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A spiral arm is a region of a galaxy made of relatively densely-packed stars and interstellar gas and dust, in a curved formation caused by the movement of density waves. Stars move in and out of spiral arms; the most common way to explain this is like a traffic jam, where cars will enter and leave but the jam persists.

Spiral galaxies, barred spiral galaxies, and irregular spiral galaxies (to some extent) have spiral arms or arm-like structures. Most galaxies have only a few; the Milky Way is thought to have four main arms plus some number of minor ones.