Editing Hydrus

{{short description|Constellation in the southern celestial hemisphere}}
{{other uses}}
{{distinguish|text=the equatorial constellation of [[Hydra (constellation)|Hydra]]}}
{{Redirect|Hyi||HYI (disambiguation){{!}}HYI}}
{{featured article}}
{{Use dmy dates|date=February 2021}}
{{Infobox constellation
| name = Hydrus
| abbreviation = Hyi
| genitive = Hydri
| pronounce = {{IPAc-en|ˈ|h|aɪ|d|r|ə|s}}, genitive {{IPAc-en|ˈ|h|aɪ|d|r|aɪ}}
| symbolism = the water snake
| RA = {{RA|00|06.1}} to {{RA|04|35.1}}<ref name="boundary"/>
| dec=  −57.85° to −82.06°<ref name="boundary"/>
| family = [[Bayer Family|Bayer]]
| quadrant = SQ1
| areatotal = 243
| arearank = 61st
| numbermainstars = 3
| numberbfstars = 19
| numberstarsplanets = 4
| numberbrightstars = 2
| numbernearbystars = 1
| brighteststarname = [[Beta Hydri|β Hyi]]
| starmagnitude = 2.82
| neareststarname = [[Beta Hydri|β Hyi]]
| stardistancely = 24.38
| stardistancepc = 7.47
| numbermessierobjects = none
| meteorshowers = none
| bordering = [[Dorado]]<br />[[Eridanus (constellation)|Eridanus]]<br />[[Horologium (constellation)|Horologium]]<br />[[Mensa (constellation)|Mensa]]<br />[[Octans]]<br />[[Phoenix (constellation)|Phoenix]] (corner)<br />[[Reticulum]]<br />[[Tucana]]
| latmax = [[8th parallel north|8]]
| latmin = [[South Pole|90]]
| month = December
| notes =
}}
'''Hydrus''' {{IPAc-en|ˈ|h|aɪ|d|r|ə|s}} is a small [[constellation]] in the deep [[Southern Celestial Hemisphere|southern sky]]. It was one of twelve constellations created by [[Petrus Plancius]] from the observations of [[Pieter Dirkszoon Keyser]] and [[Frederick de Houtman]] and it first appeared on a 35-cm (14&nbsp;in) diameter celestial globe published in late 1597 (or early 1598) in Amsterdam by Plancius and [[Jodocus Hondius]]. The first depiction of this constellation in a celestial atlas was in [[Johann Bayer]]'s [[Uranometria]] of 1603. The French explorer and astronomer [[Nicolas-Louis de Lacaille|Nicolas Louis de Lacaille]] charted the brighter [[star]]s and gave their [[Bayer designation]]s in 1756. Its name means "male water snake", as opposed to [[Hydra (constellation)|Hydra]], a much larger constellation that represents a female water snake. It remains below the horizon for most [[Northern Hemisphere]] observers.

The brightest star is the 2.8-[[apparent magnitude|magnitude]] [[Beta Hydri]], also the closest reasonably bright star to the [[pole star#Southern pole star (South Star)|south celestial pole]]. Pulsating between magnitude 3.26 and 3.33, [[Gamma Hydri]] is a variable [[red giant]] 60 times the diameter of the [[Sun]]. Lying near it is [[VW Hydri]], one of the brightest [[dwarf nova]]e in the heavens. Four star systems in Hydrus have been found to have [[exoplanet]]s to date, including [[HD 10180]], which could bear up to nine planetary companions.

