why dont use a shpere, ..
then you use some effect like bright star image /sprite that gets from the sun, overlaying the sphere... or even some coronal ejection effects but with simples particles

http://www.youtube.com/watch?v=Pm1VutgxXkw&feature=fvsr

sun from XNA
http://www.youtube.com/watch?v=gkV3raj93_o

Celestia


here