In order for that shiny halo to appear around HDR-colored materials, you need to add a Bloom post-processing effect 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:
- Create a global post-processing volume in your scene with Volume -> Global Volume.
- Add a new Profile to that volume.
- 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.
- Select your camera and enable the "Post Processing" checkbox.
- Give your sun's material an HDR color for its emission property in the desired color, and crank up its intensity
- 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 or HDRP Bloom
For the Build-in Render Pipeline:
- Install the post-processing package via the package manager.
- Create an empty gameObject (your post-processing volume)
- Set the layerLayer of that game object to a new Layerlayer "Post Processing"
- Add the Post-Processing Volume Component to the game object (Rendering -> Post-process Volume)
- Check "Is Global" on the volume
- Add a new Profile to that volume.
- 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.
- Select the game object with your camera and add the Post-Processing Layer Component to it Rendering -> Post-process Layer.
- On that post-processing layer, set the setting "volume layer" to your new layer "Post Processing"
- Give your sun's material an HDR color for its emission property in the desired color, and crank up its intensity
- 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
From now on, remember to wear sunscreen while working on this scene :)