Irregular galaxies are those that cannot be classified as either ellipsoidal or spirals. They make up roughly 3% of all galaxies. These galaxies have little symmetry in their structure, and are made up of a steadily forming supply of small clusters of young blue stars. They have masses in the range 10⁸ to 10¹⁰ solar masses, diameters of 1 to 10 kiloparsecs, and blue magnitudes from -13 to -20 (fairly faint).

Some irregulars are believed to be low mass, low density galaxies which are less evolved than their more massive spiral neighbors. With less mass, the gravitational force contracting the galaxy is less, and it stays in a more diffuse state for a longer time. Irregulars typically contain a lot of hydrogen gas, which has not been used up yet in stellar burning in stellar cores. Many of the small dwarf galaxies which surround the Milky Way are classified as irregulars, including the Magellenic Clouds.

If we look back in time at distant galaxies, we find that the proportion of irregular galaxies increases and the proportion of spirals and elliptical galaxies decreases. This supports the theory that galaxies start out their lives in a more diffuse form, and over time they evolve to the more defined, compact states.

Observable Evidence	Implications
blue colours (B - V < 0.8)	young stellar population
low surface brightness	forming
highly asymmetrical	not a relaxed system
significant amounts of dust	gas resevoir present
emission line spectrum	current star formation
ordered rotation	formation via collapse
common in field	formation via collapse
few globular clusters	formation via collapse

M82 (Cigar)	Small Magellanic Cloud	Large Magellanic Cloud	NGC 6822
NGC 1313 (starburst)	IC 5152	NGC 4449	MDS (distant)
Patchy, irregular galaxies in the nearby and distant Universe. [NASA/HST, AAO]