Spiral galaxies have three visible parts: a thin disk composed of stars, gas, and dust; a central bulge of older stars; and a spherical halo of the oldest stars and massive star clusters. The signature of these galaxies is an elegant spiral pattern in the disk. Disk stars move in nearly circular orbits along the same plane. Stars of the bulge and halo move in orbits at all angles to this plane.
Keywords: Astrophysics, Cosmic dust, Gases, Stars--Clusters, Stars--Orbits, Spiral galaxies