The effect you looking for is called “Bloom” (it’s standard name of this effect). It’s impotent to know that Bloom effect reacts to color values higher then 1, the higher color output on material emission (unlit output) the more glowy it becomes:
post processing settings also you to set intensivity of bloom, threshold as well as color tint of bloom. Play around with settings as well as material of sky to emit more brighter pixels
If default bloom is not satisfactory, you would need to make your own post processing material:
Bloom at it simplest from is blanded/mixed (with original image) blur effect, so you might look at that