I'm not 100% sure but I think it's because of the small aperture you're using (f/22), similar to when you want to get a starburst effect. Someone correct me if I'm incorrect.
I did google search and found similiar explanation as yours. Thank you very much! It is interesting to know.
Why are there more blades in the sunstar in colin's image that those of yjiang's? Surely the aperture doesn't have 14 blades.... isn't 6-8 blades the norm?
welcome to the club
http://photo.net/medium-format-photography-forum/001uPAFolks describing interference at edge of aperture are right, Light spreads out (diffracts) perpendicular to the edges of the blades, with star spikes at right angles to flat/straight edges of the aperture opening.
So (if you really care) a 5-blade iris will give a 10-point sun star, a symmetical 6 blade iris a 6-point star (because opposite edges are parallel, their points overlap), 7-blade iris a 14-point star, etc...And this effect may be less pronounced/more diffuse with rounded iris touted on some newer lenses