Dwarf galaxies are small, relatively speaking, compared to larger spiral or elliptical galaxies. They typically have a diameter between 100 and 30,000 light-years, with a low luminosity and few stars. Dwarf galaxies can be found in groups or clusters around larger galaxies, such as the Milky Way, and are often referred to as "substructures" within these systems. They have lower metallicity than their larger counterparts, meaning they contain fewer elements heavier than helium. This is thought to contribute to their unique characteristics, including a higher proportion of older stars and a lack of ongoing star formation compared to spiral galaxies.