A dwarf galaxy is a small galaxy composed of up to several billion stars, a small number compared to our own Milky Way's 200–400 billion stars. The Large Magellanic Cloud, containing over 30 billion stars, is sometimes classified as a dwarf galaxy while others consider it a full-fledged galaxy orbiting the Milky Way galaxy.
Formation of dwarf galaxies 
Current theory states that most galaxies, including dwarf galaxies, form in association with dark matter or out of gas containing metals. However, NASA's Galaxy Evolution Explorer space probe identified new dwarf galaxies forming out of gases lacking metals. These galaxies were located in the Leo Ring, a cloud of hydrogen and helium around two massive galaxies in the constellation Leo.
Conversely, dwarf galaxies because of their small size have been observed to be pulled toward and ripped by neighbouring spiral galaxies till they ultimately merge.
Local dwarfs 
There are many dwarf galaxies in the Local Group: these small galaxies frequently orbit around larger galaxies, such as the Milky Way, the Andromeda Galaxy and the Triangulum Galaxy. A 2007 paper has suggested that many dwarf galaxies were created by tidal forces during the early evolution of the Milky Way and Andromeda. Tidal dwarf galaxies are produced when galaxies collide and their gravitational masses interact. Streams of galactic material are pulled out away from the parent galaxies and the halos of dark matter that surround them.
The Milky Way has more than 20 known dwarf galaxies orbiting it, and recent observations have also led astronomers to believe the largest globular cluster in the Milky Way, Omega Centauri, is in fact the core of a dwarf galaxy with a black hole in its center, which was at some time absorbed by the Milky Way.
Common types of Dwarf galaxies 
- Elliptical galaxy: dwarf elliptical galaxy (dE)
- Dwarf spheroidal galaxy (dSph): Used to be a subtype of dwarf ellipticals, now regarded as a distinct type of dwarf galaxy.
- Irregular galaxy: dwarf irregular galaxy (dI)
- Spiral galaxy: dwarf spiral galaxy (dS) 
- Magellanic type dwarfs
Blue Compact Dwarf galaxies 
In astronomy, a blue compact dwarf galaxy (BCD galaxy) is a small galaxy which contains large clusters of young, hot, massive stars. These stars cause the galaxy to appear blue in color. Most of them are also classified as dwarf Irregular galaxies or as dwarf lenticular galaxies. Since they exist mostly in clusters, blue compact dwarf galaxies don't have an exact uniform shape. They consume gas intensely, which causes their stars to become very violent when they are forming.
Blue compact dwarf galaxies cool in the process of forming new stars. Their stars are all formed at different time periods, so the galaxies have time to cool and to build up matter to form new stars.
The galaxies aren't blue throughout - they actually have a lot of stars of different colors. But throughout them there are a lot of star-forming regions in which the most massive and brightest stars are blue. Since these stars greatly outshine their neighbors, the combined light from these galaxies appears blue from far away.
As time passes, blue compact dwarf galaxies change shape due to star formation.
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Hobbit galaxies 
Ultra-compact dwarfs 
Ultra-compact dwarf galaxies (UCD) are a recently discovered class of very compact galaxies with very high stellar population counts. They are thought to be on the order of 200 light years across, with a hundred million stars. It is theorized that these are the cores of nucleated dwarf elliptical galaxies, that have been stripped of gas and outlying stars by tidal interactions, travelling through the hearts of rich clusters. UCDs have been found in the Virgo Cluster, Fornax Cluster, Abell 1689, Coma Cluster, amongst other clusters.
