In order for that shiny halo to appear around HDR-colored materials, you need to add a Bloom [post-processing effect][1] to your scene.

How to do that and what settings that filter has available differs a bit depending on whether you use the Universal Render Pipeline, the High-Definition Render Pipeline or the build-in render pipeline.

For HDRP and URP:

1. Create a global post-processing volume in your scene with Volume -> Global Volume. 
2. Add a new Profile to that volume. 
3. Click on "Add Override..." and add the "Bloom" effect to that profile. Enable the "Intensity" property and set it to 1.0 for now. You can tweak it later.
4. Select your camera and enable the "Post Processing" checkbox.
5. Give your sun's material an HDR color for its emission property in the desired color, and crank up its intensity
6. You can tweak the emission color of your material and the bloom effect on your post-processing volume by configuring it according to the documentation for [URP Bloom][7] or [HDRP Bloom][8]

For the Build-in Render Pipeline:

1. Install the [post-processing package][3] via the [package manager][4]. 
2. Create an empty gameObject (your post-processing volume)
3. Set the [Layer][5] of that game object to a new layer "Post Processing"
4. Add the Post-Processing Volume Component to the game object (Rendering -> Post-process Volume) 
5. Check "Is Global" on the volume
6. Add a new Profile to that volume. 
7. Click on "Add Effect..." and add the "Bloom" effect to that profile. Enable the "Intensity" property and set it to 1.0 for now. You can tweak it later.
8. Select the game object with your camera and add the Post-Processing Layer Component to it [Rendering -> Post-process Layer][6].
9. On that post-processing layer, set the setting "volume layer" to your new layer "Post Processing"
10. Give your sun's material an HDR color for its emission property in the desired color, and crank up its intensity
11. You can tweak the emission color of your material and the bloom effect on your post-processing volume by configuring it according to the documentation for [Build-in RP Bloom][2]

From now on, remember to wear sunscreen while working on this scene :)


 [1]: https://docs.unity3d.com/Manual/PostProcessingOverview.html
 [2]: https://docs.unity3d.com/Packages/[email protected]/manual/Bloom.html
 [3]: https://docs.unity3d.com/Packages/[email protected]/manual/Installation.html
 [4]: https://docs.unity3d.com/Manual/Packages.html
 [5]: https://docs.unity3d.com/Manual/Layers.html
 [6]: https://docs.unity3d.com/Packages/[email protected]/manual/Quick-start.html
 [7]: https://docs.unity3d.com/Packages/[email protected]/manual/post-processing-bloom.html
 [8]: https://docs.unity3d.com/Packages/[email protected]/manual/Post-Processing-Bloom.html