And then there were those Trilithium missiles used in Star Trek: Generations to make the star go nova...one of those strange things...
So it seems planetary and solar-system destruction is a common theme of science fiction.
Of course, the nasty effect of a nova or supernova is that, because of the intense radiation it throws off, it'd essentially kill everything within lightyears of its origin. So if Gal Civ II were to make a supernova effect, it'd have to be able to kill an entire small galaxy, essentially. x_x Hence why planet-by-planet destruction is preferable...it only destroys planets, not entire starsystems and even galaxies.
Of course, you could always throw realism to the wind there... XD