Hello. Generally, stars are in orbit around the center of the galaxy. The motions of the stars can be detected in two ways. How fast a star is moving along the line of sight is shown by Doppler shifts in their spectra. Also, by comparing photos taken many yrs. apart, motion across the line of sight is detectable. Thus, proper motion is the rate at which a star moves across the sky relative to the rest of the stars. The rate of movement is quite small. It`s usually less than .1 secs. of arc/yr. For ex., photos taken 10 yrs. apart reveal the proper motion of Bernard`s star which is one of the sun`s nearest neighbors at only 1.8 pc away. Thus, the measurement of proper motion is often combined with radial velocity and distance to reveal the shape of a star`s orbit.

• Re: Where/How do I find Castor's whole space velocity vector - Strugglin(still)... - February 23, 2000 - 04:05 UTC