It's a very easy oil painting trick, but it does require some preparation;
First paint the entire canvas black with black acrylic and let it dry completely. Then add the following oil paint colours wildly (as in; it doesn't have to be precise at all!): Indian Yellow where you want the sun to be, Alyzarin Crimson (red) in a ring around the yellow, followed up by a ring of Pthalo blue around the red. Use lot's of paint! Important stuff: these oil colours are transparent colours, meaning that when you put them on the black canvas it still looks very dark. With a clean brush blend all that together in circles starting in the yellow, working all the way outwards.
Then grab a small brush full of white and just smack some on wildly, starting in the yellow and working outward. When the brush runs out of white, wipe it quickly with some paper towel, load it up with more white, apply again. When you think there's enough white on it blend it all with a clean dry brush for added effect. The transparent colours mix with the white and will start to show very brightly.
I love this trick, it's really easy and needs very little precision.
Bob Ross shows and explains it way better in this video! https://www.youtube.com/watch?v=nox3vRVZUUs