Motions of the Nearby Stars

Abstract

The motions of the nearby stars can be studied in especial detail since, if the stars are sufficiently close to admit a determination of the trigonometrical parallax, all three rectangular components of the velocity relative to the Sun can be calculated from the parallax, proper motions and radial velocity. For example if the stars are within 20 pc of the Sun, as in Gliese’s catalogue (1957), the parallaxes exceed 0″050 and are in good cases known within about 10%, and for stars as close as this the proper motions are in most cases reliable. Although a volume of radius 20 pc centred on the Sun is a small volume relative to the size of the Galaxy as a whole, the stars at present within this volume do not remain so throughout a galactic year. Many of them wander one or two kiloparsecs away from the Sun, so that the fraction of the Galaxy sampled in a survey of the nearby stars is much larger than the volume in which they find themselves temporarily, and is in fact from a statistical point of view a sampling of a reasonably large fraction of the outer parts of the Galaxy.