Editing Galaxy (section)

== Types and morphology ==
{{see also|Galactic bulge|Central massive object|Active galactic nucleus|Galaxy morphological classification}}
[[File:Hubble sequence photo.png|thumb|360px|Types of galaxies according to the [[Hubble sequence|Hubble classification scheme]] : an ''E'' indicates a type of [[elliptical galaxy]]; an ''S'' is a [[Spiral galaxy|spiral]]; and ''SB'' is a [[barred spiral galaxy]]]]

Galaxies come in three main types: ellipticals, spirals, and irregulars. A slightly more extensive description of galaxy types based on their appearance is given by the [[Hubble sequence]]. Since the Hubble sequence is entirely based upon visual morphological type (shape), it may miss certain important characteristics of galaxies such as [[star formation]] rate in [[Starburst galaxy|starburst galaxies]] and activity in the cores of [[active galaxy|active galaxies]].<ref name="IRatlas" />

{{anchor|Center}}Many galaxies are thought to contain a supermassive black hole at their center. This includes the Milky Way, whose core region is called the [[Galactic Center]].{{sfn|Fraknoi et al.|2023|pp=[https://openstax.org/books/astronomy-2e/pages/25-4-the-center-of-the-galaxy 851–856], [https://openstax.org/books/astronomy-2e/pages/27-2-supermassive-black-holes-what-quasars-really-are 907–915]}}

=== Ellipticals ===
{{Main|Elliptical galaxy}}

The Hubble classification system rates elliptical galaxies on the basis of their ellipticity, ranging from E0, being nearly spherical, up to E7, which is highly elongated. These galaxies have an [[ellipsoid]]al profile, giving them an elliptical appearance regardless of the viewing angle. Their appearance shows little structure and they typically have relatively little [[interstellar medium|interstellar matter]]. Consequently, these galaxies also have a low portion of [[open cluster]]s and a reduced rate of new star formation. Instead, they are dominated by generally older, more [[stellar evolution|evolved stars]] that are orbiting the common center of gravity in random directions. The stars contain low abundances of heavy elements because star formation ceases after the initial burst. In this sense they have some similarity to the much smaller [[globular cluster]]s.<ref name="elliptical">{{cite web
 |last1=Barstow |first1=M. A.
 |date=2005
 |title=Elliptical Galaxies
 |url=http://www.star.le.ac.uk/edu/Elliptical.shtml
 |archive-url=https://web.archive.org/web/20120729081504/http://www.star.le.ac.uk/edu/Elliptical.shtml
 |archive-date=2012-07-29
 |publisher=[[Leicester University]] Physics Department
 |access-date=June 8, 2006
}}</ref>

====Type-cD galaxies====

[[Image:Galaxy Cluster Abell 1413.jpg|thumb|right|300px|The galaxy cluster [[Abell 1413]] is dominated by this cD elliptical galaxy designated Abell 1413 BCG. It has an isophotal diameter of over 800,000 light-years across. Note the [[gravitational lens]]ing.]]

The [[List of largest galaxies|largest galaxies]] are the [[Type-cD galaxy|type-cD galaxies]].
First described in 1964 by a paper by Thomas A. Matthews and others,<ref name=Matthews>{{cite journal | url=https://ui.adsabs.harvard.edu/abs/1964ApJ...140...35M/abstract | bibcode=1964ApJ...140...35M | title=A Discussion of Galaxies Indentified &#91;sic&#93; with Radio Sources | last1=Matthews | first1=Thomas A. | last2=Morgan | first2=William W. | last3=Schmidt | first3=Maarten | journal=[[The Astrophysical Journal]] | year=1964 | volume=140 | page=35 | doi=10.1086/147890 }}</ref> they are a subtype of the more general class of D galaxies, which are giant elliptical galaxies, except that they are much larger. They are popularly known as the '''supergiant elliptical galaxies''' and constitute the largest and most luminous galaxies known. These galaxies feature a central elliptical nucleus with an extensive, faint halo of stars extending to megaparsec scales.<ref name="NASAVlog">{{cite web | url=https://www.nasa.gov/content/discoveries-highlights-tracing-the-growth-of-galaxies |publisher=[[NASA]]|title=Tracing the Growth of Galaxies |access-date=April 20, 2024| date=6 February 2017 }}</ref> The profile of their surface brightnesses as a function of their radius (or distance from their cores) falls off more slowly than their smaller counterparts.<ref name=Tonry>{{cite book | chapter-url=https://ui.adsabs.harvard.edu/abs/1987IAUS..127...89T/abstract | bibcode=1987IAUS..127...89T | last1=Tonry | first1=John L. | title=Structure and Dynamics of Elliptical Galaxies | chapter=Properties of cD Galaxies | year=1987 | volume=127 | page=89 | doi=10.1007/978-94-009-3971-4_7 | isbn=978-90-277-2586-8 | s2cid=117980521 }}</ref>

