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