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{{Short description|Constellation straddling the celestial equator}}
{{Distinguish|text= the southern constellation [[Hydrus]]}}
{{Infobox constellation
| name = Hydra
| abbreviation = Hya
| genitive = Hydrae
| pronounce =
{{plainlist |
* {{IPAc-en|ˈ|h|aɪ|d|r|ə}}
* genitive {{IPAc-en|ˈ|h|aɪ|d|r|iː}}
}}
| symbolism = the [[sea serpent]]
| RA = {{RA|8|10|56.2029}} – {{RA|15|02|31.3847}}
| dec = {{DEC|6.6302376}} – {{DEC|-35.6938896}}
| family = [[Hercules Family|Hercules]]
| quadrant = SQ2
| areatotal = 1303
| arearank = 1st
| numbermainstars = 17
| numberbfstars = 75
| numberbrightstars = 2
| numbernearbystars = 4
| brighteststarname = [[Alphard]] (α Hya)
| starmagnitude = 1.98
| neareststarname = [[LHS 3003]]
| stardistancely = 20.67
| stardistancepc = 6.34
| numbermessierobjects = 3
| meteorshowers =
{{plainlist |
* Alpha Hydrids
* Sigma Hydrids
}}
| bordering =
{{plainlist |
* [[Antlia]]
* [[Cancer (constellation)|Cancer]]
* [[Canis Minor]]
* [[Centaurus]]
* [[Corvus (constellation)|Corvus]]
* [[Crater (constellation)|Crater]]
* [[Leo (constellation)|Leo]]
* [[Libra (constellation)|Libra]]
* [[Lupus (constellation)|Lupus]] (corner)
* [[Monoceros (constellation)|Monoceros]]
* [[Puppis]]
* [[Pyxis]]
* [[Sextans]]
* [[Virgo (constellation)|Virgo]]
}}
| latmax = [[54th parallel north|54]]
| latmin = [[83rd parallel south|83]]
| month = April
}}
'''Hydra''' is the largest of the 88 modern [[constellation]]s, measuring 1303 square degrees, and also the longest at over 100 degrees. Its southern end borders [[Libra (constellation)|Libra]] and [[Centaurus (constellation)|Centaurus]] and its northern end borders [[Cancer (constellation)|Cancer]].{{sfn|Ridpath|Tirion|2001|pp=158–160}} It was included among the 48 constellations listed by the 2nd century astronomer [[Ptolemy]]. Commonly represented as a [[water snake (disambiguation)|water snake]], it straddles the celestial equator.
== History and mythology==
=== Western mythology ===
[[File:Sidney Hall - Urania's Mirror - Noctua, Corvus, Crater, Sextans Uraniæ, Hydra, Felis, Lupus, Centaurus, Antlia Pneumatica, Argo Navis, and Pyxis Nautica.jpg|left|300px|thumb|Hydra and surrounding constellations, from ''[[Urania's Mirror]]'' (1825).]]
