A dwarf galaxy is a gravitationally bound system that contains far fewer stars and less mass than a large spiral or elliptical galaxy. Whereas giant galaxies such as the Milky Way hold hundreds of billions of stars, dwarf systems typically contain from a few million up to several billion stars. For general context on galaxy classification and properties, see related resources.
Characteristics and types
Dwarf galaxies come in several morphological types. Common categories include dwarf spheroidal galaxies (small, low-luminosity, and often devoid of gas), dwarf irregulars (gas-rich and actively forming stars), and dwarf ellipticals (compact and more structured). Many are dominated dynamically by dark matter, meaning their visible stars account for only a fraction of the total mass. Stellar populations can be ancient and metal-poor, or in some dwarfs a mixture of old and younger stars. Broad surveys that count stellar populations and luminosities provide the basic measures used to classify them; for typical stellar number ranges see further information.
Origin and evolution
Dwarf galaxies are thought to be among the first galactic systems to form in the early universe and frequently act as building blocks in hierarchical models of galaxy formation. They may form in isolation or arise as satellites captured by larger galaxies. Interactions with a massive companion can strip gas and stars, transform morphology, or even lead to the dwarf's disruption and assimilation into the larger halo. Their small sizes and shallow gravitational wells make them especially vulnerable to environmental effects such as tidal forces and ram-pressure stripping.
Scientific importance
Because dwarf galaxies are numerous and relatively simple compared with giant spirals, they are critical laboratories for several areas of astrophysics. They provide clean tests of dark matter models: internal motions of stars and gas in dwarfs can reveal the distribution of unseen mass. Dwarfs also inform studies of star formation in low-metallicity environments, chemical enrichment over cosmic time, and the reionization era when the first small galaxies may have contributed ionizing photons to the intergalactic medium.
Observation and notable examples
Detecting faint dwarfs presents observational challenges; many are discovered as faint smudges in deep imaging or by identifying concentrations of resolved stars. Well-known nearby examples include the satellite dwarf spheroidals and irregulars that orbit the Milky Way and Andromeda. The Large Magellanic Cloud is sometimes treated as a dwarf galaxy and sometimes as a full galaxy, reflecting a borderline size and complexity; for relations to the Milky Way, consult additional notes. Lists of named dwarfs include the Fornax, Draco, and Sagittarius dwarf systems among others.
Distinctions and ongoing research
Notable distinctions concern size, luminosity and whether the object is a self-contained galaxy or a tidal fragment. Contemporary research uses deep imaging, spectroscopy, and numerical simulations to map dwarfs' dark matter content, star-formation histories, and interactions with larger hosts. As surveys grow more sensitive, astronomers continue to find fainter and smaller dwarfs, refining our understanding of how galaxies assemble across cosmic time.