Please help me with this sentence correction problem

were it not for the fusion-powered heat and radiation that rush from its core, instead its own weight would cause a star to collapse.

1. instead its own weight would cause a star to collapse.
2. instead a star would have collapsed under its own weight.
3. a star would have to be collapsing under its own weight.
4. a star would collapse under its own weight.
5. its own weight would have caused a star's collapse.

Somehow intuitively my answer was same as OA but i was not able to prove it grammatically.

OA is D