The Greek constellation of Hydra is an adaptation of a [[Babylonian astronomy|Babylonian constellation]]: the [[MUL.APIN]] includes a "serpent" constellation (MUL.DINGIR.MUŠ) that loosely corresponds to Hydra. It is one of two Babylonian "serpent" constellations (the other being the origin of the Greek [[Serpens]]), a [[mythological hybrid]] of serpent, lion and bird.{{sfn|White|2008|pp=89, 180}}
<!-- Deleted image removed: [[File:41-Hydra-Constellation.PNG|left|thumb|300px|Hydra as depicted on The [[Abu al-Fath Manuchihr Khan|Manuchihr]] Globe made in [[Mashhad]] 1632-33 AD. Adilnor Collection, Sweden.]] -->
The shape of Hydra resembles a twisting [[snake]], and features as such in some Greek myths. One myth associates it with a water snake that a [[crow]] served [[Apollo]] in a cup when it was sent to fetch water. Apollo saw through the fraud and angrily cast the crow, cup and snake into the sky. It is also associated with the monster [[Lernaean Hydra|Hydra]], with its many heads, killed by [[Hercules]], represented in [[Hercules (constellation)|another constellation]].{{sfn|Ridpath|Tirion|2001|pp=158–159}} According to legend, if one of the Hydra's heads was cut off, two more would grow in its place. However, Hercules' nephew, Iolaus, seared the necks with a torch to prevent them from growing back and thus enabled Hercules to overcome the Hydra.<ref>P.K. Chen (2007) A Constellation Album: Stars and Mythology of the Night Sky, p. 44 ({{ISBN|978-1931559386}}).</ref>
=== Mythology and equivalents in other cultures ===
In [[Chinese constellations|Chinese astronomy]], the stars that correspond to Hydra are located within the [[Vermilion Bird]] and the [[Azure Dragon]]. The head of Hydra was collectively known as "Min al Az'al," meaning "belonging to the uninhabited spot" in [[Arabic]].<ref>{{Cite web|title=LacusCurtius • Allen's Star Names — Hydra|url=https://penelope.uchicago.edu/Thayer/E/Gazetteer/Topics/astronomy/_Texts/secondary/ALLSTA/Hydra*.html|access-date=2021-01-08|website=penelope.uchicago.edu}}</ref>
==
[[Image:HydraCC.jpg|thumb|left|200px|The constellation Hydra as it can be seen by the naked eye.]]
=== Stars ===
{{See also|List of stars in Hydra}}
Despite its size, Hydra contains only one moderately bright [[star]], [[Alphard]], designated Alpha Hydrae. It is an [[orange giant]] of magnitude 2.0, 177 light-years from Earth. Its traditional name means "the solitary one".<ref>[http://www.ianridpath.com/startales/hydra.html Ridpath, Ian. Star Tales Hydra.] Retrieved 2018-12-28.</ref> [[Beta Hydrae]] is a blue-white star of magnitude 4.3, 365 light-years from Earth. [[Gamma Hydrae]] is a [[yellow giant]] of magnitude 3.0, 132 light-years from Earth.{{sfn|Ridpath|Tirion|2001|pp=158–160}}
Hydra has one bright [[binary star]], [[Epsilon Hydrae]], which is difficult to split in amateur telescopes; it has a period of 1000 years and is 135 light-years from Earth.<ref name=":0">{{Cite web|url=http://stars.astro.illinois.edu/sow/epshya.html|title=Epsilon Hydrae|website=stars.astro.illinois.edu|access-date=2018-12-27}}</ref> The primary is a yellow star of magnitude 3.4 and the secondary is a blue star of magnitude 6.7. However, there are several dimmer double stars and binary stars in Hydra. [[27 Hydrae]] is a triple star with two components visible in binoculars and three visible in small amateur telescopes. The primary is a white star of magnitude 4.8, 244 light-years from Earth. The secondary, a binary star, appears in binoculars at magnitude 7.0 but is composed of a magnitude 7 and a magnitude 11 star; it is 202 light-years from Earth. 54 Hydrae is a binary star 99 light-years from Earth, easily divisible in small amateur telescopes. The primary is a yellow star of magnitude 5.3 and the secondary is a purple star of magnitude 7.4.{{sfn|Ridpath|Tirion|2001|pp=158–160}} N Hydrae (N Hya) is a pair of stars of magnitudes 5.8 and 5.9. Struve 1270 (Σ1270) consists of a pair of stars, magnitudes 6.4 and 7.4.
