I'm doing a project on the Crab Nebula for my first-year astronomy class and I am finding the spectroscopy a bit puzzling. What I have read is that the normal emission line for a particular gas is split into two, one red shifted and one blue shifted. This of course is accounted for by the Dopler Shift, as parts of the gas cloud is approaching us and part is receding. What I cant fully comprehend is why the lines would be split in two and why wouldn't we see a wider band of colour. Is there not parts of the cloud that remain still, and parts that are receding or approaching faster than others?