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, 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.
Formation of dwarf galaxies Current theory states that most
galaxies, including dwarf galaxies, form in association with dark matter, or from
gas that contains 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.
Because of their small size, dwarf galaxies have been observed being pulled
toward and ripped by neighbouring spiral galaxies, resulting in galaxy merger.
Local dwarf galaxies 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. 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. Common dwarf galaxy types
Elliptical galaxy: dwarf elliptical galaxy
Dwarf spheroidal galaxy: Once a subtype of dwarf ellipticals, now regarded as a
distinct type Irregular galaxy: dwarf irregular galaxy
Spiral galaxy: dwarf spiral galaxy Magellanic type dwarfs
Blue compact dwarf galaxies Ultra-compact dwarf galaxies
Blue compact dwarf galaxies In astronomy, a blue compact dwarf
galaxy is a small galaxy which contains large clusters of young, hot, massive
stars. These stars, the brightest of which are blue, cause 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.
Nearby examples include NGC 1705, NGC 2915, NGC 3353 and UGCA 281.
Ultra-compact dwarfs Ultra-compact dwarf galaxies are a
recently discovered class of very compact galaxies with very high stellar
populations. 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. An extreme UCD example is M60-UCD1, about
54 million light years away, which contains approximately 200 million solar
masses within a 160 light year radius and 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
apparent relative magnitude of approximately −14.6. This makes M59-UCD3
the densest known galaxy. Partial list of dwarf galaxies
See also Galaxy morphological classification
List of nearest galaxies Pea galaxy
References External links
Milky Way Satellite Galaxies SPACE.com article on "hobbit galaxies"
Science article on "hobbit galaxies"