Each nebula material, as well as their volumetric stars and the stars in the world material, can be animated using the "Phase" parameter(which makes use of the 4D "W" parameter of multiple nodes within each material).

Each type outputs a Volumetric Shader node, which can be combined with nodes such as the Mix Shader node, with either a numeric mix factor, or an image or procedural texture based factor for more interesting results. 

Reference: https://docs.blender.org/manual/en/2.79/render/cycles/nodes/types/shaders/mix.html