Figure: The Supernova phenomenon

During its life, a massive star (mass greater than 6 or 7 solar masses) is becoming like an onion, where the different concentric layers correspond to different fusion reactions. The outermost layer is burning hydrogen (H) to form helium (He), next, it's helium which is changing into carbon (C), then oxygen (O) is forming, and when we go deeper to the core, we find more and more heavy elements : neon (Ne), sodium, magnesium (Mg), silicon (Si), sulphur (S), nickel, cobalt and, at last iron (Fe).

Iron can't change into any other element, simply because there is no more energy available: it accumulates in the core, which at the same time fills with electron degenerate matter.

Outer layers are contracting, so the mass of the core is getting bigger and bigger, but it has no more energy to fight again gravity. When its mass reaches the critical Chandrasekhar mass, - this name comes from an Indian physicist - whose value is around 1.4 solar masses, it suddenly collapses, dragging along the outer layers of the stars.

This collapse generates a huge mecanical energy whose transfert to the outer layers results in the explosion of the star, producing one of the most luminous events known : the supernova.

Credit: http://nrumiano.free.fr/Estars/supernova.html