Let's go back to our journey when all the core - at least
the biggest part of it- of the star has burned into carbon, but now let's assume
that the initial star has a mass greater than 6 or 7 solar masses. The carbon
core is able to collapse because of its own weight, and carbon in the star begins
fusion into magnesium. At this time, the inner temperature is greater than hundreds
of millions of degrees.
Contrary to what happens in a type Ia supernova, that we have previously seen, the dilatation and the heating of the star are able to regulate this fusion, and so to avoid an explosion.
Iron can't change into any other element, because it would need energy to transform, and there is not enough energy for this : 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 supernova is called 'type II', as opposed to the 'type I' what we have previously seen.
The fragments of the star are ejected at a speed which can be faster than 10.000 kilometres by second. They form a splendid nebula around the remnant of the star.
Such an explosion is powerful enough to briefly allow new fusion reactions in the iron core, permitting the generation of elements heavier than iron.
A great part of the elements that you can find on Earth, except hydrogen and helium, come from supernovae explosions.
The end of a massive star is a very fast process : if the fusion of hydrogen, as long as the star is on the main sequence, can last billions of years, all of the carbon is transformed in 10,000 years, all of the neon and the oxygen in one year, and the final transformation of silicon to iron requires only one day.
We must say that a type II supernova is a rare phenomenon : a rough
estimation is 0.6 supernova each year for 10 billion solar brightness, i.e.,
one supernova every 800 years in the Milky Way.
This means that, if we want to observe a hundred or so supernovae each year, we must observe a volume of about 40 cubic Megaparsec.