What is a characteristic feature of spiral galaxies?

Spiral galaxies are primarily characterized by their prominent spiral arms, which are made up of stars, gas, and dust. These arms emanate from the central bulge of the galaxy and can be seen distinctly in photographs, giving spiral galaxies their classic shape. The presence of these arms is a direct result of the density waves that propagate through the galaxy, triggering new star formation in the regions where the density of stars, gas, and dust is higher.

The other options do not describe spiral galaxies accurately. For instance, spiral galaxies are not elliptical in shape, nor do they consist predominantly of population II stars, which are typically found in older elliptical galaxies. Additionally, spiral galaxies actually contain significant amounts of gas and dust, which are essential for ongoing star formation, further distinguishing them from elliptical galaxies that may lack such materials. This characteristic of spiral arms is therefore key to identifying and understanding the structure and dynamics of spiral galaxies.