The Iron Peak

Beyond silicon, the increasing temperatures necessary to initiate fusion reactions signal a change in the way the fusion reactions take place (see the right panel).
Silicon Burning
At temperatures of ~ 3 x 109 K, silicon can burn to heavier elements in a complicated set of reactions that is similar to the way neon burns to magnesium. First the silicon is dissociated by a photon:

gamma ray + 28Si --> 24Mg + alpha particle

Then the alpha particles produced in such steps can undergo a whole sequence of reactions like

alpha particle + 28Si <--> 32S + gamma ray

alpha particle + 32S <--> 36Ar + gamma ray

.
.
.

alpha particle + 56Fe <--> 56Ni + gamma ray

where the vertical row of dots indicates that many reactions have been left out of the list. The notation <--> implies that the reactions are in near equilibrium, which means that they run at almost the same speeds when read left to right or right to left. For example, the rate for the capture of an alpha particle on silicon to produce sulfur plus a gamma ray is about the same as the rate for the reverse reaction of a gamma ray dissociating the sulfur back into silicon plus an alpha particle.

Nuclear Statistical Equilibrium
This complex sequence of reactions is called nuclear statistical equilibrium or NSE. It is also commonly just called "silicon burning", though the sequence of reactions involves many nuclei other than silicon. In such an equilibrium, the reactions are very complex and over a long period the elements that get produced in the most abundance are the ones with the largest binding energies (the most stable ones). Since these are the nuclei near iron, the net effect of silicon burning is to produce a complicated distribution of elements from silicon up to iron, with iron the most abundant. Thus, nuclear statistical equilibrium explains the concentration of elements in the iron peak of the elemental abundance curve.
Timescales for Advanced Burning
The following table illustrates the timescales, temperatures, and primary products of some of the advanced burning stages that we have just discussed in a star of 25 solar masses.

Burning Stages in a 25 Solar Mass Star
Nuclear Fuel Burning Products Ignition Temperature (K) Minimum Main Sequence Mass Period in 25 Solar Mass Star
H He 4 x 106 0.1 7 x 106 y
He C, O 1.2 x 108 0.4 5 x 105 y
C Ne,Na,Mg,O 6 x 108 4 600 y
Ne O,Mg 1.2 x 109 ~8 1 y
O Si,S,P 1.5 x 109 ~8 ~0.5 y
Si Ni-Fe 2.7 x 109 ~8 ~1 day
Masses in solar units. Table based on calculations by S. Woosley

Burning Timescales in Perspective

To set the burning timescales in the above table in perspective, imagine compressing the entire life of the star into a single year. Then it would burn hydrogen on the main sequence until December 7, it would burn its helium over the next 24 days, the carbon would burn in the 42 minutes before midnight, the neon and oxygen would burn in the last several seconds before midnight, and silicon would be converted to iron in the last 1/100th second of the year. And there would be quite a New Year's Eve fireworks display when the star went supernova on the stroke of midnight!

As indicated in the table, theory suggests that a star with about 8 solar masses or more can generate sufficient core temperature to go through all of these burning stages. Since these stages terminate in the production of an iron core that will become unstable if it exceeds the Chandrasekhar mass, we conclude that stars with 8 solar masses or more remaining at the ends of their lives are very likely to explode as type II (core collapse) supernovae.

Rapidity of Advanced Burning
Notice the remarkable speed with which a massive star such as this races through the last stages of burning. The 25 solar mass star spends 7 million years on the main sequence burning hydrogen and 500,000 years burning its helium, but it burns its carbon in 600 years, its oxygen in only 6 months, and burns its silicon to iron in about a day (before exploding as a supernova). To appreciate this timescale, consider the analogy in the above box.