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You are looking at two sets of stars and gas that have passed by each other.  The effects are complicated and material often gets drawn out of one of the galaxies. Also we are seeing a flat image of a three dimensional event. 
Jonathan Ransom 2 on 15 May 2006 at 23:14 +0000 wrote:
David A. Edwards writes:
>In this case, as you can see from the M51 images, they are physically
>connected by a trail of stars.  These two galaxies are so close they are
>messing each other up, gravitationally.

Hummm, maybe it's my eyes but I'm not sure I can see the attaraction here. I see the trail, but not sure that they are gravitaionally atracted. If they were gravitationally attracted wouldn't M51 take material from the south and east as well as the north?

Jon