Visible at latitudes between +65° and −87°. Best visible at 21:00 (9 p.m.) during the month ofOctober.
Aquarius is anequatorial constellation of thezodiac, betweenCapricornus andPisces. Its name isLatin for "water-carrier" or "cup-carrier", and its traditional astrological symbol is (♒︎), a representation of water. Aquarius is one of the oldest of the recognized constellations along the zodiac (the Sun's apparent path).[2] It was one of the 48 constellations listed by the 2nd century astronomerPtolemy, and it remains one of the88 modern constellations. It is found in a region often called theSea due to its profusion of constellations with watery associations such asCetus thewhale, Pisces thefish, andEridanus theriver.[3]
Aquarius is identified asGU.LA "The Great One" in theBabylonian star catalogues and represents the godEa himself, who is commonly depicted holding an overflowing vase. The Babylonian star-figure appears on entitlement stones andcylinder seals from thesecond millennium. It contained thewinter solstice in the Early Bronze Age.[4] InOld Babylonian astronomy, Ea was the ruler of the southernmost quarter of the Sun's path, the "Way of Ea", corresponding to the period of 45 days on either side of winter solstice. Aquarius was also associated with the destructive floods that the Babylonians regularly experienced, and thus was negatively connoted.[3] InAncient Egypt astronomy, Aquarius was associated with the annualflood of the Nile; the banks were said to flood when Aquarius put his jar into the river, beginning spring.[5]
In theGreek tradition, the constellation came to be represented simply as a single vase from which a stream poured down toPiscis Austrinus. The name in theHindu zodiac is likewisekumbha "water-pitcher".[2]
In Greek mythology, Aquarius is sometimes associated withDeucalion, the son ofPrometheus who built a ship with his wifePyrrha to survive an imminent flood. They sailed for nine days before washing ashore onMount Parnassus.[3][6] Aquarius is also sometimes identified with beautifulGanymede, a youth inGreek mythology and the son ofTrojan kingTros, who was taken toMount Olympus byZeus to act as cup-carrier to thegods.[7][8][6] NeighboringAquila represents the eagle, under Zeus' command, that snatched the young boy;[9] some versions of the myth indicate that the eagle was in fact Zeus transformed. One tradition, stated that he was carried off by Eos.[10] Yet another figure associated with the water bearer isCecrops I, a king of Athens whosacrificed water instead ofwine to the gods.[6]
A representation of Aquarius printed in 1825 as part ofUrania's Mirror (including a now-obsolete constellation,Ballon Aerostatique south of it)
In the first century,Ptolemy'sAlmagest established the common Western depiction of Aquarius. His water jar, an asterism itself, consists of Gamma, Pi, Eta, andZeta Aquarii; it pours water in a stream of more than 20 stars terminating withFomalhaut, now assigned solely to Piscis Austrinus. The water bearer's head is represented by 5th magnitude25 Aquarii while his left shoulder is Beta Aquarii; his right shoulder and forearm are represented byAlpha andGamma Aquarii respectively.[6]
InChinese astronomy, the stream of water flowing from the Water Jar was depicted as the "Army of Yu-Lin" (Yu-lim-kiun orYulinjun,Hanzi: 羽林君). The name "Yu-lin" means "feathers and forests", referring to the numerous light-footed soldiers from the northern reaches of the empire represented by these faint stars.[5][6] The constellation's stars were the most numerous of any Chinese constellation, numbering 45, the majority of which were located in modern Aquarius. The celestial army was protected by the wallLeibizhen (垒壁阵), which counted Iota, Lambda, Phi, and Sigma Aquarii among its 12 stars.[6] 88, 89, and 98 Aquarii representFou-youe, the axes used as weapons and for hostage executions. Also in Aquarius isLoui-pi-tchin, the ramparts that stretch from 29 and 27 Piscium and 33 and 30 Aquarii through Phi, Lambda, Sigma, and Iota Aquarii to Delta, Gamma, Kappa, and Epsilon Capricorni.[5] Similarly in theHindu calendar Aquarius is depicted asKumbha, andKumbha, which means a pot or a jug, stands for the zodiac sign ofAquarius.
