thin disk

The thin disk is a structure within the Milky Way consisting of a thin disk of stars near the galactic plane, basically extending across the entire galaxy. It is distinguishable from other stars for being a somewhat younger population. Overlayed is the thick disk, the names indicating the thick disk extends further from the plane than does the thin disk, consisting of stars generally predating the oldest stars of the thin disk. Thin-disk stars are also distinguished by the fact that their peculiar velocity is generally lower than the stars of the thick disk, and their metallicity is generally higher. Since the discovery of the Milky Way thin and thick disks, additional observation and analysis has revealed additional other structural divisions/subdivisions. 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. 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.

Note the term thin disk is also used for the alpha disk model, a model of disks such as accretion disks and circumstellar disks, modeling the case where they are sufficiently thin that their emitted radiation effectively control the amount of heat they hold.