Pictures: I typed "galaxy pictures" into yahoo.com search.
"If you are on a train that is traveling north at 100km/hr. And you run toward the back of the train at 10km/hr you will hear the train whistle going away at 10km/hr. The 100km/hr is not important."
Because I can not explain how light works very well I tried to move over to sound. Hopefully you have heard the sound of a train horn. (car horn)
If the train is coming toward you it sounds higher. train)f
If the train is going away the sound is lower.
You can hear this effect by standing near a train, at the crossing.
This effect also works if the train is not moving but you are driving a can toward or away from a train.
differential motion! (from CowBoyBob)
Because we cannot know if our sun is moving, we can not know if the other sun is moving. We can see the effect of "getting closer or farther away".
To explain this; I used the running on a train story.
If we are going 1,000,000 and the other sun is also going 1,000,000 in the same direction there will be no color shift. 1,000,000-1,000,000=0
**broken link removed**