The formation of these cD galaxies remains an active area of research, but the leading model is that they are the result of the mergers of smaller galaxies in the environments of dense clusters, or even those outside of clusters with random overdensities.<ref>{{Cite journal |last1=Tovmassian |first1=Hrant M. |last2=Andernach |first2=Heinz |date=2012-12-11 |title=On the formation of cD galaxies and their parent clusters: Formation of cD galaxies and parent clusters |journal=[[Monthly Notices of the Royal Astronomical Society]] |language=en |volume=427 |issue=3 |pages=2047–2056 |doi=10.1111/j.1365-2966.2012.22044.x|doi-access=free |arxiv=1212.0238 }}</ref> These processes are the mechanisms that drive the formation of fossil groups or fossil clusters, where a large, relatively isolated, supergiant elliptical resides in the middle of the cluster and are surrounded by an extensive cloud of X-rays as the residue of these galactic collisions. Another older model posits the phenomenon of [[cooling flow]], where the heated gases in clusters collapses towards their centers as they cool, forming stars in the process,<ref>{{Cite journal |last1=Bender |first1=Ralf |last2=Kormendy |first2=John |last3=Cornell |first3=Mark E. |last4=Fisher |first4=David B. |date=2015-06-30 |title=Structure and Formation of cD Galaxies: NGC 6166 in Abell 2199 |url=https://iopscience.iop.org/article/10.1088/0004-637X/807/1/56 |journal=[[The Astrophysical Journal]] |volume=807 |issue=1 |pages=56 |doi=10.1088/0004-637X/807/1/56 |arxiv=1411.2598 |bibcode=2015ApJ...807...56B |hdl=1959.3/404318 |issn=1538-4357 |quote=This idea was entertained in the heyday of the cooling-flow problem, when we observed large amounts of X-ray-emitting, hot gas in clusters but could not measure temperature profiles. [...] This possibility is now regarded as a non-starter.}}</ref> a phenomenon observed in clusters such as [[Perseus Cluster|Perseus]],<ref name=Fabian>{{cite journal | bibcode=1977MNRAS.180..479F | title=Subsonic accretion of cooling gas in clusters of galaxies | last1=Fabian | first1=A. C. | last2=Nulsen | first2=P. E. J. | journal=[[Monthly Notices of the Royal Astronomical Society]] | year=1977 | volume=180 | issue=3 | page=479 | doi=10.1093/mnras/180.3.479 | doi-access=free }}</ref> and more recently in the [[Phoenix Cluster]].<ref>{{Cite journal |last1=McDonald |first1=M. |last2=McNamara |first2=B. R. |last3=Voit |first3=G. M. |last4=Bayliss |first4=M. |last5=Benson |first5=B. A. |last6=Brodwin |first6=M. |last7=Canning |first7=R. E. A. |last8=Florian |first8=M. K. |last9=Garmire |first9=G. P. |last10=Gaspari |first10=M. |last11=Gladders |first11=M. D. |last12=Hlavacek-Larrondo |first12=J. |last13=Kara |first13=E. |last14=Reichardt |first14=C. L. |last15=Russell |first15=H. R. |date=2019-11-01 |title=Anatomy of a Cooling Flow: The Feedback Response to Pure Cooling in the Core of the Phoenix Cluster |journal=[[The Astrophysical Journal]] |volume=885 |issue=1 |pages=63 |doi=10.3847/1538-4357/ab464c |doi-access=free |arxiv=1904.08942 |bibcode=2019ApJ...885...63M |issn=0004-637X}}</ref>

