How to make a simple space skybox with sun in Unity

 

In the Unity menu click Assets > Create > Material and in the material’s inspector, select Skybox > 6 Sided as shader. For all six texture slots you assign a space textures by dragging them into each slot. You can use this CC0-licensed public domain star sky texture for all of them if you are fine with subtly repeating stars. Open the texture’s import settings and set the Wrap Mode to Clamp and Apply.

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In your camera object properties, set Clear Flags to Skybox.  In Unity the menu, select Window > Lighting and select the material you created as Skybox under the Environmental Lighting section of the Scene section of the Lighting box.

Voilà! You have a starry skybox. To add a sun, create a lens flare object with Assets > Create > Lens Flare. For Flare Texture use a sun texture without alpha channel – for example this CC0-licensed public domain sun sprite I made. Set Texture Layout to 1 Texture, Size to 10, Color to white, and check Zoom and Fade.

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Add a lens flare component to your directional light with Add Component > Effects > Lens Flare. Select the lens flare you created in the Flare property and check Directional. You can set Fade Speed to 50. Now you should have a nice looking starry skybox with a sun that follows your directional light.