Stellar Associations - Space Velocity

Space Velocity

The component of stellar motion toward or away from the Sun, known as radial velocity, can be measured from the spectrum shift caused by the Doppler effect. The transverse, or proper motion must be found by taking a series of positional determinations against more distant objects. Once the distance to a star is determined through astrometric means such as parallax, the space velocity can be computed. This is the star's actual motion relative to the Sun or the local standard of rest (LSR). The latter is typically taken as a position at the Sun's present location that is following a circular orbit around the galactic center at the mean velocity of those nearby stars with low velocity dispersion. The Sun's motion with respect to the LSR is called the "peculiar solar motion".

The components of space velocity in the Milky Way's Galactic coordinate system are usually designated U, V, and W, given in km/s, with U positive in the direction of the Galactic center, V positive in the direction of galactic rotation, and W positive in the direction of the North Galactic Pole. The peculiar motion of the Sun with respect to the LSR is (U, V, W) = (10.00 ± 0.36, 5.23 ± 0.62, 7.17 ± 0.38) km/s.

The stars in the Milky Way can be subdivided into two general populations, based on their metallicity, or proportion of elements with atomic numbers higher than helium. Among nearby stars, it has been found that population I, higher metallicity stars have generally lower velocities than older, population II stars. The latter have elliptical orbits that are inclined to the plane of the galaxy. Comparison of the kinematics of nearby stars has also led to the identification of stellar associations. These are most likely groups of stars that share a common point of origin in giant molecular clouds.

Within the Milky Way galaxy, there are three primary components of stellar kinematics: the disk, halo and bulge or bar. These kinematic groups are closely related to the stellar populations in the galaxy, forming a strong correlation between the motion and chemical composition, thus indicating different formation mechanisms. The halo may be further sub-divided into an inner and outer halo, with the inner halo having a net prograde rotation with respect to the galaxy and the outer a net retrograde movement.