Stellar kinematics

Stellar kinematics describes stars’ positions, velocities, and motions without requiring a specific account of the forces that produce them. It reveals the structure and history of stellar populations in the Milky Way.

Radial velocity: An object's velocity component along the line of sight, commonly measured from spectral Doppler shifts. It supplies the line-of-sight component missing from proper motion.
Six-dimensional phase space: A representation of an object by three spatial coordinates and three velocity components. It captures a star’s full position and velocity when distance and measurements permit.
Gaia (spacecraft): A European Space Agency space observatory that measures positions, parallaxes, and motions of stars. Its catalog provides precise astrometry for millions of Milky Way stars.
Stellar dynamics: The study of stellar systems through the gravitational forces and interactions governing their motion. It interprets the motions that stellar kinematics describes without specifying their causes.
Parallax: The apparent shift of a nearby object against distant background objects as the observer's position changes. Distance from parallax converts angular proper motion into transverse speed.
Galactic coordinates: A spherical coordinate system centered on the Sun and aligned with the Milky Way's plane. They express stellar positions and motions relative to the Galactic disk.
Stellar population: A group of stars defined by shared properties such as age, composition, or Galactic location. Distinct populations often have characteristic velocity distributions.
Galactic dynamics: The study of the structure and evolution of galaxies using gravitational physics. It connects measured stellar motions to the Milky Way’s mass and evolution.
Doppler effect: The change in observed frequency caused by relative motion between a source and an observer. Spectral Doppler shifts are the basis of stellar radial-velocity measurements.
Oort constants: Parameters describing the local angular rotation and shear of the Milky Way's disk. Nearby stellar velocities constrain the Galaxy’s local rotation pattern.

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