==History==

[[File:Johann Bayer - Uraniometria - Southern Birds.jpg|thumb|left|350px|Hydrus (lower right) in an extract from [[Johann Bayer]]'s ''[[Uranometria]]'', its first appearance in a celestial atlas.]]
Hydrus was one of the twelve constellations established by the astronomer [[Petrus Plancius]] from the observations of the southern sky by the Dutch explorers [[Pieter Dirkszoon Keyser]] and [[Frederick de Houtman]], who had sailed on the first Dutch trading expedition, known as the ''[[First Dutch Expedition to Indonesia|Eerste Schipvaart]]'', to the [[East Indies]]. It first appeared on a 35-cm (14&nbsp;in) diameter celestial globe published in late 1597 (or early 1598) in Amsterdam by Plancius with [[Jodocus Hondius]]. The first depiction of this constellation in a celestial atlas was in the German cartographer [[Johann Bayer]]'s ''[[Uranometria]]'' of 1603.<ref>{{cite web| url=http://www.ianridpath.com/startales/bayer-southern.html | title=Johann Bayer's Southern Star Chart|last1=Ridpath|first1=Ian|author-link=Ian Ridpath|publisher=self-published |work=Star Tales | access-date= 23 August 2013}}</ref><!-- cites previous three sentences --><ref>{{cite web |url = http://chandra.harvard.edu/photo/constellations/hydrus.html |title = Hydrus (Water Snake) |publisher = Chandra X-ray Observatory |access-date =10 July 2012}}</ref> De Houtman included it in his southern star catalogue the same year under the Dutch name ''De Waterslang'', "The Water Snake",<ref>{{cite web| url=http://www.ianridpath.com/startales/startales1c.html#houtman | title=Frederick de Houtman's Catalogue|last1=Ridpath|first1=Ian|publisher=self-published  |work=Star Tales | access-date= 11 September 2013}}</ref> it representing a type of snake encountered on the expedition rather than a mythical creature.<ref name=ridphydrus>{{cite web| url=http://www.ianridpath.com/startales/hydrus.html | title=Hydrus—the Lesser Water Snake|last1=Ridpath|first1=Ian|author-link=Ian Ridpath|publisher=self-published  |work=Star Tales| access-date= 29 September 2013}}</ref> The French explorer and astronomer [[Nicolas-Louis de Lacaille|Nicolas Louis de Lacaille]] called it ''l’Hydre Mâle'' on the 1756 version of his [[planisphere]] of the southern skies, distinguishing it from the feminine [[Hydra (constellation)|Hydra]]. The French name was retained by [[Jean Nicolas Fortin|Jean Fortin]] in 1776 for his ''Atlas Céleste'', while Lacaille [[Latin]]ised the name to Hydrus for his revised ''Coelum Australe Stelliferum'' in 1763.<ref>{{cite web| url=http://www.ianridpath.com/startales/lacaille.html | title=Lacaille's Southern Planisphere of 1756|last1=Ridpath|first1=Ian|author-link=Ian Ridpath|publisher=self-published  |work=Star Tales | access-date= 29 September 2013}}</ref>

