یک کهکشان کوتوله به کهکشانی اطلاق میشود که دارای فقط چند میلیارد ستاره باشد (در مقایسه با کهکشان راه شیری که ۲۰۰–۴۰۰ میلیارد ستاره دارد). ابر ماژلانی بزرگ نیز ۳۰ میلیارد ستاره دارد و به همین دلیل گاهی به عنوان کهکشان کوتوله طبقهبندی میشود.
کهکشان راه شیری چهارده کهکشان کوتوله شناخته شده دارد که به شکل اقماری دور این کهکشان میگردند و مطالعات جدیدتر نشان میدهد که امگا قنطورس (بزرگترین خوشه ستارهای شناخته شده) در واقع هسته یک کهکشان کوتوله است که در مرکزش یک سیاهچاله قرار دارد.
کهکشان کوتوله به کهکشانی گویند که کم نور و کم جرم است و تعداد ستارگان آن به مراتب کمتر از سایر کهکشان هاست و رصد آنها مشکل است. یک کهکشان به طور کلی سیستم اتصال گرانشی ستاره ای است و همچنین متشکل از گاز و غبار میان ستاره ای و ماده تاریک است. نظریات حاکی از آن است که گرانش ماده تاریک موجب شکل گیری کهکشانها شده است و کهکشانهای کوتوله آزمایشگاه طبیعی ماده تاریک هستند. به همین علت مطالعه کهکشانهای کوتوله در مسائلی نظیر چگونگی شکل گیری کهکشانهای بزرگ و اینکه چگونه ماده تاریک با ایجاد اتصال موجب شکل گیری آنها میشود، بسیار مؤثر است.
A dwarf galaxy is a small galaxy composed of about 100 million up to several billion stars, as compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is sometimes classified as a dwarf galaxy; others consider it a full-fledged galaxy. Dwarf galaxies' formation and activity are thought to be heavily influenced by interactions with larger galaxies. Astronomers identify numerous types of dwarf galaxies, based on their shape and composition.
The Phoenix Dwarf Galaxy is a dwarf irregular galaxy, featuring younger stars in its inner regions and older ones at its outskirts.
There are many dwarf galaxies in the Local Group; these small galaxies frequently orbit 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 galactic tides during the early evolutions 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 away from the parent galaxies and the halos of dark matter that surround them. A 2018 study suggests that some local dwarf galaxies formed extremely early, during the Dark Ages within the first billion years after the big bang.
More than 20 known dwarf galaxies orbit the Milky Way, 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 at its centre, which was at some time absorbed by the Milky Way.
UGC 11411 is a galaxy known as an irregular blue compact dwarf (BCD) galaxy.
In astronomy, a blue compact dwarf galaxy (BCD galaxy) is a small galaxy which contains large clusters of young, hot, massive stars. These stars, the brightest of which are blue, because the galaxy itself to appear blue in colour. Most BCD galaxies are also classified as dwarf irregular galaxies or as dwarf lenticular galaxies. Because they are composed of star clusters, BCD galaxies lack a uniform shape. They consume gas intensely, which causes their stars to become very violent when forming.
BCD galaxies cool in the process of forming new stars. The galaxies' stars are all formed at different time periods, so the galaxies have time to cool and to build up matter to form new stars. As time passes, this star formation changes the shape of the galaxies.
Ultra-compact dwarf galaxies (UCD) are a class of very compact galaxies with very high stellar densities,
discovered in the 2000s.
They are thought to be on the order of 200 light years across, containing about 100 million stars. It is theorised 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, and the Coma Cluster, amongst others.
In particular, an unprecedentedly large sample of ~ 100 UCDs has been found in the core region of the Virgo cluster by the Next Generation Virgo Cluster Survey team. The first ever relatively robust studies of the global properties of Virgo UCDs suggest that
UCDs have distinct dynamical
and structural properties from normal globular clusters. An extreme example of UCD is M60-UCD1, about 54 million light years away, which contains approximately 200 million solar masses within a 160 light year radius; its central region packs in stars about 25 times closer together than stars in Earth's region in the Milky Way.M59-UCD3 is approximately the same size as M60-UCD1 with a half-light radius, rh, of approximately 20 parsecs but is 40% more luminous with an absolute visual magnitude of approximately −14.6. This makes M59-UCD3 the densest known galaxy.
Based on stellar orbital velocities, two UCD in the Virgo Cluster are claimed to have supermassive black holes weighing 13% and 18% of the galaxies' masses.
^"Evidence for densest galaxy in nearby universe". Phys.org (Omicron Technology Ltd). 24 September 2013. Retrieved 25 September 2013. What makes M60-UCD1 so remarkable is that about half of this mass is found within a radius of only about 80 light years. The density of stars is about 15,000 times greater—meaning the stars are about 25 times closer to each other—than in Earth's region of the Milky Way galaxy.