==== Shell galaxy ====
[[File:NGC 3923 Elliptical Shell Galaxy.jpg|thumb|[[NGC 3923]] Elliptical Shell Galaxy (Hubble photograph)]]

A shell galaxy is a type of elliptical galaxy where the stars in its halo are arranged in concentric shells. About one-tenth of elliptical galaxies have a shell-like structure, which has never been observed in spiral galaxies. These structures are thought to develop when a larger galaxy absorbs a smaller companion galaxy—that as the two galaxy centers approach, they start to oscillate around a center point, and the oscillation creates gravitational ripples forming the shells of stars, similar to ripples spreading on water. For example, galaxy [[NGC 3923]] has over 20 shells.<ref>{{cite web|title = Galactic onion|url = http://www.spacetelescope.org/images/potw1519a/|publisher = [[European Space Agency]]|access-date = 2015-05-11|archive-date = August 6, 2020|archive-url = https://web.archive.org/web/20200806221639/https://www.spacetelescope.org/images/potw1519a/|url-status = live}}</ref>

=== Spirals ===
{{Main|Spiral galaxy|Barred spiral galaxy}}
[[File:M101 hires STScI-PRC2006-10a.jpg|thumb|right|The [[Pinwheel Galaxy]], NGC 5457]]

Spiral galaxies resemble spiraling [[pinwheel (toy)|pinwheels]]. Though the stars and other visible material contained in such a galaxy lie mostly on a plane, the majority of mass in spiral galaxies exists in a roughly spherical halo of [[dark matter]] which extends beyond the visible component, as demonstrated by the universal rotation curve concept.<ref name="Williams2009">{{Cite journal | last1 = Williams | first1 = M. J. | last2 = ((Bureau)) | first2 = M. | last3 = Cappellari | first3 = M. | s2cid = 17940107 | doi = 10.1111/j.1365-2966.2009.15582.x | title = Kinematic constraints on the stellar and dark matter content of spiral and S0 galaxies | journal = [[Monthly Notices of the Royal Astronomical Society]] | volume = 400 | issue = 4 | pages = 1665–1689 | year = 2010 | doi-access = free |arxiv = 0909.0680 |bibcode = 2009MNRAS.400.1665W }}</ref>

Spiral galaxies consist of a rotating disk of stars and interstellar medium, along with a central bulge of generally older stars. Extending outward from the [[bulge (astronomy)|bulge]] are relatively bright arms. In the Hubble classification scheme, spiral galaxies are listed as type ''S'', followed by a letter (''a'', ''b'', or ''c'') which indicates the degree of tightness of the spiral arms and the size of the central bulge. An ''Sa'' galaxy has tightly wound, poorly defined arms and possesses a relatively large core region. At the other extreme, an ''Sc'' galaxy has open, well-defined arms and a small core region.<ref>{{cite web
 |last1       = Smith
 |first1      = Gene
 |date        = March 6, 2000
 |url         = http://casswww.ucsd.edu/public/tutorial/Galaxies.html
 |title       = Galaxies&nbsp;— The Spiral Nebulae
 |publisher   = [[University of California, San Diego]] Center for Astrophysics & Space Sciences
 |access-date  = November 30, 2006
 |url-status  = live
 |archive-url  = https://web.archive.org/web/20230327194826/https://casswww.ucsd.edu/archive/public/tutorial/Galaxies.html
 |archive-date = March 27, 2023
 |df          = mdy-all
}}</ref> A galaxy with poorly defined arms is sometimes referred to as a [[flocculent spiral galaxy]]; in contrast to the [[grand design spiral galaxy]] that has prominent and well-defined spiral arms.<ref name=bergh1998 /> The speed in which a galaxy rotates is thought to correlate with the flatness of the disc as some spiral galaxies have thick bulges, while others are thin and dense.<ref>{{Cite journal |last1=Obreschkow |first1=D. |last2=Glazebrook |first2=K. |date=2014-02-28 |title=Fundamental Mass-Spin-Morphology Relation of Spiral Galaxies |url=https://iopscience.iop.org/article/10.1088/0004-637X/784/1/26 |journal=[[The Astrophysical Journal]] |volume=784 |issue=1 |pages=26 |arxiv=1312.4543 |bibcode=2014ApJ...784...26O |doi=10.1088/0004-637X/784/1/26 |issn=0004-637X}}</ref><ref>{{Cite news|url=http://phys.org/news/2014-02-fat-flat-galaxies.html|title=Fat or flat: Getting galaxies into shape| archive-url=https://web.archive.org/web/20210324072603/https://phys.org/news/2014-02-fat-flat-galaxies.html |work=[[Phys.org]]|archive-date=March 24, 2021|date=February 27, 2014 }}4</ref>
[[File:Hubble2005-01-barred-spiral-galaxy-NGC1300.jpg|thumb|right|[[NGC 1300]], an example of a [[barred spiral galaxy]]]]

