The galaxy rendering process is outlined in a case study. The article describes the art and design approach as well as the technical implementation.
The code lives in Assets/scripts/DrawStars.cs. It is based around 3 instances of the SpiralGalaxy (attached to the milky_way_prefab in the galaxy_view_scene) which make up the stars and clouds and are rendered into textures to generate layers with different effects.
The galaxy is the result of the following 3 layers:
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Stars rendered last but first in the hierarchy.
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Cloud shadows which create the dark spots that can be seen when looking at the Galaxy from the side.
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Clouds that make up the fluffy blue nebulas that surround the stars.
The galaxy itself is rendered through Unity's DrawProcedural via the OnPostRender method. OnPostRender is being called by the scripts attached to the camera and is used via a RenderProxy script to trigger the rendering process.