Red shift and blue shift in astronomy. Diagram of the Doppler effect, the change in the wavelength of radiation due to the motion of a source (galaxy at right) relative to the observer (Earth, left). At top, a dark vertical line shows the position of an absorption line when the Earth and the galaxy are relatively at rest. If the galaxy is moving away from Earth (middle), the wavelength of the emitted radiation stretches and the absorption line shifts towards a longer wavelength in the red part of the spectrum (red shift). If the galaxy is moving towards Earth (bottom), the wavelength is compressed and the absorption line is shifted into the blue part of the spectrum (blue shift).