If a galaxy lacks the shape of a spiral or of an elliptical, then it is usually lumped with a multitude of other odd-shaped galaxies into a group which Edwin Hubble labeled as "irregular." Despite the great variations between galaxies in the "spiral" classification and between those in the "elliptical" category, the shapes of the irregulars are undoubtedly even more randomly assorted.
Photo. The M82 irregular galaxy. Astronomers believe that the reason for this galaxy's irregular shape was perhaps a gigantic explosion in its nucleus. Courtesy of Carina Software, 12919 Alcosta Boulevard, Suite #7, San Ramon, CA 94583, (510) 355-1266. Used with permission.
Strangely enough, there are in truth several striking regularities among irregular galaxies, and most have much more in common with spiral galaxies than with ellipticals:
Photo. This is one of several "medium deep survey" images that show irregular galaxies with what NASA calls "peculiar" shapes. The blue areas indicate rapid star formation, a characteristic of these irregular galaxies. Courtesy R. Griffiths/Johns Hopkins University, Medium Deep Survey Team, NASA.