Like a real sun effect, which glows & gives a shiney effect
I'd suggest you look into dynamic lights first of all, to give you a feel of what Construct is generally capable of regarding lighting.
A glowing sun, well, that's usually done with a combination of effects and sprites. It depends on how you want it to look.
Something like the following
is simply done with sprites and the additive effect. Other useful effects for your purposes could be superBloom and Color Fusion.