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You might want to look at Graphics.DrawMesh for the asteroid field. This enables you to draw geometry in bulk without the overhead of a GameObject each time.
Hey this is what I did to simulate this effect. Basically take a 2 face plane, use a star texture with an alpha channel on the plane, then copy and paste many stars into one batched game object and batch all your stars with combine mesh script, then youd have to manually place the stars in your universe sphere 
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