This is unfinished and certainly needs some tidying up but here is a solution to produce a true spiral by rotating the object coordinates around the origin based on the distance from the origin. As I say, this needs some refinement and tidying up - and for a convincing galaxy would need added noise displacement to make it much less regular. I'll try and refine this in the near future and post updates along with a full breakdown and explanation but thought I'd include it at is in case it's useful in the meantime.
