{{Short description|Star in the constellation Hydra}} {{Starbox begin}} {{Starbox observe | epoch = J2000.0 | constell = [[Hydra (constellation)|Hydra]] | ra = {{RA|14|05|33.018}}<ref name=dr3/> | dec = {{DEC|-27|01|34.01}}<ref name=dr3/> | appmag_v = 15.3<ref name=ucac4/> }} {{Starbox character | type = [[horizontal branch]]<ref name=guyot2025/> | class = sdB+<ref name=kilkenny1997/> | variable = [[V361 Hydrae variable|V361 Hya]]<ref name=gcvs/> }} {{Starbox astrometry | radial_v = | prop_mo_ra = −7.8<ref name=ucac4/> | prop_mo_dec = −9.4<ref name=ucac4/> | parallax = −0.7258 | p_error = 0.3350 | parallax_footnote = <ref name=dr2/> | dist_ly = ~7,200<ref name=tic/> | dist_pc = 2,231<ref name=tic/> | absmag_v = }} {{Starbox detail | mass = 0.47<ref name=tic/> | radius = 0.101<ref name=tic/> | gravity= 6.10<ref name=geier2020/> | luminosity = 13.3<ref name=tic/> | temperature = 34,700<ref name=geier2020/> }} {{Starbox catalog | names = [[Edinburgh-Cape Blue Object Survey|EC]] 14026−2647, [[UCAC4]] 315-074628 }} {{Starbox reference | Simbad = V361+Hya }} {{Starbox end}}
'''V361 Hydrae''' is a [[hot subdwarf]] [[variable star]], the prototype of a class of pulsating variables. It is very approximately 2,200 [[parsec]]s away in the [[constellation]] of [[Hydra (constellation)|Hydra]]. The star is classified as an extreme [[horizontal branch]] star; it is small, with only half the [[solar mass|mass]] and a tenth the diameter of the [[Sun]], but is 13 times as [[solar luminosity|luminous as the Sun]].
==Discovery== This star was first noted in the [[Edinburgh-Cape Blue Object Survey]], a search for blue objects in the [[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 variable]]s. In 2000, this star was given the [[variable star designation]] V361 Hydrae<ref name=ibvs/> and in 2008 a new class of variable stars was formally created, the V361 Hydrae variables.<ref name=jeffery2008/>
V361 Hydrae itself varies in brightness by about a tenth of a [[magnitude (astronomy)|magnitude]]<ref name=gcvs/> with a period of 144 seconds.<ref name=kilkenny1997/>
==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]] star at or just before the onset of [[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]] stars. The reason that some stars lose so much mass is not well known.<ref name=silvotti2018/> At the [[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]].<ref name=silvotti2007/>
==Possible companion== The [[stellar spectrum|spectrum]] of V361 Hydrae is unusual and difficult to interpret. While it is clearly a hot hydrogen-deficient [[subdwarf]], the spectrum contains a full series of hydrogen [[absorption line]]s that are typical of a class F or G [[main sequence star]] and these are presumed to originate from an otherwise-unseen companion.<ref name=kilkenny1997/><ref name=geier2020/>
== References == <references>
<ref name=dr2>{{Cite Gaia DR2|6270542648889732096}}</ref>
<ref name=dr3>{{Cite Gaia DR3|6270542648889732096}}</ref>
<ref name=gcvs>{{cite journal | title=General Catalogue of Variable Stars | display-authors=1 | last1=Samus | first1=N. N. | last2=Kazarovets | first2=E. V. | last3=Durlevich | first3=O. V. | last4=Kireeva | first4=N. N. | last5=Pastukhova | first5=E. N. | version=GCVS 5.1 | journal=Astronomy Reports | year=2017 | volume=61 | issue=1 | pages=80–88 | doi=10.1134/S1063772917010085 | bibcode=2017ARep...61...80S }}</ref>
<ref name=ucac4>{{cite book | last=Zacharias | first=N. | title=The fourth US Naval Observatory CCD Astrograph Catalog (UCAC4) | series=VizieR On-line Data Catalog | volume=1322 | date=2012 | bibcode=2012yCat.1322....0Z }}</ref>
<ref name=tic>{{cite journal |last1=Stassun |first1=Keivan G. |last2=Oelkers |first2=Ryan J. |last3=Paegert |first3=Martin |last4=Torres |first4=Guillermo |last5=Pepper |first5=Joshua |last6=De Lee |first6=Nathan |last7=Collins |first7=Kevin |last8=Latham |first8=David W. |last9=Muirhead |first9=Philip S. |last10=Chittidi |first10=Jay |last11=Rojas-Ayala |first11=Bárbara |last12=Fleming |first12=Scott W. |last13=Rose |first13=Mark E. |last14=Tenenbaum |first14=Peter |last15=Ting |first15=Eric B. |last16=Kane |first16=Stephen R. |last17=Barclay |first17=Thomas |last18=Bean |first18=Jacob L. |last19=Brassuer |first19=C. E. |last20=Charbonneau |first20=David |last21=Ge |first21=Jian |last22=Lissauer |first22=Jack J. |last23=Mann |first23=Andrew W. |last24=McLean |first24=Brian |last25=Mullally |first25=Susan |last26=Narita |first26=Norio |last27=Plavchan |first27=Peter |last28=Ricker |first28=George R. |last29=Sasselov |first29=Dimitar |last30=Seager |first30=S. |display-authors=1 |title=The Revised TESS Input Catalog and Candidate Target List |journal=The Astronomical Journal |date=2019 |volume=158 |issue=4 |page=138 |doi=10.3847/1538-3881/ab3467 |doi-access=free |arxiv=1905.10694 |bibcode=2019AJ....158..138S }}</ref>
