You mean the star bloom/glare effect? Well…the thing is, Unreal doesnt really have a lot of options here. UE3 had the lens flare editor which was used for making cool sun discs together with flares as well but again, Unreal 4 doesnt support that editor yet.
I will make the sun look better in the future by using the FFT bloom (probably in the next video) but for a proper effect, you would need to build a custom sun flare particle system (something that I actually have done in the past) but thats waaaay too technical and complex for a regular lighting academy session