A galaxy's brilliant blue spiral arms look like permanent, physical structures. In reality, they are giant cosmic traffic jams sustained by the hidden mass of older, yellow stars.
This traffic jam phenomenon is described by Density Wave Theory. The arms are not rigid objects; they are regions of slightly higher gravitational density traveling through the galaxy's disk, moving at a different speed than the stars themselves. As stars and gas orbit the galactic center, they catch up to this dense region, get squeezed together, and eventually pass through the other side.
This is where yellow stars—like our Sun—and smaller red dwarfs do the heavy lifting. While the bright blue stars dominate the photographs, they are incredibly short-lived. Massive O and B-type blue stars are born when the density wave compresses gas clouds, but they burn through their fuel in just a few million years. They explode as supernovae before they ever leave the spiral arm, which is why the arms appear so brilliantly blue.
Yellow and red stars are the silent engine of the galaxy. They make up the vast majority of a galaxy's stellar mass. Because they live for billions of years, they orbit the galaxy dozens of times, routinely entering and exiting the spiral arms without dying.
It is the immense collective mass of these older, long-lived yellow and red stars bunching together that creates and maintains the gravitational density wave in the first place. Without the deep gravitational well provided by this underlying sea of older stars, the density wave would quickly dissipate, and the spiral structure would wind up and disappear. The blue stars provide the flash that makes spiral arms visible, but the steady, enduring gravity of yellow stars provides the infrastructure that keeps them stable.