Galactic disk

A flattened, rotating component of a galaxy containing stars, gas, and dust. Its rotation and ongoing star formation distinguish it from a galaxy’s central bulge and extended halo.

Differential rotation: Rotation in which different parts of an object orbit at different angular speeds. Stars and gas at different disk radii orbit the galactic center at different rates.
Galaxy rotation curve: A graph of orbital speed versus distance from a galaxy’s center. It measures the disk’s rotation and reveals how mass is distributed within and around it.
Milky Way: The barred spiral galaxy that contains the Sun and Solar System. Its stellar and gaseous disk is the nearest detailed example of a galactic disk.
Galactic bulge: A dense, roughly spheroidal concentration of stars near the center of a galaxy. The bulge is centrally concentrated and less flattened than the surrounding disk.
Galaxy formation and evolution: The processes by which galaxies assemble, change their structure, and develop over cosmic time. Disk formation is one stage in the broader assembly and evolution of galaxies.
Angular momentum: A conserved physical quantity measuring rotational motion. A galaxy’s retained angular momentum helps spread collapsing matter into a rotating disk.
Dynamical friction: The loss of momentum by a moving massive object through gravitational interactions with surrounding matter. It can transfer orbital energy and angular momentum during disk formation and satellite interactions.
Hubble sequence: A classification scheme organizing galaxies by morphology, including spiral and lenticular forms. Disk prominence and spiral structure help distinguish major galaxy types in this scheme.
Galactic halo: An extended, roughly spherical region surrounding a galaxy’s luminous central parts. The halo’s diffuse stars and dark matter contrast with the disk’s flattened rotating matter.
Cold dark matter: A form of dark matter whose particles moved slowly relative to light in the early universe. In cosmological models, dark matter halos provide the gravitational setting in which disks assemble.

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