Astronomers classify galaxies by their shape into elliptical, spiral and irregular categories. Each group is further subdivided into more specific types, such as dwarf ellipticals or barred spirals.
Spiral galaxies are organized into distinctive whirlpool shapes with a central bulge, noticeable spiral arms and an irregular-shaped halo around them. Elliptical galaxies are oval in shape, and they tend to be brightest at their centers and gradually dim toward their edges. Elliptical galaxies are subdivided according to how elongated they are. Irregular galaxies are the most eclectic of all. They have no discernible interior organization or structure and their interiors are generally obscured by large amounts of dust.