(long, winding visible structure within a spiral galaxy)
A spiral galaxy's spiral arm is one of the long structures
visible with stars that wind around, forming the spiral.
They lead from the galaxy's central region (which may include
a bulge or bar) outward toward the edge of the galaxy's
disk.
They typically host star formation, thus include
very bright blue (early) stars,
which is what makes them stand out when the galaxy is observed.
A grand design spiral galaxy is one with 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:
Sagittarius-Carina Arm (aka Carina-Sagittarius Arm)
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 sometimes 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.