Editing Aquarius (constellation) (section)

==Features==
{{See also|List of stars in Aquarius}}

===Stars===
[[Image:AquariusCC.jpg|thumb|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 than {{nobr|[[stellar magnitude|magnitude]] 2.8 .}}{{sfn|Moore|2000}} (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 possess [[extrasolar planet|planetary systems]].

The two brightest stars, [[Alpha Aquarii|α&nbsp;Aquarii]] and β&nbsp;Aquarii, are luminous yellow supergiants, of spectral types G0Ib and G2Ib respectively,<ref name=mnras402_2_1369/> that were once hot blue-white B-class main sequence stars 5 to 9&nbsp;times as massive as the Sun. The two are also moving through space perpendicular to the plane of the Milky Way.<ref name=apj627_1_L53>{{cite journal | last1=Ayres | first1=Thomas R. | last2=Brown | first2=Alexander | last3=Harper | first3=Graham M. | date=July 2005 | title=Chandra observations of coronal emission from the early G supergiants α and β&nbsp;Aquarii | journal=[[The Astrophysical Journal]] | volume=627 | issue=1 | pages=L53–L56 | doi=10.1086/431977 | bibcode=2005ApJ...627L..53A | doi-access=free }}</ref> β&nbsp;Aquarii is the brightest star in Aquarius with apparent {{nobr|magnitude 2.91}} – only slightly brighter than α&nbsp;Aquarii. It also has the proper name of Sadalsuud. Having cooled and swollen to around 50&nbsp;times the Sun's diameter, it is around 2200&nbsp;times as luminous as the Sun.<ref name=kalerbeta>{{cite web | first=James B. | last=Kaler | title=Sadalsuud (Beta Aquarii) | website=Stars |publisher=[[University of Illinois|U. Illinois]] | series=Astronomy Department | url=http://stars.astro.illinois.edu/sow/sadalsuud.html | access-date=11 October 2016 }}</ref> It is around 6.4&nbsp;times as massive as the Sun and around 56&nbsp;million years old.<ref name=leonid>{{cite journal |last1=Lyubimkov |first1=Leonid S. |last2=Lambert |first2=David L. |last3=Korotin |first3=Sergey A. |last4=Rachkovskaya |first4=Tamara M. |last5=Poklad |first5=Dmitry B. |year=2015 |title=Carbon abundance and the N/C ratio in atmospheres of A-, F- and G-type supergiants and bright giants |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=446 |issue=4 |page=3447 |doi=10.1093/mnras/stu2299 |doi-access=free |s2cid=118473779 |bibcode=2015MNRAS.446.3447L |arxiv=1411.2722}}</ref> Sadalsuud is {{nobr|540 ± 20 light-years}} from Earth.<ref name=vanLeeuwen2007>{{cite journal | first=F. | last=van&nbsp;Leeuwen | year=2007 | title=Validation of the new Hipparcos reduction | journal=[[Astronomy and Astrophysics]] | volume=474 | issue=2 | pages=653–64 | bibcode=2007A&A...474..653V | doi=10.1051/0004-6361:20078357 | arxiv=0708.1752 | s2cid=18759600 }}</ref> α&nbsp;Aquarii, also known as ''Sadalmelik'', has apparent {{nobr|magnitude 2.94 .}} It is {{nobr|520 ± 20 [[light year]]s}} distant from Earth,<ref name=vanLeeuwen2007/> and is around 6.5&nbsp;times as massive as the Sun, and 3000&nbsp;times as luminous. It is 53&nbsp;million years old.<ref name=mnras402_2_1369>{{cite journal | last1=Lyubimkov | first1=Leonid S. | last2=Lambert | first2=David L. | last3=Rostopchin | first3=Sergey I. | last4=Rachkovskaya | first4=Tamara M. | last5=Poklad | first5=Dmitry B. | year=2010 | title=Accurate fundamental parameters for A-, F- and G-type supergiants in the solar neighbourhood | journal=[[Monthly Notices of the Royal Astronomical Society]] | volume=402 | issue=2 | pages=1369–1379 | doi=10.1111/j.1365-2966.2009.15979.x | doi-access=free | bibcode=2010MNRAS.402.1369L |arxiv = 0911.1335 | s2cid=119096173 }}</ref>

