Merajul Islam at 14:59, 28 January 2022

2022-01-28T14:59:51Z

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{{short description|Group of galaxies that includes the Milky Way}}
{{Use dmy dates|date=April 2020}}
{{Infobox galaxy cluster
| name = Local Group
| image = Local Group and nearest galaxies.jpg
| image_size = 300
| alt = Local Group of galaxies, including the massive members [[Messier 31]] (Andromeda Galaxy) and Milky Way, as well as other nearby galaxies.
| caption = Local Group of galaxies, including the massive members [[Messier 31]] (Andromeda Galaxy) and Milky Way, as well as other nearby galaxies.
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| epoch = [[Epoch (astronomy)#Julian years and J2000|J2000]]
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| parent = [[Virgo Supercluster]]
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}}
[[Image:VLT Shows Milky Way's Neighbouring Galaxies Have Different History.jpg|thumb|Distribution of the iron content (in logarithmic scale) in four neighbouring dwarf galaxies of the Milky Way]]

The '''Local Group''' is the [[galaxy group]] that includes the [[Milky Way]].
It has a total diameter of roughly {{convert|3|Mpc|e6ly m|lk=on|abbr=off|sigfig=1}},{{citation needed|date=December 2021}} and a total mass of the order of {{convert|2e12|solar mass|kg|lk=in|sigfig=1}}.<ref>The mass of the Local Group is essentially accounted for by the mass of the Milky Way and the Andromeda Galaxy.
Estimates for the mass of each galaxy are compatible with {{val|e=12|u={{solar mass}}}},
and Peñarrubia et al. (2014) estimate {{val|2.3|0.7|e=12|u={{solar mass}}}} for the Local Group,
but Karachentsev and Kashibadze (2006) estimate the somewhat lower value of {{val|1.29|0.14|e=12|u={{solar mass}}}}.</ref>
It consists of two collections of galaxies in a "[[dumbbell]]" shape: the Milky Way and its satellites form one lobe, and the [[Andromeda Galaxy]] and its satellites constitute the other. The two collections are separated by about {{convert|800|kpc|e6ly m|abbr=on|sigfig=1}} and are moving toward one another with a velocity of {{val|123|u=km/s}}.<ref name="Karachentsevetal2006">{{Cite journal |last1=Karachentsev |first1=I. D. |last2=Kashibadze |first2=O. G. |year=2006 |title=Masses of the local group and of the M81 group estimated from distortions in the local velocity field |journal=Astrophysics |volume=49 |issue=1 |pages=3–18 |bibcode=2006Ap.....49....3K |doi=10.1007/s10511-006-0002-6|s2cid=120973010 }}</ref> The group itself is a part of the larger [[Virgo Supercluster]], which may be a part of the [[Laniakea Supercluster]].
The exact number of galaxies in the Local Group is unknown as some are occluded by the Milky Way; however, at least 80 members are known, most of which are [[dwarf galaxy|dwarf galaxies]].

The two largest members, the Andromeda Galaxy and the Milky Way, are both [[spiral galaxy|spiral galaxies]] with masses of about {{val|e=12}} [[solar mass]]es each. Each has its own system of [[Satellite galaxy|satellite galaxies]]:
* The Andromeda Galaxy's satellite system consists of [[Messier 32]] (M32), [[Messier 110]] (M110), [[NGC 147]], [[NGC 185]], [[Andromeda I]] (And I), [[Andromeda II|And II]], [[Andromeda III|And III]], [[Andromeda V|And V]], [[Pegasus Dwarf Spheroidal Galaxy|And VI]] (also known as the Pegasus Dwarf Spheroidal Galaxy, or Pegasus dSph), [[Cassiopeia Dwarf|And VII]] (also known as the Cassiopeia Dwarf Galaxy), [[Andromeda VIII|And VIII]], [[Andromeda IX|And IX]], [[Andromeda X|And X]], [[Andromeda XI|And XI]], [[Andromeda XIX|And XIX]], [[Andromeda XXI|And XXI]] and [[Andromeda XXII|And XXII]], plus several additional [[dwarf galaxy#Ultra-faint dwarf galaxies| ultra-faint dwarf]] spheroidal galaxies.<ref>{{Cite journal |last1=Kalirai |first1=Jason S. |last2=Beaton |first2=Rachael L. |last3=Geha |first3=Marla C. |last4=Gilbert |first4=Karoline M. |last5=Guhathakurta |first5=Puragra |last6=Kirby |first6=Evan N. |last7=Majewski |first7=Steven R. |last8=Ostheimer |first8=James C. |last9=Patterson |first9=Richard J. |date=17 February 2010 |title=The Splash Survey: Internal Kinematics, Chemical Abundances, and Masses of the Andromeda I, Ii, III, Vii, X, and Xiv Dwarf Spheroidal Galaxies |journal=The Astrophysical Journal |volume=711 |issue=2 |pages=671–692 |arxiv=0911.1998 |bibcode=2010ApJ...711..671K |doi=10.1088/0004-637X/711/2/671 |s2cid=43188686 |issn=0004-637X}}</ref>
* The Milky Way's [[List of Milky Way's satellite galaxies|satellite galaxies system]] comprises the [[Sagittarius Dwarf Spheroidal Galaxy|Sagittarius Dwarf Galaxy]], [[Large Magellanic Cloud]], [[Small Magellanic Cloud]], [[Canis Major Overdensity|Canis Major Dwarf Galaxy]] (disputed, considered by some not a galaxy), [[Ursa Minor Dwarf]] Galaxy, [[Draco Dwarf]] Galaxy, [[Carina Dwarf]] Galaxy, [[Sextans Dwarf Spheroidal|Sextans Dwarf Galaxy]], [[Sculptor Dwarf Galaxy]], [[Fornax Dwarf]] Galaxy, [[Leo I (dwarf galaxy)|Leo I]] (a dwarf galaxy), [[Leo II (dwarf galaxy)|Leo II]] (a dwarf galaxy), [[Ursa Major I Dwarf]] Galaxy and [[Ursa Major II Dwarf]] Galaxy, plus several additional ultra-faint [[dwarf spheroidal galaxy|dwarf spheroidal galaxies]].<ref name="Koposov2015">{{Cite journal |last1=Sergey E. Koposov |last2=Vasily Belokurov |last3=Gabriel Torrealba |last4=N. Wyn Evans |date=10 March 2015 |title=Beasts of the Southern Wild. Discovery of a large number of Ultra Faint satellites in the vicinity of the Magellanic Clouds |journal=The Astrophysical Journal |volume=805 |issue=2 |page=130 |arxiv=1503.02079 |bibcode=2015ApJ...805..130K |doi=10.1088/0004-637X/805/2/130|s2cid=118267222 }}</ref>