In spiral galaxies, the spiral arms do have the shape of approximate [[logarithmic spiral]]s, a pattern that can be theoretically shown to result from a disturbance in a uniformly rotating mass of stars. Like the stars, the spiral arms rotate around the center, but they do so with constant [[angular velocity]]. The spiral arms are thought to be areas of high-density matter, or "[[Density wave theory|density waves]]".<ref name=bertin_lin1996 /> As stars move through an arm, the space velocity of each stellar system is modified by the gravitational force of the higher density. (The velocity returns to normal after the stars depart on the other side of the arm.) This effect is akin to a "wave" of slowdowns moving along a highway full of moving cars. The arms are visible because the high density facilitates star formation, and therefore they harbor many bright and young stars.<ref name=belkora355 />
[[File:Hoag's object.jpg|thumb|right|[[Hoag's Object]], an example of a [[ring galaxy]]]]

==== Barred spiral galaxy ====
A majority of spiral galaxies, including the [[Milky Way]] galaxy, have a linear, bar-shaped band of stars that extends outward to either side of the core, then merges into the spiral arm structure.<ref>{{cite journal
 |last1=Eskridge |first1=P. B.
 |last2=Frogel |first2=J. A.
 |s2cid=189840251
 |date=1999
 |title=What is the True Fraction of Barred Spiral Galaxies?
 |journal=[[Astrophysics and Space Science]]
 |volume=269/270 |pages=427–430
 |bibcode=1999Ap&SS.269..427E
 |doi=10.1023/A:1017025820201
}}</ref> In the Hubble classification scheme, these are designated by an ''SB'', followed by a lower-case letter (''a'', ''b'' or ''c'') which indicates the form of the spiral arms (in the same manner as the categorization of normal spiral galaxies). Bars are thought to be temporary structures that can occur as a result of a density wave radiating outward from the core, or else due to a [[Galactic tide|tidal interaction]] with another galaxy.<ref>{{cite journal
 |last1=Bournaud |first1=F.
 |last2=Combes |first2=F.
 |s2cid=17562844
 |date=2002
 |title=Gas accretion on spiral galaxies: Bar formation and renewal
 |journal=[[Astronomy & Astrophysics]]
 |volume=392
 |issue=1 |pages=83–102
 |bibcode=2002A&A...392...83B
 |doi=10.1051/0004-6361:20020920
|arxiv = astro-ph/0206273 }}</ref> Many barred spiral galaxies are active, possibly as a result of gas being channeled into the core along the arms.<ref>{{cite journal
 |last1=Knapen |first1=Johan H.
 |last2=Perez-Ramirez |first2=Daniel
 |last3=Laine |first3=Seppo
 |s2cid=10845683
 |date=2002
 |title=Circumnuclear regions in barred spiral galaxies&nbsp;— II. Relations to host galaxies
 |journal=[[Monthly Notices of the Royal Astronomical Society]]
 |volume=337 |issue=3 |pages=808–828
 |bibcode=2002MNRAS.337..808K
 |doi=10.1046/j.1365-8711.2002.05840.x
|doi-access=free
 |arxiv = astro-ph/0207258 }}</ref>