[[Gamma Aquarii|γ&nbsp;Aquarii]], also called ''Sadachbia'',<ref name=IAU-LSN>{{cite web | title=Star Names |publisher=[[International Astronomical Union]] |website=IAU.org | url=https://www.iau.org/public/themes/naming_stars/ |access-date=30 July 2018}}</ref> is a white main sequence star of spectral type star of spectral type A0V that is between 158 and 315&nbsp;million years old and is around 2.5&nbsp;times the Sun's mass ({{solar mass|2.5}}),<ref name=David2015>{{cite journal | last1=David | first1=Trevor J. | last2=Hillenbrand | first2=Lynne A. | year=2015 | title=The ages of early-type stars: Strömgren photometric methods calibrated, validated, tested, and applied to hosts and prospective hosts of directly imaged exoplanets | journal=[[The Astrophysical Journal]] | volume=804 | issue=2 | pages=146 | s2cid=33401607 | bibcode=2015ApJ...804..146D | doi=10.1088/0004-637X/804/2/146 | arxiv=1501.03154 }}<br/>{{cite web |title=Hipparcos&nbsp;102395 |website=[[VizieR]] |type=star data catalog entry |publisher=[[Centre de données astronomiques de Strasbourg|CDS]] |place=Strasbourg, FR |via=[[Harvard University]]–[[Smithsonian Institution]], [[Center for Astrophysics]] |url=https://vizier.cfa.harvard.edu/viz-bin/VizieR?-source=J/ApJ/804/146/table5&HIP=102395&-ref=VIZ649a83a23ca11c}}</ref> and double its radius.<ref name=aaa367_521>{{cite journal | last1=Pasinetti Fracassini | first1=L.E. | last2=Pastori | first2=L. | last3=Covino | first3=S. | last4=Pozzi | first4=A. | title=Catalogue of apparent diameters and absolute radii of stars (CADARS) – third edition – comments and statistics | journal=[[Astronomy and Astrophysics]] | volume=367 | issue=2 | pages=521–524 |date=February 2001 | doi=10.1051/0004-6361:20000451 | bibcode=2001A&A...367..521P | arxiv=astro-ph/0012289| s2cid=425754 }}</ref> Its magnitude is&nbsp;3.85, and it is {{nobr|164 ± 9 light years}} away,<ref name=vanLeeuwen2007/> hence its luminosity is {{Solar luminosity|50}}.{{sfn|Moore|2000}} The name ''Sadachbia'' comes from the Arabic for "lucky stars of the tents", ''sa'd al-akhbiya''.{{sfn|Star Tales}}

[[delta Aquarii|δ&nbsp;Aquarii]], also known as ''Skat''<ref name=IAU-LSN/>{{sfn|Ridpath|2001}} or ''Scheat''{{sfn|Moore|2000}} is a blue-white spectral type A2 star with apparent magnitude&nbsp;3.27 and [[luminosity]] {{Solar luminosity|105}}.{{sfn|Moore|2000}}

[[epsilon Aquarii|ε&nbsp;Aquarii]], also known as ''Albali'',{{sfn|Staal|1988|pp=42–44}} is a blue-white spectral type A1 star with apparent magnitude&nbsp;3.77, [[absolute magnitude]]&nbsp;1.2, and a luminosity of {{Solar luminosity|28}}.{{sfn|Moore|2000}}{{sfn|Ridpath|2001}}

[[zeta Aquarii|ζ&nbsp;Aquarii]] is a spectral type F2 [[double star]]; both stars are white.{{sfn|Ridpath|2001}} In combination, they appear to be magnitude&nbsp;3.6 with luminosity {{Solar luminosity|50}}. The primary has magnitude&nbsp;4.53 and the secondary's magnitude is&nbsp;4.31, but both have absolute {{nobr|magnitude 0.6.}}{{sfn|Moore|2000}} The system's orbital period is 760&nbsp;years; currently the two components are moving farther apart.{{sfn|Ridpath|2001}}

