Which type of galaxy is characterized by a flat disk with spiral arms?

A galaxy characterized by a flat disk with spiral arms is classified as a spiral galaxy. This type of galaxy features a central bulge surrounded by a distinct disk where the spiral arms extend outward. These arms are made up of stars, gas, and dust, and they tend to have ongoing star formation, which gives them a bright appearance. The flat disk structure is fundamental to spiral galaxies, allowing us to distinguish them from other types such as elliptical or irregular galaxies.

Elliptical galaxies, in contrast, do not have the flat disk and spiral structure; they are more rounded and lack the distinct arms. Irregular galaxies, as the name suggests, do not have a defined shape like spirals and often appear chaotic. Lenticular galaxies possess a central bulge and a disk as well but lack the prominent spiral arms characteristic of spiral galaxies. These differences highlight why the correct answer focuses on spiral galaxies specifically for their defining feature of having a flat disk with spiral arms.