Near the border with Cetus, the axeFuyue was represented by three stars; its position is disputed and may have instead been located inSculptor.Tienliecheng also has a disputed position; the 13-star castle replete with ramparts may have possessed Nu and Xi Aquarii but may instead have been located south in Piscis Austrinus. The Water Jar asterism was seen to the ancient Chinese as the tomb,Fenmu. Nearby, the emperors'mausoleumXiuliang stood, demarcated by Kappa Aquarii and three other collinear stars.Ku ("crying") andQi ("weeping"), each composed of two stars, were located in the same region.[6]
Three of the Chineselunar mansions shared their name with constellations.Nu, also the name for the 10th lunar mansion, was ahandmaiden represented by Epsilon, Mu, 3, and 4 Aquarii. The 11th lunar mansion shared its name with the constellationXu ("emptiness"), formed by Beta Aquarii and Alpha Equulei; it represented a bleak place associated with death and funerals.Wei, the rooftop and 12th lunar mansion, was a V-shaped constellation formed by Alpha Aquarii, Theta Pegasi, and Epsilon Pegasi; it shared its name with two other Chinese constellations, in modern-dayScorpius andAries.[6]
The constellation Aquarius as it can be seen by the naked eye
Despite both its prominent position on the zodiac and its large size, Aquarius has no particularly bright stars, its four brightest stars being less bright thanmagnitude 2.8 .[7] (The Apparent Magnitude scale is reverse logarithmic, with increasingly bright objects having lower and lower (more negative) magnitudes.) Recent research has shown that there are several stars lying within its borders that possessplanetary systems.
The two brightest stars,α Aquarii and β Aquarii, are luminous yellow supergiants, of spectral types G0Ib and G2Ib respectively,[11] that were once hot blue-white B-class main sequence stars 5 to 9 times as massive as the Sun. The two are also moving through space perpendicular to the plane of the Milky Way.[12] β Aquarii is the brightest star in Aquarius with apparentmagnitude 2.91 – only slightly brighter than α Aquarii. It also has the proper name of Sadalsuud. Having cooled and swollen to around 50 times the Sun's diameter, it is around 2200 times as luminous as the Sun.[13] It is around 6.4 times as massive as the Sun and around 56 million years old.[14] Sadalsuud is540 ± 20 light-years from Earth.[15] α Aquarii, also known asSadalmelik, has apparentmagnitude 2.94 . It is520 ± 20light years distant from Earth,[15] and is around 6.5 times as massive as the Sun, and 3000 times as luminous. It is 53 million years old.[11]
γ Aquarii, also calledSadachbia,[16] is a white main sequence star of spectral type star of spectral type A0V that is between 158 and 315 million years old and is around 2.5 times the Sun's mass (2.5 M☉),[17] and double its radius.[18] Its magnitude is 3.85, and it is164 ± 9 light years away,[15] hence its luminosity is 50 L☉.[7] The nameSadachbia comes from the Arabic for "lucky stars of the tents",sa'd al-akhbiya.[6]
δ Aquarii, also known asSkat[16][8] orScheat[7] is a blue-white spectral type A2 star with apparent magnitude 3.27 andluminosity 105 L☉.[7]
ε Aquarii, also known asAlbali,[5] is a blue-white spectral type A1 star with apparent magnitude 3.77,absolute magnitude 1.2, and a luminosity of 28 L☉.[7][8]
ζ Aquarii is a spectral type F2double star; both stars are white.[8] In combination, they appear to be magnitude 3.6 with luminosity 50 L☉. The primary has magnitude 4.53 and the secondary's magnitude is 4.31, but both have absolutemagnitude 0.6.[7] The system's orbital period is 760 years; currently the two components are moving farther apart.[8]
θ Aquarii, sometimes calledAncha,[5] is spectral type G8 with apparent magnitude 4.16 and an absolutemagnitude 1.4.[7]
κ Aquarii, also calledSitula, has an apparentmagnitude 5.03.[16]
