A spiral galaxy's spiral arm is one of the long structures visible with stars that wind around, forming the spiral. They lead from a central bulge or bar outward toward the edge of the galaxy's disk. They typically host star formation, thus include very bright, early, blue stars, which is what makes them stand out when the galaxy is observed.
A grand design spiral galaxy appears to have two symmetric spiral arms. Our limited view of the Milky Way (from the inside) has led to challenges in determining its arm structure. It is currently thought to have four major arms. Among the names used:
The Sun resides in a smaller arm called the Orion Arm, Orion-Cygnus Arm, Orion Spur, or Local Arm.
The term spiral arm is also used for spiral-shaped structures of disks such as circumstellar disks. These suggest gravitational instability, which may also be described as a factor in the development of galaxy spiral arms.