Transcript: The component of a star’s velocity to and away from the observer is called the radial velocity, and it’s measured using the Doppler Effect. In the Doppler Effect, when a source of waves is moving towards the observer the waves are bunched up in the direction of motion causing a blueshift. When the source of waves is moving away from the observer the waves are stretched out causing a redshift. The typical size of a radial velocity of a star in the solar neighborhood is about ten to twenty kilometers per second. As a fraction of the velocity of light, which is the way the Doppler Effect must be measured, this is a small percentage, less than one part in 104. So the measurement of radial velocity by the Doppler Effect requires a high precision measurement of less than 0.01 percent. If stars were moving randomly near the Sun, half of the stars would show blueshifts and half would show redshifts.