Editing Hydrus (section)

===Stars===
{{see also|List of stars in Hydrus}}
Keyzer and de Houtman assigned fifteen stars to the constellation in their Malay and Madagascan vocabulary, with a star that would be later designated as [[Alpha Hydri]] marking the head, Gamma the chest and a number of stars that were later allocated to Tucana, Reticulum, Mensa and Horologium marking the body and tail.<ref>{{cite journal | title = On Frederick de Houtman's Catalogue of Southern Stars, and the Origin of the Southern Constellations | author= Knobel, Edward B. |journal = [[Monthly Notices of the Royal Astronomical Society]] | volume= 77| issue= 5 | pages=414–32 [422] | bibcode = 1917MNRAS..77..414K |date=1917 | doi=10.1093/mnras/77.5.414| author-link= Edward Knobel | doi-access= free }}</ref> Lacaille charted and designated 20 stars with the [[Bayer designation]]s Alpha through to Tau in 1756. Of these, he used the designations Eta, Pi and Tau twice each, for three sets of two stars close together, and omitted Omicron and Xi. He assigned Rho to a star that subsequent astronomers were unable to find.<ref name=wagman>{{cite book | last = Wagman | first = Morton | date = 2003 | title = Lost Stars: Lost, Missing and Troublesome Stars from the Catalogues of Johannes Bayer, Nicholas Louis de Lacaille, John Flamsteed, and Sundry Others | publisher = The McDonald & Woodward Publishing Company | location = Blacksburg, [[Virginia|VA]] | isbn = 978-0-939923-78-6 |pages=176–77}}</ref>

Beta Hydri, the brightest star in Hydrus, is a yellow star of [[apparent magnitude]] 2.8, lying 24 [[light-year]]s from Earth.<ref>{{cite web |url = http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Beta+hyi&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id |title = Beta Hydri |work = SIMBAD Astronomical Database|publisher=Centre de Données astronomiques de Strasbourg |access-date = 11 September 2013}}</ref> It has about 104% of the [[Solar mass|mass of the Sun]] and 181% of the Sun's radius, with more than three times the Sun's [[luminosity]].<ref name=aaa527_A37>{{Cite journal | last1=Brandão | first1=I. M. | last2=Doğan | first2=G. | last3=Christensen-Dalsgaard | first3=J. | last4=Cunha | first4=M. S. | last5=Bedding | first5=T. R. |author5-link= Tim Bedding| last6=Metcalfe | first6=T. S. | last7=Kjeldsen | first7=H. | last8=Bruntt | first8=H. | last9=Arentoft | first9=T.| display-authors = 8 | title=Asteroseismic Modelling of the Solar-type Subgiant Star β Hydri | journal=Astronomy & Astrophysics | volume=527 | page=A37 |date=March 2011 | doi=10.1051/0004-6361/201015370 | bibcode=2011A&A...527A..37B |arxiv = 1012.3872| s2cid=37441284 }}</ref> The [[stellar spectrum|spectrum]] of this star matches a [[stellar classification]] of G2&nbsp;IV, with the [[luminosity class]] of 'IV' indicating this is a [[Subgiant|subgiant star]]. As such, it is a slightly more [[stellar evolution|evolved]] star than the Sun, with the supply of hydrogen fuel at its [[stellar structure|core]] becoming exhausted. It is the nearest subgiant star to the Sun and one of the oldest stars in the solar neighbourhood. Thought to be between 6.4 and 7.1 billion years old, this star bears some resemblance to what the Sun may look like in the far distant future, making it an object of interest to astronomers.<ref name=aaa527_A37/> It is also the closest bright star to the [[pole star#Southern pole star (South Star)|south celestial pole]].<ref name="motz"/>

