To do a good spiral galexy, you will want to have rig of emitters all rotating as a group. What this means is you will create a wheel of sorts where the emitters, perhaps 6 or so are all pointing out from the center, like a wagon wheel. As you emit, the particle streams will look like 6 streams all going out straight from the center. Then, if you group the emitters and rotate the group, you will get a good spiral effect. For stars, somehting that really really helps is to have varying sizes of particles. nParticles work best for this as there are easier controls in the nParticle than regular dynamics to do size randomness. I would also use your dynamics relationship to pick ONE of the particle nodes and reconnect it to all the emitters so you don't have to make any adjustments to all particle nodes. delete the other 5 particles.
Now, to up the ante a little bit, you could have "secondary" particles which act as even finer "dust" in the spiral. These particles can emit fluids to get that gassy look. There are tutorials on ways to do this which will take less time to explain than it will take to figure out what I'm typing. YouTube isn't coming up for me at the moment, but it should be easy to find a tutorial for emitting fluids from particles.
Learn the dynamics relationship box to add different particle types to the same emitter and you can create vast galaxies which will look great!
Good luck and save often.
Tom
