The brightest star is the 2.8-magnitudeBeta Hydri, also the closest reasonably bright star to thesouth celestial pole. Pulsating between magnitude 3.26 and 3.33,Gamma Hydri is a variablered giant 60 times the diameter of theSun. Lying near it isVW Hydri, one of the brightestdwarf novae in the heavens. Four star systems in Hydrus have been found to haveexoplanets to date, includingHD 10180, which could bear up to nine planetary companions.
Hydrus (lower right) in an extract fromJohann Bayer'sUranometria, its first appearance in a celestial atlas.
Hydrus was one of the twelve constellations established by the astronomerPetrus Plancius from the observations of the southern sky by the Dutch explorersPieter Dirkszoon Keyser andFrederick de Houtman, who had sailed on the first Dutch trading expedition, known as theEerste Schipvaart, to theEast Indies. It first appeared on a 35-cm (14 in) diameter celestial globe published in late 1597 (or early 1598) in Amsterdam by Plancius withJodocus Hondius. The first depiction of this constellation in a celestial atlas was in the German cartographerJohann Bayer'sUranometria of 1603.[2][3] De Houtman included it in his southern star catalogue the same year under the Dutch nameDe Waterslang, "The Water Snake",[4] it representing a type of snake encountered on the expedition rather than a mythical creature.[5] The French explorer and astronomerNicolas Louis de Lacaille called itl’Hydre Mâle on the 1756 version of hisplanisphere of the southern skies, distinguishing it from the feminineHydra. The French name was retained byJean Fortin in 1776 for hisAtlas Céleste, while LacailleLatinised the name to Hydrus for his revisedCoelum Australe Stelliferum in 1763.[6]
Irregular in shape,[7] Hydrus is bordered byMensa to the southeast,Eridanus to the east,Horologium andReticulum to the northeast,Phoenix to the north,Tucana to the northwest and west, andOctans to the south; Lacaille had shortened Hydrus' tail to make space for this last constellation he had drawn up.[5] Covering 243 square degrees and 0.589% of the night sky, it ranks 61st of the 88 constellations in size.[8] The three-letter abbreviation for the constellation, as adopted by theInternational Astronomical Union in 1922, is "Hyi".[9] The official constellation boundaries, as set by Belgian astronomerEugène Delporte in 1930, are defined by a polygon of 12 segments. In theequatorial coordinate system, theright ascension coordinates of these borders lie between00h 06.1m and04h 35.1m, while thedeclination coordinates are between −57.85° and −82.06°.[1] As one of the deep southern constellations, it remains below the horizon at latitudes north of the30th parallel in theNorthern Hemisphere, and is circumpolar at latitudes south of the50th parallel in theSouthern Hemisphere.[7]Herman Melville mentions it andArgo Navis inMoby Dick "beneath effulgent Antarctic Skies", highlighting his knowledge of the southern constellations from whaling voyages.[10] A line drawn between the long axis of theSouthern Cross toBeta Hydri and then extended 4.5 times will mark a point due south.[11] Hydrusculminates at midnight around 26 October.[12]
Keyzer and de Houtman assigned fifteen stars to the constellation in their Malay and Madagascan vocabulary, with a star that would be later designated asAlpha 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.[13] Lacaille charted and designated 20 stars with theBayer designations 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.[14]
Beta Hydri, the brightest star in Hydrus, is a yellow star ofapparent magnitude 2.8, lying 24light-years from Earth.[15] It has about 104% of themass of the Sun and 181% of the Sun's radius, with more than three times the Sun'sluminosity.[16] Thespectrum of this star matches astellar classification of G2 IV, with theluminosity class of 'IV' indicating this is asubgiant star. As such, it is a slightly moreevolved star than the Sun, with the supply of hydrogen fuel at itscore 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.[16] It is also the closest bright star to thesouth celestial pole.[7]
Located at the northern edge of the constellation and just southwest ofAchernar isAlpha Hydri,[17] a white sub-giant star of magnitude 2.9, situated 72 light-years from Earth.[18] Of spectral type F0IV,[19] 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.[18] A line drawn between Alpha Hydri andBeta Centauri is bisected by the south celestial pole.[12]
In the southeastern corner of the constellation isGamma Hydri,[7] ared giant of spectral type M2III located 214 light-years from Earth.[20] It is asemi-regular variable star, pulsating between magnitudes 3.26 and 3.33. Observations over five years were not able to establish itsperiodicity.[21] 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.[22] Located 3° northeast of Gamma is theVW Hydri, adwarf nova of the SU Ursae Majoris type. It is a closebinary system that consists of awhite dwarf and another star, the former drawing off matter from the latter into a brightaccretion 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.[23] One of the brightest dwarf novae in the sky,[24] it has a baseline magnitude of 14.4 and can brighten to magnitude 8.4 during peak activity.[23]BL Hydri is another close binary system composed of a low-mass star and a strongly magnetic white dwarf. Known as apolar or AM Herculis variable, these producepolarized optical andinfrared emissions and intense soft and hardX-ray emissions to the frequency of the white dwarf'srotation period—in this case 113.6 minutes.[25]
There are two notable optical double stars in Hydrus. Pi Hydri, composed ofPi1 Hydri andPi2 Hydri, is divisible in binoculars.[7] Around 476 light-years distant,[26] Pi1 is a red giant of spectral type M1III that varies between magnitudes 5.52 and 5.58.[27] Pi2 is an orange giant of spectral type K2III and shining with a magnitude of 5.7, around 488 light-years from Earth.[28]
Eta Hydri is the other optical double, composed ofEta1 andEta2.[7] Eta1 is a blue-white main sequence star of spectral type B9V that was suspected of being variable,[29] and is located just over 700 light-years away.[30] Eta2 has a magnitude of 4.7 and is a yellow giant star of spectral type G8.5III around 218 light-years distant,[31] 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 ofJupiter was discovered in 2005, orbiting around Eta2 every 711 days at a distance of 1.93astronomical units (AU).[32]
Three other systems have been found to have planets, most notably theSun-like starHD 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.[33] Lying around 127 light-years (39 parsecs) from the Earth,[34] it has an apparent magnitude of 7.33.[35]
GJ 3021 is asolar twin—a star very like the Sun—around 57 light-years distant with a spectral type G8V and magnitude of 6.7.[36] It has aJovian planet companion (GJ 3021 b). Orbiting about 0.5 AU from its star, it has a minimum mass 3.37 times that of Jupiter and a period of around 133 days.[37] 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.[38]
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.[39]
Hydrus contains only faintdeep-sky objects.IC 1717 was a deep-sky object discovered by the Danish astronomerJohn 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 faintcomet.[40]PGC 6240, known as the White Rose Galaxy, is a giantspiral 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.[41] The constellation also contains a spiral galaxy,NGC 1511, which lies edge on to observers on Earth and is readily viewed in amateur telescopes.[12]
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^Wagman, Morton (2003).Lost Stars: Lost, Missing and Troublesome Stars from the Catalogues of Johannes Bayer, Nicholas Louis de Lacaille, John Flamsteed, and Sundry Others. Blacksburg,VA: The McDonald & Woodward Publishing Company. pp. 176–77.ISBN978-0-939923-78-6.
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