Our own galaxy, the [[Milky Way]], is a large disk-shaped barred-spiral galaxy<ref>{{cite journal
 |last1=Alard |first1=Christophe
 |s2cid=18018228
 |date=2001
 |title=Another bar in the Bulge
 |journal=[[Astronomy and Astrophysics Letters]]
 |volume=379 |issue=2 |pages=L44–L47
 |bibcode=2001A&A...379L..44A
 |doi=10.1051/0004-6361:20011487
|arxiv = astro-ph/0110491 }}</ref> about 30&nbsp;kiloparsecs in diameter and a kiloparsec thick. It contains about two hundred billion (2×10<sup>11</sup>)<ref>{{cite press release
 |last1=Sanders
 |first1=Robert
 |date=January 9, 2006
 |title=Milky Way galaxy is warped and vibrating like a drum
 |publisher=[[University of California, Berkeley]]
 |url=http://www.berkeley.edu/news/media/releases/2006/01/09_warp.shtml
 |access-date=May 24, 2006
 |archive-date=January 18, 2014
 |archive-url=https://web.archive.org/web/20140118051946/http://www.berkeley.edu/news/media/releases/2006/01/09_warp.shtml
 |url-status=live
 }}</ref> stars and has a total mass of about six hundred billion (6×10<sup>11</sup>) times the mass of the Sun.<ref>{{cite journal
 |last1=Bell |first1=G. R.
 |last2=Levine |first2=S. E.
 |date=1997
 |title=Mass of the Milky Way and Dwarf Spheroidal Stream Membership
 |journal=[[Bulletin of the American Astronomical Society]]
 |volume=29 |issue=2 |page=1384
 |bibcode=1997AAS...19110806B
}}</ref>

==== Super-luminous spiral ====
Recently, researchers described galaxies called super-luminous spirals. They are very large with an upward diameter of 437,000 light-years (compared to the Milky Way's 87,400 light-year diameter). With a mass of 340 billion solar masses, they generate a significant amount of ultraviolet and mid-infrared light. They are thought to have an increased star formation rate around 30 times faster than the Milky Way.<ref>{{cite web|url=http://futurism.com/just-discovered-new-type-colossal-galaxy/|title=We Just Discovered a New Type of Colossal Galaxy|last=Santos|first=Miguel|website=[[Futurism]]|language=en-US|access-date=2016-03-21|date=2016-03-21|archive-date=March 24, 2021|archive-url=https://web.archive.org/web/20210324071443/https://futurism.com/just-discovered-new-type-colossal-galaxy|url-status=live}}</ref><ref>{{Cite journal|last1=Ogle|first1=Patrick M.|last2=Lanz|first2=Lauranne|last3=Nader|first3=Cyril|last4=Helou|first4=George|s2cid=35287348|date=2016-01-01|title=Superluminous Spiral Galaxies|journal=[[The Astrophysical Journal]]|language=en|volume=817|issue=2|page=109|doi=10.3847/0004-637X/817/2/109|issn=0004-637X|arxiv = 1511.00659 |bibcode = 2016ApJ...817..109O |doi-access=free }}</ref>

