disk galaxy

Disk galaxy is a term for disk-shaped galaxies (e.g., as opposed to elliptical galaxies), including spiral galaxies, and including galaxies that are generally disk-shaped, even if they have a bulge, such as lenticular galaxies. The term galactic disk refers to the galaxy's disk-like feature, often not intended to include the protrusions of any bulge. The disks of galaxies sometimes consist of overlaid disks of distinctive sets of stars, each such "sub disk" with its distinctive gas, dust, and kinematic characteristics. Within the Milky Way, a thick disk (of thick disk stars) has within it, a thin disk (of thin disk stars), the latter apparently younger, the result of more recent star formation.