Stellar kinematics

Lore & Background

Stellar kinematics involves measuring the component of stellar motion toward or away from the Sun, known as radial velocity, via the Doppler effect. The transverse, or proper motion, is found by taking a series of positional determinations against more distant objects. Once distance is determined through astrometric means such as parallax, the space velocity can be computed, representing the star's actual motion relative to the Sun or the local standard of rest (LSR). The Sun's motion with respect to the LSR is called the peculiar solar motion, with components (U, V, W) = (11.1, 12.24, 7.25) km/s, with noted statistical and systematic uncertainties. Kinematic measurements can identify exotic phenomena such as hypervelocity stars escaping from the Milky Way, interpreted as the result of gravitational encounters of binary stars with the supermassive black hole at the Galactic Center. Stellar kinematics is related to but distinct from stellar dynamics, which involves the theoretical study or modeling of motions under gravity. Stellar-dynamical models are often compared with stellar-kinematic data to study evolutionary history, mass distributions, and to detect dark matter or supermassive black holes. Examples of using kinematics combined with modeling include determining the rotation of the Milky Way's disc, identifying structural components (disc, halo, bulge/bar) each with distinct kinematics, and measuring mass distributions of galaxies through tracer objects like globular clusters.

Reader's Guide

Stellar kinematics yields important astrophysical information about stars and the galaxies in which they reside. By combining kinematic data with astrophysical modeling, astronomers can infer the gravitational potential and mass distribution of galactic systems. Measured stellar velocities in the innermost regions of galaxies have provided evidence that many galaxies host supermassive black holes at their center, while velocity measurements of globular clusters in halo regions provide evidence for dark matter. The field's significance was greatly enhanced by the Gaia Data Release 2, which provided a rich dataset of precise stellar kinematic and parallax data, contributing to a more nuanced understanding of the Milky Way's structure. Stellar kinematics remains essential for testing dynamical models and understanding the evolutionary history of galaxies.

Did You Know?

• Hypervelocity stars escaping the Milky Way are interpreted as the result of gravitational encounters of binary stars with the supermassive black hole at the Galactic Center.