Answer:
The radial velocity of a star is measured by the Doppler Effect its motion produces in its spectrum, and unlike the tangential velocity or proper motion, which may take decades or millennia to measure, is more or less instantly determined by measuring the wavelengths of absorption lines in its spectrum.
Using a wavelength
The change in wavelength is proportional to the relative velocity v in the line of sight according to the formula: (λ − λ) λ = v c where λ is the rest wavelength observed when there is no relative motion of the source, λ’ is the wavelength from the moving source and c is the speed of light.
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