Historically, galaxies have been categorized according to their
apparent shape (usually referred to as their visual morphology). A
common form is the elliptical galaxy, which has an ellipse-shaped light profile. Spiral galaxies are disk-shaped assemblages with dusty, curving arms. Galaxies with irregular or unusual shapes are known as irregular galaxies,
and typically result from disruption by the gravitational pull of
neighboring galaxies. Such interactions between nearby galaxies, which
may ultimately result in galaxies merging, may induce episodes of
significantly increased star formation, producing what is called a starburst galaxy. Small galaxies that lack a coherent structure could also be referred to as irregular galaxies.