Question

A star's motion towards or away from us is detected by the ________ of the spectral lines.

Answer 1

A star's motion towards or away from us is detected by the wavelength of the spectral lines.
Because when a star is moving toward us or away from us, the apparent color does not change but there is a change in the wavelength of spectral lines, when the stars are moving towards us, the spectral lines are towards red end and when they are moving away from us the spectral lines are towards the blue end.