[[theta Aquarii|θ&nbsp;Aquarii]], sometimes called ''Ancha'',{{sfn|Staal|1988|pp=42–44}} is spectral type G8 with apparent magnitude&nbsp;4.16 and an absolute {{nobr|magnitude 1.4.}}{{sfn|Moore|2000}}

[[Kappa Aquarii|κ&nbsp;Aquarii]], also called ''Situla'', has an apparent {{nobr|magnitude 5.03.}}<ref name=IAU-LSN/>

[[lambda Aquarii|λ&nbsp;Aquarii]], also called ''Hudoor'' or ''Ekchusis'',{{sfn|Staal|1988|pp=42–44}} is [[M-type star|spectral type M2]] with magnitude&nbsp;3.74 and luminosity {{Solar luminosity|120}}.{{sfn|Moore|2000}}

[[xi Aquarii|ξ&nbsp;Aquarii]], also called ''Bunda'',<ref name=IAU-LSN/> is [[A-type star|spectral type A7]] with an apparent magnitude&nbsp;4.69 and an absolute {{nobr|magnitude 2.4.}}{{sfn|Moore|2000}}

[[pi Aquarii|π&nbsp;Aquarii]], also called ''Seat'', is [[B-type star|spectral type B0]] with apparent magnitude&nbsp;4.66 and absolute {{nobr|magnitude −4.1.}}{{sfn|Moore|2000}}

===Planetary systems===
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,{{sfn|APOD Gliese 876}} was the first [[red dwarf]] star to be found to possess a [[planetary system]]. It is orbited by four planets, including one [[terrestrial planet]] 6.6 times the mass of Earth. The planets vary in orbital period from 2 days to 124 days.{{sfn|Exoplanet Encyclopedia Gliese 876}} [[91 Aquarii]] is an [[orange giant]] star orbited by one planet, [[91 Aquarii b|91&nbsp;Aquarii&nbsp;b]]. The planet's mass is 2.9 times the mass of Jupiter, and its orbital period is 182 days.{{sfn|Exoplanet Encyclopedia 91 Aqr}} [[Gliese 849]] is a red dwarf star orbited by the first known long-period Jupiter-like planet, [[Gliese 849 b|Gliese&nbsp;849&nbsp;b]]. The planet's mass is 0.99 times that of Jupiter and its orbital period is 1,852 days.{{sfn|Exoplanet Encyclopedia Gj 849}}

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 307 [[parsec]]s from Earth and has a mass of 0.888 [[solar mass]]es and a radius of 0.87 [[solar radii]]. WASP-6 b was discovered in 2008 by the [[transit method]]. It orbits its parent star every 3.36 days at a distance of 0.042 [[astronomical unit]]s (AU). It is 0.503 [[Jupiter mass]]es but has a proportionally larger radius of 1.224 [[Jupiter radius|Jupiter radii]].{{sfn|Exoplanet Encyclopedia WASP-6}} [[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.{{sfn|Exoplanet Encyclopedia HD 206610 b}} Much closer to its sun is [[WASP-47 b]], which orbits every 4.15 days only 0.052&nbsp;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.{{sfn|Exoplanet Encyclopedia WASP-47 b}}

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{{mdash}}1.1&nbsp;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 similar [[spectral class]]; it has a radius of 1.1 solar radii and a mass of 1.09 solar masses.{{sfn|Exoplanet Encyclopedia HD 210277 b}} [[HD 212771 b]], a larger planet at 2.3 Jupiter masses, orbits host star [[HD 212771]] at a distance of 1.22&nbsp;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{{mdash}}1.15 solar masses{{mdash}}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.{{sfn|Exoplanet Encyclopedia HD 212771 b}}

As of 2013, there were only two known multiple-planet systems within the bounds of Aquarius: the Gliese 876 and [[HD 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 planets [[HD 215152 b]] and [[HD 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. The [[error]] in the mass measurements (0.0032 and {{Jupiter mass|0.0049}} 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&nbsp;AU compared to 0.0652.{{sfn|Exoplanet Encyclopedia HD 215152 b}}{{sfn|Exoplanet Encyclopedia HD 215152 c}}

On 23 February 2017, NASA announced that [[Ultra-cool dwarf|ultracool dwarf star]] [[TRAPPIST-1]] in Aquarius has seven [[Exoplanet|Earth-like rocky planets]].<ref name="Trappist-1">{{cite web|title=NASA Telescope Reveals Largest Batch of Earth-Size, Habitable-Zone Planets Around Single Star|url=https://www.nasa.gov/press-release/nasa-telescope-reveals-largest-batch-of-earth-size-habitable-zone-planets-around|website=NASA|access-date=23 February 2017|date=22 February 2017}}</ref> Of these, as many as four may lie within the system's [[habitable 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.

