Hi Phil,
The spiral arms start as density waves in the galactic disc, caused by gravitational effects (watch out: not to be confused with “gravitational waves”, which are ripples in spacetime itself, something totally different!). A local increase in density tends to attract more mass (mostly in the form of gas and dust clouds, but also already formed stars), further increasing the density. That increased density spurs on the formation of new stars, which are very bright and blue of color, which is why spiral arms are generally bright and blue (seen in the picture). However, in simulations these stars don’t usually “die” in the spiral arm where they are born. Instead, they move on and travel ahead of the spiral arms, just as cars manage to leave behind the traffic jam on the highway at some point. Hope that helps to make sense of it.
Casper