5D Mk III, EF 24-70 f/2.8 L II, f/13, 70mm, 1/80th sec
Adding a sun star or a star from a street light: Basically, you need to stop your lens down, if you have the sun in your scene you need to stop down to f/16 to get the full effect, unless you have some structure like a branch or mountain obstructing the surrounding area of the sun, in the case of the cross above, I positioned the sun right behind the opening in the cross and stopped down to f/13 and this gave me the best of both worlds.
Having a lens with a nine bladed aperture helps as well, the above star has 18 points due to the nine bladed aperture, another benefit of a quality lens.
You can see the effect on the bridge lights, I didn't have to stop all the way down to f/16 because of the dark background.
A partial star here at f/11
Images In Light