spiral arm

spiral galaxy NGC 2997

The spiral galaxy NGC 2997.

A spiral arm is a curved feature, containing young stars, open clusters, HII regions, and dust, that winds outward from the nucleus of a spiral galaxy into the disk. Usually two arms are present that wrap around in a well-defined pattern; however, some spirals have four or, in rare cases, three arms, in an arrangement that may complex and fragmentary. The arms are visible because of bright, massive stars, the formation of which is triggered by the movement through the disk of a density wave.