 <unk> velocity is measured by the <unk> shift of the star 's spectral lines , and is given in units of km / s . The proper motion of a star , its parallax , is determined by precise <unk> measurements in units of <unk> @-@ arc seconds ( <unk> ) per year . With knowledge of the star 's parallax and its distance , the proper motion velocity can be calculated . Together with the radial velocity , the total velocity can be calculated . Stars with high rates of proper motion are likely to be relatively close to the Sun , making them good candidates for parallax measurements . 
