What shape is characteristic of a spiral galaxy?

A spiral galaxy is characterized by its distinct 'swirly' shape, which consists of a central bulge surrounded by a flat disk of stars, gas, and dust extending outward in spiral arms. These arms curve outward and are reminiscent of a swirl, which is where that descriptor comes from. This unique structure is the result of galaxy formation and evolution processes, including the gravitational forces acting on the stars and gas as well as angular momentum.

Round, flat, and irregular shapes do not adequately capture the essence of spiral galaxies. While a galaxy may have a central bulge, it is the spiral arms that give a galaxy its characteristic swirly appearance, which is not reflected in those other options. Thus, the best descriptor for the shape of a spiral galaxy is indeed 'swirly.'