For stars with masses of about 10 times that of the Sun this continues for about ten million years. The core again contracts until it is hot enough for the conversion of carbon into neon, sodium and magnesium. This lasts for about 10 thousand years. No further energy can be obtained by fusion once the core has reached iron and so there is now no radiation pressure to balance the force of gravity. The crunch comes when the mass of iron reaches 1.4 solar masses.
Gravitational compression heats the core to a point where it endothermically decays into neutrons.
The core collapses from half the Earth's diameter to about 100 kilometres in a few tenths of a second and in about one second becomes a 10 kilometre diameter neutron star. A shock wave is produced which passes, in 2 hours, through the outer layers of the star causing fusion reactions to occur. These form the heavy elements. In particular the silicon and sulphur, formed shortly before the collapse, combine to give radioactive nickel and cobalt which are responsible for the shape of the light curve after the first two weeks. This rapidly expanding envelope is seen as the initial rapid rise in brightness.