The [[Triangulum Galaxy]] is the third-largest member of the Local Group, with a mass of approximately {{convert|5e10|solar mass|kg|sigfig=1|abbr=on}}, and is the third spiral galaxy.<ref>{{Cite web |url=https://heasarc.gsfc.nasa.gov/docs/cosmic/local_group_info.html |title=The Local Group |website=NASA's High Energy Astrophysics Science Archive Research Center (HEASARC) |publisher=[[NASA]] |access-date=5 May 2015}}</ref> It is unclear whether the Triangulum Galaxy is a companion of the Andromeda Galaxy; the two galaxies are 750,000 light years apart,<ref>{{Cite web|title=Messier Object 33|url=https://www.messier.seds.org/m/m033.html|access-date=2021-05-21|website=www.messier.seds.org}}</ref> and experienced a close passage 2–4 billion years ago which triggered star formation across Andromeda's disk. The [[Pisces Dwarf]] Galaxy is equidistant from the Andromeda Galaxy and the Triangulum Galaxy, so it may be a satellite of either.<ref name="apj562_2">{{Cite journal |last1=Miller |first1=Bryan W. |last2=Dolphin |first2=Andrew E. |last3=Lee |first3=Myung Gyoon |last4=Kim |first4=Sang Chul |last5=Hodge |first5=Paul |display-authors=1 |date=December 2001 |title=The Star Formation History of LGS 3 |url=http://stacks.iop.org/0004-637X/562/i=2/a=713 |journal=The Astrophysical Journal |volume=562 |issue=2 |pages=713–726 |arxiv=astro-ph/0108408 |bibcode=2001ApJ...562..713M |doi=10.1086/323853 |s2cid=119089499 |issn=0004-637X}}</ref>

The membership of [[NGC 3109]], with its companions [[Sextans A]] and the [[Antlia Dwarf]] Galaxy, is uncertain due to extreme distances from the center of the Local Group.<ref name="Karachentsevetal2006" />
The other members of the group are likely gravitationally secluded from these large subgroups: [[IC 10]], [[IC 1613]], [[Phoenix Dwarf]] Galaxy, [[Leo A]], [[Tucana Dwarf]] Galaxy, [[Cetus Dwarf]] Galaxy, [[Pegasus Dwarf Irregular Galaxy]], [[Wolf–Lundmark–Melotte]], [[Aquarius Dwarf]] Galaxy, and [[Sagittarius Dwarf Irregular Galaxy]].<ref>{{Cite journal |last1=Kalirai |first1=Jason S. |last2=Beaton |first2=Rachael L. |last3=Geha |first3=Marla C. |last4=Gilbert |first4=Karoline M. |last5=Guhathakurta |first5=Puragra |last6=Kirby |first6=Evan N. |last7=Majewski |first7=Steven R. |last8=Ostheimer |first8=James C. |last9=Patterson |first9=Richard J. |last10=Wolf |first10=Joe |year=2012 |title=The observed properties of dwarf galaxies in and around the Local Group |journal=The Astronomical Journal |volume=144 |issue=1 |pages=4 |arxiv=1204.1562 |doi=10.1088/0004-6256/144/1/4|bibcode=2012AJ....144....4M |s2cid=118515618 }}</ref>

[[Merajul Islam]]

Group - Revision history

Merajul Islam at 14:59, 28 January 2022