Figure 1
Next time you hear a fire engine siren, listen carefully to how high the note is (its pitch). When the fire engine overtakes you, there will be a sudden drop in the note. This is caused by something known as the Doppler Effect . When the siren is coming towards you, the sound waves are squashed up, making the sound higher (Figure 1). When it is going away, the waves are stretched out and the sound is lower (Figure 2).
Figure 1
Figure 2
The same idea applies to light from galaxies, except it is the colour that is changed. For galaxies coming towards you, the light appears slightly blue. For galaxies going away, it appears slightly red. By looking at the spectrum of a galaxy, astronomers can work out exactly how much the light has been changed and so determine the speed of the galaxy away from the Earth.
* Click here to find out how the speed of a galaxy is related to its distance