A curved structure in the disk of spiral (and some irregular) galaxies where young stars, clusters, nebulae (H II regions), and dust are concentrated. Some galaxies have a well-defined, two-arm spiral pattern, while in others the arms may be three or four in number and sometimes fragmentary. The arms are visible because of the recent formation of bright, massive, short-lived stars within them. This star-forming activity is periodic, corresponding to the movement through the disk of a density wave.
Subjects: Astronomy and Astrophysics.