What ever method you choose, make sure that the method for destroying the star ties in with something else in the story that is NOT about destroying the star. For example, if the "bomb" is small enough to fit in a hand, then it could also be a (misunderstood) child's toy that figures in the story in a plot line that is not directly tied into the plot line in which the star is destroyed. But of course, when the star is destroyed with the child's toy, then this provides an opportunity to tie the two otherwise independent plot lines together. Oh. I think I'm going to cry. :)
Oh. That doesn't explain HOW to destroy the star.
How's this:
The star's destruction was assured when, a long time ago, the star was engineered (the engineering marvel remains unexplained) to remain stable in spite of being so supermassive that it should have immediately collapsed into a black hole. But -- thanks to the engineers who stabilized it -- it's a star. The also engineered a "thermostat" that needs an adjustment every 150 Million years. It got lost. It was recovered. It became a toy. Somebody figured it out and used it to destabilize the star. It collapsed. Ta Da!...
Oh darn. That's not "a bomb"