Twelve exoplanet systems have been found in Aquarius as of 2013.Gliese 876, one of the nearest stars to Earth at a distance of 15 light-years,[19] was the firstred dwarf star to be found to possess aplanetary system. It is orbited by four planets, including oneterrestrial planet 6.6 times the mass of Earth. The planets vary in orbital period from 2 days to 124 days.[20]91 Aquarii is anorange giant star orbited by one planet,91 Aquarii b. The planet's mass is 2.9 times the mass of Jupiter, and its orbital period is 182 days.[21]Gliese 849 is a red dwarf star orbited by the first known long-period Jupiter-like planet,Gliese 849 b. The planet's mass is 0.99 times that of Jupiter and its orbital period is 1,852 days.[22]
There are also less-prominent systems in Aquarius.WASP-6, a type G8 star of magnitude 12.4, is host to one exoplanet,WASP-6 b. The star is 307parsecs from Earth and has a mass of 0.888solar masses and a radius of 0.87solar radii. WASP-6 b was discovered in 2008 by thetransit method. It orbits its parent star every 3.36 days at a distance of 0.042astronomical units (AU). It is 0.503Jupiter masses but has a proportionally larger radius of 1.224Jupiter radii.[23]HD 206610, a K0 star located 194 parsecs from Earth, is host to one planet,HD 206610 b. The host star is larger than the Sun; more massive at 1.56 solar masses and larger at 6.1 solar radii. The planet was discovered by the radial velocity method in 2010 and has a mass of 2.2 Jupiter masses. It orbits every 610 days at a distance of 1.68 AU.[24] Much closer to its sun isWASP-47 b, which orbits every 4.15 days only 0.052 AU from its sun, yellow dwarf (G9V)WASP-47. WASP-47 is close in size to the Sun, having a radius of 1.15 solar radii and a mass even closer at 1.08 solar masses. WASP-47 b was discovered in 2011 by the transit method, like WASP-6 b. It is slightly larger than Jupiter with a mass of 1.14 Jupiter masses and a radius of 1.15 Jupiter masses.[25]
There are several more single-planet systems in Aquarius.HD 210277, a magnitude 6.63 yellow star located 21.29 parsecs from Earth, is host to one known planet:HD 210277 b. The 1.23 Jupiter mass planet orbits at nearly the same distance as Earth orbits the Sun—1.1 AU, though its orbital period is significantly longer at around 442 days. HD 210277 b was discovered earlier than most of the other planets in Aquarius, detected by the radial velocity method in 1998. The star it orbits resembles the Sun beyond their similarspectral class; it has a radius of 1.1 solar radii and a mass of 1.09 solar masses.[26]HD 212771 b, a larger planet at 2.3 Jupiter masses, orbits host starHD 212771 at a distance of 1.22 AU. The star itself, barely below the threshold of naked-eye visibility at magnitude 7.6, is a G8IV (yellow subgiant) star located 131 parsecs from Earth. Though it has a similar mass to the Sun—1.15 solar masses—it is significantly less dense with its radius of 5 solar radii. Its lone planet was discovered in 2010 by the radial velocity method, like several other exoplanets in the constellation.[27]
As of 2013, there were only two known multiple-planet systems within the bounds of Aquarius: the Gliese 876 andHD 215152 systems. The former is quite prominent; the latter has only two planets and has a host star farther away at 21.5 parsecs. The HD 215152 system consists of the planetsHD 215152 b andHD 215152 c orbiting their K0-type, magnitude 8.13 sun. Both discovered in 2011 by the radial velocity method, the two tiny planets orbit very close to their host star. HD 215152 c is the larger at 0.0097 Jupiter masses (still significantly larger than the Earth, which weighs in at 0.00315 Jupiter masses); its smaller sibling is barely smaller at 0.0087 Jupiter masses. Theerror in the mass measurements (0.0032 and0.0049MJ respectively) is large enough to make this discrepancy statistically insignificant. HD 215152 c also orbits further from the star than HD 215152 b, 0.0852 AU compared to 0.0652.[28][29]
On 23 February 2017, NASA announced thatultracool dwarf starTRAPPIST-1 in Aquarius has sevenEarth-like rocky planets.[30] Of these, as many as four may lie within the system'shabitable zone, and may have liquid water on their surfaces. The discovery of the TRAPPIST-1 system is seen by astronomers as a significant step toward finding life beyond Earth.
