Redshift blueshift

High pitch The sound waves that are compressed are pushed together and therefore have a higher pitch.

Moving car As a police car drives past you with its siren on the sound waves are stretched and compressed.

Low pitch The sound waves that are stretched have a lower pitch, as the waves are further apart.

Redshift As the galaxy moves away, its light is more stretched as it moves to the red end of the spectrum.

Blueshift As a galaxy moves towards us its light is compressed and becomes bluer.

Moving galaxy A similar effect happens with a galaxy as it moves towards or away from us.