What is #43: Iron?

The last two elements discussed in this series were hydrogen and helium, the two most abundant elements in the Universe, and building blocks to the other elements through star formation and other energetic events in space. Iron is the 26th element on the periodic table but is interesting in that it is the last element that can be produced in the core of a star. Given there are more than 100 known elements, less than a quarter are produced through fusion.

In the core of stars, hydrogen fuses to helium for most of a star's life. Near the end, helium can fuse to carbon in low mass stars. In high mass stars, carbon and oxygen can fuse into heavier elements through several steps, all the way up to iron. Iron cannot fuse into a heavier element. Why not?

Iron has the lowest mass per nuclear particle, meaning it can't fuse into a heavier element. For fusion to release energy, the product must have a greater mass per nuclear particle, and that isn't possible if iron is the lowest.

So how do we get elements heavier than iron? Where do silver and gold come from? The heavier elements come from even more energetic events such as a supernova explosion. But even that may not be enough energy for the heavier elements. It's thought the collision of two neutron stars is needed to produce the heaviest elements.

Cool stuff!