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dwarf galaxyA galaxy that is unusually faint either because of its very small size, its very low surface brightness, or both. Since galaxies exist in a continuous range of sizes from the giant ellipticals downward, the dividing line between average and dwarf is somewhat arbitrary. Since no spiral or S0 galaxies have been observed with total magnitudes below –16, this is often used as a convenient demarcation line. They contain only a few million stars and are very difficult to observe against foreground stars because they are almost completely transparent. Dwarf galaxies may make up the bulk of the cosmic population and occur in all morphologies except as spiral galaxies.
The dwarf irregulars (dI) are the most numerous of these galaxies, and contain a significant fraction of their mass as neutral hydrogen gas in a dark halo (see also LSB galaxies). Dwarf ellipticals (dE) are dominated by metal-poor halo stars, and their lack of gas or dust suggests that any star formation occurred a long time ago in these systems. In contrast, the gas-rich blue compact dwarf (BCD) galaxies are undergoing active star formation with sizes and spectra resembling giant H II regions. See also extragalactic H II region.