Trendy galactic poster design

Merge this pink spot with a new blank layer (CMD+E) to reset the blending mode back to Normal, then change the mode to Color Dodge. Change the Opacity to around 60% to tone down the effect.

On a new layer, draw a spot of white using a soft brush. Grab the mask tool and delete away half of the circle.

Press CMD+T to transform the semi-circle, then stretch and squash it into a cast of light. Rotate it by 45% (hold Shift) and move into position on the design.