The other main named star in Hydra is [[Sigma Hydrae|Sigma Hydrae (σ Hydrae)]], which also has the name of Minchir,<ref name="IAU-LSN">{{cite web | url=https://www.iau.org/public/themes/naming_stars/ | title=Naming Stars |publisher=IAU.org |access-date=30 July 2018}}</ref> from the [[Arabic]] for snake's nose. At magnitude 4.54, it is rather dim. The head of the snake corresponds to the [[Āshleshā]] [[Nakshatra]], the lunar zodiacal constellation in Indian astronomy. The name of Nakshatra (Ashlesha) became the proper name of Epsilon Hydrae since 1 June 2018 by IAU.<ref name="IAU-LSN"/>
Hydra is also home to several [[variable star]]s. [[R Hydrae]] is a [[Mira variable]] star 2000 light-years from Earth; it is one of the brightest Mira variables at its maximum of magnitude 3.5. It has a minimum magnitude of 10 and a period of 390 days. [[V Hydrae]] is an unusually vivid red variable star 20,000 light-years from Earth. It varies in magnitude from a minimum of 9.0 to a maximum of 6.6.{{sfn|Levy|2005|p={{page needed|date=August 2012}}}} [[U Hydrae]] is a semi-regular variable star with a deep red color, 528 light-years from Earth. It has a minimum magnitude of 6.6 and a maximum magnitude of 4.2; its period is 115 days.{{sfn|Ridpath|Tirion|2001|pp=158–160}}
Hydra includes [[GJ 357]], an M-type main sequence star located only 31 light-years from the Solar System. This star has three confirmed exoplanets in its orbit, one of which, [[GJ 357 d]], is considered to be a "Super-Earth" within the circumstellar habitable zone.
The constellation also contains the radio source Hydra A galaxy as well as nearby [[WISE 0855−0714]] brown dwarf being the closest (sub)stellar object of the constellation.
=== Deep-sky objects ===
[[File:The planetary nebula Abell 33 captured using ESO's Very Large Telescope.jpg|thumb|Planetary nebula [[Abell 33]] captured using [[ESO]]'s [[Very Large Telescope]].<ref>{{cite news|title=Chance Meeting Creates Celestial Diamond Ring|url=http://www.eso.org/public/news/eso1412/|access-date=10 April 2014|newspaper=ESO Press Release}}</ref> ]]
[[File:NGC 3923 Elliptical Shell Galaxy.jpg|thumb|left|[[Elliptical galaxy]] [[NGC 3923]].<ref>{{cite web|title=Galactic onion|url=http://www.spacetelescope.org/images/potw1519a/|access-date=14 June 2015}}</ref>]]
Hydra contains three [[Messier object]]s. [[Messier 83|M83]], also known as the Southern Pinwheel Galaxy, is located on the border of Hydra and [[Centaurus]], [[Messier 68|M68]] is a [[globular cluster]] near M83, and [[Messier 48|M48]] is an [[open star cluster]] in the western end of the serpent.{{sfn|Ridpath|Tirion|2001|pp=158–160}}
[[NGC 3242]] is a [[planetary nebula]] of magnitude 7.5, 1400 light-years from Earth.<ref name="apod-1997-03-31">{{Cite APOD
| title=NGC 3242: The 'Ghost of Jupiter' Planetary Nebula
| date=31 March 1997
| access-date=2010-12-03}}</ref> Discovered in 1785 by [[William Herschel]], it has earned the nickname "Ghost of [[Jupiter]]" because of its striking resemblance to the giant planet.{{sfn|Levy|2005|pp=132–133}} Its blue-green disk is visible in small telescopes and its halo is visible in larger instruments.{{sfn|Ridpath|Tirion|2001|pp=158–160}}
[[Messier 48|M48]] (NGC 2548) is an open cluster that is visible to the naked eye under dark skies. Its shape has been described as "triangular"; this 80-star cluster is unusually large, more than half a degree in diameter, larger than the diameter of the [[full Moon]].{{sfn|Ridpath|Tirion|2001|pp=158–160}}
