Spiral arm

A curved region of enhanced stars, gas, and dust extending through a spiral galaxy’s disk. Spiral arms are patterns in the disk, not rigid structures made of the same stars.

Density wave theory: A model in which spiral structure is a wave of higher density moving through a galaxy’s disk. It explains how a spiral arm can remain prominent as its constituent matter passes through.
Spiral galaxy: A galaxy with a flattened rotating disk and a spiral pattern of stars, gas, and dust. Spiral arms are defining structures within this broader galaxy type.
Milky Way: The barred spiral galaxy that contains the Solar System. Its spiral arms are mapped indirectly from our position inside the disk.
Flocculent spiral galaxy: A spiral galaxy whose arms are patchy and fragmented rather than continuous. Its broken pattern contrasts with the long, coherent arms of grand-design galaxies.
Differential rotation: Rotation in which different parts of a system have different angular speeds. A galaxy’s inner disk rotates faster than its outer disk, shaping how spiral patterns wind.
Galactic disk: The flattened, rotating component of a galaxy containing stars, gas, and dust. Spiral arms extend through this component.
Andromeda Galaxy: A large spiral galaxy about 2.5 million light-years from the Milky Way. Its disk offers a nearby external view of spiral structure.
Barred spiral galaxy: A spiral galaxy whose central region contains an elongated bar of stars. A bar can organize or drive spiral structure, but it is distinct from an arm.
Swing amplification: A process that amplifies leading density disturbances into trailing structures in a differentially rotating disk. It can strengthen spiral features in a galaxy without requiring a long-lived wave.
Interstellar medium: The gas and dust occupying the space between stars in a galaxy. Its clouds and dust lanes trace material concentrated in spiral arms.

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