Milky Way Galaxy Space motionsastronomy

Space motions comprise a three-dimensional determination of stellar motion. They may be divided into a set of components related to directions in the Galaxy: U, directed away from the galactic centre; V, in the direction of galactic rotation; and W, toward the north galactic pole. For the nearby stars the average values for these galactic components are as follows: U = −8 km/sec, V = −28 km/sec, and W = −12 km/sec. These values are fairly similar to those for the galactic circular velocity components, which give U = −9 km/sec, V = −12 km/sec, and W = −7 km/sec. Note that the largest difference between these two sets of values is for the average V, which shows an excess of 16 km/sec for the nearby stars as compared with the circular velocity. Since V is the velocity in the direction of galactic rotation, this can be understood as resulting from the presence of stars in the local neighbourhood that have significantly elliptical orbits for which the apparent velocity in this direction is much less than the circular velocity. This fact was noted long before the kinematics of the Galaxy was understood and is referred to as the asymmetry of stellar motion.

The average components of the velocities of the local stellar neighbourhood also can be used to demonstrate the so-called stream motion. Calculations based on the Dutch-born American astronomer Peter van de Kamp’s table of stars within 17 light-years, excluding the star of greatest anomalous velocity, reveal that dispersions in the V direction and the W direction are approximately half the size of the dispersion in the U direction. This is an indication of a commonality of motion for the nearby stars; i.e., these stars are not moving entirely at random but show a preferential direction of motion—the stream motion—confined somewhat to the galactic plane and to the direction of galactic rotation.