There are several globular clusters in Hydra. [[Messier 68|M68]] (NGC 4590) is a globular cluster visible in binoculars and resolvable in medium amateur telescopes. It is 31,000 light-years from Earth and of the 8th magnitude.{{sfn|Ridpath|Tirion|2001|pp=158–160}} [[NGC 5694]] is a globular cluster of magnitude 10.2, 105,000 light-years from Earth. Also called "Tombaugh's Globular Cluster", it is a Shapley class VII cluster; the classification indicates that it has intermediate concentration at its nucleus. Though it was discovered as a non-stellar object in 1784 by [[William Herschel]], its status as a globular cluster was not ascertained until 1932, when [[Clyde Tombaugh]] looked at photographic plates taken of the region near [[Pi Hydrae]] on 12 May 1931.{{sfn|Levy|2005|p=173}}
[[Messier 83|M83]] (NGC 5236), the Southern Pinwheel Galaxy, is an 8th magnitude face-on [[spiral galaxy]].{{sfn|Ridpath|Tirion|2001|pp=158–160}} It is easily observed in skies south of 40°N latitude, found by using [[1 Centauri|1]], [[2 Centauri|2]], [[3 Centauri|3]], and [[4 Centauri]] as guide stars.{{sfn|Dalrymple|2013|p=38-40}} It has been host to six [[supernova]]e. Large amateur telescopes - above 12 inches [[aperture]] - reveal its spiral arms, [[barred spiral galaxy|bar]], and small, bright nucleus.{{sfn|Ridpath|Tirion|2001|pp=158–160}}{{sfn|Dalrymple|2013|p=38-40}} In a medium-sized amateur instrument, around 8 inches in aperture, the spiral arms become visible under good conditions. It is not perfectly symmetrical in the eyepiece, rather, the northwest side is flattened and the nucleus has a southwest-to-northeast bar. A smaller sister to the Milky Way, it is a [[grand design spiral galaxy]] 40,000 light-years across.{{sfn|Dalrymple|2013|p=38-40}}
[[File:Young and dynamic NGC 2865.jpg|thumb|[[NGC 2865]] is relatively youthful and dynamic, with a rapidly rotating disc full of young stars and metal-rich gas.<ref>{{cite web|title=Young and dynamic|url=http://www.spacetelescope.org/images/potw1515a/|website=www.spacetelescope.org|publisher=ESA/Hubble|access-date=14 April 2015}}</ref>]]
There are many other galaxies located in Hydra. [[NGC 3314]], usually delineated as NGC 3314a and NGC 3314b, is a pair of galaxies that appear superimposed, despite the fact that they are not related or interacting in any way. The foreground galaxy, NGC 3314a, is at a distance of 140 million light-years, and is a face-on spiral galaxy. The background galaxy, NGC 3314b, is an oblique spiral galaxy, and has a nucleus that appears reddened because of NGC 3314a's dusty disk.{{sfn|Wilkins|Dunn|2006|p={{page needed|date=August 2012}}}} [[ESO 510-G13]] is a warped [[spiral galaxy]] located 150 million light-years from Earth. Though most galactic disks are flat because of their rate of rotation, their conformation can be changed, as is the case with this galaxy. Astronomers speculate that this is due to [[galaxy merger|interactions]] with other galaxies.{{sfn|Wilkins|Dunn|2006|p={{page needed|date=August 2012}}}} [[NGC 5068]] may be a member of the M83 group, but its identity is disputed. It has a low surface brightness and has a diameter of 4.5 arcminutes. It is not perfectly circular, rather, it is elliptical and oriented on a west-northwest/east-southeast axis. However, it is of fairly uniform brightness throughout.{{sfn|Dalrymple|2013|p=41}}
Another notable galaxy is [[NGC 4993]], an elliptical galaxy which was the source of events [[GW170817]], [[GRB 170817A]] and [[SSS17a]] from the merger of two [[neutron star]]s.<ref name="SM-20171016">{{cite news |last=Cho |first=Adrian |title=Merging neutron stars generate gravitational waves and a celestial light show |url=https://www.science.org/content/article/merging-neutron-stars-generate-gravitational-waves-and-celestial-light-show |date=16 October 2017 |journal=[[Science (magazine)|Science]] |access-date=16 October 2017 |doi=10.1126/science.aar2149}}</ref><ref name="PhysRev2017">{{cite journal
|last1=Abbott |first1=B. P.