=== Other morphologies ===
* [[Peculiar galaxy|Peculiar galaxies]] are galactic formations that develop unusual properties due to tidal interactions with other galaxies.
** A [[ring galaxy]] has a ring-like structure of stars and interstellar medium surrounding a bare core. A ring galaxy is thought to occur when a smaller galaxy passes through the core of a spiral galaxy.<ref>{{cite journal
 |last1=Gerber |first1=R. A.
 |last2=Lamb |first2=S. A.
 |last3=Balsara |first3=D. S.
 |date=1994
 |title=Ring Galaxy Evolution as a Function of "Intruder" Mass
 |journal=[[Bulletin of the American Astronomical Society]]
 |volume=26 |page=911
 |bibcode=1994AAS...184.3204G
}}</ref> Such an event may have affected the [[Andromeda Galaxy#Structure|Andromeda Galaxy]], as it displays a multi-ring-like structure when viewed in [[infrared]] radiation.<ref>{{cite press release
 |publisher=[[European Space Agency]]
 |date=October 14, 1998
 |title=ISO unveils the hidden rings of Andromeda
 |url=http://www.iso.vilspa.esa.es/outreach/esa_pr/andromed.htm
 |access-date=May 24, 2006
 |url-status=dead
 |archive-url=https://web.archive.org/web/20060505051509/http://www.iso.esac.esa.int/outreach/esa_pr/andromed.htm
 |archive-date=May 5, 2006
 |df=mdy-all
 }}</ref>
* A [[lenticular galaxy]] is an intermediate form that has properties of both elliptical and spiral galaxies. These are categorized as Hubble type S0, and they possess ill-defined spiral arms with an elliptical halo of stars<ref>{{cite press release
 |date=May 31, 2004
 |title=Spitzer Reveals What Edwin Hubble Missed
 |url=http://www.cfa.harvard.edu/press/pr0419.html
 |archive-url=https://web.archive.org/web/20060907042809/http://www.cfa.harvard.edu/press/pr0419.html
 |archive-date=2006-09-07
 |publisher=[[Harvard-Smithsonian Center for Astrophysics]]
 |access-date=December 6, 2006
}}</ref> ([[Barred lenticular galaxy|barred lenticular galaxies]] receive Hubble classification SB0).
* [[Irregular galaxy|Irregular galaxies]] are galaxies that can not be readily classified into an elliptical or spiral morphology.
** An Irr-I galaxy has some structure but does not align cleanly with the Hubble classification scheme.
** Irr-II galaxies do not possess any structure that resembles a Hubble classification, and may have been disrupted.<ref>{{cite web
 |last1=Barstow |first1=M. A.
 |date=2005
 |title=Irregular Galaxies
 |url=http://www.star.le.ac.uk/edu/Irregular.shtml
 |archive-url=https://web.archive.org/web/20120227172628/http://www.star.le.ac.uk/edu/Irregular.shtml
 |archive-date=2012-02-27
 |publisher=[[University of Leicester]]
 |access-date=December 5, 2006
}}</ref> Nearby examples of (dwarf) irregular galaxies include the [[Magellanic Clouds]].{{sfn|Fraknoi et al.|2023|pp=[https://openstax.org/books/astronomy-2e/pages/26-2-types-of-galaxies 879]}}

* A [[dark galaxy|dark]] or "ultra diffuse" galaxy is an extremely-low-luminosity galaxy. It may be the same size as the Milky Way, but have a visible star count only one percent of the Milky Way's. Multiple mechanisms for producing this type of galaxy have been proposed, and it is possible that different dark galaxies formed by different means.<ref>{{Cite journal |last1=Pandya |first1=Viraj |last2=Romanowsky |first2=Aaron J. |last3=Laine |first3=Seppo |last4=Brodie |first4=Jean P. |last5=Johnson |first5=Benjamin D. |last6=Glaccum |first6=William |last7=Villaume |first7=Alexa |last8=Cuillandre |first8=Jean-Charles |last9=Gwyn |first9=Stephen |last10=Krick |first10=Jessica |last11=Lasker |first11=Ronald |last12=Martín-Navarro |first12=Ignacio |last13=Martinez-Delgado |first13=David |last14=van Dokkum |first14=Pieter |date=2018-05-01 |title=The Stellar Populations of Two Ultra-diffuse Galaxies from Optical and Near-infrared Photometry |journal=[[The Astrophysical Journal]] |volume=858 |issue=1 |pages=29 |doi=10.3847/1538-4357/aab498 |doi-access=free |arxiv=1711.05272 |bibcode=2018ApJ...858...29P |issn=0004-637X}}</ref> One candidate explanation for the low luminosity is that the galaxy lost its star-forming gas at an early stage, resulting in old stellar populations.<ref name="NYT-20240126">{{cite news |last=Overbye |first=Dennis |authorlink=Dennis Overbye|title=What Do You Call a Galaxy Without Stars?|url=https://www.nytimes.com/2024/01/26/science/space/dark-galaxy-stars.html |date=26 January 2024 |url-access=registration |work=[[The New York Times]] |url-status=live |archiveurl=https://web.archive.org/web/20240202001203/https://www.nytimes.com/2024/01/26/science/space/dark-galaxy-stars.html |archivedate=2 February 2024 }}</ref><ref>{{Cite journal |last1=van Dokkum |first1=Pieter G. |last2=Abraham |first2=Roberto |author-link2=Roberto Abraham |last3=Merritt |first3=Allison |last4=Zhang |first4=Jielai |last5=Geha |first5=Marla|author5-link= Marla Geha |last6=Conroy |first6=Charlie |date=January 2015 |title=Forty-Seven Milky Way-Sized, Extremely Diffuse Galaxies in the Coma Cluster |journal=The Astrophysical Journal Letters |language=en |volume=798 |issue=2 |pages=L45 |arxiv=1410.8141 |bibcode=2015ApJ...798L..45V |doi=10.1088/2041-8205/798/2/L45 |issn=2041-8205}}</ref>

