Doppler Effect
Waves of all kinds - light, sound, radar - appear to change their frequency as their source moves relative to an observer. Austrian physicist Christian Doppler was the first to analyze this property.

When waves are traveling toward you, they are compressed. When they are moving away, they are stretched out. You've probably heard evidence of the Doppler effect when a siren screeches by you; as it approaches the siren sounds higher pitched because the wavelength of the sound wave is getting shorter. As the siren recedes into the distance, it appears to have a lower pitch because the sound waves are getting longer. The Doppler effect can be perceived both when a siren is rushing by you and when you are hurrying past a stationary alarm.

When a wave source, such as a siren, is moving, the distance between waves appears to an observer to change even though the waves are emitted at constant intervals.

Astronomers use the Doppler effect to help determine how quickly stars are moving away from us. It is a vital tool as they analyze the expansion of the universe. It's also the basis for one technique planet hunters use as they search for planets orbiting other distant suns.

Red Shift
Astronomers use the redshift phenomenon to map the expansion of the universe. The idea is based on the Doppler effect. As space expands, the distance between all objects is stretched further and further. The expansion also stretches the wavelengths of starlight emitted millions of years ago. So the light from a distant star will appear redder to an earthly observer than it otherwise would. Remember, the longest wavelengths in the visible spectrum are for red light and the shortest are for blue.

The redshift phenomenon was discovered in the early 1900s, when astronomers noticed spectra for entire galaxies that differed from their expectations. They observed absorption lines on spectra that were shifted toward the longer wavelengths at the red end of the spectrum - so-called redshifted. The best explanation for this redshift was that the universe is expanding and so the wavelengths the photons were traveling on were stretching.

In this figure the top spectrum is for a more distant galaxy than the one below. Observe how the absorption lines are shifted toward the longer, red wavelength.

Astronomers are able to use the redshift to calculate how much the cosmos has expanded since the light left its source. The expansion (z) is exactly equal to the change in the expected wavelength of the light divided by the expected wavelength.

z = (observed wavelength - emitted wavelength) / emitted wavelength.

By looking at the redshifts for a number of supernovae, scientists can calculate a rate for the expansion of the universe and figure out how that rate has changed over the history of the cosmos.
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