irregular galaxy

NGC 4490

The irregular galaxy NGC 4490. Image courtesy Richard Crisp.

An irregular galaxy is a galaxy, with a poorly-defined structure, that falls outside the categories of disk galaxy or elliptical galaxy. Only about 3% of observed galaxies fall into this category.


Two main types are recognized in the Hubble classification scheme. Type Irr I galaxies are less massive (108 to 1010 solar masses) than large ellipticals or spirals and often have a high gas content and show evidence of extensive star formation. Type Irr II galaxies make up a catchall group that may, in some cases, include interacting and merging systems.


Small irregular galaxies that appear to be completely non-uniform in shape are known as dwarf irregular galaxies.


A nearby example of an irregular galaxy is the Sagittarius Dwarf Irregular (SagDIG).