What primary feature distinguishes spiral galaxies from other galaxy types?

The defining characteristic of spiral galaxies is their prominent spiral arms. These arms are regions where stars, gas, and dust are more concentrated, leading to a vivid, spiral structure that can be observed from a distance. This formation contrasts with other types of galaxies, such as elliptical galaxies, which lack such defined structures and are generally more rounded and featureless.

In spiral galaxies, the arms are sites of active star formation, which makes them visually striking and rich in luminous young stars. The presence of these arms is a result of differential rotation within the galaxy, where different parts of the galaxy rotate at varying speeds, creating the spiral pattern.

The existence of a central black hole is common in many galaxy types, not just spiral galaxies, making it not a distinguishing feature. Additionally, while older stars are present in spiral galaxies, they coexist with young, bright stars, which are more prominent in the spiral arms. As for quasars, they are extremely bright objects found at the centers of some galaxies, but their presence is unrelated to whether a galaxy is spiral or not. Hence, the most distinctive trait of spiral galaxies remains their prominent spiral arms.