Irregular Galaxy: Definition, Facts & Names

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  • 0:01 Definition of an…
  • 1:00 Types of Galaxies
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Lesson Transcript
Instructor: Jeff Fennell

Jeff has a master's in engineering and has taught Earth science both domestically and internationally.

Galaxies come in many forms and shapes, and irregular galaxies are those that have no form or structure to them. Making up roughly 25% of all galaxies in the universe, they are important in understanding the evolution of galaxies. After this lesson, there will be a quiz to test your knowledge.


Galaxies come in many shapes and sizes. In 1921, Edwin Hubble set out to classify galaxies based on their shape. In doing so, two main categories of galaxies were created: spiral and elliptical.

Unlike a spiral or elliptical galaxy, irregular galaxies lack a distinct structure or shape. They are thought to make up about a quarter of all galaxies. Most irregular galaxies were once spiral or elliptical galaxies but were deformed by disorders in gravitational pull. Irregular galaxies contain large amounts of hydrogen that, in some cases, indicates new star formation.

The shape of an irregular galaxy does not fall into any of the regular classes of the Hubble sequence. While not classified in the Hubble Classification System, irregular galaxies are classified into three categories: Irr-I (Irregular one), Irr-II (Irregular two) and dIrr (dwarf irregular).


Irr-I type galaxies have an irregular shape, but contain some structure. However, their structure is not clean enough to place them into the Hubble sequence. These galaxies have a large abundance of hydrogen gas and many young or forming stars.

Irr-II galaxies have an irregular shape that does not appear to feature any structure that can place it into the Hubble sequence. Like Irr-I galaxies, Irr-II systems also contain large amounts of hydrogen gas but little to no apparent signs of star formation.

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