Hubble classified galaxies that do not fit neatly into his scheme of ellipticals and spirals as irregular (Irr) galaxies. Irregular galaxies as a class have no particular shape, and have no spherical or circular symmetries as the ellipticals and spirals do. There is a range of sizes, but irregulars tend to be small. They average about 20,000 light years in diameter. The smallest irregulars, dwarf irregulars, are only about 1,000 light years in diameter.
Because they are relatively small, irregular galaxies have small masses (typically about one million times the mass of the sun) and therefore relatively few stars. Astronomers now classify irregular galaxies into two groups, Irr I and Irr II. In Irr I galaxies, we can resolve young stars and evidence of ongoing star formation. In Irr II galaxies, we cannot resolve individual stars. They also have no distinct shape. Both types of irregular galaxies contain a large percentage of young stars and interstellar gas and dust.