Complex sentence: Theoretical studies of many supernovae indicate that most are triggered by one of two basic mechanisms: the sudden re-ignition of nuclear fusion in a degenerate star or the sudden gravitational collapse of a massive star's core.
Split into simpler sentences: Theoretical studies of many supernovae show that they can be triggered by one of two basic mechanisms. This can be by sudden re-ignition of nuclear fusion in a degenerate star; or by the sudden gravitational collapse of the massive star's core.