Positions in the spectrum are referred to by colour, or more precisely by wavelength. Just as our ears interpret the different lengths of sound waves as pitches, so our eyes interpret the different lengths of light waves as colours, light waves being longer towards the red end of the spectrum.
In 1842 Christian Johann Doppler (1803-1853) of the Prague Polytechnic (est.1707) postulated the Doppler Effect – the change that occurs in the pitch of sound or the colour of light when the source is approaching or receding from the hearer or observer. The frequency of light is measured as the number of wave peaks that pass a reference point each second. The light from an approaching star has its waves squeezed together, which increases its frequency and shifts it towards the blue end of the spectrum. Similarly, light from a receding star has its waves stretched, reducing its frequency and shifting it towards the red end of the spectrum. Redshift and blueshift were first successfully described in 1848 by Armand-Hippolyte-Louis Fizeau (1819-96).
The amount of shift is directly related to the speed at which a star is increasing or reducing its distance from us (radial velocity). A star is virtually certain to be moving across the line-of-sight as well (proper motion) so its true velocity is found by adding the two components together vectorially.
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