Editing Andromeda (constellation) (section)

=== Deep-sky objects ===
[[File:M31bobo.jpg|thumb|250px|right|M31, the Great Galaxy of Andromeda.]]

Andromeda's borders contain many visible distant galaxies.{{sfn|Thompson|Thompson|2007|pp=66–73}} The most famous [[deep-sky object]] in Andromeda is the [[spiral galaxy]] cataloged as Messier 31 (M31) or NGC 224 but known colloquially as the [[Andromeda Galaxy]]  for the constellation.{{sfn|Pasachoff|2000|p=244}} M31 is one of the most distant objects visible to the naked eye, 2.2&nbsp;million light-years from Earth (estimates range up to 2.5 million light-years).{{sfn|Wilkins|Dunn|2006|pp=348, 366}}  It is seen under a dark, transparent sky as a hazy patch in the north of the constellation.{{sfn|Wilkins|Dunn|2006|pp=348, 366}} M31 is the largest neighboring galaxy to the Milky Way and the largest member of the [[Local Group]] of galaxies.{{sfn|Pasachoff|2000|p=244}}{{sfn|Wilkins|Dunn|2006|pp=348, 366}} In absolute terms, M31 is approximately 200,000 light-years in diameter, twice the size of the Milky Way.{{sfn|Wilkins|Dunn|2006|pp=348, 366}} It is an enormous&nbsp;– 192.4 by 62.2 arcminutes in [[apparent size]]{{sfn|Thompson|Thompson|2007|pp=66–73}}&nbsp;– [[barred spiral galaxy]] similar in form to the Milky Way and at an approximate magnitude of 3.5, is one of the brightest deep-sky objects in the northern sky.{{sfn|Bakich|1995|p=51}} Despite being visible to the naked eye, the "little cloud" near Andromeda's figure was not recorded until AD 964, when the Arab astronomer [[al-Sufi]] wrote his ''[[Book of Fixed Stars]]''.{{sfn|Ridpath, ''Star Tales'' Andromeda}}{{sfn|Higgins|2002}} M31 was first observed telescopically shortly after its invention, by [[Simon Marius]] in 1612.{{sfn|Rao|2011}}

The future of the Andromeda and Milky Way galaxies may be interlinked: in about five billion years, the two could potentially begin an [[Andromeda–Milky Way collision]] that would spark extensive new star formation.{{sfn|Wilkins|Dunn|2006|pp=348, 366}}

[[File:Andromeda Galaxy M31 - Heic1502a Full resolution.jpg|thumbnail|250px|left|''[[Zooming In on the Andromeda Galaxy]] –'' A mosaic of the Andromeda Galaxy and the largest images ever taken using the [[Hubble Space Telescope]]<ref>{{cite web|title=Sharpest ever view of the Andromeda Galaxy|url=http://www.spacetelescope.org/images/heic1502a/|website=www.spacetelescope.org|publisher=ESA/Hubble|access-date=14 January 2015}}</ref>]]

American astronomer [[Edwin Hubble]] included M31 (then known as the Andromeda Nebula) in his groundbreaking 1923 research on galaxies.{{sfn|Higgins|2002}} Using the 100-inch [[Hooker Telescope]] at [[Mount Wilson Observatory]] in California, he observed [[Cepheid variable]] stars in M31 during a search for [[nova]]e, allowing him to determine their distance by using the stars as [[standard candle]]s.{{sfn|Hoskin|Dewhirst|1999|pp=292–296}} The distance he found was far greater than the size of the Milky Way, which led him to the conclusion that many similar objects were "island universes" on their own.{{sfn|ESA, ''Edwin Powell Hubble''}}{{sfn|PBS, ''Edwin Hubble''|1998}}{{sfn|HubbleSite, ''About Edwin Hubble''|2008}} Hubble originally estimated that the Andromeda Galaxy was 900,000 light-years away, but [[Ernst Öpik]]'s estimate in 1925 put the distance closer to 1.5 million light-years.{{sfn|Hoskin|Dewhirst|1999|pp=292–296}}

