As I recall, the reason a star is a star is because all the hydrogen and helium and other stuff that's in there are reacting, and exploding constantly. However, the amount of stuff blowing up in the middle is so immense that its own weight pulls it in. As soon as that mass is no longer heavy enough, the explosion expands. I don't fully understand it, but I think basically if you remove enough mass from the core, you can cause a premature supernova. This was of course learned from an episode of Stargate SG-1 and is therefore not totally likely to be real, though a good amount of that show's science has proven credible in the past...