From what I've seen, you'll need an actual VDB file to get access to the kind of Emission Mapping you probably want for a nebula. As far as I can tell, a simple Volume object won't do it. Take a look at the second half of this to get an idea of how emission mapping of VDBs works:
https://www.youtube.com/watch?v=eh3qG0JpMuE&t=892sThere are lots of free VDB files out there to experiment with while you're getting started. If you want a free VDB generator to make your own, you can try the free version of Houdini, or various paid particle/volumetric tools can generate them, like X-Particles and Real Flow.
Good luck!
(Edit) I just realized that we are still using 4.x here in our shop, and there have been a lot of volumetric rendering updates in V 2018, and 2019, so take what I said with a big pinch of salt.