 Most people have had the experience of hearing a pitch of a car horn, train whistle, or ambulance siren drop off as the source moves past. As the sound source moves towards the observer, the sound waves are compressed, making the pitch of the sound higher. As the sound source moves away from the observer, the sound waves are stretched out, making the pitch of the sound lower. The same effect works for light. Here we have the visible spectrum from a star. Hydrogen in the star's atmosphere creates absorption lines with a unique pattern. Here's the pattern for a star at rest with respect to the observer. Light from an approaching star has its wavelengths shortened. We see that the lines shift to the blue. They are said to be blue-shifted. And light from a receding star has its wavelengths lengthened. We see the lines shift to the red. They are said to be red-shifted. The key to measuring the Doppler effect is to measure the change in position of the spectral lines. The further the shift, the faster the radial velocity. With this Doppler effect we can determine three important things about stars and the gases surrounding them. We can determine how fast stars and star material are moving towards or away from us. We can detect and measure orbital motion of binary stars. And we can even determine how fast a star is rotating.