What characteristic distinguishes a spiral galaxy?

A spiral galaxy is characterized primarily by the presence of distinct arms that spiral outward from a central bulge. This structure is a result of the swirling motion of stars and gas within the galaxy, which creates the beautiful, well-defined spiral patterns that we often observe in images of spiral galaxies. The central bulge typically contains older stars, while the arms are often sites of ongoing star formation, inhabited by younger, blue stars and clusters of gas and dust.

The central region, often referred to as the bulge, is surrounded by these prominent spiral arms which extend outward and create the galaxy's recognizable shape. This distinguishing feature sets spiral galaxies apart from other types of galaxies, such as elliptical galaxies, which lack these defined arms and exhibit a more rounded, ellipsoidal shape.