irregular galaxy

The irregular galaxy NGC 4490. Image courtesy Richard Crisp

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).

Related category

• GALAXIES

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