|collaboration=[[LIGO Scientific Collaboration]] & [[Virgo interferometer|Virgo Collaboration]]
|title=GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral
|journal=Physical Review Letters |date=16 October 2017 |volume=119 |issue=16
|pages=161101
|doi=10.1103/PhysRevLett.119.161101 |pmid=29099225
|doi-access=free
|arxiv=1710.05832 |bibcode=2017PhRvL.119p1101A}}</ref>
=== Meteor showers ===
The [[Sigma Hydrids]] peak on December 6 and are a very active shower. The parent body of the Sigma Hybrids is believed to be Comet Nishimura (C/2023 P1), which was discovered in 2023 by the Japanese astronomical photographer Hideo Nishimura.[https://earthsky.org/tonight/new-comet-c-2023-p1-nishimura-bright-august-september-october-2023/] The [[Alpha Hydrids]] are a minor shower that peaks between January 1 and 7.{{sfn|Jenniskens|2012|p=24}}
==See also==
* [[Hydra (Chinese astronomy)]]
== References ==
===Citations===
{{Reflist}}
===References===
* {{cite journal
| last = Dalrymple
| first = Les
| journal = Sky & Telescope
| date = May 2013
| title = Exploring the M83 Galaxy Group
| volume = 125
| issue = 5
| page = 38
| bibcode = 2013S&T...125e..38D
}}
* {{cite journal
| last = Jenniskens
| first = Peter
| date = September 2012
| journal = Sky & Telescope
| title = Mapping Meteoroid Orbits: New Meteor Showers Discovered
}}
* {{cite book
| last = Levy
| first = David H.
| title = Deep Sky Objects
| date = 2005
| publisher = [[Prometheus Books]]
| isbn = 978-1-59102-361-6
| url-access = registration
| url = https://archive.org/details/deepskyobjects00davi
}}
* {{cite book
| last1 = Ridpath
| first1 = Ian
| last2 = Tirion
| first2 = Wil
| date = 2001
| title = Stars and Planets Guide
| publisher = [[Princeton University Press]]
| isbn = 978-0-691-08913-3
}}
* Ian Ridpath and Wil Tirion (2007). ''Stars and Planets Guide'', Collins, London. {{ISBN|978-0-00-725120-9}}. Princeton University Press, Princeton. {{ISBN|978-0-691-13556-4}}.
* {{cite book
| last = White
| first = Gavin
| date = 2008
| title = Babylonian Star-lore
| publisher = Solaria Pubs
}}
* {{cite book
| last1 = Wilkins
| first1 = Jamie
| last2 = Dunn
| first2 = Robert
| date = 2006
| title = 300 Astronomical Objects: A Visual Reference to the Universe
| publisher = Firefly Books
| ___location = Buffalo, New York
| isbn = 978-1-55407-175-3
}}
== External links ==
{{Commons and category|Hydra (constellation)|Hydra (constellation)}}
{{EB1911 poster|Hydra (constellation)|Hydra}}
* [http://www.allthesky.com/constellations/hydra/ The Deep Photographic Guide to the Constellations: Hydra]
* [http://www.ianridpath.com/startales/hydra.html Ian Ridpath's Star Tales – Hydra]
* [https://iconographic.warburg.sas.ac.uk/category/vpc-taxonomy-017075 Warburg Institute Iconographic Database (medieval and early modern images of Hydra)]
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{{Stars of Hydra}}
{{Constellations}}
{{Portal bar|Astronomy|Stars|Outer space}}
{{Authority control}}
{{DEFAULTSORT:Hydra (Constellation)}}
[[
[[Category:Constellations]]
[[Category:Constellations listed by Ptolemy]]
[[Category:Equatorial constellations]]
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