=== Dwarfs ===
{{Main|Dwarf galaxy}}

Despite the prominence of large elliptical and spiral galaxies, most galaxies are dwarf galaxies.<ref name=Mateo19989/> They are relatively small when compared with other galactic formations, being about one hundredth the size of the Milky Way, with only a few billion stars. [[Blue compact dwarf]] galaxies contains large clusters of [[OB star|young, hot, massive stars]]. Ultra-compact dwarf galaxies have been discovered that are only 100&nbsp;parsecs across.<ref>{{cite journal
 |last1=Phillipps |first1=S.
 |last2=Drinkwater |first2=M. J.
 |last3=Gregg |first3=M. D.
 |last4=Jones |first4=J. B.
 |s2cid=18297376
 |date=2001
 |title=Ultracompact Dwarf Galaxies in the Fornax Cluster
 |journal=[[The Astrophysical Journal]]
 |volume=560 |issue=1 |pages=201–206
 |bibcode=2001ApJ...560..201P
 |doi=10.1086/322517
|arxiv = astro-ph/0106377 }}</ref>

Many dwarf galaxies may orbit a single larger galaxy; the Milky Way has at least a dozen such satellites, with an estimated 300–500 yet to be discovered.<ref>{{cite magazine
 |last1=Groshong
 |first1=Kimm
 |date=April 24, 2006
 |title=Strange satellite galaxies revealed around Milky Way
 |magazine=[[New Scientist]]
 |url=https://www.newscientist.com/article/dn9043-strange-satellite-galaxies-revealed-around-milky-way.html
 |access-date=January 10, 2007
 |archive-date=July 2, 2015
 |archive-url=https://web.archive.org/web/20150702024442/http://www.newscientist.com/article/dn9043-strange-satellite-galaxies-revealed-around-milky-way.html
 |url-status=live
 }}</ref>
Most of the information we have about dwarf galaxies come from observations of the [[local group]], containing two spiral galaxies, the Milky Way and Andromeda, and many dwarf galaxies. These dwarf galaxies are classified as either [[irregular galaxy|irregular]] or [[dwarf elliptical galaxy|dwarf elliptical]]/[[dwarf spheroidal galaxy|dwarf spheroidal galaxies]].<ref name=Mateo19989>{{Cite journal |last=Mateo |first=Mario |date=1998 |title=Dwarf Galaxies of the Local Group |url=https://www.annualreviews.org/doi/10.1146/annurev.astro.36.1.435 |journal=[[Annual Review of Astronomy and Astrophysics]] |language=en |volume=36 |issue=1 |pages=435–506 |doi=10.1146/annurev.astro.36.1.435 |arxiv=astro-ph/9810070 |bibcode=1998ARA&A..36..435M |s2cid=119333888 |issn=0066-4146}}</ref>

A study of 27 Milky Way neighbors found that in all dwarf galaxies, the central mass is approximately 10 million [[solar mass]]es, regardless of whether it has thousands or millions of stars. This suggests that galaxies are largely formed by [[dark matter]], and that the minimum size may indicate a form of [[warm dark matter]] incapable of gravitational coalescence on a smaller scale.<ref>{{cite web
 |last1=Schirber
 |first1=M.
 |date=August 27, 2008
 |url=https://www.science.org/content/article/no-slimming-down-dwarf-galaxies
 |title=No Slimming Down for Dwarf Galaxies
 |publisher=[[ScienceNOW]]
 |access-date=August 27, 2008
 |archive-date=May 30, 2020
 |archive-url=https://web.archive.org/web/20200530044532/https://www.sciencemag.org/news/2008/08/no-slimming-down-dwarf-galaxies
 |url-status=live
 }}</ref>