===Deep sky objects===
[[File:The green bean galaxy J2240.jpg|thumb|The [[green bean galaxy]] J2240 lies in the constellation of Aquarius<ref>{{cite press release |title= Galaxy-wide Echoes from the Past |url= http://www.eso.org/public/news/eso1249/ |access-date = 7 December 2012 |website= ESO Press Release}}</ref>]]

Because of its position away from the galactic plane, the majority of deep-sky objects in Aquarius are galaxies, globular clusters, and planetary nebulae.{{sfn|Thompson|Thompson|2007}} Aquarius contains three [[deep sky object]]s that are in the [[Messier catalog]]: the globular clusters [[Messier 2|Messier&nbsp;2]], [[Messier 72|Messier&nbsp;72]], and the asterism [[Messier 73|Messier&nbsp;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.<ref>{{Cite journal|last1=Odenkirchen|first1=M.|last2=Soubiran|first2=C.|date=2002-02-01|title=NGC 6994 – clearly not a physical stellar ensemble|url=https://www.aanda.org/articles/aa/abs/2002/07/aa1843/aa1843.html|journal=Astronomy & Astrophysics|language=en|volume=383|issue=1|pages=163–170|doi=10.1051/0004-6361:20011730|arxiv=astro-ph/0111601|bibcode=2002A&A...383..163O|s2cid=15545816|issn=0004-6361}}</ref> Two well-known [[planetary nebula]]e are also located in Aquarius: the [[Saturn Nebula]] (NGC&nbsp;7009), to the southeast of [[μ Aquarii]]; and the famous [[Helix Nebula]] (NGC&nbsp;7293), southwest of [[δ Aquarii|δ&nbsp;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&nbsp;mm aperture [[telescope]] 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.{{sfn|Ridpath|2001}} 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 astronomer [[Lord Rosse]] for its resemblance to the planet [[Saturn]] in a telescope; it has faint protrusions on either side that resemble [[Saturn's rings]]. It appears blue-green in a telescope and has a central star of magnitude 11.3.{{sfn|Ridpath|2001}} Compared to the Helix Nebula, another planetary nebula in Aquarius, it is quite small.{{sfn|Levy|2005|p=132}} [[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,{{sfn|Ridpath|2001}} though it has a fairly high [[integrated magnitude]] of 6.0.{{sfn|Levy|2005|p=131}}

One of the visible galaxies in Aquarius is [[NGC 7727]], of particular interest for amateur astronomers who wish to discover or observe [[supernovae]]. A [[spiral galaxy]] ([[galaxy morphological classification|type S]]), it has an [[integrated magnitude]] of 10.7 and is 3 by 3 arcseconds.{{sfn|Sherrod|Koed|2003|p=222}} [[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.{{sfn|APOD Atoms-for-Peace Galaxy}}

===Meteor showers===
There are three major meteor showers with [[radiant (meteor shower)|radiant]]s in Aquarius: the [[Eta Aquariids]], the [[Delta Aquariids]], and the Iota Aquariids.

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.{{sfn|Ridpath|2001}} 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. The [[parent body]] of the shower is [[Halley's Comet]], a [[periodic comet]]. [[Meteoroid#Fireball|Fireball]]s are common shortly after the peak, approximately between 9 May and 11 May. The normal meteors appear to have yellow trails.{{sfn|Sherrod|Koed|2003|p=52}}

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.{{sfn|Ridpath|2001}}

The Iota Aquariids is a fairly weak meteor shower that peaks on 6 August, with a rate of approximately 8 meteors per hour.{{sfn|Ridpath|2001}}