journal*. **158** (4): 138. [arXiv](/source/ArXiv_(identifier)):[1905.10694](https://arxiv.org/abs/1905.10694). [Bibcode](/source/Bibcode_(identifier)):[2019AJ....158..138S](https://ui.adsabs.harvard.edu/abs/2019AJ....158..138S). [doi](/source/Doi_(identifier)):[10.3847/1538-3881/ab3467](https://doi.org/10.3847%2F1538-3881%2Fab3467).

1. ^ [***a***](#cite_ref-geier2020_8-0) [***b***](#cite_ref-geier2020_8-1) [***c***](#cite_ref-geier2020_8-2) Geier, S. (2020). "The population of hot subdwarf stars studied with Gaia. III. Catalogue of known hot subdwarf stars: Data Release 2". *Astronomy and Astrophysics*. **635**: A193. [arXiv](/source/ArXiv_(identifier)):[2002.10896](https://arxiv.org/abs/2002.10896). [Bibcode](/source/Bibcode_(identifier)):[2020A&A...635A.193G](https://ui.adsabs.harvard.edu/abs/2020A&A...635A.193G). [doi](/source/Doi_(identifier)):[10.1051/0004-6361/202037526](https://doi.org/10.1051%2F0004-6361%2F202037526).

1. **[^](#cite_ref-ibvs_9-0)** Kazarovets, E. V.; Samus, N. N.; Durlevich, O. V. (2000). "The 75th Name-List of Variable Stars". *Information Bulletin on Variable Stars*. **4870**: 1. [Bibcode](/source/Bibcode_(identifier)):[2000IBVS.4870....1K](https://ui.adsabs.harvard.edu/abs/2000IBVS.4870....1K).

1. **[^](#cite_ref-jeffery2008_10-0)** Jeffery, C. Simon (2008). "Variable Star Designations for Extreme Helium Stars". *Information Bulletin on Variable Stars*. **5817**: 1. [Bibcode](/source/Bibcode_(identifier)):[2008IBVS.5817....1J](https://ui.adsabs.harvard.edu/abs/2008IBVS.5817....1J).

1. **[^](#cite_ref-silvotti2018_11-0)** Silvotti, R.; Schuh, S.; Kim, S.-L.; Lutz, R.; Reed, M.; Benatti, S.; Janulis, R.; Lanteri, L.; Østensen, R.; Marsh, T. R.; Dhillon, V. S.; Paparo, M.; Molnar, L. (2018). "The SDB pulsating star V391 Peg and its putative giant planet revisited after 13 years of time-series photometric data". *Astronomy and Astrophysics*. **611**: A85. [arXiv](/source/ArXiv_(identifier)):[1711.10942](https://arxiv.org/abs/1711.10942). [Bibcode](/source/Bibcode_(identifier)):[2018A&A...611A..85S](https://ui.adsabs.harvard.edu/abs/2018A&A...611A..85S). [doi](/source/Doi_(identifier)):[10.1051/0004-6361/201731473](https://doi.org/10.1051%2F0004-6361%2F201731473).

1. **[^](#cite_ref-silvotti2007_12-0)** Silvotti, R.; Schuh, S.; Janulis, R.; Solheim, J.-E.; Bernabei, S.; Østensen, R.; Oswalt, T. D.; Bruni, I.; Gualandi, R.; Bonanno, A.; Vauclair, G.; Reed, M.; Chen, C.-W.; Leibowitz, E.; Paparo, M.; Baran, A.; Charpinet, S.; Dolez, N.; Kawaler, S.; Kurtz, D.; Moskalik, P.; Riddle, R.; Zola, S. (2007). ["A giant planet orbiting the 'extreme horizontal branch' star V391 Pegasi"](https://ruj.uj.edu.pl/xmlui/handle/item/156623). *Nature*. **449** (7159): 189–191. [Bibcode](/source/Bibcode_(identifier)):[2007Natur.449..189S](https://ui.adsabs.harvard.edu/abs/2007Natur.449..189S). [doi](/source/Doi_(identifier)):[10.1038/nature06143](https://doi.org/10.1038%2Fnature06143). [PMID](/source/PMID_(identifier)) [17851517](https://pubmed.ncbi.nlm.nih.gov/17851517).

## Sources

- C. S. Jeffery (2005). "Pulsations in Subdwarf B Stars". *[Journal of Astrophysics and Astronomy](/source/Journal_of_Astrophysics_and_Astronomy)*. **26** (2–3): 261–271. [Bibcode](/source/Bibcode_(identifier)):[2005JApA...26..261J](https://ui.adsabs.harvard.edu/abs/2005JApA...26..261J). [doi](/source/Doi_(identifier)):[10.1007/BF02702334](https://doi.org/10.1007%2FBF02702334). [S2CID](/source/S2CID_(identifier)) [13814916](https://api.semanticscholar.org/CorpusID:13814916).

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 [V361 Hydrae](https://en.wikipedia.org/wiki/V361_Hydrae) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/V361_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.