Because of its position away from the galactic plane, the majority of deep-sky objects in Aquarius are galaxies, globular clusters, and planetary nebulae.[3] Aquarius contains threedeep sky objects that are in theMessier catalog: the globular clustersMessier 2,Messier 72, and the asterismMessier 73. While M73 was originally catalogued as a sparsely populated open cluster, modern analysis indicates the 6 main stars are not close enough together to fit this definition, reclassifying M73 as an asterism.[32] Two well-knownplanetary nebulae are also located in Aquarius: theSaturn Nebula (NGC 7009), to the southeast ofμ Aquarii; and the famousHelix Nebula (NGC 7293), southwest ofδ Aquarii.
M2, also catalogued as NGC 7089, is a rich globular cluster located approximately 37,000 light-years from Earth. At magnitude 6.5, it is viewable in small-aperture instruments, but a 100 mm aperturetelescope is needed to resolve any stars. M72, also catalogued as NGC 6981, is a small 9th magnitude globular cluster located approximately 56,000 light-years from Earth.[8] M73, also catalogued as NGC 6994, is an open cluster with highly disputed status.
Aquarius is also home to several planetary nebulae.NGC 7009, also known as the Saturn Nebula, is an 8th magnitude planetary nebula located 3,000 light-years from Earth. It was given its moniker by the 19th century astronomerLord Rosse for its resemblance to the planetSaturn in a telescope; it has faint protrusions on either side that resembleSaturn's rings. It appears blue-green in a telescope and has a central star of magnitude 11.3.[8] Compared to the Helix Nebula, another planetary nebula in Aquarius, it is quite small.[33]NGC 7293, also known as the Helix Nebula, is the closest planetary nebula to Earth at a distance of 650 light-years. It covers 0.25 square degrees, making it also the largest planetary nebula as seen from Earth. However, because it is so large, it is only viewable as a very faint object,[8] though it has a fairly highintegrated magnitude of 6.0.[34]
One of the visible galaxies in Aquarius isNGC 7727, of particular interest for amateur astronomers who wish to discover or observesupernovae. Aspiral galaxy (type S), it has anintegrated magnitude of 10.7 and is 3 by 3 arcseconds.[35]NGC 7252 is a tangle of stars resulting from the collision of two large galaxies and is known as the Atoms-for-Peace galaxy because of its resemblance to a cartoon atom.[36]
The Eta Aquariids are the strongest meteor shower radiating from Aquarius. It peaks between 5 and 6 May with a rate of approximately 35 meteors per hour.[8] Originally discovered by Chinese astronomers in 401, Eta Aquariids can be seen coming from the Water Jar beginning on 21 April and as late as 12 May. Theparent body of the shower isHalley's Comet, aperiodic comet.Fireballs are common shortly after the peak, approximately between 9 May and 11 May. The normal meteors appear to have yellow trails.[37]
The Delta Aquariids is a double radiant meteor shower that peaks first on 29 July and second on 6 August. The first radiant is located in the south of the constellation, while the second radiant is located in the northern circlet of Pisces asterism. The southern radiant's peak rate is about 20 meteors per hour, while the northern radiant's peak rate is about 10 meteors per hour.[8]
The Iota Aquariids is a fairly weak meteor shower that peaks on 6 August, with a rate of approximately 8 meteors per hour.[8]
As of 2002[update], the Sun appears in the constellation Aquarius from 16 February to 12 March. Intropical astrology, the Sun is considered to be in the signAquarius from 20 January to 19 February, and insidereal astrology, from 15 February to 14 March.
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