together, which made the pitch get higher. as the siren moved away, the sound waves got "stretched" a little, which makes them further apart, which makes the sound lower. a real good way to get a picture of this is to use a slinky toy. hold one end of the slinky and give the other end to someone else and have them move a short distance away from you. you will notice that as the slinky stretches out, the loops get further and further apart. if the other person then moves back towards you, the loops get closer together. sound and light behave in the same way.
if a star is 