Located at the northern edge of the constellation and just southwest of [[Achernar]] is [[Alpha Hydri]],<ref name=moore2005>{{cite book | author=Moore, Patrick | date=2005 | title=The Observer's Year: 366 Nights of the Universe | url=https://archive.org/details/observersyearnig00moor | url-access=limited | page=[https://archive.org/details/observersyearnig00moor/page/n149 4] | edition=2nd | publisher=Springer |location=New York City | isbn=978-1-85233-884-8 | author-link=Patrick Moore }}</ref> a white sub-giant star of magnitude 2.9, situated 72 light-years from Earth.<ref name="kaleralpha">{{cite web|url=http://stars.astro.illinois.edu/sow/alphahyi.html |title=Alpha Hydri |last=Kaler|first=Jim|author-link= James B. Kaler |work=Stars|publisher=University of Illinois|access-date=12 September 2013}}</ref> Of spectral type F0IV,<ref>{{cite web |url = http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Alpha+hydri&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id |title = LTT 1059—High Proper-motion Star |work = SIMBAD Astronomical Database|publisher=Centre de Données astronomiques de Strasbourg |access-date = 12 September 2013}}</ref> it is beginning to cool and enlarge as it uses up its supply of hydrogen. It is twice as massive and 3.3 times as wide as the Sun and 26 times more luminous.<ref name="kaleralpha"/> A line drawn between Alpha Hydri and [[Beta Centauri]] is bisected by the south celestial pole.<ref name=hartungs/>

In the southeastern corner of the constellation is [[Gamma Hydri]],<ref name="motz"/> a [[red giant]] of spectral type M2III located 214 light-years from Earth.<ref>{{cite web |url = http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Gamma+hydri&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id |title = Gamma Hydri—Variable Star |work = SIMBAD Astronomical Database|publisher=Centre de Données astronomiques de Strasbourg |access-date = 12 September 2013}}</ref> It is a [[Semiregular variable star|semi-regular variable]] star, pulsating between magnitudes 3.26 and 3.33. Observations over five years were not able to establish its [[wikt:periodicity|periodicity]].<ref>{{cite journal | title = Long-term Photometry and Periods for 261 Nearby Pulsating M Giants | last1 = Tabur | first1 = V. | last2 = Bedding | first2 = T. R. |author2-link= Tim Bedding| date = 2009 | journal = Monthly Notices of the Royal Astronomical Society | volume = 400 | issue = 4 | pages = 1945–61 | doi = 10.1111/j.1365-2966.2009.15588.x |arxiv = 0908.3228 |bibcode = 2009MNRAS.400.1945T | last3 = Kiss | first3 = L. L. | last4 = Moon | first4 = T. T. | last5 = Szeidl | first5 = B. | last6 = Kjeldsen | first6 = H. | doi-access = free | s2cid = 15358380 }}</ref> It is around 1.5 to 2 times as massive as the Sun, and has expanded to about 60 times the Sun's diameter. It shines with about 655 times the luminosity of the Sun.<ref name="kalergamma">{{cite web|url=http://stars.astro.illinois.edu/sow/gammahyi.html |title=Gamma Hydri |last=Kaler|first=Jim |work=Stars|publisher=University of Illinois|access-date=12 September 2013}}</ref> Located 3° northeast of Gamma is the [[VW Hydri]], a [[dwarf nova]] of the SU Ursae Majoris type. It is a close [[Binary star|binary]] system that consists of a [[white dwarf]] and another star, the former drawing off matter from the latter into a bright [[accretion disk]]. These systems are characterised by frequent eruptions and less frequent supereruptions. The former are smooth, while the latter exhibit short "superhumps" of heightened activity.<ref name=AAVSOVW>{{cite web|url=http://www.aavso.org/vsots_vwhyi|title=VW Hydri|last=BSJ|date=19 July 2010|work=AAVSO Website|publisher=American Association of Variable Star Observers|access-date=12 September 2013}}</ref> One of the brightest dwarf novae in the sky,<ref>{{cite journal|date=1974|title=Photometric Study of the Dwarf Nova VW Hydri|journal=Astronomy and Astrophysics |author= Vogt, N. |volume= 36| pages= 369–78 |bibcode= 1974A&A....36..369V }}</ref> it has a baseline magnitude of 14.4 and can brighten to magnitude 8.4 during peak activity.<ref name=AAVSOVW/> [[BL Hydri]] is another close binary system composed of a low-mass star and a strongly magnetic white dwarf. Known as a [[Polar (cataclysmic variable star)|polar]] or AM Herculis variable, these produce [[Polarization (waves)|polarized]] optical and [[infrared]] emissions and intense soft and hard [[X-ray]] emissions to the frequency of the white dwarf's [[rotation period]]—in this case 113.6 minutes.<ref>{{cite journal|last1=Middleditch|last2=Imamura|last3=Steiman-Cameron|first1=John|first2=James N.|first3=Thomas Y.|date=1997|title=Discovery of Quasi-periodic Oscillations in the AM Herculis Object BL Hydri|journal=Astrophysical Journal|volume=489|issue=2|pages=912–16 |doi=10.1086/304834 |bibcode=1997ApJ...489..912M|doi-access=free}}</ref>

