In the Effects tab of the Environment and Effects dialog, scroll up to the Lens Effects Parameters rollout. Choose Ring and move it to the list on the right. A ring appears around the model/object in the Effects Preview window.
Scroll down to the Ring Element rollout and make the following settings to define the ring:
Set Size to 22.0.
Set Thickness to 33.0, giving the ring more of a glowing perimeter.
Set Use Source Color to 50.0.
Adjust the ring effect:
By increasing the intensity of the main glow and juggling the size and thickness of the ring, you can control the size of the white-hot center of the sun.
Increase the intensity of the Ring Element to 133.0.