<ref name=ibvs>{{cite journal |last1=Kazarovets |first1=E. V. |last2=Samus |first2=N. N. |last3=Durlevich |first3=O. V. |title=The 75th Name-List of Variable Stars |journal=Information Bulletin on Variable Stars |date=2000 |volume=4870 |page=1 |bibcode=2000IBVS.4870....1K }}</ref>
<ref name=jeffery2008>{{cite journal |last1=Jeffery |first1=C. Simon |title=Variable Star Designations for Extreme Helium Stars |journal=Information Bulletin on Variable Stars |date=2008 |volume=5817 |page=1 |bibcode=2008IBVS.5817....1J }}</ref>
<ref name=geier2020>{{cite journal |last1=Geier |first1=S. |title=The population of hot subdwarf stars studied with Gaia. III. Catalogue of known hot subdwarf stars: Data Release 2 |journal=Astronomy and Astrophysics |date=2020 |volume=635 |pages=A193 |doi=10.1051/0004-6361/202037526 |arxiv=2002.10896 |bibcode=2020A&A...635A.193G }}</ref>
<ref name=silvotti2007>{{cite journal |last1=Silvotti |first1=R. |last2=Schuh |first2=S. |last3=Janulis |first3=R. |last4=Solheim |first4=J.-E. |last5=Bernabei |first5=S. |last6=Østensen |first6=R. |last7=Oswalt |first7=T. D. |last8=Bruni |first8=I. |last9=Gualandi |first9=R. |last10=Bonanno |first10=A. |last11=Vauclair |first11=G. |last12=Reed |first12=M. |last13=Chen |first13=C.-W. |last14=Leibowitz |first14=E. |last15=Paparo |first15=M. |last16=Baran |first16=A. |last17=Charpinet |first17=S. |last18=Dolez |first18=N. |last19=Kawaler |first19=S. |last20=Kurtz |first20=D. |last21=Moskalik |first21=P. |last22=Riddle |first22=R. |last23=Zola |first23=S. |title=A giant planet orbiting the 'extreme horizontal branch' star V391 Pegasi |journal=Nature |date=2007 |volume=449 |issue=7159 |pages=189–191 |doi=10.1038/nature06143 |pmid=17851517 |bibcode=2007Natur.449..189S |url=https://ruj.uj.edu.pl/xmlui/handle/item/156623 }}</ref>
<ref name=silvotti2018>{{cite journal |last1=Silvotti |first1=R. |last2=Schuh |first2=S. |last3=Kim |first3=S.-L. |last4=Lutz |first4=R. |last5=Reed |first5=M. |last6=Benatti |first6=S. |last7=Janulis |first7=R. |last8=Lanteri |first8=L. |last9=Østensen |first9=R. |last10=Marsh |first10=T. R. |last11=Dhillon |first11=V. S. |last12=Paparo |first12=M. |last13=Molnar |first13=L. |title=The SDB pulsating star V391 Peg and its putative giant planet revisited after 13 years of time-series photometric data |journal=Astronomy and Astrophysics |date=2018 |volume=611 |pages=A85 |doi=10.1051/0004-6361/201731473 |arxiv=1711.10942 |bibcode=2018A&A...611A..85S }}</ref>
<ref name=guyot2025>{{cite journal |last1=Guyot |first1=N. |last2=Van Grootel |first2=V. |last3=Charpinet |first3=S. |last4=Farnir |first4=M. |last5=Dupret |first5=M.-A. |last6=Brassard |first6=P. |title=The theoretical pulsation spectra of hot B subdwarfs: Static and evolutionary STELUM models |journal=Astronomy and Astrophysics |date=2025 |volume=696 |doi=10.1051/0004-6361/202452423 |arxiv=2503.16046 |bibcode=2025A&A...696A..13G }}</ref>
<ref name=kilkenny1997>{{Cite journal |last1=Kilkenny |first1=D. |last2=O'Donoghue |first2=D. |last3=Koen |first3=C. |last4=Stobie |first4=R. S. |last5=Chen |first5=A. |date=1997-06-01 |title=The Edinburgh-Cape Blue Object Survey - II. Zone 1 - the North Galactic CAP |bibcode=1997MNRAS.287..867K |journal=Monthly Notices of the Royal Astronomical Society |volume=287 |issue=4 |pages=867–893 |doi=10.1093/mnras/287.4.867 |issn=0035-8711|doi-access=free }}</ref>
</references>
==Sources== * {{cite journal|author=C. S. Jeffery|title=Pulsations in Subdwarf B Stars|journal=[[Journal of Astrophysics and Astronomy]]|volume=26|issue=2–3|pages=261–271|date=2005|doi=10.1007/BF02702334|bibcode = 2005JApA...26..261J |s2cid=13814916}}
{{Stars of Hydra}}
[[Category:B-type subdwarfs]] [[Category:Hydra (constellation)]] [[Category:Very rapidly pulsating hot stars]] [[Category:Objects with variable star designations|Hydrae, V361]] [[Category:Horizontal-branch stars]]