Final answer:
Spiral arms are not permanent features of a spiral galaxy and can dynamically change over time.
Step-by-step explanation:
Spiral arms are usually the most prominent features in the disk of a spiral galaxy. However, one statement about spiral arms that is false is that they last for a very long time. Spiral arms are not static structures and can change and evolve over time. They are formed due to density waves that propagate through the galaxy's disk, causing regions of increased star formation and interstellar matter density.
These regions can eventually disperse and fade away, while new spiral arms can form. Therefore, spiral arms are not permanent features of a spiral galaxy and can dynamically change over time.
For example, the Milky Way galaxy, which is a spiral galaxy, has shown evidence of having different numbers and configurations of spiral arms throughout its history.