Astronomy 121 Basics   The Doppler Effect

Electromagnetic radiation always travels through diffuse space at the same speed, the speed of light:
300,000 kilometers/second


The observed speed will not depend on relative motion.

However, the wavelength of the light does change with relative motion.

Stars moving away from us appear red-shifted and stars moving toward us appear blue-shifted.

This can be illustrated by considering the wave crests of the wave motion.