You want to give your sun a bright, emissive color, and you may want to give it a color value that’s greater than 1.0. Values greater than 1 will cause bloom in post process, which makes it look brighter than it is.
So, for example, you could create a sphere, and give it the constant emissive color red=5, green=3, blue=0.5. Make the material itself be unlit, and make the mesh not cast shadows.