Do you mean you bounce off it? There is no shield, it's an object. The reason you can bounce off it (just like planets, stations, ships etc) is because there is no death zone around it (so normally you'd die before you could hit a suns core).

Freelancer specifies areas of damage - around plants, suns and also radiation damage zones too. That particular sun just doesn't have it's damage zone - hence why you can fly right up to it.