irregular galaxy

irregular galaxy A type of galaxy with ill-defined structure; Hubble type Irr or Ir. There are two main types in the Hubble classification. Irr I galaxies are not as massive as large ellipticals or spirals, often have a high gas content, and are undergoing star formation. The categories Irr⁺ and Irr⁻ are sometimes used to denote the degree of resolution of the structure of the galaxy into star clusters, H II regions, and other features, with Irr⁺ showing higher resolution than Irr⁻. The high gas content of Irr I galaxies implies that they may not have undergone much evolution since their formation. A subdivision of Irr I, the so-called Magellanic irregulars, symbol Im, resemble the Magellanic Clouds in the Local Group. The Hubble type Irr II denotes galaxies of unusual appearance that are classified as irregular simply because they do not fit into any other class, and may in many cases represent interacting or merging systems.