Question:
In class I compared the red shift of distant stars to the drop in sound pitch when a car drives past you. Explain why these two phenomena are related.

Answer:
Both are due to the Doppler shift in wave frequencies for objects moving either toward or away from you.

With sound, the waves are closer together when the car is moving toward you, so the pitch is higher. The waves are farther apart if the car is moving away from you, so the pitch is lower.

With light, the same thing happens, but with color. If stars are moving toward you, the light waves are closer together and their color is shifted toward the blue. If stars are moving away from you, the light waves are farther apart and their color is shifted toward the red.