thick disk

The thick disk is a structure within the Milky Way consisting of a distinguishable set of stars that extends further from the galactic plane than the majority. Overlayed is the thin disk, a larger set of stars that forms a thinner disk-shaped region that doesn't extend as far from the galactic plane. Thick-disk stars are distinguishable for being lower in metallicity and with a higher peculiar velocity, and for being generally older stars. The Milky Way thick disk was first distinguished as a population of stars with a distinct set of characteristics and the term thin disk is for those stars across the disk that are not part of it. Since the discovery of the Milky Way thin and thick disks, additional observation and analysis has revealed other distinct structural divisions/subdivisions. Similar distinctions, including thin and thick disks, have been detected in other disk galaxies as well.

A general theory is that at one time, the Milky Way consisted of a single disk, and an event such as a galaxy merger with a dwarf galaxy caused the stars to speed up and spread further from the galactic plane, and the gas added by the merger triggered star formation of a new disk of stars at the plane.