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 (known as Grand Design spirals), while in others the arms may be three or four in number and sometimes fragmentary (termed flocculent). The arms are visible because of the recent formation of bright, massive, short-lived stars within them. This star-forming activity is periodic, possibly corresponding to the movement through the disk of a density wave.