The Andromeda Galaxy's two main companions, [[Messier 32|M32]] and [[Messier 110|M110]] (also known as NGC 221 and NGC 205, respectively) are faint elliptical galaxies that lie near it.{{sfn|Bakich|1995|p=54}}{{sfn|Pasachoff|2000|p=244}} M32, visible with a far smaller size of 8.7 by 6.4 arcminutes,{{sfn|Thompson|Thompson|2007|pp=66–73}} compared to M110, appears superimposed on the larger galaxy in a telescopic view as a hazy smudge, M110 also appears slightly larger and distinct from the larger galaxy;{{sfn|Pasachoff|2000|p=244}} M32 is 0.5° south of the core, M110 is 1° northwest of the core.{{sfn|Ridpath|2001|pp=72–74}} M32 was discovered in 1749 by French astronomer [[Guillaume Le Gentil]] and has since been found to lie closer to Earth than the Andromeda Galaxy itself.{{sfn|Block|2003}} It is viewable in binoculars from a dark site owing to its high surface brightness of 10.1 and overall magnitude of 9.0.{{sfn|Thompson|Thompson|2007|pp=66–73}} M110 is classified as either a [[dwarf spheroidal galaxy]] or simply a generic elliptical galaxy. It is far fainter than M31 and M32, but larger than M32 with a surface brightness of 13.2, magnitude of 8.9, and size of 21.9 by 10.9 arcminutes.{{sfn|Thompson|Thompson|2007|pp=66–73}}

The Andromeda Galaxy has a total of 15 [[List of Andromeda's satellite galaxies|satellite galaxies]], including M32 and M110. Nine of these lie in a plane, which has caused astronomers to infer that they have a common origin. These satellite galaxies, like the [[List of Milky Way's satellite galaxies|satellites of the Milky Way]], tend to be older, gas-poor [[dwarf elliptical galaxy|dwarf elliptical]] and [[dwarf spheroidal galaxy|dwarf spheroidal galaxies]].{{sfn|Koch|Grebel|2006}}

[[File:NGC 7662 Hubble WikiSky.jpg|thumb|250px|left|The Blue Snowball Nebula as seen through the [[Hubble Space Telescope]].]]

Along with the Andromeda Galaxy and its companions, the constellation also features [[NGC 891]] (Caldwell 23), a smaller galaxy just east of [[gamma Andromedae|Almach]]. It is a barred spiral galaxy seen edge-on, with a dark [[dust lane]] visible down the middle. NGC 891 is incredibly faint and small despite its magnitude of 9.9,{{sfn|Moore|2000|pp=328–330}} as its surface brightness of 14.6 indicates;{{sfn|Thompson|Thompson|2007|pp=66–73}} it is 13.5 by 2.8 arcminutes in size.{{sfn|Moore|2000|pp=328–330}} NGC 891 was discovered by the brother-and-sister team of [[William Herschel|William]] and [[Caroline Herschel]] in August 1783.{{sfn|Wilkins|Dunn|2006|pp=348, 366}} This galaxy is at an approximate distance of 30 million light-years from Earth, calculated from its [[redshift]] of 0.002.{{sfn|Wilkins|Dunn|2006|pp=348, 366}}

Andromeda's most celebrated [[open cluster]] is [[NGC 752]] ([[Caldwell Catalogue|Caldwell]] 28) at an overall magnitude of 5.7.{{sfn|Moore|2000|pp=328–330}} It is a loosely scattered cluster in the Milky Way that measures 49 arcminutes across and features approximately twelve bright stars, although more than 60 stars of approximately 9th magnitude become visible at low magnifications in a telescope.{{sfn|Thompson|Thompson|2007|pp=66–73}}{{sfn|Ridpath|2001|pp=72–74}} It is considered to be one of the more inconspicuous open clusters.{{sfn|Moore|Tirion|1997|pp=116–117}} The other open cluster in Andromeda is [[NGC 7686]], which has a similar magnitude of 5.6 and is also a part of the Milky Way. It contains approximately 20 stars in a diameter of 15 arcminutes, making it a tighter cluster than NGC 752.{{sfn|Moore|2000|pp=328–330}}

There is one prominent [[planetary nebula]] in Andromeda: [[NGC 7662]] (Caldwell 22).{{sfn|Moore|2000|pp=328–330}} Lying approximately 3 degrees southwest of [[Iota Andromedae]] at a distance of about 4,000 light-years from Earth, the "Blue Snowball Nebula"{{sfn|Thompson|Thompson|2007|pp=66–73}} is a popular target for amateur astronomers.{{sfn|Pasachoff|2000|p=270}} It earned its popular name because it appears as a faint, round, blue-green object in a telescope, with an overall magnitude of 9.2.{{sfn|Thompson|Thompson|2007|pp=66–73}}{{sfn|Pasachoff|2000|p=270}} Upon further magnification, it is visible as a slightly elliptical annular disk that gets darker towards the center, with a magnitude 13.2 central star.{{sfn|Thompson|Thompson|2007|pp=66–73}}{{sfn|Ridpath|2001|pp=72–74}} The nebula has an overall magnitude of 9.2 and is 20 by 130 arcseconds in size.{{sfn|Moore|2000|pp=328–330}}