The sun orbits around the galaxy. It takes about 225,000,000 years to complete one orbit at a distance from the centre of about 30,000 light years.
But there is something odd about the arrangement.
All the stars in the spiral arms take about the same amount of time to complete one orbit.
For planets orbiting a star, the further the planet is from the star, the slower the orbital velocity.
In the galaxy,
It makes little difference if the star is near the core or much further out than we are.
The reason for this is the galaxy is an extended object.
For any given star in the galaxy, there is a gravitational force due to the central bulge.
But there is also gravitational force due to stars in nearby galactic arms.
Because the arms are so much closer than the core, the arms actually contribute a big effect.
The result is all stars orbit the core in roughly the same amount of time.
This means that the extended arms tend to rotate as a unit. If the outer parts rotated more slowly, then the inner parts would rapidly "wind up" and leave the outer parts of the arms disconnected.
But what we observe (particularly in other spiral galaxies) is that the structure of individual arms is able to be followed from the core to the outer most part of the arm.