Partial list of dwarf galaxies 
- Aquarius Dwarf
- Canis Major Dwarf Galaxy
- Henize 2-10
- I Zwicky 18
- Irregular Galaxy IC 10
- Large Magellanic Cloud
- NGC 1569
- NGC 1705
- Pegasus Dwarf Irregular Galaxy
- Phoenix Dwarf
- Sagittarius Dwarf Elliptical Galaxy
- Sagittarius Dwarf Irregular Galaxy
- Sculptor Dwarf Galaxy
- Sculptor Dwarf Irregular Galaxy
- Sextans A
- Sextans Dwarf
- Small Magellanic Cloud
- Tucana Dwarf
- Ursa Minor Dwarf
- Willman 1
- Carina Dwarf
- Draco Dwarf
- Fornax Dwarf
- Leo II (dwarf galaxy)
See also 
- "New Recipe For Dwarf Galaxies: Start With Leftover Gas", Science Daily, 19 February 2009
- Jaggard, Victoria (9 September 2010). "Pictures: New Proof Spiral Galaxies Eat, Digest Dwarfs". National Geographic Daily News. National Geographic Society. Retrieved 11 February 2012.
- "Hubble Sizes up a Dwarf Galaxy". Picture of the Week. ESA/Hubble. Retrieved 25 October 2011.
- Metz, M.; Kroupa (2007). "Dwarf-spheroidal satellites: are they of tidal origin?". Monthly Notices of the Royal Astronomical Society 376: 387–392. arXiv:astro-ph/0701289. Bibcode:2007MNRAS.376..387M. doi:10.1111/j.1365-2966.2007.11438.x.
- New Recipe for Dwarf Galaxies: Start with Leftover Gas Newswise, Retrieved on February 20, 2009.
- Noyola, E. and Gebhardt, K. and Bergmann, M. (2008). "Gemini and Hubble Space Telescope Evidence for an Intermediate-Mass Black Hole in ω Centauri". The Astrophysical Journal 676 (2): 1008–1015. arXiv:0801.2782. Bibcode:2008ApJ...676.1008N. doi:10.1086/529002.
- J. M. Schombert, R. A. Pildis, J. A. Eder, A. Oelmer, Jr. (1995). "Dwarf Spirals". Astronomical Journal 110: 2067–2074. Bibcode:1995AJ....110.2067S. doi:10.1086/117669.
- "WISE Discovers Baby Galaxies in the Nearby Universe". Wide-Field Infrared Explorer. U.C. Berkeley. September 2, 2011.
- Angel R. Lopez-Sanchez, Barbel Koribalski, Janine van Eymeren, Cesar Esteban, Attila Popping, and John Hibbard. "The environment of nearby Blue Compact Dwarf Galaxies". Internet.
- Polychronis Papaderos. "Blue Compact Dwarf Galaxy". Internet.
- K. Noeske, P. Papaderos, L. M. Cairos. "New insights to the photometric structure of Blue Compact Dwarf Galaxies from deep Near-Infrared Studies". Internet.
- G.R. Meurer, G. Mackie, C. Carignan (1994). "Optical observations of NGC 2915: A nearby blue compact dwarf galaxy". The Astronomical Journal 107 (6): 2021–2035. Bibcode:1994AJ....107.2021M. doi:10.1086/117013.
- New 'Hobbit' Galaxies Discovered Around Milky Way. SPACE.com (2007-01-15). Retrieved on 2012-02-03.
- Hobbits in Space – ScienceNOW. Sciencenow.sciencemag.org. 9 January 2007. Retrieved on 2012-02-03.
- Anglo-Australian Observatory Astronomers discover dozens of mini-galaxies 0100 AEST Friday 2 April 2004
- Stelios Kazantzidis; Ben Moore; Lucio Mayer (2003). "Galaxies and Overmerging: What Does it Take to Destroy a Satellite Galaxy?". arXiv:astro-ph/0307362 [astro-ph].
- Mieske; Infante; Benitez; Coe; Blakeslee; Zekser; Ford; Broadhurst et al. (2004). "Ultra Compact Dwarf galaxies in Abell 1689: a photometric study with the ACS". The Astronomical Journal 128 (4): 1529–1540. arXiv:astro-ph/0406613. Bibcode:2004AJ....128.1529M. doi:10.1086/423701.
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- Milky Way Satellite Galaxies
- SPACE.com article on "hobbit galaxies"
- Science article on "hobbit galaxies"