Stellar kinematics

Stellar kinematics is the branch of astronomy that observes and measures how stars move through space. This includes tracking stellar velocities within the Milky Way and its satellite galaxies, as well as the internal motions of stars in more distant galaxies. By measuring the kinematics of stars in different parts of the Milky Way—such as the thin disk, thick disk, bulge, and stellar halo—astronomers gather key clues about how our galaxy formed and evolved. Kinematic data can also reveal unusual objects, like hypervelocity stars that are escaping the Milky Way, likely flung out by gravitational interactions with the supermassive black hole at the Galactic Center.

This field is related to, but distinct from, stellar dynamics, which uses theoretical models to study how gravity shapes stellar motions. These models of galaxies or star clusters are often tested against real kinematic measurements to understand their history, map their mass, and detect dark matter or supermassive black holes through their gravitational pull on stars.

**Space velocity**

A star’s motion toward or away from the Sun is called radial velocity, measured by the Doppler shift in its spectrum. Its sideways motion, or proper motion, is found by tracking its position over time against more distant objects. Once a star’s distance is known—via astrometric methods like parallax—its space velocity can be calculated. This is the star’s true motion relative to the Sun or the local standard of rest (LSR), a point at the Sun’s location moving in a circular orbit around the Galactic Center at the average speed of nearby low-dispersion stars. The Sun’s own motion relative to the LSR is called the peculiar solar motion.

In the Milky Way’s Galactic coordinate system, space velocity components are labeled U, V, and W in km/s. U is positive toward the Galactic Center, V positive in the direction of galactic rotation, and W positive toward the North Galactic Pole. The Sun’s peculiar motion relative to the LSR is (U, V, W) = (11.1, 12.24, 7.25) km/s, with statistical uncertainties of (+0.69−0.75, +0.47−0.47, +0.37−0.36) km/s and systematic uncertainties of (1, 2, 0.5) km/s. (Note that V is 7 km/s larger than the 1998 estimate by Dehnen et al.)

**Use of kinematic measurements**

Stellar kinematics provides essential astrophysical information about stars and their host galaxies. When combined with modeling, these measurements reveal the structure of entire galactic systems. For instance, stellar velocities in the innermost regions of galaxies—including the Milky Way—have shown that many contain supermassive black holes. In outer regions like the galactic halo, the motions of globular clusters point to the presence of dark matter. Both cases rely on the fact that stellar kinematics is tied to the gravitational potential binding the stars: accurate velocity measurements of stars or clusters orbiting in a given region allow astronomers to infer the gravitational potential and mass distribution that drives their orbits.

Examples of using kinematics with modeling to understand astrophysical systems include:

- **Rotation of the Milky Way’s disk:** By combining proper motions and radial velocities of disk stars, astronomers observe differential rotation. Careful modeling of these measurements yields a picture of the disk’s rotation, with local rotation in the solar neighborhood described by the Oort constants. - **Structural components of the Milky Way:** Kinematic data help build models that explain the galaxy’s overall structure as distinct populations of stars. These populations often occupy specific regions—for example, the Milky Way has three main components (disk, halo, and bulge or bar), each with unique kinematics. These groups correlate strongly with stellar chemistry, suggesting different formation histories. For the disk, the average rotation speed is V = 220 km/s, with an RMS velocity relative to this of V_RMS = 50 km/s. Bulge stars have randomly oriented velocities and a larger RMS velocity.