Stellar kinematics

Stellar kinematics is the observational study of how stars move through space. Astronomers use this field to measure the velocities of stars within the Milky Way and its satellite galaxies. They also study the internal motions of more distant galaxies. By tracking these movements, scientists can understand how galaxies form and evolve over billions of years. This study is different from stellar dynamics. Stellar dynamics is the theoretical modeling of how gravity influences motion. While dynamics uses math to predict movement, kinematics uses direct observations to see what is actually happening.

To find a star's full space velocity, astronomers must measure its motion in different directions. First, they measure proper motion, which is the star's movement across the sky. They find this by comparing the star's position against much more distant objects over time. Second, they measure radial velocity, which is the speed at which a star moves toward or away from the Sun. This is found by looking at the Doppler effect, which causes shifts in a star's light spectrum. Once the distance is known through parallax, the total space velocity can be calculated. This speed is measured relative to the Sun or the Local Standard of Rest (LSR).

The Milky Way is organized into distinct parts with unique motions. The stellar disk contains stars that move in a predictable way. The bulge or bar at the center contains stars with randomly oriented velocities. The stellar halo consists of stars with orbits that reach the outer edges of the galaxy. Some halo stars belong to globular clusters that formed a long time ago. The halo can even be split into an inner and outer section. The inner halo has a net prograde motion, while the outer halo moves in a net retrograde direction.

Measuring these motions helps scientists discover hidden parts of our universe. If we know the velocity of a star, we can infer the gravitational potential of its region. This allows us to map the mass distribution of a galaxy. For instance, fast-moving stars near galactic centers provide evidence for supermassive black holes. In the outer halo, the movement of globular clusters provides evidence for dark matter. Even in distant galaxies, astronomers use the motion of entire stellar populations to find mass distributions. They look for redshifted or blueshifted regions to understand how a galaxy rotates.

Stars can also be classified by their chemical makeup, known as metallicity. Population I stars have higher metallicity and are usually found in the stellar disk. Population II stars are older and have lower metallicity. These older stars often follow elliptical orbits that are tilted away from the disk. Astronomers also find stellar associations, which are groups of stars sharing a common origin. These stars likely formed together in giant molecular clouds. Studying these groups helps us understand the history of star formation in our galaxy.

Some stars are much faster than their neighbors, known as high-velocity stars. A star is considered high-velocity if it moves between 65 km/s and 100 km/s relative to its surroundings. One type is the runaway star, which moves abnormally fast through the interstellar medium. These stars might be ejected from a group due to gravitational interactions in a three-body system. Another type is the hypervelocity star. These stars may be moving so fast that they escape the Milky Way entirely. Such stars can result from gravitational encounters with the supermassive black hole at the Galactic Center.

Modern technology has revolutionized this field through the Gaia mission. In 2018, the Gaia Data Release 2 provided a massive dataset of precise measurements. This release helped determine the proper motions of 75 globular clusters at great distances. The Gaia DR3 release further improved this data with detailed astrophysical parameters. These advancements help us study white dwarfs and the merger history of our galaxy. New data allows us to see how stars will move even hundreds of thousands of years into the future.