{{clear|left}}

==Characteristics==
Irregular in shape,<ref name="motz"/> Hydrus is bordered by [[Mensa (constellation)|Mensa]] to the southeast, [[Eridanus (constellation)|Eridanus]] to the east, [[Horologium (constellation)|Horologium]] and [[Reticulum]] to the northeast, [[Phoenix (constellation)|Phoenix]] to the north, [[Tucana]] to the northwest and west, and [[Octans]] to the south; Lacaille had shortened Hydrus' tail to make space for this last constellation he had drawn up.<ref name=ridphydrus/> Covering 243 square degrees and 0.589% of the night sky, it ranks 61st of the 88 constellations in size.<ref name=bagnall251>{{cite book |last=Bagnall |first=Philip M. |title=The Star Atlas Companion: What You Need to Know about the Constellations |publisher=Springer |location=New York City |date=2012 |isbn=978-1-4614-0830-7 |page=251|url=https://books.google.com/books?id=KcIg02TKW6QC&pg=PA251}}</ref> The three-letter abbreviation for the constellation, as adopted by the [[International Astronomical Union]] in 1922, is "Hyi".<ref name="pa30_469">{{cite journal | last=Russell | first=Henry Norris |author-link=Henry Norris Russell | title=The New International Symbols for the Constellations | journal=[[Popular Astronomy (US magazine)|Popular Astronomy]] | volume=30 | page=469 | bibcode=1922PA.....30..469R | date=1922 }}</ref> The official constellation boundaries, as set by Belgian astronomer [[Eugène Joseph Delporte|Eugène Delporte]] in 1930, are defined by a polygon of 12 segments. In the [[equatorial coordinate system]], the [[right ascension]] coordinates of these borders lie between {{RA|00|06.1}} and {{RA|04|35.1}}, while the [[declination]] coordinates are between −57.85° and −82.06°.<ref name="boundary">{{Cite journal | title=Hydrus, Constellation Boundary | journal=The Constellations | url=https://www.iau.org/public/themes/constellations/#hyi | access-date=11 September 2013}}</ref> As one of the deep southern constellations, it remains below the horizon at latitudes north of the [[30th parallel north|30th parallel]] in the [[Northern Hemisphere]], and is circumpolar at latitudes south of the [[50th parallel south|50th parallel]] in the [[Southern Hemisphere]].<ref name="motz">{{cite book|last=Motz|first=Lloyd|author2=Nathanson, Carol|title=The Constellations: An Enthusiast's Guide to the Night Sky|publisher=Aurum Press|location=London, United Kingdom|date=1991|pages=385, 391|isbn=978-1-85410-088-7}}</ref> [[Herman Melville]] mentions it and [[Argo Navis]] in ''[[Moby Dick]]'' "beneath effulgent Antarctic Skies", highlighting his knowledge of the southern constellations from whaling voyages.<ref>{{cite book|last=Zimmerman|first=Brett |title=Herman Melville: Stargazer|publisher=McGill-Queen's University Press|location=Montreal, Quebec|date=1998|pages=[https://archive.org/details/hermanmelvillest0000zimm/page/5 5]|isbn=978-0-7735-1786-8|url=https://archive.org/details/hermanmelvillest0000zimm|url-access=registration}}</ref> A line drawn between the long axis of the [[Crux|Southern Cross]] to [[Beta Hydri]] and then extended 4.5 times will mark a point due south.<ref name="Stilwell 2010">{{cite book |author=Stilwell. Alexander | title=Crisis Survival |series=SAS and Elite Forces Survival Guide | date=2010 |url=https://books.google.com/books?id=ZKRfAu1xfOMC&pg=PT201 | publisher=Amber Books |location=London, United Kingdom |isbn=978-1-908696-06-9 |page=201}}</ref> Hydrus [[culmination|culminates]] at midnight around 26 October.<ref name=hartungs/>

==Features==
[[File:Constellation Hydrus.jpg|thumb|250px|left|The constellation Hydrus as it can be seen by the naked eye]]

===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>

===Deep-sky objects===
Hydrus contains only faint [[deep-sky object]]s. [[IC 1717]] was a deep-sky object discovered by the Danish astronomer [[John Louis Emil Dreyer]] in the late 19th century. The object at the coordinate Dreyer observed is no longer there, and is now a mystery. It was very likely to have been a faint [[comet]].<ref name=bagnall251-2>{{cite book |last=Bagnall |first=Philip M. |title=The Star Atlas Companion: What You Need to Know about the Constellations |publisher=Springer |location=New York City |date=2012 |isbn=978-1-4614-0830-7 |pages=251–52|url=https://books.google.com/books?id=KcIg02TKW6QC&pg=PA251}}</ref> [[PGC 6240]], known as the White Rose Galaxy, is a giant [[spiral galaxy]] surrounded by shells resembling rose petals, located around 345 million light years from the Solar System. Unusually, it has cohorts of globular clusters of three distinct ages suggesting bouts of post-starburst formation following a merger with another galaxy.<ref>{{cite journal|last1=Maybhate|first1=Aparna|title=Evidence for Three Subpopulations of Globular Clusters in the Early-Type Poststarburst Shell Galaxy AM|journal=Astronomical Journal|date=2007|volume=134|series=1729|pages=139–655|doi=10.1086/521817|arxiv = 0707.3133 |bibcode = 2007AJ....134.1729M|first2=Paul|last3=Schweizer|first3=François|last4=Puzia|first4=Thomas|last5=Carter|first5=David|issue=5 |last2=Goudfrooij|s2cid=12557470}}</ref> The constellation also contains a spiral galaxy, [[NGC 1511]], which lies edge on to observers on Earth and is readily viewed in amateur telescopes.<ref name=hartungs/>

