sirdnA

Fusion occurs when atoms are pushed together with such force that they 'fuse'. It takes energy to do this, which is supplied by the pressure of gravity inside a star. This causes Hydrogen (the lightest element) to fuse to Helium (the second lightest). A byproduct of this is more energy, which pushes against the gravity and creates light, heat, etc. Just as burning wood releases energy to create heat and light as well (but burning is a different process than fusion). The energy released by fusion is greater than the energy needed to create it. Fusion is one of the most efficient forms of energy release we know of.

Eventually, the Hydrogen runs out, and the energy to push against the gravity of the star weakens. The gravity pushes harder, and this more powerful energy is enough to begin fusing helium, which takes slightly more energy than Helium. The star gets a new lease on life.

Finally, after several cycles of this fusion of heavier and heavier elements, the star is left with only iron in the core. Iron is so heavy it takes more energy to fuse it than you liberate from the process. So the gravity takes over and the star collapses. The pressure waves 'bounce' off the core and cause the upper layers (which are not iron) to explode in a supernova. Depending on the mass of the star, it eventually ends up as a dwarf, neutron star or black hole.

So iron to a star is a lot like celery - it takes more energy to digest than you get from eating it. If you lived on nothing but celery, you'd eventually starve, no matter how much of it you consumed. Same thing with a star and iron.