Stars live on the edge. Throughout their life, they constantly walk a line between blowing themselves apart and gravitational collapse. Gravity is constantly trying to pull inwards on the mass of the star. Without a counter force, the star would collapse. As gravity causes collapse, the gas in the core is heated to tremendous temperatures. Eventually, the temperature is so high, and atoms are moving so fast that the natural repulsion of protons, the Coulomb force, is overcome and the strong nuclear force dominates and allows fusion to occur. The energy released by these reactions pushes outwards and balances the force of gravity which keeps the star stable.

As stars age, the fusion reactions that keep the star stable get increasingly complex. Starting with the most simple form of fusion, hydrogen fusion, new elements are created and build up in the core. As the reactions progress, their by-products begin to build up in the core and eventually choke the reaction like soot in a combustion reaction. As the reaction is choked off, the core cools because the reactions are no longer as efficient as they once were. Gravity gains a foothold and the fragile balance is broken. The core begins to collapse.

As the core starts to collapse, it squeezes the matter in the core causing the temperature to rise. Eventually, the temperature rises so high that the by-products from the previous reaction is able to fuse. This in turn causes the cycle to begin again whereby heavier elements are forged, they eventually build-up, choke the reaction, collapse ensues, heating results, and new fusion occurs. The cycle continues until the core produces iron which is incapable of fusion. Towards the end, the core resembles the layers of an onion with different fusion reacions taking place in shells around the iron core. Created by Jeff Bryant (jeffb@wolfram.com)

Some References
www.earth.uni.edu
www.phys.lsu.edu
ircamera.as.arizona.edu