What is the immediate trigger mechanism causing the supernova explosion?

The gravitational collapse of the star's iron core

The supernova explosion itself is initiated by a specific, catastrophic event occurring deep within the star's interior. When the massive star exhausts its nuclear fuel, its core becomes composed of iron, which cannot undergo further energy-releasing fusion. This lack of outward thermal pressure allows gravity to utterly dominate. The core then undergoes a rapid and intense gravitational collapse, compressing matter to extreme densities in mere seconds. This violent compression generates a massive shockwave that rebounds outward, becoming the mechanism that violently ejects the star's outer layers into space, leading directly to the visible supernova explosion.