There are two notable optical double stars in Hydrus. Pi Hydri, composed of [[Pi1 Hydri|Pi<sup>1</sup> Hydri]] and [[Pi2 Hydri|Pi<sup>2</sup> Hydri]], is divisible in binoculars.<ref name="motz"/> Around 476 light-years distant,<ref>{{cite web |url = http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Pi1+hydri&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id |title = HR 667 |work = SIMBAD Astronomical Database|publisher=Centre de Données astronomiques de Strasbourg |access-date = 12 September 2013}}</ref> Pi<sup>1</sup> is a red giant of spectral type M1III that varies between magnitudes 5.52 and 5.58.<ref>{{cite web |url = http://www.aavso.org/vsx/index.php?view=detail.top&oid=39391 |title = NSV 767 |publisher = AAVSO |work = International Variable Star Index |date = 18 January 2010 |access-date = 12 September 2013}}</ref> Pi<sup>2</sup> is an orange giant of spectral type K2III and shining with a magnitude of 5.7, around 488 light-years from Earth.<ref>{{cite web |url =http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Pi2+hydri&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id |title = HR 678 |work = SIMBAD Astronomical Database|publisher=Centre de Données astronomiques de Strasbourg |access-date = 12 September 2013}}</ref>

Eta Hydri is the other optical double, composed of [[Eta1 Hydri|Eta<sup>1</sup>]] and [[Eta2 Hydri|Eta<sup>2</sup>]].<ref name="motz"/> Eta<sup>1</sup> is a blue-white main sequence star of spectral type B9V that was suspected of being variable,<ref>{{cite web |url = http://www.aavso.org/vsx/index.php?view=detail.top&oid=16482 |title = Eta1 Hydri  |publisher = AAVSO |work = International Variable Star Index |date = 4 January 2010 |access-date = 12 September 2013}}</ref> and is located just over 700 light-years away.<ref>{{cite web |url = http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Eta1+Hyi&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id |title = Eta 1Hydri—Variable Star |work = SIMBAD Astronomical Database|publisher=Centre de Données astronomiques de Strasbourg |access-date = 12 September 2013}}</ref> Eta<sup>2</sup> has a magnitude of 4.7 and is a yellow giant star of spectral type G8.5III around 218 light-years distant,<ref>{{cite web |url = http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Eta2+Hyi&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id |title = HR 570—Star |work = SIMBAD Astronomical Database|publisher=Centre de Données astronomiques de Strasbourg |access-date = 27 September 2013}}</ref> which has evolved off the main sequence and is expanding and cooling on its way to becoming a red giant. Calculations of its mass indicate it was most likely a white A-type main sequence star for most of its existence, around twice the mass of the Sun. A planet, [[Eta2 Hydri b]], greater than 6.5 times the mass of [[Jupiter]] was discovered in 2005, orbiting around Eta<sup>2</sup> every 711 days at a distance of 1.93 [[astronomical unit]]s (AU).<ref>{{cite journal | last1 =Setiawan | first1=J. | title=A Substellar Companion Around the Intermediate-mass Giant star HD 11977 | journal=[[Astronomy & Astrophysics]] | date=2005 | volume=437 | issue=2 | pages=L31–L34 | doi=10.1051/0004-6361:200500133 | last2=Rodmann | first2=J. | last3=Da Silva | first3=L. | last4=Hatzes | first4=A. P. | last5=Pasquini | first5=L. | last6=Von Der Lühe | first6=O. | last7=De Medeiros | first7=J. R. | last8=Döllinger | first8=M. P. | last9=Girardi | first9=L.| display-authors = 8 | bibcode=2005A&A...437L..31S|arxiv = astro-ph/0505510 | s2cid=6362483 }}</ref><!-- cites previous three sentences -->

