What component of a star's motion is measured by the Doppler shift, known as radial velocity?
Answer
Motion directly along the line of sight connecting the star and the observer.
The Doppler effect measurement yields the radial velocity, which specifically quantifies the motion occurring directly along the line of sight between the star and the observer. This means it measures how quickly the distance between the two points is increasing or decreasing. It does not capture any motion that occurs perpendicular to this line of sight. That perpendicular motion, referred to as proper motion, must be calculated separately by tracking the star's angular position across the sky over extended periods, such as decades or centuries.
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