Thanks wilmorhu,
For the last shot the planet is a textured sphere and all the nebulae content is a texture with alpha, which I painted based on images of milky way. This texture is placed on cards with additive mode in the shader.
I placed a bunch of these cards, some in front of the sphere and rest behind it. Then I also changed colors of the same texture through my shader instance to add variation and also create sort of an explosion effect on the sphere planet.
In the middle of all this is a light which creates the volumetric rays when camera reveals the whole scene from behind the dish of Voyager.
I was planning to create some making but my motherboard crashed just one day before I had planned to release the film, so I will get to the making at some point once I can find an alternative.
Let me know if you have any more questions, glad to share what I learned 
Hope this helps.
Cheers