Three other systems have been found to have planets, most notably the [[Solar analog#Solar-type|Sun-like]] star [[HD 10180]], which has seven planets, plus possibly an additional two for a total of nine—as of 2012 more than any other system to date, including the Solar System.<ref name=Tuomi>{{cite journal|last=Tuomi|first=Mikko|title=Evidence for 9 planets in the 10180 system|journal=Astronomy & Astrophysics|date=April 2012|arxiv=1204.1254|doi=10.1051/0004-6361/201118518|volume=543|pages=A52|bibcode = 2012A&A...543A..52T |s2cid=15876919}}</ref> Lying around {{Convert|127|ly|pc|abbr=off}} from the Earth,<ref name=BBC11070991>{{Cite news| first=Victoria | last=Gill | date=24 August 2010 | url=https://www.bbc.co.uk/news/science-environment-11070991 | title=Rich exoplanet system discovered | journal=BBC News | access-date=24 August 2010}}</ref> it has an apparent magnitude of 7.33.<ref>{{Cite web| title=HD 10180—Star | work=SIMBAD | publisher=Centre de Données astronomiques de Strasbourg | url=http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=HD+10180 | access-date=13 September 2013}}</ref>

[[HD 1237|GJ 3021]] is a [[Solar analog|solar twin]]—a star very like the Sun—around 57 light-years distant with a spectral type G8V and magnitude of 6.7.<ref>{{Cite web| title=HD 1237—Pre-main sequence Star | work=SIMBAD | publisher=Centre de Données astronomiques de Strasbourg | url=http://simbad.u-strasbg.fr/simbad/sim-id?Ident=GJ+3021&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id | access-date=28 September 2013}}</ref> It has a [[Giant planet|Jovian planet]] companion ([[Gliese 3021 b|GJ 3021 b]]). Orbiting about 0.5&nbsp;AU from its star, it has a minimum mass 3.37 times that of Jupiter and a period of around 133 days.<ref name="Naef2001">{{cite journal | title=The CORALIE survey for southern extrasolar planets V. 3 new extrasolar planets | last1=Naef | first1=D. | last2=Mayor | first2=M. | last3=Pepe | first3=F. | last4=Queloz | first4=D. | last5=Santos | first5=N. C. | last6=Udry | first6=S. | last7=Burnet | first7=M. | journal=Astronomy and Astrophysics | volume=375 | issue=1 | pages=205–18 | date=2001 | arxiv=astro-ph/0106255 | bibcode=2001A&A...375..205N | doi=10.1051/0004-6361:20010841 | s2cid=16606841 }}</ref> The system is a complex one as the faint star GJ 3021B orbits at a distance of 68 AU; it is a red dwarf of spectral type M4V.<ref name="Chauvin2007">{{Cite journal | title=Characterization of the Long-period Companions of the Exoplanet Host Stars: HD 196885, HD 1237 and HD 27442| last1=Chauvin | first1=G. | last2=Lagrange | first2=A.-M. | last3=Udry | first3=S. | last4=Mayor | first4=M. | journal=Astronomy and Astrophysics | volume=475 | issue=2 | pages=723–27 | date=2007 | arxiv=0710.5918 | bibcode=2007A&A...475..723C | doi=10.1051/0004-6361:20067046 | s2cid=16950822 }}</ref>

[[HD 20003]] is a star of magnitude 8.37. It is a yellow main sequence star of spectral type G8V a little cooler and smaller than the Sun around 143 light-years away. It has two planets that are around 12 and 13.5 times as massive as the Earth with periods of just under 12 and 34 days respectively.<ref>{{cite arXiv |eprint=1109.2497 |class=astro-ph.EP|last1=Brandão|first1=I. M.|title=The HARPS search for southern extra-solar planets XXXIV. Occurrence, mass distribution and orbital properties of super-Earths and Neptune-mass planets|last2=Dogan|first2=G.|last3=Christensen-Dalsgaard|first3=J.|last4=Cunha|first4=M. S.|last5=Bedding|first5=T. R.|last6=Metcalfe|first6=T. S.|last7=Kjeldsen|first7=H.|last8=Bruntt|first8=H.|last9=Arentoft|first9=T.|year=2011}}</ref>