If observed from above, what would happen to stars in spiral arms over 500 million years?

The correct answer highlights the dynamic nature of stars within spiral arms of galaxies. Over a span of 500 million years, stars in spiral arms are not stationary; instead, they are in motion and pursue their orbital paths around the galaxy's center. As they orbit, stars encounter the density waves that create the visible structure of the spiral arms.

As stars move through these arms, they experience gravitational interactions that cause them to temporarily cluster together. This results in the "bunching up" effect observed in the spiral arms, as stars of various ages and types move in and out of these regions. The spiral arms are essentially areas of enhanced density where star formation is more likely due to the interplay of gravitational forces influencing the gas and dust.

Additionally, it's important to note that the nature of spiral arms is transient; because stars are continually moving, they will eventually pass through the arms and disperse into the surrounding areas of the galaxy. This movement contributes to the overall evolution of the galaxy’s structure over time.

Thus, the behavior of stars in spiral arms over a period of 500 million years illustrates the dynamic processes at play within galaxies, demonstrating that stars do not remain static but rather engage in continual orbital motion that impacts their distribution.