Answer: They are typically smaller than spiral galaxies.
Some show evidence for prior collision or close encounter with another galaxy.
They exhibit vigorous star-forming activity.
Irregular galaxies are typically smaller than spiral or normal ellipticals, of irregular form, and without spiral arms. Irr I galaxies are often characterized by lots of gas, dust, and associated star formation. Irr II galaxies have forms that indicate prior collisions or close encounters with other galaxies. Irregular galaxies are one of the most common types of galaxies.
