re: Besides, it's already been done...
Clarke ignited Jupiter by dumping gigatons of mass into it in the form of millions of magically-produced monoliths, which--if you ignore the source of the mass--is at least scientifically possible. Forty pounds of plutonium ain't going to do the trick, no matter how much you compress it.
I forget how much mass you'd have to add to Jupiter to "ignite" it, but I know its many times the current mass of the planet.