Located mostly in [[Dorado]], the [[Large Magellanic Cloud]] extends into Hydrus.<ref>{{cite book|author1=Ellyard, David |author2=Tirion, Wil |date=2008 |title=The Southern Sky Guide|edition = 3rd|orig-year=1993|page=48 |publisher=Cambridge University Press |location=Port Melbourne, Victoria |isbn= 978-0-521-71405-1|url=https://books.google.com/books?id=5Gn0ivM4EK0C&pg=PA48}}</ref> The globular cluster [[NGC 1466]] is an outlying component of the galaxy, and contains many [[RR Lyrae variable|RR Lyrae-type variable stars]]. It has a magnitude of 11.59 and is thought to be over 12 billion years old.<ref name="westerlund">{{cite book|last=Westerlund|first=Bengt E. |title=The Magellanic Clouds|publisher=Cambridge University Press|location=Cambridge, United Kingdom|date=1997|pages=57|chapter=The Age Distribution of Cloud Clusters|isbn=978-0-521-48070-3|chapter-url=https://books.google.com/books?id=OSiPrgHFOYUC&pg=PA57}}</ref> Two stars, HD 24188 of magnitude 6.3 and HD 24115 of magnitude 9.0, lie nearby in its foreground.<ref name=hartungs>{{cite book|author1=Malin, David |author2=Frew, David J. |title=Hartung's Astronomical Objects for Southern Telescopes: A Handbook for Amateur Observers|publisher=Cambridge University Press|location = Cambridge, United Kingdom |date=1995|page=259 | isbn = 978-0-521-55491-6 |url=https://books.google.com/books?id=EQAYGmZT0o8C&pg=PA259}}</ref> [[NGC 602]] is composed of an [[emission nebula]] and a young, bright [[open cluster]] of stars that is an outlying component on the eastern edge of the [[Small Magellanic Cloud]],<ref>{{cite book |title=1001 Celestial Wonders to See Before You Die: The Best Sky Objects for Star Gazers |first=Michael E. |last=Bakich |publisher=Springer |location=New York City |date=2010 |isbn=978-1-4419-1777-5 |page=373|url=https://books.google.com/books?id=qEhpS7d5ZdAC&pg=PA373}}</ref> a [[satellite galaxy]] to the [[Milky Way]]. Most of the cloud is located in the neighbouring constellation Tucana.<ref name="cambridge">{{cite book|author1=Moore, Patrick |author2=Tirion, Wil |title=Cambridge Guide to Stars and Planets|publisher=Cambridge University Press|location=Cambridge, United Kingdom |date=1997|page=[https://archive.org/details/cambridgeguideto00moor/page/194 194]|isbn=978-0-521-58582-8|url=https://archive.org/details/cambridgeguideto00moor|url-access=registration }}</ref>

==See also==
* [[Hydrus (Chinese astronomy)]]

==References==
{{Reflist}}

==External links==
{{Commons}}
* [http://chandra.harvard.edu/photo/constellations/hydrus.html Chandra information about Hydrus]
* [http://www.allthesky.com/constellations/hydrus/constell.html The Deep Photographic Guide to the Constellations: Hydrus]
* [http://astrojan.nhely.hu/hydrus.htm The clickable Hydrus]

{{Stars of Hydrus}}
{{Constellations}}
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[[Category:Hydrus| ]]
[[Category:Southern constellations]]
[[Category:Constellations listed by Petrus Plancius]]